A R T I C U L O T E C N I C O ,

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY A C R O S S THE K/T BOUNDARY FROM THE

GULF C O A S T A L PLAIN OF MEXICO: IMPLICATIONS FOR T IM ING THE EXTRATERRESTRIAL BOLIDE

IMPACT IN Y U C A T A N

J.F. LONGORIA and M.A. GAMPER Department of Geology, Florida International University,

University Park, Miami, Florida 33199

ABSTRACT

Planktonic foraminijera from the Arroyo El Mimbral section at the Tampico region, including the ' 'K/T boundary clastic unit" are indicative of Biozone K27 (middle Maastrichtian). Samples taken immediately below and above the K/T boundary clastic unit lack A. mayaroensis which is the planktic index for the uppermost Maastrichtian. Foraminiferal fauna in this calcarenite unit includes only taxa observed in the underlying Méndez. If the physical stratigraphy (e.g., tektite layer, shock glass, iridium enrichment) of El Mimbral section is proven to be directly associated with an extraterrestrial impact on Yucatán, our biochronologic results from this section do not support the idea that this impact occurred precisely "at the K/T boundary", but rather in the middle Maastrichtian, before the chronostratigraphic ally defined K/T boundary. The anomalous iridium abun­dance occurs between samples EM30—20 and EM31D—8 assignable to Biozone K27, 36 cm below the P. eugubina—FAD (First Appearance Datum) On the other hand, the base of the spherule bed is 2.60 m below the K/T boundary. The uppermost Maastrichtian K 29 biozone of the Gulf Coastal Plain of Mexico is further divided into K29Aj, K29A2, K29Bj, K29B2 and K29Bj based on the following planktonic foraminiferal biohorizons: 1) A. mayaroensis—FAD, 2) small globigerinas— FAD, 3) A. mayaroensis-LAD (Last Appearance Datum), 4) "G. " fringa-FAD, 5) "G." minutula-FAD; and 6) P. eugubina—FAD. Inconsistences in planktonic foraminifera taxonomy, and discrepancies in biostratigraphic practice across the K/T boundary sections as applied by previous authors to different regions impede precise biocorrelation of the Mexican data with other sections such as El Kef in northern Africa. Indeed, the aforementionedfactors may have led to a diachronous definition of the chronostratigraphic K/T boundary.

R E S U M E N

Los foraminiferos planctónicos encontrados en la sección del Arroyo El Mimbral en la región de Tampico, incluyendo la llamada unidad clástica del límite Cretácico/Terciario, son indicativos de la Biozona K27 (Maastrichtiense medio). Las muestras tomadas inmediatamente abajo y arriba de esa unidad clástica carecen de A. mayaroensis que es la especie planctónica índice del Maastrichtiense más alto. La fauna de planctónicos de esta unidad clástica incluye únicamente los taxa observados en la infrayaciente Formación Méndez. Si la estratigrafía física (e.g., capa de tectitas, vidrio de impacto, enriquecimiento de Iridio) de la sección de El Mimbral se llega a comprobar que está directamente asociada con un impacto extraterrestre en Yucatán, entonces, nuestros resultados biocronológicos derivados de esta sección no apoyan la idea de que este impacto haya ocurrido precisamente en el límite Cretácico/Terciario, sino más bien en el Maastrichtiense medio, antes del límite cronoestratigráfico. En El Mimbral,

BOL. AMPG, VOL. XLII, NUM. 2, 1992, p. 19-40

20 LONGORIA AND GAMPER

el contenido anómalo del Iridio ocurre entre las muestras ЕМЗО—20y la EM31D—8; este intervalo es asignable a la Biozona K27 y yace 36 cm abajo del límite dado por la primera aparición de P. eugubina. Por otra parte, la base de la capa de esferuli-

tas se localiza a 2.60 m abajo del límite cronoestratigráfico. La biozona más alta del Maastrichtiense de la Planicie Costera

del Golfo es subdivisible en cinco subzonas, ellas son: K29Aj, K29A2, K29Bj, K29B2, K29B^, las cuales están basadas

en los biohorizontes siguientes: 1) primera aparición de A. mayaroensis, 2) primera aparición de las globigerinas pequeñas,

3) última aparición de A. mayaroensis, 4) primera aparición de "G. "fringa, 5) primera aparición de "G. " minutula,

6) primera aparición de P. eugubina. Las inconsistencias en la taxonomía de los foraminiferos planctónicos, por una parte,

y las discrepancias en la forma de practicar la bioestratigrafía por parte de otros autores, impide que podamos hacer la biocorrela-

ción de los datos mexicanos con otras secciones, tales como la de El Kef en Africa del Norte. En efecto, los factores arriba señala­

dos han conducido a una definición diacrònica del límite cronoestratigráfico del Cretácico/Terciario.

INTRODUCTION

P l a n k t o n i c f o r a m i n i f e r a a r e c l a s s i c a l l y u s e d t o d e ­

l i n e a t e t h e M a a s t r i c h t i a n / D a n i a n b o u n d a r y . I n t h e

a b s e n c e o f a n a c c e p t e d c h r o n o s t r a t i g r a p h i c b o u n d a r y

( i . e . , a c c e p t e d b y a n I n t e r n a t i o n a l G e o l o g i c a l C o n ­

g r e s s ) t h e Parvularugoglobigerina eugubina—FAD h a s

b e e n c u s t o m a r y a c c e p t e d a s t h e b a s e o f t h e T e r t i a r y

( L u t e r b a c h e r a n d P r e m o l i S i l v a , 1 9 6 4 ; B o U i , 1 9 6 6 ;

P r e m o l i S i l v a a n d B o l l i , 1 9 7 3 ; P r e m o H S i l v a , 1 9 7 7 ;

C a m p e r , 1 9 7 7 ; L o n g o r i a , 1 9 7 7 ; 1 9 8 4 ; Alvarez et al.,

1980; T o u m a r k i n e a n d L u t e r b a c h e r , 1 9 8 5 ; B e r g g r e n

et al., 1 9 8 5 ) . A m a r k e d c h a n g e i n f o r a m i n i f e r a l f a u ­

n a is k n o w n t o o c c u r at t h e e n d o f t h e C r e t a c e o u s

w h e r e b y d i v e r s i f i e d , l a r g e — s i z e d ( a v e r a g e 8 0 0

m i c r o n s ) M a a s t r i c h t i a n H e d b e r g e l l a c e a s e n s u L o n ­

g o r i a a n d G a m p e r ( 1 9 7 5 ) a r e r e p l a c e d b y l e s s d i v e r ­

s i f i e d , m i n u t e ( a v e r a g e s i z e 8 0 m i c r o n s ) f o r m s c o l l e c ­

t i v e l y k n o w n i n f o r m a l l y as " s m a l l g l o b i g e r i n a s " . T h i s

d r a m a t i c c h a n g e in p l a n k t o n i c f o r a m i n i f e r a l f a u n a s

w a s first r e p o r t e d b y L u t e r b a c h e r a n d P r e m o l i S i l v a

( 1 9 6 4 ) f r o m t h e G u b b i o s e c t i o n o f I t a l y . S u b s e q u e n t ­

l y , s e v e r a l a u t h o r s h a v e r e p o r t e d a s i m i l a r s u c c e s s i o n

f r o m o t h e r r e g i o n s ( G a m p e r , 1 9 7 7 ; S m i t , 1 9 7 7 ; 1 9 8 2 ;

Merrn et al., 1 9 8 1 ; S m i t a n d R o m e i n , 1 9 8 5 ) ; n o n e t h e ­

l e s s , t h e r e is a g e n e r a l c o n s e n s u s a m o n g f o r a m i n i f e r ­

al w o r k e r s t o c o n s i d e r t h e p r e s e n c e o f Abathomphalus

mayaroensis t o c o i n c i d e w i t h t h e h i g h e s t M a a s t r i c h t i ­

a n ( B o l l i , 1 9 5 7 ; P e s s a g n o , 1 9 6 7 ; L o n g o r i a , 1 9 8 4 )

w h e r e a s t h e P. eugubina—FAD a s m a r k i n g t h e b a s e o f

t h e D a n i a n s t a g e . G a m p e r ( 1 9 7 7 ) first n o t e d t h a t a

t i m e i n t e r v a l b e t w e e n b o t h b i o h o r i z o n s w a s i g n o r e d

in m o s t b i o s t r a t i g r a p h i c z o n a t i o n s s i n c e t h e u p p e r m o s t

M a a s t r i c h t i a n b i o z o n e , the A. mayaroensis b i o z o n e , w a s

r e g a r d e d a s a t a x o n — r a n g e — z o n e a n d t h e l o w e r

b o u n d a r y o f t h e P. eugubina b i o z o n e w a s d e f i n e d b y

t h e first a p p e a r a n c e d a t u m ( F A D ) o f t h e n o m i n a l t a x -

o n . S h e ( G a m p e r , 1 9 7 7 ) p r o p o s e d the A. mayaroensis/P.

eugubina I n t e r v a d — Z o n e ( s e n s u I n t e r n a t i o n a l S u b -

c o m m i s s i o n o n S t r a t i g r a p h i e C l a s s i f i c a t i o n , 1 9 7 6 ) t o

i n c l u d e t h i s i n t e r v a l o f t i m e w i t h i n t h e u p p e r m o s t C r e ­

t a c e o u s ( F i g . 1 ) . U s u a l l y , t h i s i n t e r v a l i s r e p r e s e n t e d

b y a f e w m e t e r s o f s e d i m e n t s , t h e m a x i m u m t h i c k ­

n e s s i s a b o u t 3 m .

I n a d d i t i o n , G a m p e r ( 1 9 7 7 ) d e f i n e d five b i o ­

h o r i z o n s i n t h e M é n d e z S h a l e o f e a s t e r n M e x i c o , f r o m

o l d e r t o y o u n g e r : 1 ) t h e A. mayaroensis—Y AD, 2 ) t h e

s m a l l g l o b i g e r i n a s — F A D , 3 ) t h e A. mayaroensis—

L A D , 4 ) t h e P. eugubina-YAD, a n d 5 ) t h e G.

daubjergensis—F AD. T h u s , s h e d e m o n s t r a t e d t h e

p r e s e n c e o f a s h o r t i n t e r v a l s e p a r a t i n g t h e A.

mayaroensis—L,AD f r o m t h e P. eugubina—FAD. F u r ­

t h e r m o r e , t h e o v e r l a p o f A. mayaroensis a n d t h e s m a l l

g l o b i g e r i n a s w a s a l s o r e c o r d e d . T h i s i n t e r v a l w a s l a t e r

r e f e r r e d t o a s t h e A. mayaroensis/P. eugubina I n t e r v a l -

Z o n e ( B i o z o n e K 2 9 s e n s u L o n g o r i a , 1 9 8 4 ) c o r ­

r e s p o n d i n g t o t h e h i g h e s t M a a s t r i c h t i a n .

T h e l o w e r p a r t o f ^ . mayaroensis/P. eugubina b i o z o n e

( s u b z o n e K 2 9 A ) c o n t a i n s a n a b u n d a n t a n d d i v e r s i ­

fied p l a n k t o n i c f o r a m i n i f e r a l a s s e m b l a g e t y p i c a l l y i n ­

c l u d i n g m o r e t h a n 3 5 s p e c i e s ( L o n g o r i a a n d G a m p e r ,

1 9 7 4 ) ; h o w e v e r , a s e r i e s o f b i o l o g i c a l c h a n g e s t o o k

p l a c e d u r i n g t h i s i n t e r v a l l e a d i n g t o t h e s u b s t i t u t i o n

o f t h e l a r g e r t a x a b y s m a l l — s i z e d f a u n a i n s u b z o n e

K 2 9 B r e s u l t i n g in a p r o b l e m a t i c d e l i n e a t i o n o f t h e C r e ­

t a c e o u s / T e r t i a r y b o u n d a r y . U p t o d a t e t h e r e h a s n o t

b e e n a c h r o n o s t r a t i g r a p h i c a l l y a c c e p t e d d e f i n i t i o n o f

t h e M a a s t r i c h t i a n / D a n i a n b o u n d a r y .

T h e a b r u p t c h a n g e i n f o r a m i n i f e r a l f a u n a s b e t w e e n

t h e u p p e r m o s t M a a s t r i c h t i a n a n d t h e l o w e r m o s t D a n ­

i a n is h e r e i n r e f e r r e d t o a s t h e K / T b o u n d a r y p l a n k t i c

f o r a m i n i f e r a l b i o e v e n t ( F i g . 1 ) . I t h a s b e e n a t t r i b u t e d

t o a c a t a s t r o p h i c e v e n t o f t h e e n v i r o n m e n t . B o t h ter ­

r e s t r i a l ( G a r t n e r a n d K e a n y , 1 9 7 8 ; T h i e r s t e i n a n d

B e r g e r , 1 9 7 8 ) a n d e x t r a t e r r e s t r i a l ( G a n a p a t h y , 1 9 8 0 ;

A l v a r e z et al., 1 9 8 0 ; A l v a r e z et al., 1 9 9 2 ) c a u s e s h a v e

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY ACROSS THE K/T BOUNDARY 21

b e e n i n v o k e d t o e x p l a i n t h e a b r u p t c h a n g e in f a u n a . N o n e t h e l e s s , t h e t h e o r y that a n e x t r a t e r r e s t r i a l b o l i d e i m p a c t e d o n t h e E a r t h a b o u t 6 5 m y a g o h a s r e c e i v e d c o n s i d e r a b l e a c c e p t a n c e in r e c e n t y e a r s . T h e i m p a c t site is still u n d e r d e b a t e ; h o w e v e r , r e c e n t g e o p h y s i ­cal w o r k in the n o r t h e r n c o a s t o f Y u c a t á n t o g e t h e r w i t h c i r c u m s t a n t i a l e v i d e n c e i n d i c a t e that the C h i c x u l u b cir­cu lar s t r u c t u r e m a y b e t h e b e s t c a n d i d a t e for the i m ­pact s i te ( P e n f i e l d a n d C a m a r g o , 1 9 8 1 ; H i l d e b r a n d et a/., 1 9 9 1 ; P o p e i ^ a / . , 1 9 9 1 ) . A C a r i b b e a n s i te h a s a l so b e e n a d v o c a t e d b y M a u r r a s s e a n d S e n ( 1 9 9 1 ) .

Datum Planes

Parvularugoglobigerina eugubina-FAD

pV. mayaroensis/P. eugubina IntervaJ-Zone (sensu Gamper, 1977)

A. mayaroensis-LAD Fig. 1.— Biohorizons defining the K/T boundary planktic

foraminiferal bioevent. This interval corresponds to the time through which Hedbergellacea sensu Longoria and Gamper (1975) underwent a rapid biological transformation herein linked to neoteny.

S m i t et al. ( 1 9 9 2 ) s t u d i e d t h e A r r o y o E l M i m b r a l s e c t i o n in S i e r r a d e T a m a u l i p a s o r i g i n a l l y d e s c r i b e d b y M u i r ( 1 9 3 6 , p . 8 3 ) a n d l a t e r b y H a y ( 1 9 6 0 , p . 7 1 ) as r e p r e s e n t i n g a n u n c o n f o r m a b l e c o n t a c t b e t w e e n the M é n d e z S h a l e ( b e l o w ) a n d t h e V e l a s c o S h a l e ( a b o v e ) . A c c o r d i n g t o S m i t et al. ( 1 9 9 2 , p . 9 9 ) "The marls were found to be unsuitable for determining magneto stratigraphy,

but foraminiferal biostratigraphy places the clastic unit precisely at the K/T boundary. We interpret this clastic unit as the deposit of a megawave or tsunami produced by an extra­terrestrial impact". T h e y o n l y c i t e d t w o t a x a , G. con­tusa a n d R. fructicosa, a s d i a g n o s t i c for the ,̂ 4. mayaroensis Z o n e l e a d i n g to b e l i e v e that t h e i r d e f i n i t i o n o f the K / T b o u n d a r y w a s m e r e l y b a s e d o n t h e p r e s e n c e o f t h e " c l a s t i c u n i t " . S m i t f í a / . ( 1 9 9 2 , p . 1 0 2 ) c l a i m e d t h a t t h e p r e s e n c e o f g l a s s y t e k t i t e s , s h o c k e d m i n e r a l s a n d a n o m a l o u s i r i d i u m a b u n d a n c e at the i n t e r v a l b e t w e e n t y p i c a l M é n d e z a n d V e l a s c o s u p p o r t s t h e h y p o t h e s i s o f a K / T i m p a c t o n n e a r b y Y u c a t á n . T h i s h y p o t h e s i s is c o n t r o v e r s i a l a n d h a s r e c e i v e d v i v i d a t t a c k s b y Of ­f i cer ( 1 9 9 2 ) , w h o a r g u e s for a t e r r e s t r i a l o r i g i n o f t h e a f o r e m e n t i o n e d f e a t u r e s . T h u s , t w o c o n t r a s t i n g v i e w s o f t h e o r i g i n o f the p h y s i c a l f e a t u r e s a n d t w o c o n ­t r a s t i n g b i o c h r o n o l o g i e s ( c o m p l e t e v s i n c o m p l e t e ) o f t h e El M i m b r a l s e c t i o n h a v e d e v e l o p e d .

It is o u r g o a l to d o c u m e n t t h e p l a n k t o n i c f o r a m i ­n i f e r a l b i o c h r o n o l o g y at A r r o y o El M i m b r a l a n d d i s ­c u s s its s i g n i f i c a n c e o n t h e t i m i n g o f t h e b o l i d e i m ­p a c t . H o w e v e r , b e f o r e f o c u s i n g o n A r r o y o El M i m ­b r a l , w e w i l l v e n t i l a t e s o m e p r o b l e m s r e l a t e d to t h e b i o s t r a t i g r a p h y , t a x o n o m y a n d e v o l u t i o n a r y t r e n d s o f p l a n k t o n i c f o r a m i n i f e r a t h a t , i n o u r v i e w , c o n t i n u e t o h a m p e r t h e d e f i n i t i o n o f the K / T b o u n d a r y . F i n a l l y , w e w i l l d i s c u s s s o m e o f t h e p r o b l e m s c o n c e r n i n g t h e n a t u r e o f f a u n a l c h a n g e s o b s e r v e d in p l a n k t o n i c f o r a m i n i f e r a a c r o s s t h e K / T b o u n d a r y .

BIOCHRONOLOGY OF THE CRETACEOUS/TERTIARY BOUNDARY

S e v e r a l a s p e c t s o f p l a n k t o n i c f o r a m i n i f e r a , i n ­c l u d i n g t h e t a x o n o m y o f i n d e x t a x a a n d t h e i r e v o l u ­t i o n a r y t r e n d s b e f o r e a n d a f t e r t h e p o s t u l a t e d K / T b o u n d a r y , h a v e i n f l u e n c e d t h e b i o c h r o n o l o g y a n d u l t i m a t e l y t h e d e l i n e a t i o n o f t h e b o u n d a r y . T h e s e a s p e c t s a r e d i s c u s s e d in l i g h t o f t h e i r r o l e in p l a c i n g t h e c h r o n o s t r a t i g r a p h y o f t h e K / T b o u n d a r y .

T a x o n o m y

T w o c o n t r a s t i n g f a u n a l a s s e m b l a g e s a r e k n o w n t o o c c u r in B i o z o n e K 2 9 o f M e x i c o : A s s e m b l a g e I c o n ­s i s t s o f a b o u t 3 5 s p e c i e s o f r o b u s t ( l a r g e s i z e , a v e r a g e 8 0 0 m i c r o n s ) p l a n k t o n i c f o r a m i n i f e r a . M o s t o f t h e s e t a x a h a v e b e e n d e s c r i b e d a n d i l l u s t r a t e d in n u m e r o u s p u b l i c a t i o n s t h r o u g h w h i c h t h e i r t a x o n o m i c s t a -

22 LONGORIA AND GAMPER

b i l i t y h a s b e e n e s t a b l i s h e d ( L o n g o r i a a n d G a m p e r , 1 9 7 4 ) . I n c o n t r a s t . A s s e m b l a g e II c o n s i s t i n g o f t h e " s m a l l g l o b i g e r i n a s " is p o o r l y k n o w n . A s s t a t e d a-b o v e , t h e t e r m " s m a l l g l o b i g e r i n a s " is a n i n f o r m a l , d e s c r i p t i v e t e r m , a n d it is e m p l o y e d h e r e in a s i m i l a r w a y a s " l a r g e r f o r a m i n i f e r a " is u s e d t o i m p l y a n artif i­c i a l g r o u p i n g o f t a x a s h o w i n g a b o u t t h e s a m e s i z e ; h o w e v e r , it i s e v i d e n t t h a t t h e y a r e n o t d i r e c t l y i n t e r ­r e l a t e d . T h e t e r m " s m a l l g l o b i g e r i n a s " a p p l i e s t o t h e b i o l o g i c a l p h e n o m e n o n o f s i z e r e d u c t i o n o f p l a n k t i c s at t h e e n d o f t h e C r e t a c e o u s a n d w i l l b e u s e d a s s u c h t h r o u g h o u t t h e t e x t .

A m a j o r f a c t o r l i m i t i n g t h e t a x o n o m i c s t a b i l i t y a s w e l l a s t h e s t r a t i g r a p h i c p o s i t i o n a n d e v o l u t i o n a r y t r e n d s o f t h i s g r o u p o f s m a l l g l o b i g e r i n a s h a s b e e n t h e l a c k o f a c a r e f u l a n a l y s i s o f t y p e s p e c i m e n s a n d t h e s t u d y o f c o m p l e t e a n d u n d i s t u r b e d s e c t i o n s ( T o u m a r ­k i n e a n d L u t e r b a c h e r , 1 9 8 5 ) . L e s s t h a n a — d o z e n t a ­x a h a v e b e e n d e s c r i b e d f r o m t h e s m a l l g l o b i g e r i n a s a s s e m b l a g e i n c l u d i n g : "G. "globigerinelloides, "G. "eugu­bina, "G."sabina, "G. " umbrica, "G." aconitana, "G."

Jringa, "G. " minutula, a n d "G. " longiapertura; h o w ­e v e r , m o s t o f t h e m a r e incertae sedis. T h e m a j o r i t y o f t h e a u t h o r s l i m i t t h e i r s t u d y t o t a x o n o m i c n o t e s o n o n e o f t w o o f t h e s e s p e c i e s t h a t t h e y c o n s i d e r " r e a d ­i ly r e c o g n i z a b l e a n d s t r a t i g r a p h i c a l l y u s e f u l s p e c i e s " . U n f o r t u n a t e l y , e v e n s p e c i e s s u c h a s "Globigerina" eugubina w h i c h f i ts t h e a b o v e c r i t e r i a h a s b e e n m i s s -i d e n t i f i e d a n d it h a s b e e n g i v e n a b r o a d m o r p h o l o g i c v a r i a t i o n . F o r e x a m p l e , in c o n t r a s t w i t h t h e s i x s p e ­c i e s o f t h e s m a l l g l o b i g e r i n a s r e c o g n i z e d b y L u t e r b a ­c h e r a n d P r e m o l i S i l v a ( 1 9 6 4 ) , o n l y o n e w a s r e g a r d ­e d a s v a l i d b y S m i t ( 1 9 8 2 ) ; h o w e v e r , t h e s p e c i m e n s i l l u s t r a t e d b y S m i t ( 1 9 8 2 , p l a t e s 1, 2 ) c o n t r a s t w i t h t h e m o r p h o t y p e s i l l u s t r a t e d b y L u t e r b a c h e r a n d P r e ­m o l i S i l v a ( 1 9 6 4 , p l a t e 1, f i g . 8 a — c ) t o t h e e x t r e m e t h a t it i s h a r d t o k n o w if i n d e e d al l t h o s e m o r p h o ­t y p e s i l l u s t r a t e d b y S m i t r e p r e s e n t t h e s a m e s p e c i e s . T h e r e c e n t p a p e r b y O l s s o n et al. ( 1 9 9 2 ) h a s g r e a t l y c o n t r i b u t e d t o t h e t a x o n o m i c s t a b i l i t y o f m a n y l o w e r D a n i a n s p e c i e s s h o w i n g t h a t i n d e e d l o w e r D a n i a n t a x a d i s p l a y d i a g n o s t i c w a l l s t r u c t u r e s u n i q u e t o t h i s g r o u p o f p l a n k t i c s . A c o m p a r i s o n o f t h e f i g u r e d s p e c i m e n s b y O l s s o n et al. ( 1 9 9 2 ) w i t h t h e s p e c i m e n s h e r e i n i l l u s ­t r a t e d i n f i g u r e 2 r e a d i l y s h o w s t h e d i s t i n c t i o n .

The Evolutionary Trends of Planktonic Foraminifera

D u r i n g t h e g l o b o t r u n c a n i d e v o l u t i o n a r y e p i s o d e

( C a m p a n i a n — M a a s t r i c h t i a n ) , t h e H e d b e r g e l l a c e a

( = H e d b e r g e l l o i d e a s e n s u L o n g o r i a a n d G a m p e r , 1 9 7 5 ) r e a c h e d a d i v e r s i t y p e a k w h e r e b y t r o c h o i d a l , p l a n i s p i r a l a n d s e r i a l t a x a w e r e h i g h l y d i v e r s i f i e d ( L o n g o r i a a n d G a m p e r , 1 9 7 5 ) . T r o c h o i d a l t a x a d e ­v e l o p e d s p e c i a l i z e d u m b i l i c a l s t r u c t u r e s s u c h a s t e g i -11a a n d t e c h o s w h i c h a r e l a c k i n g i n T e r t i a r y f o r m s . T h e s e s t r u c t u r e s t o g e t h e r w i t h t h e w a l l p o r e p a t t e r n a r e r e l i a b l e p h y l o g e n e t i c c h a r a c t e r s ( L o n g o r i a a n d G a m p e r , 1 9 7 5 ; H e m l e b e n , 1 9 9 0 ; O l s s o n f< a / . , 1 9 9 2 ) t h a t r e l a t e t h e p l a n k t i c s p e c i e s f r o m t h e K / T b o u n d ­a r y t r a n s i t i o n p r e — / * . eugubina F A D t o t h e i r C r e t a ­c e o u s a n c e s t o r s ( F i g . 2 ) .

P l a n k t o n i c f o r a m i n i f e r a p r o l i f e r a t e d i n M a a s t r i c h t ­i a n o p e n m a r i n e f a c i e s d i s p l a y i n g a m u l t i t u d e o f b i o ­l o g i c a l e x p e r i m e n t a t i o n s . F o r e x a m p l e , 1 ) a b a t h o m -p h a l i d s e v o l v e d f r o m g l o b o t r u n c a n e l l a s ; 2 ) r u g o g l o -b i g e r i n i d s r e a c h e d a m a s s i v e d i v e r s i f i c a t i o n ; 3 ) s e r i a l f o r m s d e v e l o p e d u n i q u e c o m p l e x m u l t i s e r i a l t e s t s ; 4 ) h e l v e t i e l l i d s a p p e a r e d , a n d 5 ) s m a l l g l o b i g e r i n a s d e ­v e l o p e d . A s a w h o l e , al l t h e s e f e a t u r e s i m p l y t h a t p l a n k t i c s , a s a g r o u p , w e r e e x p e r i e n c i n g a m a j o r b i o ­l o g i c a l c h a n g e b y t h e e n d o f t h e M a a s t r i c h t i a n . T h e a p p e a r a n c e o f t h e s m a l l g l o b i g e r i n a s is r e f e r r e d t o a s t h e p l a n k t i c b i o e v e n t ; a s s t a t e d a b o v e , t h i s b i o l o g i c a l t r a n s f o r m a t i o n h a s b e e n t i e d t o a c a t a s t r o p h i c e v e n t o f t h e e n v i r o n m e n t c a u s e d b y a n e x t r a t e r r e s t r i a l i m ­p a c t o n E a r t h ( A l v a r e z et al., 1 9 8 0 ; 1 9 9 2 ) . H o w e v e r , e v i d e n c e f r o m t h e A. mayaroensis/P. eugubina I n t e r v a l — Z o n e o f M e x i c o i n d i c a t e s t h a t s o m e o f t h e s m a l l g l o b i g e r i n a s a p p e a r b e f o r e t h e " m a s s i v e e x t i n c ­t i o n " o f l a r g e r p l a n k t i c f o r a m i n i f e r a o f A s s e m b l a g e I ( G a m p e r , 1 9 7 7 , p . 2 5 , f ig . 2 ; L o n g o r i a , 1 9 7 7 ) . S i m ­i l a r o v e r l a p s b e l o w t h e K / T b o u n d a r y h a v e b e e n r e p o r t e d b y K e l l e r ( 1 9 8 8 a , b ; 1 9 8 9 ) . C o n s e q u e n t l y , t h e d i r e c t c o n n e c t i o n b e t w e e n t h e b i o l o g i c a l t r a n s f o r ­m a t i o n e x p e r i m e n t e d b y p l a n k t i c s a t t h e e n d o f t h e M a a s t r i c h t i a n a n d t h e b o l i d e i m p a c t n e e d s t o b e p r o v ­e n .

A b u n d a n t s m a l l g l o b i g e r i n a s w e r e f o u n d i n s a m ­p l e s c o n t a i n i n g Abathomphalus mayaroensis ( L o n g o r i a , 1 9 8 4 ) ; h o w e v e r , a s s t a t e d a b o v e , t h e l a c k o f d e t a i l m o r p h o l o g i c s t u d i e s i m p e d e s a t t h i s t i m e t o k n o w if i n d e e d t h e s m a l l g l o b i g e r i n a s c o n c u r r i n g w i t h A. ma­

yaroensis a r e a t al l r e l a t e d , o r e v e n t h e s a m e , a s t h o s e f o u n d a f t e r t h e A. mayaroensis L A D . M o s t o f t h e s e ta­x a i d e n t i f i e d a r e incertae sedis; h o w e v e r , t h e y d i s p l a y u m b i l i c a l s t r u c t u r e s ( F i g . 4 ) c o n s i s t i n g o f t e c h o a n d t e g i l l u m s i m i l a r t o t h o s e f o u n d i n a b a t h o m p h a l i d s a n d g l o b o t r u n c a n i d s , a l l o w i n g u s t o p o s t u l a t e t h a t t h e y are p h y l o g e n e t i c a l l y r e l a t e d t o C r e t a c e o u s f a u n a s . T h u s ,

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY ACROSS THE K/T BOUNDARY 23

their e x i s t e n c e d o n o t s u p p o r t t h e t h e o r y o f a c a t a ­

strophic m a s s i v e e x t i n c t i o n b u t r a t h e r a g r a d u a l ? —

biological t r a n s f o r m a t i o n , l i k e l y n e o t e n y ( G a m p e r —

Longoria a n d L o n g o r i a , 1 9 8 4 ) . A c c o r d i n g t o t h e n e o t ­

eny h y p o t h e s i s , p l a n k t i c f o r a m i n i f e r a a t t a i n e d s e x u a l

matur i ty in a y o u t h f u l ( j u v e n i l e ) s t a g e . W e e n v i s i o n

that m o r e t h a t o n e g r o u p o f M a a s t r i c h t i a n H e d b e r ­

ge l lacea u n d e r w e n t n e o t e n y at t h e e n d o f t h e C r e t a ­

ceous; t h u s b e c a m e t h e a n c e s t o r s o f t h e n e w T e r t i a r y

stocks ( F i g . 3 ) .

T h e e v o l u t i o n a r y t r e n d s o f ser ia l p l a n k t o n i c f o r a m i ­

nifera ( H e t e r o h e l i x o i d e a s e n s u L o n g o r i a a n d G a m ­

per, 1 9 7 5 ) is n o t w e l l k n o w n i n s p i t e o f s e v e r a l a t ­

tempts ( B r o w n , 1 9 6 9 ; M a r t i n , 1 9 7 2 ; N a s h , 1 9 8 1 ; N e -

derbrast , 1 9 9 1 ) . I n a d d i t i o n , h e t e r o h e l i c i d s i n c l u d i n g

G. cretacea s e e m t o b e f a c i e s — c o n t r o l l e d , t h u s t h e y a re

rarely u s e d a s i n d e x t a x a i n b i o c h r o n o l o g i c s c h e m e s .

It is u n f o r t u n a t e t h a t s o m e a u t h o r s ( S m i t , 1 9 8 2 ; K e l ­

ler, 1988) h a v e u s e d t h e p r e s e n c e o f G. cretacea as a n o m -

minal t a x o n for t h e s o c a l l e d PO b i o z o n e b e c a u s e it h a s

led to a n i m p r e c i s e d e f i n i t i o n o f t h e K / T b o u n d a r y

in s e c t i o n s c o n t a i n i n g t h e a f o r e m e n t i o n e d s p e c i e s .

The Planktic Foraminiferal Bioevent

T h e C r e t a c e o u s / T e r t i a r y b o u n d a r y p l a n k t o n i c fo ­

ramin i f era l b i o e v e n t r e p r e s e n t s a n i n t e r v a l o f t i m e

m a r k e d b y a b r u p t c h a n g e s i n p l a n k t i c f a u n a . M a n y

of the b i o l o g i c a l t r a n s f o r m a t i o n s t a k i n g p l a c e d u r i n g

this b i o e v e n t r e m a i n u n c l e a r . A n u n e q u i v o c a l o b s e r ­

v a t i o n is t h a t d u r i n g t h i s e v e n t , l a r g e — s i z e d p l a n k ­

tic t a x a are r e p l a c e d b y smadl g l o b i g e r i n a s . N o n e t h e ­

less , the n a t u r e o f t h i s f a u n a l c h a n g e is c o n t r o v e r s i a l

(Ke l l er , 1 9 8 8 a, b ; 1 9 8 9 ) . H o w e v e r , t h e e v i d e n c e o b ­

tained f r o m t h e s t u d y o f t h e u m b i l i c a l s t r u c t u r e s ( F i g .

4) clearly i n d i c a t e s t h a t t h e s m a l l g l o b i g e r i n a s are n e o t -

enic f o r m s a n d n o t r e w o r k e d o r c o n t a m i n a t e d a s s o m e

authors i m p l y . I n a d d i t i o n , o u r s t u d i e s f r o m t h e M é n ­

dez S h a l e o f M e x i c o ( G a m p e r , 1 9 7 7 ; L o n g o r i a , 1 9 8 4 )

s e e m to i n d i c a t e t h a t t h e s m a l l g l o b i g e r i n a s a p p e a r

before t h e t o t a l d i s a p p e a r a n c e o f t h e l a r g e — s i z e d

fauna . S e v e r a d w o r k i n g h y p o t h e s e s h a v e b e e n a d ­

v a n c e d t o a c c o u n t for t h i s f o r a m i n i f e r a l c h a n g e . F o r

e x a m p l e , B e r g g r e n ( 1 9 6 0 ) c o n s i d e r e d t h a t t h e s m a l l

forms w e r e j u v e n i l e f o r m s o f t h e C r e t a c e o u s p l a n k ­

tics; B l o w ( 1 9 7 9 ) r e g a r d e d t h e m a s i m p o v e r i s h e d C r e ­

t a c e o u s f a u n a s ; S m i t ( 1 9 8 2 ) a n d S m i t a n d R o m e i n

( 1 9 8 5 ) c o n s i d e r e d a m a s s e x t i n c t i o n o f t h e m a i n C r e ­

t a c e o u s f o r m s a n d t h e s u r v i v a l o f o n e s p e c i e s —G.

cretacea— a s t h e a n c e s t o r o f all T e r t i a r y g l o b i g e r i n i d s

( S m i t , 1 9 8 2 , p . 3 3 5 ) . S m i t ' s t h e o r y o n t h e o r i g i n o f

T e r t i a r y p l a n k t i c s c o n t r a s t s w i t h p r e v i o u s s c h e m e s :

B e r g g r e n ( 1 9 6 5 ) c o n s i d e r e d t w o c l a d e s in t h e r a d i a ­

t i o n o f P a d e o g e n e t a x a . G a m p e r — L o n g o r i a a n d L o n ­

g o r i a ( 1 9 8 4 ) c o n s i d e r e d that the s m a l l g l o b i g e r i n a s r e p ­

r e s e n t a n e o t e n i c t r a n s f o r m a t i o n o f t h e l a r g e — s i z e d

t a x a , t h u s s e v e r a l c l a d e s w e r e c o n s i d e r e d a s t h e basad s t o c k o f t h e P a l e o g e n e r a d i a t i o n s ( F i g . 3 ) . M o r e r e ­

c e n t l y , H e m l e b e n ( 1 9 9 0 ) a n d O l s s o n et al. ( 1 9 9 2 ) a d ­

v a n c e d t h e i d e a t h a t t h e w a l l s t r u c t u r e o f l o w e r m o s t

P a l e o g e n e p l a n k t i c s i n d i c a t e s t h a t t h e y o r i g i n a t e d f r o m

a " h e d b e r g e l l o i d " c o m m o n a n c e s t o r . It is o u r o p i n ­

i o n t h a t t h i s " h e d b e r g e l l o i d " a c t u a l l y r e p r e s e n t s a

r e c a p i t u l a t i o n o f t h e a n c e s t r a l m o r p h o t y p e s b u t i n ­

d e e d "Hedbergella" monmouthensis a n d " / / . " holmdelemis

b e l o n g i n t o a s e p a r a t e g e n u s w h i c h is d e s c r i b e d b e ­

l o w . F u r t h e r m o r e , w e c o n s i d e r e d t h a t l o w e r D a n i a n

p l a n k t i c s a r e a p o l y p h y l e t i c g r o u p o r i g i n a t i n g f r o m

d i f f e r e n t C r e t a c e o u s a n c e s t o r s . A t l e a s t t h r e e d i f f er ­

e n t l i n e a g e s a r e r e c o g n i z e d ( F i g . 3 ) .

P r o p o n e n t s o f t h e e x t r a t e r r e s t r i a l h y p o t h e s i s ( A l ­

v a r e z et al., 1 9 8 0 , 1 9 8 4 ; S m i t , 1 9 8 2 ; S m i t a n d R o ­

m e i n , 1 9 8 5 ; A l v a r e z el al., 1 9 9 2 ) ins i s t in r e l a t i n g t h i s

p h e n o m e n o n to t h e b i o l o g i c a l c h a n g e in s p i t e o f s e ­

r i o u s o p p o s i t i o n b a s e d o n c h e m i c a l a n a l y s i s o f b o u n d ­

a r y s e c t i o n s ( R a m p i n o , 1 9 8 2 ) . A t t h i s p o i n t , w e d o

n o t d i s p u t e t h e s i g n i f i c a n c e o f t h e p h y s i c a l e v i d e n c e

— s h o c k e d m i n e r a l s , i r i d i u m a n o m a l y , d i s t u r b e d

b e d s — p r e s e n t e d b y t h o s e a u t h o r s in f a v o r o f a n e x ­

t r a t e r r e s t r i a l b o l i d e i m p a c t o n E a r t h in t h e s a m e s e c ­

t i o n s c o n t a i n i n g t h e K / T b o u n d a r y b i o e v e n t ; h o w ­

e v e r , t h e s y n c h r o n e i t y o f t h e p l a n k t i c b i o e v e n t a n d

t h e p o s t u l a t e d i m p a c t wi l l b e d i s c u s s e d in l i g h t o f o u r

b i o c h r o n o l o g i c a l d a t a f r o m d i s t u r b e d a n d u n d i s t u r b ­

e d s e c t i o n s t h r o u g h o u t M e x i c o . S p e c i a l e m p h a s i s is

h e r e i n g i v e n to the A r r o y o El M i m b r a l s e c t i o n r e c e n d y

d e s c r i b e d b y S m i t et al. ( 1 9 9 2 ) a s c o n t a i n i n g a c o m ­

p l e t e K / T b o u n d a r y b i o c h r o n o l o g i c a l r e c o r d .

Description of New Taxa

G e n u s Hossiella L o n g o r i a a n d G a m p e r , n . g e n .

F i g u r e 2

T y p e s p e c i e s . — Globorotalia monmouthensis O l s s o n , 1 9 6 0 .

Description. — T e s t t r o c h o s p i r a l , b i c o n v e x t o p l a n o ­

c o n v e x , u m b i l i c a t e ; p e r i p h e r y r o u n d e d , p e r f o r a t e d

241 LONGORIA AND GAMPER

Fig. 2.— Morphological similarities between Lower Cenomanian and uppermost Cretaceous taxa. Specimens 1—3, and 11, Hossiella archeocompressa (Blow) from Biozone K29.

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY ACROSS THE K/T BOUNDARY 25

Hedbergella delrioensis (Carsey) from Biozone K18. Specimen 4—9, Hossiella monmouthensis (Olsson) and specimen 10

26 LONGORIA AND GAMPER

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m a y a r o e n s i s F A D

Fig. 3. — Phylogenetic interpretation of planktic foraminifera across the K/T boundary. G. cretacea, H. monmouthensis and R. hexacamerata are regarded as the basal stocks of three major lower Tertiary groups: Globoconusa, Turborotalia and eoglobigerinids, respectively.

w i t h o u t a n y i n d i c a t i o n o f i m p e r f o r a t e p e r i p h e r a l b a n d , p e r i p h e r a l o u t l i n e s t r o n g l y l o b a t e . C h a m b e r s s p h e r i ­c a l t o o v a t e . S u t u r e s d e p r e s s e d a n d r a d i a l o n b o t h s p i r a l a n d u m b i l i c a l s i d e s . W a l l s m o o t h l a c k i n g r u ­g o s i t i e s o r s p i n e s , d e n s e l y p e r f o r a t e d . P r i m a r y a p e r ­t u r e i n t e r i o m a r g i n a l , e x t r a u m b i l i c a l , c o v e r e d b y a n i m p e r f o r a t e d f l a p .

Remarks.— T h e t e r m Hossiella ( f r o m t h e H e b r e w H o s h i e a i m p l y i n g a n e w b e g i n n i n g ) w a s c r e a t e d a s

a nomen nudum ( G a m p e r — L o n g o r i a a n d L o n g o r i a , 1 9 8 4 ) t o i n c l u d e s o m e o f t h e s m a l l g l o b i g e r i n a s p r e s e n t i n t h e K 2 9 B b i o z o n e o f M e x i c o . T h e t e r m is h e r e i n e s t a b l i s h e d f o r a g e n u s — g r o u p t a k i n g Globorotalia monmouthensis O l s s o n a s t y p e s p e c i e s . T h u s , Hossiella L o n g o r i a a n d G a m p e r , n . g e n . , is h e r e i n c r e a t e d to i n c l u d e t h e g r o u p o f u p p e r m o s t C r e t a c e o u s t a x a that m o r p h o l o g i c a l l y is a n a l m o s t h o m e o m o r p h o f Hedber-gella ( F i g . 2 ) . B o t h g e n e r a d i s p l a y t h e s a m e o v e r a l l

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY ACROSS THE K/T BOUNDARY 27

Fig. 4.— Small globigerinas of uncertain taxonomic posi­tion (incertae sedis) from the A. mayaroensis/P. eugubina Interval—Zone (subzone K29B) of Mexico. Note the um­bilical structure (techo?) covering the umbilical area char­acteristic of Cretaceoris Hedbergellacea. Maximum diameter 85 microns for both specimens.

tes t m o r p h o l o g y , i n c l u d i n g s i m i l a r u m b i l i c a l s t r u c ­t u r e s ; h o w e v e r , t h e y s h o u l d b e s e p a r a t e d o n t h e b a s i s o f t h e i r p h y l o g e n e t i c d e v e l o p m e n t . T h e o r i g i n o f Hossiella L o n g o r i a a n d G a m p e r , n . g e n . is u n c e r t a i n ; it i s l i k e l y t h a t it d e v e l o p e d f r o m a n a b a t h o m p h a l i d o r g l o b o t r u n c a n i d a n c e s t o r d u r i n g t h e n e o t e n i c s t a g e i n t h e u p p e r m o s t M a a s t r i c h t i a n . Hossiella a l s o d i f f ers f r o m Hedbergella b y h a v i n g a s m o o t h w a l l s t r u c t u r e i n ­s t e a d o f a s p i n y s u r f a c e ( F i g . 2 ) . F u r t h e r m o r e , m e m b e r s o f the g e n u s Hossiella L o n g o r i a c i n d G a m p e r , n . g e n . , c a n n o t b e d e r i v e d f r o m Hedbergella b e c a u s e t h i s g e n u s w a s e x t i n c t b y t h e e a r l y C a m p a n i a n . Hossiella d i f f e r s f r o m Globotruncanella b y h a v i n g a p e r ­f o r a t e d flap i n s t e a d o f a t e c h o . It is l i k e l y t h a t Hossiella g a v e r i se t o a m a j o r s p e c i a t i o n in t h e l o w e r D a n i a n ( F i g . 3 ) a n d l a t e r e v o l v e d i n t o Turborotalia s e n s u B l o w ( 1 9 7 9 ) .

T h e t y p e s p e c i e s o f Hossiella L o n g o r i a a n d G a m p e r , n . g e n . , w a s o r i g i n a l l y p l a c e d in t h e g e n u s Globorotalia i n 1 9 6 0 . B e r g g r e n ( 1 9 6 2 ) p l a c e d G. monmouthensis O l s s o n i n Globotruncanella. T h u s , i ts t a x o n o m i c p o s i ­t i o n h a s c h a n g e d re f l ec t ing its u n c e r t a i n g e n e r i c s t a t u s .

Hossiella is a n i m p o r t a n t e l e m e n t in the p h y l o g e n e t i c d e v e l o p m e n t o f l o w e r D a n i a n teixa. It i s l i k e l y t h a t Hossiella r e p r e s e n t s t h e r e c a p i t u l a t i o n o f L o w e r C r e t a ­c e o u s a n c e s t o r s . A c c o r d i n g t o O l s s o n et al. ( 1 9 9 2 , p . 2 1 0 ) Hossiella monmouthensis ( O l s s o n ) is t h e C r e t a ­c e o u s a n c e s t o r o f l o w e r m o s t D a n i a n " Globanomalina" ( = Turborotalia s e n s u B l o w , 1 9 7 9 ) . W e h a d h y p o t h ­e s i z e d ( G a m p e r — L o n g o r i a a n d L o n g o r i a , 1 9 8 4 ) t h a t Hossiella is o n e o f t h e s e v e r a l s t o c k s a m o n g t h e s m a l l g l o b i g e r i n a s t h a t u n d e r w e n t n e o t e n i c t r a n s f o r m a t i o n ( F i g . 3 ) . T h e c l o s e s i m i l a r i t y o f i ts wadl p o r e p a t t e r n t o t h a t o f Globotruncanella s e e m s t o i n d i c a t e s t h a t t h e y a r e p h y l o g e n e t i c c i l l y r e l a t e d . H o w e v e r , a s s t a t e d e l s e w h e r e ( L o n g o r i a a n d G e i m p e r , 1 9 7 5 ) t h e u m b i l i c a l a p p a r a t u s o f C r e t a c e o u s p l a n k t o n i c f o r a m i n i f e r a is c o n s i d e r e d a s a p h y l o g e n e t i c c h a r a c t e r . T h u s , Hossiella L o n g o r i a a n d G a m p e r , n . g e n . , c a n n o t b e p l a c e d in

A b a t h o m p h a l i d a e b e c a u s e t h e l a c k o f u m b i l i c a l t e c h o , h a v i n g i n s t e a d a n u m b i l i c a l flap. T h e l a t t e r f e a t u r e is o n l y o b s e r v e d in m e m b e r s o f H e d b e r g e l l i d a e ( s e n ­s u L o n g o r i a a n d G a m p e r , 1 9 8 5 ) ; h o w e v e r , a s s t a t e d a b o v e , t h e r e is n o t a d i r e c t c o n n e c t i o n b e t w e e n Hedbergella a n d Hossiella. F o r n o w , Hossiella i s r e g a r d ­e d a s a n incertae sedis g e n u s — g r o u p .

M e m b e r s o f Hossiella L o n g o r i a a n d G a m p e r , n . g e n . , s e e m t o l i n k t h e p a s s a g e f r o m t h e u p p e r m o s t C r e t a c e o u s ( K 2 9 B ) t o t h e l o w e r m o s t D a n i a n ( P a d p h a Z o n e ) . T h e l o w e r r a n g e oí Hossiella is u n k n o w n ; S m i t h

28 LONGORIA AND GAMPER

a n d P e s s a g n o ( 1 9 7 3 ) i l l u s t r a t e d a s p e c i m e n o f mon­mouthensis f r o m C o r s i c a n a F o r m a t i o n o f T e x a s a t t r i ­b u t e d t o K 2 7 b i o z o n e . A n u m b e r o f s p e c i e s d i s p l a ­y i n g u m b i l i c a l s t r u c t u r e s a n d w a l l s u r f a c e s i m i l a r t o Hossiella w e r e d e s c r i b e d b y B l o w ( 1 9 7 9 ) f r o m h i s P a l -p h a Z o n e o f t h e l o w e r m o s t D a n i a n . It i s l i k e l y t h a t Hossiella r a n g e s f r o m t h e m i d d l e M a a s t r i c h t i a n ( K 2 7 z o n e ) t o t h e l o w e r D a n i a n ( P a l p h a z o n e ) . T h e s p e ­c i e s Globorotalia (Turborotalia) archeocompressa B l o w a n d G. (T.) planocompressa S h u t s k a y a a r e a s s i g n e d t o Hos­siella a s t h e y s h o w u m b i l i c a l flaps ( = p o r t i c a l s t r u c ­t u r e s o f B l o w , 1 9 7 9 ) a n d s i m i l a r w a l l p e r f o r a t i o n .

Biostrat igraphy

T h e K / T b o u n d a r y p l a n k t i c b i o e v e n t t o o k p l a c e d u ­r i n g t h e t i m e i n t e r v a l b o u n d e d b y t w o d i s t i n c t i v e p l a n k t o n i c f o r a m i n i f e r a l b i o h o r i z o n s : ' 1) t h e A. maya­roensis L A D , a n d 2 ) t h e P. eugubina F A D . A s s t a t e d a b o v e , G a m p e r ( 1 9 7 7 , p . 2 6 ) n o t e d t h a t e a r l i e r b i o -z o n a t i o n s h a d n e g l e c t e d t h i s s h o r t i n t e r v a l s e p a r a t i n g t h e s e b i o h o r i z o n s ; t h u s , it w a s p o s t u l a t e d t h a t a h i a ­t u s w a s p r e s e n t in s e c t i o n s c o n t a i n i n g t h e eugubina— F A D i m m e d i a t e l y a b o v e t h e ^ l . mayaroensis L A D . T h e ­se s e c t i o n s i n c l u d e d t h e G u b b i o a n d S c a n d i n a v i a n s u c ­c e s s i o n s d e s c r i b e d b y L u t e r b a c h e r a n d P r e m o l i S i l v a ( 1 9 6 4 ) , a n d B e r g g r e n ( 1 9 6 5 ) , r e s p e c t i v e l y . R e c e n t l y , M a c L e o d a n d K e l l e r ( 1 9 9 1 ) d i s c u s s e d t h e e x i s t e n c e o f a h i a t u s i n K / T b o u n d a r y s u c c e s s i o n s o f deep—sea b a s i n s ; h o w e v e r , it i s e v i d e n t t h a t t h e A. mayaroensis 'LAD/P. eugubina F A D i n t e r v a l i s v i r t u a l l y i m p o s s i ­b l e t o i d e n t i f y i n d e e p s e a s e d i m e n t s m a i n l y d u e t o i t s c o n d e n s e d n a t u r e .

O u r o b s e r v a t i o n s f r o m e x p a n d e d l a n d s e c t i o n s s u c h a s t h e M é n d e z at A r r o y o P e d r e g o s o ( G a m p e r , 1 9 7 7 ; L o n g o r i a , 1 9 7 7 ) s h o w e d t h a t a c h a n g e i n f o r a m i n i f e ­ral f a u n a w a s t a k i n g p l a c e b e f o r e t h e e x t i n c t i o n o f t h e l a r g e — s i z e d t a x a . S i x b i o h o r i z o n s a r e d e f i n e d w i t h i n t h e b o u n d a r i e s o f B i o z o n e K 2 9 : I) A. mayaroensis— F A D , 2 ) s m a l l g l o b i g e r i n a s — F A D —likely c o r r e s p o n d ­i n g t o t h e "G. " globigerinelloides g r o u p , 3 ) A. maya­roensis-LAD, 4 ) "G. "fringa-YAD, 5 ) "G." minutu­la—Y AD, a n d 6 ) P. eugubina—Y AD. T h e s e d a t u m p l a ­nes allow a h i g h e r r e s o l u t i o n b i o c h r o n o l o g y f o r t h i s i n t e r v a l ( F i g . 5 ) c o r r e s p o n d i n g t o s u b z o n e s K 2 9 A j , K 2 9 A 2 , K 2 9 B i , K 2 9 B 2 a n d K 2 9 B 3 , r e s p e c t i v e l y . T h e s e c t i o n r e p r e s e n t e d b y t h e B u r g o s B a s i n of n o r t h e a s t M e x i c o t y p i f i e s t h i s s u c c e s s i o n ( F i g . 6 ) and has also been d o c u m e n t e d a t A r r o y o P e d r e g o s o ( L o n g o r i a , 1 9 7 7 , 1 9 8 4 ; G a m p e r , 1 9 7 7 ) .

Region:

Biozone

O

P - 1

Gulf Coastal Plain of Mexico

Gamper, 1977; Longoria, 1977; 1984

Globigerina pseudobul loides-FAD

P - a l p h a

(15 cm)

CU

o

o

P C

B

3 (20 cm)

2 (SO cm)

2 (IS cm)

1

(30 cm)

P. cugubina-FAD

"G." minutula-FAD

G. fringa-FAD

(30 cm)

Abathomphalus mayaroensis-LAD

Small Giobigerinas-P'AD

cm from base

- 1 5 5 -

-14Ü-

-120-

- 7 0 -

- 4 5 -

k - 3 0 -

Aba thompha lus mayaroens i s -FAD -0 -

Fig. 5.— Planktic foraminiferal biochronology across the K/T succession of the Burgos Basin at suboutcrop shown in figure 6.

Z o n e M 1 8 (Rugoglobigerina hexacamerata p a r t i a l — r a n ­g e - z o n e ) d e f i n e d b y B l o w ( 1 9 7 9 , p . 2 5 0 - 2 5 2 ) f r o m t h e S i r t e e m b a y m e n t , s o u t h e r n C y r e n a i c a , L y b i a , c o n t a i n s a f a u n a l a s s e m b l a g e s i m i l a r t o t h a t f o u n d i n B i o z o n e K 2 9 B , t h u s it i s r e g a r d e d a s b i o c o r r e l a t i v e w i t h t h e l a t t e r . H o w e v e r , B l o w ( 1 9 7 9 , p . 2 5 0 - 2 5 2 ) r e g a r d e d L u t e r b a c h e r a n d P r e m o l i S i l v a ' s "Globige­rina" eugubina Z o n e a s a j u n i o r s y n o n y m o f h i s z o n e M l 8 . A g a i n , t h i s c o n f u s i o n r e s u l t e d f r o m t h e l a c k o f c o m p a r a t i v e s t u d i e s b a s e d o n t h e I t a l i a n m a t e r i a l . B l o w m i s s i n t e r p r e t e d t h e m o r p h o l o g y o f "Globigeri­na '' eugubina a s d e s c r i b e d b y L u t e r b a c h e r a n d P r e m o l i S i l v a ( 1 9 6 4 ) . O n t h e o t h e r h a n d , t h e Globorotalia (Tur­borotalia) longiapertura P a r t i a l — R a n g e — Z o n e ( B i o z o n e P a l p h a ) d e s c r i b e d b y B l o w ( 1 9 7 9 , p . 2 5 2 - 2 5 5 ) fa l l s w i t h i n t h e "G. " eugubina B i o z o n e ( T o u m a r k i n e a n d L u t e r b a c h e r , 1 9 8 5 , p . 9 2 ) .

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY ACROSS THE K/T BOUNDARY 29

• . • ' / - S I

5»..

Fig. 6. — Suboutcrop of the K/T boundary succession in the Burgos Basin. Trench is 155 cm deep. The biochronology and thickness of each biozone is given in figure 5.

A s i m i l a r i m e r v a l c o n t a i n i n g a f a u n a l a s s e m b l a g e

before the a p p e a r a n c e o f P. eugubina, t h e 'Jringa—

Zone", w a s d e s c r i b e d f r o m t h e c a l c a r e o u s A l p s o f G e r ­

many b y H e r m etal. ( 1 9 8 1 ) . L i k e w i s e , t h e Guembelitria

cretacea Z o n e (PO Z o n e ) d e s c r i b e d f r o m s o u t h e a s t e r n

Spain a n d E l - K e f i n T u n i s i a b y S m i t ( 1 9 8 2 , p . 3 3 5 )

and S m i t a n d R o m e i n ( 1 9 8 5 ) c o n t a i n s a f a u n a l a s ­

semblage t y p i c a l l y f o u n d a s s o c i a t e d w i t h ^ . mayaroen­

sis; thus it fa l l s w i t h i n t h e A. mayaroensis/P. eugubina

In terva l—Zone . U n f o r t u n a t e l y , t h e s y n c h r o n e i t y o f

the a s s e m b l a g e s d e s c r i b e d b y t h o s e a u t h o r s w i t h t h e

A. mayaroensis/P. eugubina I n t e r v a l — Z o n e s e n s u G a m ­

per ( 1 9 7 7 ) a n d L o n g o r i a ( 1 9 7 7 , 1 9 8 4 ) r e m a i n s t o b e

defined ( F i g . 7 ) .

T h e K 2 9 B i o z o n e i n M e x i c o is k n o w n t o c o n t a i n

a b u n d a n t p l a n k t o n i c f o r a m i n i f e r a l a s s e m b l a g e s ( H a y ,

1 9 6 0 ; P e s s a g n o , 1 9 6 9 ; L o n g o r i a a n d G a m p e r , 1 9 7 4 ;

G a m p e r , 1 9 7 7 ; L o n g o r i a , 1 9 7 7 ; G a m p e r — L o n g o r i a

a n d L o n g o r i a , 1 9 8 4 ) a n d n e w t a x a h a v e b e e n

d e s c r i b e d f r o m s o m e o f t h o s e l o c a l i t i e s w i t h i n t h e

T a m p i c o a r e a o f n o r t h e a s t e r n M e x i c o ( L o n g o r i a a n d

G a m p e r , 1 9 8 4 ) . B a s i c a l l y , t h e m a j o r i t y o f t h e s e t a x a

a re a l s o p r e s e n t in t h e y o u n g e r K 2 7 B i o z o n e . T h e o n l y

r e l i a b l e f e a t u r e s s e p a r a t i n g b o t h b i o z o n e s a r e : 1) t h e

p r e s e n c e o f / / , gansseri a n d Guembelitria cretacea i n K 2 7 ;

a n d 2 ) t h e p r e s e n c e oi A. mayaroensis i n K 2 9 A . W e

r e g a r d th i s in t erva l as r e p r e s e n t i n g t h e u p p e r m o s t C r e ­

t a c e o u s s i n c e t h e m a j o r i t y o f t h e f o r a m i n i f e r a l w o r ­

k e r s a c c e p t t h e P. eugubina—FAD a s d e f i n i n g t h e b a s e

o f t h e D a n i a n . O t h e r a u t h o r s ( S m i t , 1 9 8 2 ; S m i t a n d

R o m e i n , 1 9 8 5 ; K e l l e r , 1 9 8 8 a , p . 1 6 2 ) d e f i n e t h e b a s e

o f t h e P a l e o g e n e a b o v e t h e m a s s e x t i n c t i o n o f C r e t a ­

c e o u s t a x a . H o w e v e r , w e re jec t t h i s p r o c e d u r e m a i n ­

l y b e c a u s e c h r o n o s t r a t i g r a p h i c b o u n d a r i e s s o d e f i n e d

a re di f f icul t to p r o v e i s o c h r o n o u s . F u r t h e r m o r e , K e l l e r

( 1 9 8 9 , p . 1 4 1 6 ) s h o w e d t h a t e x t i n c t i o n s p a t t e r n s a r e

d i f ferent b e t w e e n the B r a z o s R i v e r a n d El K e f s e c t i o n s .

T h e b i o c o r r e l a t i o n o f o u r z o n a l s c h e m e w i t h m o r e

r e c e n t b i o z o n a t i o n s s u c h a s t h a t p r e s e n t e d b y S m i t

( 1 9 8 2 ) , S m i t a n d R o m e i n ( 1 9 8 5 ) a n d m o d i f i e d b y

K e O e r ( 1 9 8 8 a ) is r e n d e r dif f icult b e c a u s e t h e a m b i g u i t y

o f t h e d e f i n i t i o n o f t h e i r z o n e s ( F i g . 7 ) . F o r e x a m p l e ,

S m i t ( 1 9 8 2 ) d e f i n e d h i s Abathomphalus mayaroensis Z o n e

a s t h e " R a n g e z o n e o f t h e z o n a l m a r k e r " ( t a x o n —

r a n g e — z o n e s e n s u I S S C , 1 9 7 6 , p . 5 3 ) ; h o w e v e r , h e

d e f i n e d i t s u p p e r b o u n d a r y " a t t h e m a i n e x t i n c t i o n

l e v e l o f C r e t a c e o u s p l a n k t o n i c F o r a m i n i f e r a " , c o n ­

t r a d i c t i n g h i s c o n c e p t o f t o t a l — r a n g e — z o n e ; f u r t h e r ­

m o r e , t h e A. mayaroensis—"LAD h a s r e s u l t e d i n a c o n ­

t r o v e r s y a n d it m a y n o t c o i n c i d e u n i v e r s a l l y w i t h t h e

e x t i n c t i o n o f t h e res t o f t h e t a x a . F o r e x a m p l e , K e l l e r

( 1 9 8 8 a , p . 1 5 5 ) e x p l a i n e d t h a t t h e a b s e n c e o f A.

mayaroensis in c o n t i n e n t a l s h e l f r e g i o n s i n d i c a t e s its d i s ­

a p p e a r a n c e i n t h e s e e n v i r o n m e n t s b e f o r e t h e K / T

b o u n d a r y . T h e o v e r l a p o f t h e s m a l l g l o b i g e r i n a s ( n o n

P. eugubina) FAD w i t h t h e u p p e r m o s t A. mayaroensis—

T o t a l — R a n g e — Z o n e s e e m s t o b e a s i g n i f i c a n t b i o -

h o r i z o n w h i c h is b e y o n d t h e r e w o r k i n g o r d o w n c a v ­

i n g o f f a u n a s s i n c e it o n l y c o n t a i n s t h e m o r e p r i m i ­

t i v e o f t h e s m a l l g l o b i g e r i n a s . I t i s l i k e l y t h a t S m i t ' s

( 1 9 8 2 ) "Abathomphalus mayaroensis Zone" is b i o c o r r e l a ­

t i v e w i t h t h e l o w e r K 2 9 A B i o z o n e .

T h e "Guembelitria cretacea (PO) Z o n e " a s d e f i n e d b y

S m i t ( 1 9 8 2 , p . 3 3 5 ) - i . e . , t h e p a r t i a l r a n g e o f t h e

30 LONGORIA AND GAMPER

n o m i n a l s p e c i e s , f r o m t h e e x t i n c t i o n o f t h e m a i n m a s s o f C r e t a c e o u s s p e c i e s t o t h e e n t r y o f G. minutula— i s e v e n m o r e d i f f i cu l t t o p l a c e w i t h i n o u r b i o c h r o n o l o g i ­c a l s c h e m e . G. cretácea i s k n o w n t o o c c u r i n t h e l o w e r K 2 7 B i o z o n e a n d "G. "minutula h a s a n u n s t a b l e t a x ­o n o m i c p o s i t i o n . F u r t h e r m o r e , a c c o r d i n g t o S m i t ( 1 9 8 2 , p . 3 3 5 ) "Globotruncanella" monmouthensis, Globigerinelloides áspera a n d G. messinae a r e a l s o p r e s e n t i n h i s PO b i o z o n e . A l l t h e a f o r e m e n t i o n e d t a x a o c c u r a s s o c i a t e d w i t h A. mayaroensis i n B i o z o n e K 2 9 A 2 o f M e x i c o ; t h u s it is l i k e l y t h a t a t l e a s t t h e l o w e r p a r t o f S m i t ' s G. cretacea Z o n e i s b i o c o r r e l a t i v e w i t h o u r B i o z o n e K 2 9 A . K e l l e r ( 1 9 8 8 a , p . 1 6 2 ) a d d e d a f u r ­t h e r c o m p l i c a t i o n t o t h e r e c o g n i t i o n o f t h e G. cretacea Z o n e ( P O ) ; c o n t r a r y t o S m i t ' s ( 1 9 8 2 , p . 3 3 5 ) d e f i n i ­t i o n o f t h e z o n e , s h e s t a t e d " T h e Guembelitria cretacea

Z o n e o r PO o f S m i t ( 1 9 8 2 ) m a r k s t h e l o w e r m o s t Pci leo-c e n e . S m i t ( 1 9 8 2 ) d e f i n e d t h e i n t e r v a l o f t h i s b i o z o n e f r o m t h e K / T b o u n d a r y t o t h e first a p p e a r a n c e o f "Globigerina eugubina", w h i c h c l e a r l y d i f f e r s f r o m S m i t ' s o r i g i n a l d e f i n i t i o n o f t h e "G. cretacea Z o n e " . L a t e r , s h e {op. cit., 1 9 8 8 b , p . 2 4 6 ) e m e n d e d S m i t ' s z o n a l u n i t t o p l a c e i t s t o p a t t h e b i o h o r i z o n d e f i n e d b y t h e P. eugubina—VAD. S h e a l s o r e c o g n i z e d a t w o f o l d d i v i s i o n o f B i o z o n e PO i n t o PO^ a n d PO^ b a s e d o n t h e "G. " conusa—YAD. A c c o r d i n g t o t h e m , B i o ­z o n e PO is c h a r a c t e r i z e d b y a n a b u n d a n c e o f G. cretacea a n d t h e a b s e n c e oiA. mayaroensis. B o t h f e a t u r e s m a k e PO d i f f e r e n t f r o m K 2 9 A , b u t t h e i r b i o c o r r e l a t i o n is u n c e r t a i n b e c a u s e G. cretacea i s a l s o a b u n d a n t i n B i o ­z o n e K 2 7 .

Region:

Biozone

Gulf Coastal Plain of Mexico

Camper, 1977; Longoria, 1977; 1984

Caribbean Sea

Premoli-Silva & Bolli, 1973

El Kef, Tunisia

Smit, 1982; Smit & Romein, 1985

Keller, 1988

I CO

o

E -

o

P - 1 Globigerina pseudobuIIoides-FAD

Globigerina pseudobulloides; FAD P l b

P -alpha p. eugubina-FAD

O N O

CO

B

A

"G." minutula-FAD

"G." fringa-FAD

Abathomphalus mayaroensis -LAD

Small Globigerinas-FAD ^

Abathomphalus mayaroensis-FAD

Globigerina eugubina-FAD P i a

Globigerina pseudobul lo ides-FAD

P l C

G. minutula-FAD

P l b

Globigeriaa pseudobul lo ides-FAD

P i a

P O

Abathomphalus mayaroensis

M 3

extinction of the main mass of Cretaceous species

P O

M 3

1

G. táurica

Eoglobigcrina spp.

Globigerina eugubina-FAD

G. c o D u a a FAD

P. deformis

extinction of the main mass of Cretaceous species

Abathomphalus mayaroens i s -LAD

Fig. 7.— Planktic foraminiferal biochronology across the K/T boundary. Several biostratigraphic schemes according to dif­ferent authors. Synchroneity of different biozones is not implied. Datum planes (FAD's and LAD's) defined by Gamper (1977) and Longoria (1977, 1984) are suggested as the standard biochronology for the Gulf Coastal Plain of North Amer­ica,

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY ACROSS THE K/T BOUNDARY 3 1

T h e a b s e n c e o f A. mayaroensis i n t h o s e s e c t i o n s c a n be due to e i t h e r a h i a t u s ( P e s s a g n o et al., 1 9 9 0 ) , o r an abrupt c h a n g e i n b a t h y m e t r y . O n t h e o t h e r h a n d , Keller ( 1 9 8 8 a , b ) c o n s i d e r e d t h a t t h e a b s e n c e o f t h i s taxon f r o m t h e u p p e r m o s t s e d i m e n t s i n E l K e f i n d i ­cates that it d i s a p p e a r e d p r i o r t o t h e K / T b o u n d a r y in shelf f a c i e s ; t h u s s h e p r o p o s e d t h e P. deformis b i o ­zone as t h e u p p e r m o s t M a a s t r i c h t i a n u n i t . It i s l i k e l y that K e l l e r ' s "Pseudoguembelitria deformis" a n d Pseudo-lextularia deformis b i o z o n e s ( K e l l e r , 1 9 8 8 a a n d 1 9 8 8 b , respectively) a re b a s e d o n r e w o r k e d f a u n a u n l e s s p r o v ­en o t h e r w i s e . F u r t h e r m o r e , it i s u n f o r t u n a t e t h a t K e l ­ler ( 1 9 8 8 a , b ) h a d u s e d t h e P. deformis-FAD a s a z o n ­al b i o h o r i z o n b e c a u s e t h i s t a x o n h a s b e e n c o n s i d e r ­ed as a j u n i o r s y n o n y m o f Pseudotextularia elegans ( N a s h ,

1981, p . 7 1 ; N e d e r b r a s t , 1 9 9 1 ) ; c o n s e q u e n d y , its F A D is uncer ta in . I t s t i m e e q u i v a l e n c y w i t h t h e u p p e r K 2 9 B Biozone r e m a i n s t o b e d o c u m e n t e d . O u r o b s e r v a t i o n s from the f o r a m i n i f e r a l f a u n a s o f M e x i c o c l e a r l y d e ­monstrate that K e l l e r ' s h y p o t h e s i s o n t h e d i f f e r e n t p a -l e o b a t h y m e t r i c d i s t r i b u t i o n o f ^ . mayaroensis is n o t v a ­lid. W e h a v e s e e n a b u n d a n t s p e c i m e n s o f A. maya­roensis a s s o c i a t e d w i t h b e n t h i c a s s e m b l a g e s o f n e r i t i c facies in n o r t h e a s t e r n M e x i c o .

A n e v e n m o r e d i f f i c u l t t a s k is a t t e m p t i n g t o p l a c e Smit's "Globigerina eugubina Z o n e ( P j J " in t h e c lass ica l biochronological s c h e m e o f t h e G u l f C o a s t a l P l a i n s e n s u Longor ia ( 1 9 8 4 ) . T h e " G . eugubina Z o n e " a s d e f i n e d by L u t e r b a c h e r a n d P r e m o H S i l v a ( 1 9 6 4 ) , B o l l i ( 1 9 6 6 , p. 13), P r e m o l i S i l v a a n d B o l l i ( 1 9 7 3 , p . 5 0 2 ) , P r e ­moli S i l v a ( 1 9 7 7 , p . 3 7 2 ) a n d T o u m a r k i n e a n d L u ­terbacher ( 1 9 8 5 , p . 9 2 ) r e p r e s e n t s e i t h e r a t o t a l -range—zone o r a n i n t e r v a d — z o n e d e f i n e d b y t h e b i o ­horizons m a r k e d b y t h e first o c c u r r e n c e o f P. eugubina and the first o c c u r r e n c e o f "G. " pseudobulloides; o n t h e other h a n d , S m i t ' s "G. eugubina Z o n e " w a s d e f i n e d as a partial r a n g e z o n e e x t e n d i n g f r o m t h e e n t r y o f Glo­bigerina minutula t o t h e e n t r y o f G. pseudobulloides ( S m i t ,

1982, p . 3 3 5 ) . C o n s e q u e n d y , S m i t ' s " G . eugubina Z o n e Pl j j" is n o t b i o c o r r e l a t i v e w i t h n e i t h e r t h e G. eugu­bina b i o z o n e a s d e s c r i b e d b y L u t e r b a c h e r a n d P r e m o ­li Silva ( 1 9 6 4 ) a n d T o u m a r k i n e a n d L u t e r b a c h e r ( 1 9 8 5 ) nor w i t h t h e G. eugubina B i o z o n e o f P r e m o l i S i l v a a n d Bolli ( 1 9 7 3 , p . 5 0 2 ) . It i s l i k e l y t h a t S m i t ' s " G . eugu­bina Z o n e P i g " r e p r e s e n t s t h e t i m e i n c l u d e d b e t w e e n the b i o h o r i z o n s m a r k e d b y t h e "G. " minutula—FAG and the "G. "pseudobulloides-FAD.

In s u m m a r y , t a x o n o m i c u n c e r t a i n t i e s o f b i o z o n a l markers a n d i n c o n s i s t e n c i e s i n t h e u s e o f b i o s t r a t i g r a ­phic un i t s b y p r e v i o u s a u t h o r s a d d s e r i o u s c o m p l e x i t i e s

t o t h e e v a l u a t i o n o f t e m p o r a d c o m p l e t e n e s s o f t h e b o u n d a r y s u c c e s s i o n s . T h e s e t w o f a c t o r s n e e d t o b e a d d r e s s e d a s t h e y m a y r e s u l t i n t h e a p p a r e n t d i s c r e p ­a n c i e s o f t h e d i s t r i b u t i o n o f b i o s t r a t i g r a p h i c d a t u m p l a n e s r e c e n d y a r g u e d b y M c L e o d a n d K e l l e r ( 1 9 9 1 ) . It is h e r e i n s u g g e s t e d t h a t a r e — e x a m i n a t i o n o f b o t h t h e t a x o n o m y a n d b i o s t r a t i g r a p h i c p r o c e d u r e i m p l e ­m e n t e d b y t h o s e a u t h o r s s h o u l d b e d o n e b e f o r e q u e s ­t i o n i n g t h e v a l i d i t y o f b i o s t r a t i g r a p h i c a l l y b a s e d e s t i ­m a t e s o f t e m p o r a l c o m p l e t e n e s s o f K / T b o u n d a r y s e c ­t i o n s a s c l a i m e d b y M c L e o d a n d K e l l e r ( 1 9 9 1 ) . I n ­d e e d , t h e m a j o r p - o b l e m is b y i t s e l f t h e d e f i n i t i o n o f t h e K / T b o u n d a r y . O n c e it i s d e f i n e d o n c h r o n o s t r a ­t i g r a p h i c g r o u n d s , w e s h o u l d b e a b l e t o d e t e c t t h e p r e ­c i s i o n , o r i m p r e c i s i o n for t h a t m a t t e r , o f b i o c o r r e ­l a t i o n .

STRATIGRAPHY OF ARROYO EL MIMBRAL SECTION

T h e K / T b o u n d a r y s u c c e s s i o n w a s s t u d i e d i n f o u r m a j o r r e g i o n s a l o n g t h e G u l f o f M e x i c o r i m ( F i g . 8 ) . T h e s e i n c l u d e d L a P o p a a n d B u r g o s B a s i n s o f t h e n o r t h e a s t ; t h e T a m p i c o — M i s a n t l a B a s i n ; t h e P u e ­b l a — V e r a c r u z c o a s t a l p l a i n ; a n d t h e C h i a p a s a n d O a x a c a f o l d e d b e l t s . T w e l v e o u t c r o p s w e r e a n a l y z e d for t h e p r e s e n t s t u d y r e p r e s e n t i n g t w o c o n t r a s t i n g t y p e o f K / T b o u n d a r y s u c c e s s i o n s : 1) t y p e I , i n c l u d e s p h y s i c a l l y d i s t u r b e d s e c t i o n s a n d d i s p l a y s c l e a r e v i ­d e n c e o f u n c o n f o r m i t i e s ; t h e s e c o m p r i s e E l M i m b r a l , E l P o r v e n i r , L a L a j a , L a S i e r r i t a , L o s R a m o n e s , a n d R a n c h o N u e v o i n t h e s t a t e s o f N u e v o L e ó n a n d T a ­m a u l i p a s . T y p e I s u c c e s s i o n is c h a r a c t e r i z e d b y t h e p r e s e n c e o f a t h i c k c a l c a r e n i t e b e d o v e r l a i n a s p h e r ­u l e b e d o f v a r i a b l e t h i c k n e s s w h i c h S m i t et al. ( 1 9 9 2 ) a n d A l v a r e z et al. ( 1 9 9 2 ) a t t r i b u t e t o a t s u n a m i g e n e r ­a t e d b y t h e e x t r a t e r r e s t r i a l i m p a c t a t C h i c x u l u b ; 2 ) t y p e I I K / T b o u n d a r y s u c c e s s i o n s s h o w n o e v i d e n t f e a t u r e s a t t r i b u t e d t o t h e t s u n a m i a n d l a c k p h y s i c a l d i s r u p t i o n o r d i s c o n f o r m i t i e s . T h i s t y p e i n c l u d e s t h e C h i l d , V e r a c r u z , P u e b l a , A r r o y o P e d r e g o s o , M a g i s -c a t z i n a n d B u r g o s s e c t i o n s . T h e f a u n a l c o n t e n t a n d t h e p h y s i c a l s t r a t i g r a p h y o f t h e s e s e c t i o n s w i l l b e d e ­s c r i b e d i n a f u t u r e p a p e r . H o w e v e r , t h e A r r o y o El M i m b r a l s e c t i o n is h e r e i n d i s c u s s e d i n l i g h t o f i t s s i g n i f i c a n c e o n t h e e x t r a t e r r e s t r i a l i m p a c t i n M e x i ­c o .

LONGORIA AND GAMPER

32

(^P/B . B B

rT/M MZ-—3. AM

AP

Fig. 8. — Index map of Mexico showing major regions in­vestigatedfor the present research. LP/B: La Popa and Bur­gos Basins. T/M: Tampico Misantla Basin. P/VR. Puebla and Veracruz coastal plain. CH: Chiapas fold belt. Sections included in this paper are: CH—Chilil; VR = Veracruz; P = Puebla: AP = Arroyo Pedregoso; MZ = Magiscat-zin; AM = Arroyo El Mimbral; BB - Burgos Basin. Other stars represent additional outcrops studied. Circular area on northern Yucatán represents the Chicxulub site.

T h e Arroyo El Mimbra l Sect ion

O u r g e o l o g i c s t u d i e s of t h e S i e r r a d e T a m a u l i p a s d a t e b a c k t o t h e s e v e n t i e s ( L o n g o r i a a n d G a m p e r , 1 9 7 4 ; L o n g o r i a , 1 9 7 7 ; G a m p e r , 1 9 7 7 ) . O n e o f u s ( M A G ) s t u d i e d t h e b i o c h r o n o l o g y o f t h e C r e t a ­c e o u s / T e r t i a r y b o u n d a r y f r o m t h e M é n d e z S h a l e e x ­p o s e d i n A r r o y o P e d r e g o s o , a b o u t 2 0 k m s o u t h e a s t f r o m El M i m b r a l s e c t i o n . I n c o n t r a s t w i t h t h e u n c o n ­f o r m a b l e M é n d e z — V e l a s c o c o n t a c t o b s e r v e d b y M u i r ( 1 9 3 6 , p . 8 3 ) a n d H a y ( 1 9 6 0 , p . 7 1 ) f r o m t h e A r r o y o E l M i m b r a l , t h e M é n d e z at A r r o y o P e d r e g o s o o f t h e s o u t h w e s t e r n f l a n k o f t h e S i e r r a d e T a m a u l i p a s was f o u n d to c o n t a i n e d a c o n t i n u o u s b i o c h r o n o l o g i c r e c o r d a c r o s s t h e K / T b o u n d a r y , w i t h o u t p h y s i c a l e v i d e n c e o f d i s c o n f o r m i t y .

T h e s u c c e s s i o n s t u d i e d is e x p o s e d a l o n g t h e w e s t e r n m a r g i n o f t h e Arroyo E l M i m b r a l ( l a t i t u d e 2 3 ° 1 3 ' N , l o n g i t u d e 9 8 ° 4 0 ' W ) c o r r e s p o n d i n g t o M u i r ' s ( 1 9 3 6 , p . 8 3 ) l o c a l i t y . T h i s o u t c r o p w a s a l s o r e c e n d y s t u d i e d b y S m i t et al. ( 1 9 9 2 ) w h o c o n c l u d e d t h a t t h e c l a s t i c u n i t is t h e dep>osit o f a t s u n a m i p r o d u c e d b y a n e x ­t r a t e r r e s t r i a l i m p a c t o n Y u c a t á n . T h e y a l s o a d d e d t h a t , contrary^ t o M u i r ( 1 9 6 3 ) a n d H a y ' s ( 1 9 6 0 ) i n ­

t e r p r e t a t i o n , t h e c a l c a r e n i t e b e d f a l l s " p r e c i s e l y a t t h e

K / T b o u n d a r y " b a s e d o n f o r a m i n i f e r a l b i o c h r o n o l o g y .

T h e s t r a t i g r a p h i c s u c c e s s i o n e x p o s e d a l o n g t h e A r r o ­

y o E l M i m b r a l c o n s i s t s o f t h e f o l l o w i n g u n i t s ( F i g . 9 ) :

5 ) Q u a t e r n a r y c o n g l o m e r a t e

U n c o n f o r m i t y

4 ) M a s s i v e , t a n c o l o r , m a r l s 3 ) T h i c k — b e d d e d , i n t e r n a l l y l a u n i n a t e d , c o n g l o m ­

e r a t i c c a l c a r e n i t e . V a r i e s i n t h i c k n e s s f r o m 3 0 t o 2 0 0 c m

2 ) A l t e r e d c a l c a r e n i t e a n d c a l c a r e o u s c o n g l o m e r a t e , v a r y i n g i n t h i c k n e s s f r o m 1 0 t o m o r e t h a n 8 0 c m

U n c o n f o r m i t y

1) T h i c k t o m e d i u m — b e d d e d , g r a y , c l a y e y , i n d u ­r a t e d l i m e s t o n e w i t h s p o r a d i c i n t e r c a l a t i o n s o f t h i n m a r l s . A b o u t 4 0 m e t e r s c o n t i n u o u s l y e x p o s e d .

Lithocorrelat ion

U n i t 1 c o r r e s p o n d s t o w h a t M u i r ( 1 9 3 6 , p . 8 3 ) re ­f e r r e d t o a s t h e P i e r r e ( M é n d e z ) s h a l e . R o c k s o f th i s u n i t b r e a k i n c o n c o i d a l f r a c t u r e s a n d w e a t h e r b y e x ­f o l i a t i o n . S m i t etal. ( 1 9 9 2 , p . 1 0 1 , f i g . 2 ) r e f e r r e d t o t h i s u n i t a s t h e M é n d e z F o r m a t i o n c o n s i s t i n g o f p e ­l a g i c m a r l y l i m e s t o n e . L o c a l l y , U n i t 1 i s d i s s e c t e d b y i g n e o u s d i k e s w h i c h m a y b e t h e c a u s e o f i t s i n d u r a ­t i o n a n d c h a n g e i n c o l o r a t i o n a c c o u n t i n g f o r i t s d i f fer ­e n t g e n e r a l a s p e c t f r o m t h e t y p i c a l M é n d e z S h a l e . U n i t 2 c o n t a i n s m i x e d m a t e r i a l s i n c l u d i n g f r a g m e n t s o f t h e u n d e r l y i n g U n i t 1 a n d l a r g e c a l c a r e o u s f r a g ­m e n t s . I n t h i n s e c t i o n , t h i s r o c k c o n s i s t s o f b i o p s e u -d o s p a r u d i t e s . U n i t 2 w a s c o n s i d e r e d s u b u n i t I o f t h e K / T b o u n d a r y c l a s t i c u n i t b y S m i t etal. ( 1 9 9 2 , p 1 0 1 ) . M u i r ( 1 9 3 6 , p . 8 3 ) r e g a r d e d t h i s u n i t a s t h e i n f i l l i n g o f t h e i r r e g u l a r i t i e s a t t h e u n c o n f o r m i t y b e t w e e n t h e M é n d e z a n d t h e V e l a s c o . U n i t 3 c o r r e s p o n d s t o M u i r ' s s a n d s t o n e u n i t a b o v e t h e M é n d e z a n d t o s u b u n i t s I a n d I I i n t h e p r o f i l e b y S m i t et al. ( 1 9 9 2 , fig. 2 ) . U n i t 4 c o r r e s p o n d s t o t h e V e l a s c o s e n s u M u i r ( 1 9 3 6 , p . 8 3 ) , a n d w a s c o n s i d e r e d a s c o m p o s e d o f p e l a g i c m a r l s b y S m i t et al. ( 1 9 9 2 , fig. 2 ) . T h e s e m a r l s a l s o m a r k e d l y c o n t r a s t w i t h t h e r e d d i s h m u d s t o n e s o f t h e t y p i c a l V e l a s c o e x p o s u r e s i n t h e T a m p i c o r e g i o n .

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY ACROSS THE K/T BOUNDARY 33

EM29A-0

EM30-20

^ EM31Dj_

^ EM31CJ6

^^_EM31B-40

^ EM31A-56

Pig. 9.— Stratigraphic profile of the K/T boundary section exposed at Arroyo El Mimbral (AM in figure 8), Sierra de Tamaulipas, northeast Mexico. Lithology boxes: 1) quaternary conglomerate; 2) calcarenite bed; 3) spherule bed; 4) marls; and 5) micritic limestone. Column on ghe right is an enlargement of the physically disturbed interval atributed to the bolide impact by Smit et al. (1992).

Sampling and Sample Process ing

S a m p l e s c o l l e c t e d f r o m t h e A r r o y o E l M i m b r a l s e c ­tion in 1 9 7 4 ( L o n g o r i a a n d G a m p e r , 1 9 7 4 ) w e r e r e ­p r o c e s s e d a n d t h e f o r a m i n i f e r a s t u d i e d b o t h i n t h i n sect ion a n d w a s h e d r e s i d u e s . S a m p l e s w e r e p r o c e s s e d using the H2O2 m e t h o d . A m i n i m u m o f f i ve t h i n s e c ­tions p e r e a c h l i m e s t o n e s a m p l e w a s m a d e . I n a d d i ­t ion, a n e w se t o f s a m p l e s w a s c o l l e c t e d a n d n e w samples w e r e p r o c e s s e d a n d t h i n — s e c t i o n e d . O u r o b ­

s e r v a t i o n s a r e b a s e d o n t h e t w o s e t s o f s a m p l e s a n d o n t h e a n a l y s i s o f b o t h w a s h e d m a t e r i a l a n d t h i n -s e c t i o n e d s a m p l e s . T h e l o w e r t h i r t y t w o m e t e r s o f t h e s e c t i o n , s a m p l e s E M I t h r o u g h E M 2 8 ( F i g . 9 ) , c o n ­t a i n a n a s s e m b l a g e o f p l a n k t o n i c f o r a m i n i f e r a c h a r a c ­t e r i s t i c o f B i o z o n e K 2 7 s e n s u L o n g o r i a ( 1 9 8 4 ) . T h e u p p e r 3 m e t e r s o f t h e s e c t i o n c o n t a i n i n g t h e p h y s i c z d d i s t u r b a n c e a t t r i b u t e d b y S m i t et al. ( 1 9 9 2 ) t o t h e e x ­t r a t e r r e s t r i a l i m p a c t w a s s a m p l e d in d e t a i l e d ( F i g . 9 ) . S a m p l e s a c r o s s t h e K / T b o u n d a r y w e r e t a k e n f r o m

3 4 LONGORIA AND GAMPER

t h e b a s e o f s a m p l e E M 2 9 A d o w n t o E M 3 4 - 3 5 ; t h e

v e r t i c a l d i s t a n c e b e t w e e n e a c h s a m p l e is g i v e n i n c e n ­

t i m e t e r s b e l o w t h e i m m e d i a t e l y p r e c e d i n g s a m p l e . T h e

f o r a m i n i f e r a f o u n d i n t h i s i n t e r v a l ar e l i s ted i n f igure 10 .

Fig. 10.— Foraminiferal faunal content and planktic bio­

chronology of the impact internal. Last digits in sample numbers

indicate cm below the chronostratigraphic boundary defined at

sample E1M29A—0 at the first occurrence of P. eugubina.

Foraminiferal Data

T a x o n o m i c s t u d i e s o f p l a n k t o n i c f o r a m i n i f e r a f r o m

t h e A r r o y o E l M i m b r a l S e c t i o n a r e r e n d e r d i f f i c u l t

because: 1) t h e s e faunas c o n s i s t o f r e c r y s t a l i z e d t e s t s ,

a n d 2 ) t h e f a u n a i s i n g e n e r a l p o o r , a n d l o w d i v e r s i ­

f i e d a s c o m p a r e d w i t h t y p i c a l M é n d e z s a m p l e s . T h i s

m a y b e d u e t o t h e c o m p a c t i o n o f t h e m i c r i t i c m u d

a m d / o r o t h e r d i a g e n e t i c c h a n g e s . I g n e o u s i n t r u s i o n s

in the a d j a c e n t v i c i n i t y m a y h a v e i n f l u e n c e d t h e

p r e s e r v a t i o n o f t h e f a u n a o b s e r v e d i n t h e s e s a m p l e s .

C o n s e q u e n t l y , t h e t h i n s e c t i o n a n a l y s i s o f p l a n k t i c s

g r e a d y e n h a n c e d t h e e v a l u a t i o n o f f a u n a l a s s e m b l a g e s .

L a r g e — s i z e d p l a n k t o n i c f o r a m i n i f e r a w e r e i d e n t i ­

f i e d i n U n i t s 1 a n d 2 . A s i g n i f i c a n t f r a c t i o n o f t h e

p l a n k t o n i c p o p u l a t i o n i n t h i s i n t e r v a l o f t h e E l M i m ­

b r a l s e c t i o n a r e t r o c h o i d a l ta ixa . A f a u n a l l i s t o f t a x a

i d e n t i f i e d i n t h i s i n t e r v a l i s s h o w n i n f i g u r e 8 a n d c o n ­

t r a s t e d w i t h t h e f a u n a l a s s e m b l a g e p r e s e n t i n t h e A.

mayaroensis T o t a l — R a n g e — Z o n e o f t h e n e a r b y A r r o y o

P e d r e g o s o s e c t i o n ( M a n u e l — A l d a m a a r e a ) . S p e c i a l

e m p h a s i s w a s g i v e n t o o n l y l i s t t a x a t h a t a r e i d e n t i f i ­

a b l e o n p o o r l y p r e s e r v e d , r e c r y s t a l i z e d s p e c i m e n s .

T h e s m a l l g l o b i g e r i n c i s o f A s s e m b l a g e I I w e r e f o u n d

i n U n i t 4 ( V e l a s c o F o r m a t i o n , s a m p l e s E 2 9 — E 2 9 A ) .

U s i n g t h e s a m e b i o c h r o n o l o g i c c r i t e r i a a s a p p l i e d t o

t h e G u l f C o a s t a l P l a i n o f M e x i c o b y L o n g o r i a ( 1 9 8 4 ) ,

F A D s a n d L A D s w e r e u s e d a s r e l i a b l e b i o h o r i z o n s to

e s t a b l i s h t h e c o m p l e t e n e s s o f t h e E l M i m b r a l s e c t i o n .

T h u s , t h e l a c k o f ^ . mayaroensis, a n d t h e p r e s e n c e o f

H. gansseri i n s a m p l e s E M 3 0 t h r o u g h E M 3 4 i n d i c a t e s

t h a t u n i t s 1 a n d 2 a r e a s s i g n a b l e t o B i o z o n e K 2 7 ( m i d ­

d l e M a a s t r i c h t i a n ) . T h e p r e s e n c e o f P . eugubina i n U n i t

4 i n d i c a t e s t h e b a s e o f t h e T e r t i a r y . T h e s e f a u n a s w i l l

b e d e s c r i b e d a n d i l l u s t r a t e d i n a sepaurate p a p e r .

D I S C U S S I O N

1.— Standard reference section for the K/T boundary event in the Gulf Coastal Plain of North America. T h e A r r o y o

E l M i m b r a l s e c t i o n o f S i e r r a d e T a m a u l i p a s , M e x i c o

b e a r s t h e s a m e b i o c h r o n o l o g i c p o s i t i o n a s t h a t o f the

B r a z o s R i v e r s e c t i o n o f T e x a s . I n b o t h s e c t i o n s , p l a n k ­

t o n i c f o r a m i n i f e r a i m m e d i a t e l y b e l o w t h e s e d i m e n t s

a t t r i b u t e d t o t h e i m p a c t w a v e a r e a s s i g n a b l e t o B i o ­

z o n e K 2 7 . H o w e v e r , t h e A r r o y o E l M i m b r a l s e c t i o n

c o n t a i n s a f a r l e s s d i v e r s i f i e d a s s e m b l a g e a s c o m p a r e d

w i t h t h a t f o u n d b y S m i t h a n d P e s s a g n o ( 1 9 7 3 ) i n the

B r a z o s R i v e r s e c t i o n . T h e m o s t c o m p l e t e b i o c h r o n o ­

l o g y f o r t h e K / T l i m i t r o p h e s u c c e s s i o n o f t h e G u l f

C o a s t a l P l a i n i s r e p r e s e n t e d b y t h e c o n t i n u o u s b i o ­

c h r o n o l o g y f o u n d a t A r r o y o P e d r e g o s o ( A l d a m a —

M a n u e l a r e a . S i e r r a d e T a m a u l i p a s ) a n d B u r g o s B a s i n

s e c t i o n s .

A c c o r d i n g l y , t h e K / T b o u n d a r y e v e n t a s d e f i n e d

a b o v e ( G a m p e r , 1 9 7 7 ) i s r e p r e s e n t e d b y B i o z o n e K 2 9

w h i c h c o n t a i n s t h e f o l l o w i n g b i o h o r i z o n s : 1 ) ^ . ma­

yaroensis—¥ PJ); 2 ) s m a l l g l o b i g e r i n a s — F A D ; 3 ) A. ma­

yaroensis -LAY); 4 ) "G. "fringa-FAD; 5 ) "G. " minu-

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY ACROSS THE K/T BOUNDARY 35

tula-FAD; a n d 6) P. eugubina—FAD. T h e s e b i o h o r i z o n s subdivide B i o z o n e K 2 9 i n t o five i n t e r v a l — s u b z o n e s : K29A, , K 2 9 A 2 , K 2 9 B i , K 2 9 B 2 a n d K 2 9 B 3 ( F i g . 5 ) . The b i o h o r i z o n s i d e n t i f i e d i n s o m e o f t h e s t u d i e d s e c ­tions are s h o w n i n figure 1 1 . I n c o n t r a s t , t h e B r a z o s River s e c t i o n o f T e x a s a n d t h e A l a b a m a s u c c e s s i o n lack K 2 9 A ] , K 2 9 A 2 , K 2 9 B , , K 2 9 B 2 a n d p o s s i b l y K29B3 ( P e s s a g n o et al., 1 9 9 0 ; O l s s o n a n d L i u , 1 9 9 0 ) .

2.— Biochronological framework. T h e l a c k o f a s t a b l e b iochrono logy a c r o s s t h e K / T b o u n d a r y i n t e r v a l h a s become m o r e e v i d e n t i n r e c e n t y e a r s , r e f l e c t i n g t h e poor u n d e r s t a n d i n g o f t h e t i m e e q u i v a l e n c y o f t h e b i ­ozonal s c h e m e s p r o p o s e d b y d i f f e r e n t a u t h o r s . A l s o , it has b e c o m e e v i d e n t t h a t t w o o f t h e b e s t k n o w n K / T boundary s e c t i o n s . E l K e f a n d B r a z o s R i v e r , h a v e marked f a u n a l d i f f e r e n c e s ( K e l l e r , 1 9 8 8 a , b ; 1 9 8 9 ) .

T h e p l a n k t o n i c f o r a m i n i f e r a l f a u n a o b t a i n e d at Arroyo El M i m b r a l l a c k s o f Abathomphalus mayaroen­sis. B a s e d o n t h e f a u n a l a s s e m b l a g e ( p r e s e n c e o f Hel-vetiella gansseri a n d t h e a b s e n c e o f Abathomphalus mayaroensis), a n d i t s c o r r e l a t i o n w i t h s i m i l a r s e t t i n g s in the c i r c u m - G u l f , w e c o n c l u d e t h a t t h e u p p e r m o s t Méndez ( U n i t 1) u n d e r l y i n g t h e t e k t i t e — b e a r i n g u n i t (Unit 2) is r e f e r a b l e t o B i o z o n e K 2 7 . T h e r e m o v a l of B i o z o n e K 2 8 a n d K 2 9 m a t e r i a l s d u e t o a n i m p a c t wave is a p o s s i b i l i t y ; h o w e v e r , t h e c a l c a r e n i t e l a y e r (Unit 2) a l so l a c k s a b a t h o m p h a l i d s a n d c o n t a i n s a f a u ­na s imi lar a s t h e M é n d e z b e l o w . I n a d d i t i o n , s a m p l e EM30—20 , i m m e d i a t e l y a b o v e U n i t 3 , c o n t a i n s t h e same f a u n a l a s s e m b l a g e ( F i g . 9 a n d 1 0 ) i m p l y i n g a continuous K 2 7 b i o z o n e t h r o u g h o u t t h e c l a s t i c , t s u n a ­mi bed. T h u s , m i x i n g o f t h e m i c r o f o s s i l s i s n o t a s e v i ­dent s ince t h e " r e w o r k e d " t a x a b a s i c a l l y c o r r e s p o n d to a b i o c h r o n o l o g i c a l l y c o n t i n u o u s K 2 7 s u c c e s s i o n .

T h e p r e s e n c e o f H. gansseri i n t h e u p p e r o n e m e t e r section, b e l o w t h e p o s t u l a t e d K / T b o u n d a r y ( t h e c l a y unit) at El K e f ( K e f l e r , 1 9 8 8 b , t a b l e 1) m a y b e a n i n ­dication o f s e d i m e n t o l o g i c a l / s t r a t i g r a p h i c a l c o m p l i c a ­tions in t h i s i n t e r v a l a t E l K e f s e c t i o n . H. gansseri i s known to r a n g e f r o m K 2 7 t o l o w e r K 2 9 A ; b e c a u s e of the a b s e n c e o f ^4. mayaroensis it i s p o s s i b l e t h a t s a m ­ples 5 2 9 - 5 3 5 o f E l K e f a r e a s s i g n a b l e t o B i o z o n e K27, or e l s e t h e r e i s r e w o r k i n g o f m a t e r i a l f r o m t h i s biozone i n t o t h e K 2 9 A . L i k e w i s e , t h e u p p e r m o s t M a a s t r i c h t i a n s e d i m e n t s o f t h e B r a z o s R i v e r s e c t i o n are a s s i g n a b l e t o B i o z o n e K 2 7 , b e c a u s e o f t h e s a m e criteria.

3 . - Chronostratigraphy. T h i r t y — s i x y e a r s a g o , a t t h e Geo log ica l C o n g r e s s i n M e x i c o c i t y , B r o t z e n ( 1 9 5 6 , p. 6) p r e s e n t e d t h i s q u e s t i o n : I s t h e D a n i a n C r e t a ­

c e o u s o r T e r t i a r y ? I n s p i t e o f m a j o r e f f o r t s t o r e s o l v e t h e M a a s t r i c h t i a n / D a n i a n b o u n d a r y ( B e r g g r e n , 1 9 6 2 , 1 9 6 4 ; B i r k e l u n d a n d B r o m l e y , 1 9 7 9 ; C h r i s t e n s e n a n d B i r k e l u n d , 1 9 7 9 ) , t h e c h r o n o s t r a t i g r a p h i c p o s i t i o n o f t h e D a n i a n s t a g e r e m a i n e d u n r e s o l v e d u n t i l r e c e n t l y ( P e r c h - N i e l s e n , 1 9 8 2 , 1 9 8 4 ) . T h e fac t t h a t D e s o r ( 1 8 4 7 , p . 1 8 0 ) o r i g i n a l l y p l a c e d t h e D a n i a n i n t h e C r e ­t a c e o u s i n f l u e n c e d B r o t z e n ( 1 9 5 6 , p . 1) t o r e t a i n t h e s a m e p o s i t i o n ; h o w e v e r , J e l e t z k y a n d S y l v e s t e r -B r a d l e y {in B r o t z e n , 1 9 5 6 , p . 8 ) a r g u e d a g a i n s t B r o t -z e n ' s p o s i t i o n a n d s u g g e s t e d a p p e a l i n g t o t h e S t r a t i g ­r a p h i c C o m m i s s i o n t o c o r r e c t D e s o r m i s t a k e a n d t o p l a c e t h e D a n i a n i n t o t h e l o w e r m o s t C e n o z o i c . A c ­c o r d i n g l y , t h e b a s e o f t h e D a n i a n s h o u l d b e p l a c e d at t h e first e v o l u t i o n a r y a p p e a r a n c e o f t a x a , i . e . , l a c k ­i n g u m b i l i c a l s t r u c t u r e s p r e s e n t i n C r e t a c e o u s H e d b e r g e l l a c e a ( s e n s u L o n g o r i a a n d G a m p e r , 1 9 7 5 ) o r n e w t y p e s o f w a l l s t r u c t u r e .

P l a n k t o n i c f o r a m i n i f e r a c h a n g e s t h r o u g h o u t B i o z o n e K 2 9 i n d i c a t e t h a t a m a j o r e v o l u t i o n a r y e v e n t t o o k p l a c e b e t w e e n t h e A. mayaroensis-LAD a n d t h e P. eugubina—FAD. T h i s e v e n t , w h e t h e r c a t a s t r o p h i c o r n o t , w h e t h e r t e r r e s t r i a l o r e x t r a t e r r e s t r i a l in n a t u r e , t o o k p l a c e i n t h e C r e t a c e o u s b e c a u s e a c c o r d i n g t o m o s t T e r t i a r y w o r k e r s t h e P. eugubina—FAD d e f i n e s t h e b a s e o f t h e T e r t i a r y . T h i s p r o c e d u r e a g r e e s w i t h t h e r e c o m m e n d a t i o n o f t h e I S S C ( 1 9 7 6 ) o f p l a c i n g c h r o ­n o s t r a t i g r a p h i c b o u n d a r i e s a t n a t u r a l b r a k e s s u c h a s t h o s e g i v e n b y t h e first e v o l u t i o n a r y a p p e a r a n c e o f n e w t a x a . O n t h e o t h e r h a n d , w e r e j e c t t h e p r a c t i c e o f p l a c i n g t h e K / T b o u n d a r y a t t h e " e x t i n c t i o n o f C r e t a c e o u s t a x a " a s a d v o c a t e d b y o t h e r w o r k e r s ( S m i t , 1 9 8 2 ; K e l l e r , 1 9 8 8 a , b ) m a i n l y b e c a u s e it r e p r e s e n t s d i a c h r o n o u s s u r f a c e s . S i m i l a r l y , t h e s o l e p r e s e n c e o f b a r r e n i n t e r v a l s s u c h a s d i a g n o s t i c c l a y h o r i z o n s s h o u l d n o t b e u s e d a s s y n c h r o n o u s c h r o n o s t r a t i g r a p h i c m a r k ­e r s t o s u p p l a n t b i o h o r i z o n s ; t h e y s h o u l d b e r e g a r d e d a s o f l o c a l , o r a t t h e m o s t , o f r e g i o n a l s y n c h r o n e i t y u n l e s s p r o v e n o t h e r w i s e ( I n t e r n a t i o n a l S u b c o m m i s ­s i o n o n S t r a t i g r a p h i c C l a s s i f i c a t i o n , 1 9 7 6 , p . 9 2 a n d 9 3 ) . U n f o r t u n a t e l y , it h a s b e c o m e a p r a c t i c e t o c o n ­s i d e r t h e s e l i t h o h o r i z o n s a s a c c u r a t e c h r o n o s t r a t i g ­r a p h i c i n d e x e s t o d e f i n e t h e K / T b o u n d a r y a s i n d i ­c a t e d b y S m i t ( 1 9 8 2 , p . 3 7 5 ) : "Whether the C/T bound­ary should be placed above or below G. cretacea Zone was discussed by Smit and ten Kate (1982) who concluded that the boundary should be placed at the major extinction event at the base of the C/T boundary clay and not at the entry of the first true Tertiary species at the top of the C/T clay. Is it already a common practice where no fauna is found in the

36 LONGORIA AND GAMPER

c l a y t o p l a c e t h e C / T b o u n d a r y a t t h e b a s e o f t h e c l a y ( A l v a r e z a n d o t h e r s , 1 9 8 0 ) " . T h e u s e o f t h e a f o r e ­m e n t i o n e d c r i t e r i a h a s l e d t o a m i s s u n d e r s t a n d i n g o f t h e c o n c e p t o f c h r o n o s t r a t i g r a p h i c b o u n d a r y a s a p ­p l i e d t o t h e K / T b o u n d a r y . T h o s e a u t h o r s m a d e a f l a g r a n t v i o l a t i o n t o t h e s t r a t i g r a p h i c r u l e s b y u s i n g b a r r e n l i t h o h o r i z o n s t o d e f i n e c h r o n o s t r a t i g r a p h i c b o u n d a r i e s . I t h a s b e e n a c o m m o n p r a c t i c e f o r n e a r ­l y t w o c e n t u r i e s t o p l a c e s t a g e a n d s e r i e s b o u n d a r i e s a t f o s s i l i f e r o u s h o r i z o n s .

T h e n , t h e G. cretacea Z o n e o r PO a s d e f i n e d b y S m i t ( 1 9 8 2 ) s h o u l d b e p l a c e d i n t h e u p p e r m o s t C r e t a c e o u s b e c a u s e : 1 ) i t s f a u n a l a s s e m b l a g e is m a d e o f C r e t a ­c e o u s t a x a , a n d 2 ) i t s c h r o n o s t r a t i g r a p h i c p o s i t i o n i s b e l o w t h e P. eugubina—FAD. T h e p r a c t i c e o f p l a c i n g t h e K / T b o u n d a r y at t h e b a r r e n c l a y s h o u l d b e a v o i d ­e d . W e r e c o g n i z e t h a t i n s o m e c a s e s l i t h o h o r i z o n s m a y c o r r e s p o n d t o t h e c h r o n o s t r a t i g r a p h i c — p a l e o n t o l o g i -c a l b o u n d a r y , b u t t h e i r s o l e o c c u r r e n c e i n a s e c t i o n s h o u l d n o t b e u s e d a s t h e b o u n d a r y m a r k e r . F o r e x ­a m p l e , K e l l e r ( 1 9 8 9 ) h a s s h o w n t h a t t h e l i t h i c , c h e m ­i c a l , a n d b i o e v e n t s a r e n o t s y n c h r o n o u s b e t w e e n t h e El K e f a n d B r a z o s R i v e r s e c t i o n s . I n a d d i t i o n , S m i t ' s a r g u m e n t i n f a v o r o f p l a c i n g h i s PO z o n e i n t h e T e r ­t i a r y ( S m i t , 1 9 8 2 , p . 3 3 5 ) s h o u l d b e r e j e c t e d b e c a u s e n o t o n l y it c o n t r a d i c t s t h e r e c o m m e n d a t i o n s b y t h e I n t e r n a t i o n a l S t r a t i g r a p h i c G u i d e , b u t a l s o t h e P. eugu­bina—FAD h a s p r e c e d e n c e . T h i s p r a c t i c e h a s l e d S m i t eial. ( 1 9 9 2 , p . 9 9 , 1 0 3 ) t o s t a t e t h a t c h a n g e s i n p h y s ­i c a l s t r a t i g r a p h y s u c h a s t h e c l a s t i c u n i t a t A r r o y o E l M i m b r a l s e c t i o n " i s o f K / T b o u n d a r y a g e " (op. cit., p . 1 0 0 ) , i n s p i t e o f t h e f a c t t h a t t h e y o n l y l i s t e d t w o l o n g r a n g i n g p l a n k t i c t a x a . H o w e v e r , f r o m a b i o c h r o n o l o g i c a l v i e w — p o i n t , i . e . t h e P. eugubina-FAD, t h e c h r o n o s t r a t i g r a p h i c K / T b o u n d a r y i n t h e E l M i m b r a l s e c t i o n o c c u r s w i t h i n t h e V e l a s c o F o r ­m a t i o n , a b o v e t h e c l a s t i c u n i t a s s h o w n i n figure 1 0 .

I n s u m m a r y , e v o l u t i o n a r y t r e n d s o f p l a n k t o n i c f o r a m i n i f e r a r e v e a l t h a t : 1 ) t a x a a s s o c i a t e d w i t h PO z o n e o f S m i t ( = B i o z o n e K 2 9 B s e n s u L o n g o r i a , 1 9 7 7 , 1 9 8 4 ) b e a r m o r p h o l o g i c f e a t u r e s t y p i c a l o f C r e t a c e o u s p l a n k t o n i c f o r a m i n i f e r a ; t h u s , t h e s m a l l g l o b i g e r i n a s f r o m t h e u p p e r m o s t p a r t o f t h e K 2 9 B i o z o n e s h o u l d b e p l a c e d i n t h e M a a s t r i c h t i a n ; 2 ) t h e K / T b o u n d a r y s h o u l d b e p l a c e d a t t h e first e v o l u t i o n a r y a p p e a r a n c e o f n e w t a x a , i . e . , t h e P. eugubina—FAD.

4 . — T i m i n g o f t h e b o l i d e i m p a c t i n t h e r e g i o n . Pope etal. ( 1 9 9 1 ) a n d H i l d e b r a n d a/. ( 1 9 9 1 ) a r g u e d f o r e v i d e n c e o f a m a j o r i m p a c t c r a t e r a t t h e K / T b o u n d a r y i n n o r t h e r n Y u c a t á n a n d s u g g e s t e d t h a t t h e

C h i c x u l u b c i r c u l a r s t r u c t u r e o n t h e n o r t h c o a s t o f Y u ­c a t á n r e p r e s e n t s t h e a c t u a l i m p a c t s t r u c t u r e . F u r t h e r ­m o r e , H i l d e b r a n d a n d S t a n s b e r r y ( 1 9 9 2 ) h a v e h y p o t h e s i z e d t h e s i z e o f t h e b o l i d e i m p a c t t o h a v e b e e n i n t h e o r d e r o f 2 5 k m i n d i a m e t e r c a u s i n g a n o c e a n s p l a s h o f g l o b a l d i m e n s i o n .

O n t h e o t h e r h a n d , p r o p o n e n t s o f t h e i m p a c t h y ­p o t h e s i s h a v e f a c e d s e r i o u s o p p o s i t i o n . F o r e x a m p l e . O f f i c e r ( 1 9 9 2 ) h a s e x p r e s s e d t h a t m o s t o f t h e g e o l o g ­i c a l f e a t u r e s a t t r i b u t e d b y S m i t et al. ( 1 9 9 2 ) a n d A l ­v a r e z et al. ( 1 9 9 2 ) t o a n e x t r a t e r r e s t r i a l b o l i d e i m p a c t c o u l d b e a c c o u n t e d f o r i n v o k i n g b y t e r r e s t r i a l p r o c e s s ­e s . A l t h o u g h w e a r e n o t i n t h e p o s i t i o n t o a r g u e for , n e i t h e r i n f a v o r o r a g a i n s t of , t h e n a t u r e o f t h e p h y s ­i ca l d i s t u r b a n c e , o u r s t u d i e s o n t h e f o r a m i n i f e r a l f a u ­n a s o f t h e A. mayaroensis/P. eugubina I n t e r v a l — Z o n e o f d i f f e r e n t r e g i o n s o f M e x i c o ( F i g s . 8 , 1 1 ) s e e m t o i n ­d i c a t e t h a t d e e p e r w a t e r s u c c e s s i o n s s h o w l e s s e f f e c t s o f t h e b o l i d e i m p a c t . F o r e x a m p l e , t y p i c a l M é n d e z s e c t i o n s o f E a s t e r n M e x i c o ( M é n d e z S t a t i o n a n d A r r o y o P e d r e g o s o ) c o n t a i n i n g m o r e t h a n 9 0 p e r c e n t o f p l a n k ­t o n i c f o r a m i n i f e r a , a n d t h u s a t t r i b u t e d t o b a t h y a l d e p t h s , c o n t a i n l e s s p h y s i c a l p e r t u r b a t i o n i f c o m p a r e d w i t h A r r o y o E l M i m b r a l a n d B r a z o s R i v e r s e c t i o n s a s w e l l a s R a n c h o N u e v o a n d a d j a c e n t s e c t i o n s i n c e n ­tra l N u e v o L e ó n s t a t e .

I f t h e p h y s i c a l f e a t u r e s s u c h a s t h e c l a s t i c u n i t o b ­s e r v e d i n t y p e I ( p h y s i c a l l y d i s t u r b e d ) s e c t i o n s a r e p r o v e n t o b e a d e p o s i t d u e t o a n e a r b y b o l i d e i m p a c t a s i n t e r p r e t e d b y A l v a r e z et al. ( 1 9 9 2 ) , t h e n , a n i m ­p o r t a n t f a c t r e m a i n s t o b e a d d r e s s e d : W h y c l o s e l y l o ­c a t e d s e c t i o n s d o n o t u n i f o r m l y s h o w e v i d e n c e o f t h e t s u n a m i ? F o r e x a m p l e , t h e A r r o y o E l M i m b r a l , A r r o ­y o P e d r e g o s o a n d M a g i s c a t z i n s e c t i o n s i n t h e T a m p i c o M i s a n t l a b a s i n ( F i g . 1 2 ) s h o w c o n t r a s t i n g K / T s u c c e s s i o n s . O f t h e t h r e e s e c t i o n s , o n l y t h e A r r o ­y o E l M i m b r a l d i s p l a y s p h y s i c a l e v i d e n c e o f d i s t u r b ­a n c e . T h e a n s w e r m a y b e t h a t , e i t h e r t h e i m p a c t d i d n o t o c u r r e d , o r e l s e , t h e C h i c x u l u b i m p a c t t o o k p l a c e b e f o r e B i o z o n e K 2 9 A ; c o n s e q u e n t l y , t h e b o l i d e i m ­p a c t d i d n o t c o i n c i d e p r e c i s e l y w i t h t h e c h r o n o s t r a ­t i g r a p h i c K / T b o u n d a r y . F u r t h e r m o r e , e v e n i f w e a c ­c e p t t h e i m p a c t a s h a v i n g o c c u r e d , t h e e f f e c t o f t h e m e g a w a v e p r o d u c e d b y t h e b o l i d e w a s m i n o r i n c o m ­p a r i s o n w i t h t h e g l o b a l e f f e c t e x p e c t e d i n s u c h a n i m ­p a c t . A t A r r o y o E l M i m b r a d w e f o u n d l i t t l e e v i d e n c e o f m i x i n g o f s e v e r a l f o s s i l h o r i z o n s a s w o u l d b e e x ­p e c t e d a c c o r d i n g t o t h e i n t e r p r e t a t i o n b y M a u r r a s s e a n d S e n ( 1 9 9 1 ) w h o a d v a n c e d t h e i d e a t h a t t h e C r e t a ­c e o u s / T e r t i a r y b o u n d a r y c o r r e s p o n d s t o a m i x i n g o f

PLANKTONIC FORAMINIFERAL BIOCHRONOLOGY ACROSS THE K/T BOUNDARY 37

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Fig. 11.— Biocorrelation of K/T boundary sections in Mexico. Outcrop names same as in figure 8. Biohorizons include: bh2 = small globigerinas-FAD; bh3 = A. mayaroensis-LAD; bh4 = "G. "fringa-FAD; bh5 = "G. " minutula-FAD; bh6 = P. eugubina—FAD. The K/T boundary is placed at the latter biohorizon. Lithic units include: 1) micritic limestone; 2) sandy shale; 3) calcareous shale; 4) chalky mudstone; 5) flysch; 6) calcareous nodules; 7) disconformity.

several foss i l h o r i z o n s d u e t o t h e g e n e r a t i o n o f a

m e g a - w a v e . T h e r o c k t e x t u r e a n d t h e " r e w o r k i n g "

of p lankton ic f o r a m i n i f e r a o b s e r v e d i n U n i t s 2 a n d

3 of El M i m b r a l c a n e a s i l y b e e x p l a i n e d a s t h e r e s u l t

of a turbidity c u r r e n t w h i c h c o u l d h a v e b e e n p r o d u c e d

by other c a u s e s s u c h a s s e a l e v e l f l u c t u a t i o n s . I n a d ­

dition, t h e o b s e r v e d b i o c h r o n o l o g y i m p l i e s t h a t t h e

extraterrestrial i m p a c t i s n o t c o i n c i d e n t w i t h t h e K / T

boundary b i o e v e n t . A l t h o u g h m a j o r b i o l o g i c a l a n d

chemical c h a n g e s a r e l i k e l y t o c o n c u r w i t h t h e p o s t u l a t ­

ed arrival o f t h i s b o l i d e , t h e b i o c h r o n o l o g i c d a t a o b ­

tained i n d i c a t e t h a t t h e r e is a d i a c h r o n i c r e l a t i o n s h i p

between t h e p h y s i c a l e v i d e n c e o f t h e i m p a c t a n d t h e

changes in p l a n k t o n i c f o r a m i n i f e r a l a s s e m b l a g e s o b ­

served at t h e K / T b o u n d a r y s e c t i o n e x p o s e d at A r r o ­

yo El M i m b r a l o f S i e r r a d e T a m a u l i p a s . C o n s e ­

quendy, t h e d i r e c t r e l a t i o n s h i p b e t w e e n t h e i m p a c t

and the c h a n g e s i n p l a n k t o n i c foraminiferal f a u n a

needs to b e r e — e v a l u a t e d .

5.— Characterizing the type of mass extinction. T h e

a n a l y s i s o f t h e e n t i r e A. mayaroensis/P. eugubina I n t e r ­

v a l — Z o n e i n c o n t i n u o u s s e c t i o n s t h r o u g h o u t t h e

C o a s t a l P l a i n o f M e x i c o l a c k i n g p h y s i c a l d i s t u r b a n c e

o f s t r a t i g r a p h y , i n d i c a t e s t h a t t h e a b r u p t c h a n g e o f

f o r a m i n i f e r a l f a u n a w a s a n t e d a t e d b y t h e first a p p e a r ­

a n c e o f s m a l l g l o b i g e r i n a s ( n o n P. eugubina), b e f o r e

t h e " m a s s i v e e x t i n c t i o n " o f l a r g e i n d i v i d u a l s ( i . e . ,

t h e A. mayaroensis L A D ) . T h u s , t h e e x t i n c t i o n s a n d

a p p e a r a n c e s o f n e w t a x a w e r e t a k i n g p l a c e b e f o r e t h e

K / T b o u n d a r y . S e r i o u s d i s c r e p a n c i e s i n p l a n k t o n i c

f o r a m i n i f e r a l b i o c h r o n o l o g y , a n d m a l p r a c t i c e i n

c h r o n o s t r a t i g r a p h y i m p e d e at t h i s t i m e t o d r a w d e f i n i ­

t i v e c o n c l u s i o n s o n t h e i n f l u e n c e o f a n e x t r a t e r r e s t r i ­

al i m p a c t a s t h e d i r e c t c a u s e o f f o r a m i n i f e r a l c h a n g e s

a t t h e e n d o f t h e C r e t a c e o u s . C o n s e q u e n t l y , o t h e r

c a u s e s b e s i d e s t h e e x t r a t e r r e s t r i a l s h o u l d b e c o n s i d e r e d

t o e x p l a i n t h e p a t t e r n s o f f a u n a l t r a n s f o r m a t i o n

b e t w e e n t h e M a a s t r i c h t i a n a n d t h e D a n i a n .

3 8 LONGORIA AND GAMPER

T A M_A U L I P A S f ti-opjco de cancer

N v o . i P r o g r e s o microondas * ^ ^^o,

C a l l e

M a q i

2 3 ' 0 0

A l d a m a

9 8 ' 4 0

anuel 9 8 ' 0 0

Fig. 12.— Schematic index map of the Sierra de Tamauli­

pas area showing the close geographic location of the Arroyo

El Mimbral, Arroyo Pedregoso and Magiscatzin K/T boun­

dary sections.

W e c o n s i d e r t h a t m o r e t h a n o n e g r o u p o f C r e t a ­

c e o u s p l a n k t i c s u n d e r w e n t n e o t e n y a n d t h u s , D a n i a n

p l a n k t i c s a r e r e g a r d e d a s a p o l y p h y l e t i c g r o u p ( F i g . 3 ) .

W e a l s o r e g a r d t h e s m a l l g l o b i g e r i n a s a s r e p r e s e n t i n g

a j u v e n i l e s t a g e ( n e o t e n i c ? ) o f h i g h l y e v o l v e d

C r e t a c e o u s H e d b e r g e l l a c e a a s p r e v i o u s l y p o s t u l a t e d

b y B e r g g r e n ( 1 9 6 2 ) . I n a n y c a s e , w e h y p o t h e s i z e d t h a t

t h e " e x t i n c t i o n " o f t h e f o r a m i n i f e r a l t a x a d u r i n g t h e

K / T b o u n d a r y e v e n t c o r r e s p o n d s t o a b i o l o g i c a l t r a n s ­

f o r m a t i o n — i . e . , n e o t e n y ( G a m p e r — L o n g o r i a a n d

L o n g o r i a , 1 9 8 4 ) . H o w e v e r , t h i s h y p o t h e s i s n e e d s t o

b e t e s t e d i n s e c t i o n s w i t h w e l l p r e s e r v e d f o r a m i n i f e r a l

f a u n a s . N o n e t h e l e s s , o u r d a t a d o n o t s u p p o r t a c a t a s ­

t r o p h i c e v e n t b u t r a t h e r , a g r a d u a l c h a n g e f o r a b o u t

5 0 0 , 0 0 0 y r .

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ACKNOWLEDGMENTS

Supported by a faculty award from the School of Arts and Sciences, Florida International University.

W e gready thank the comments and review of two anonymous members of the A M G P ' s "Comisión de Estudios Técnicos".

Manuscrito recibido por la Asociación: N o v i e m b r e 10, 1992 Manuscrito revisado y recibido: N o v i e m b r e 24 , 1992 Mtinuscrito aceptado: N o v i e m b r e 27 , 1992