Transcript
Page 1: The Toarcian Aalenian boundary in the Betic Cordillera (Southern Spain)

THE TOARCIAN AAI,ENIAN BOUNDARY IN THE BETIC CORDII,I,ERA

(SOUTHERN SPAIN)

RAFAEL GARCILA-GOMEZ, ANTONIO P. JIMENEZ, ASUNCION LINARES, PASCUAL RIVAS & JOSE SANDOVAL.

GARC~A-G(~MEZ R., JIMI~NEZ A.P., LINARES A., RIVAS P. & SANDOVAL J. 1994 - The Toarc ian Aa len i an boundary in the Betic Cordi l lera (Southern Spain) [La t rans i t ion Toarcien-Aal~nien dans les Cha~nes B~tiques (Sud de l 'Espagne)] . GEOBIOS, M. S. 17 : 211-222.

ABSTRACT

The Toarc ian-Aalen ian boundary has been s tudied accurately for the f irst t ime, in three a reas of the Subbet ic domain (Southern Spain). The successive ammoni te assemblages have pe rmi t t ed to d i s t inguish the uppe rmos t Toarcian, Aalens is Zone (Mactra, Aalens i s and Buckmani Subzones), and Lower Aalenian , Opa l inum Zone, (Opali- num and Comptum Subzones). In the Aalens is Zone, ammoni te fauna is main ly represen ted by Pleydellia toge ther wi th some Catulloceras and Hamrnatoceras. Lytocera t idae and Phyl locera t idae also occur. In the lower Opa l inum Zone, the fauna is a lways scarce ; on the contrary, Comptum Subzone has yie lded an abundan t fauna of Leioceras, Tmetoceras, Erycites, Hamrnatoceras, Spinamrnatoceras, Phylloceras and Lytoceras.The l imit Toarc ian-Aalen ian is clearly defined by the rep lacement of Pleydellia and Catulloceras by Leioceras and Tmetoceras.

KEY-WORDS : SUBBETIC, SOUTHERN SPAIN, TOARCIAN - AALENIAN BOUNDARY, AMMONITE - FAUNA, BIOSTRATIGRAPHY.

RI~SUMt~

On a ~tudi~ la t r ans i t ion Toarcien - Aaldnien dans quat re coupes de diffdrentes rdgions du Subb~tique Moyen : Cerro M~ndez (deux coupes) et Zegr£ Norte (Province de Granada) et S ie r ra de Ricote (Province de Murcia). Le passage Toarcien - Aal~nien montre des facies marno ca]caires dans deux coupes, t andis que ]es deux au t r e s mont ren t des faci6s "ammonitico rosso". I1 n 'existe pas de discontinui t6s s t r a t ig raph iques apparen tes , bien que les sed iments du Toarcien sup6r ieur - Aal6nien infer ieur peuvent ~tre tr~s condensds, voire m~me manque r pa r sui te de lacunes s t r a t i g r aph iques dans d ' au t res points du Subb6tique (Subb~tique externe). Les associat ions d 'ammoni- tes trouv6es pe rme t t en t une ident i f icat ion claire de la zone h Aalensis (sous-zones h Mactra , Aalensis et Buckma- ni), du Toarcien sup~r ieur e t de la zone h Opal inum (sous-zones h Opa l inum et Comptum) de l 'Aal6nien infdrieur. La faune de la zone g Aalens i s est dominde pa r des formes pla~yc6nes a p p a r t e n a n t au genre Pleydellia, encore que la faune soit tr~s pauvre dans la sous-zone fi Buckmani ; de plus, on trouve de fr6quentes formes serpent icSnes du genre Catulloceras, et quelques Hammatocera t inae , Eryci t inae, PhyUocerat inae et Lytocerat inae. Remarquons que dans les faci6s ammoni t ico rosso la faune est moins abondante . Dans les p remiers n iveaux de l 'Aal~nien (sons- zone & Opal inum) la faune es t tr~s peu abondante . On ne t rouve que quelques Leioceras et de ra res Hammatoce ra - t idae, a ins i que des Phyl locera t idae et Lytocerat idae. Dans la sous-zone h Comptum, la faune devient tr~s abon- dan te mais tr6s peu vari6e, domin~e pa r des formes oxycSnes du genre Leioceras, coexistant avec des platyc6nes (Tmetoceras et Hammatoce ra t i nae ) et avec des Phyl locera t inae et des Lytocerat inae. La l imite Toarcien - Aal~nien se t rouve c la i rement d~fmie par le r emplacement des dern ie rs Pleydellia et Catulloceras par les p remie r s Leioce- ras et Tmetoceras.

MOTS-CL]~S : SUBBt~TIQUE, SUD DE L'ESPAGNE, TRANSITION TOARCIEN - AALt~NIEN, AMMONITES, BIOSTRATIGRAPHIE

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TOJ A E N

O

TO G R A N A D A

Figure 1 - Geographical and geological location of the studied sections : A. Sierra de Ricote. B. Cerro M6ndez. C. Zegri Norte. 1, Guadalquivir Unit ; 2, Postorogenic and volcanic rocks ; 3, Nevado-Filabride ; Alpujfirride ; Malfiguide and Rondaide ; 4, Campo de Gibraltar Complex ; 5, Hercinian Massif ; 6, Tabular cover ; 7, Prebetic ; 8, Intermediate Units ; 9, External Subbetic ; 10, Middle Subbetic ; 11, Internal Subbetic ; 12, Penibetic. Localisation gdographique et gdologique des sections dtudides : ,4, Sierra de Ricote ; B. Cerro Mgndez ; C. Zegri Norte ; 1, Unitd du Guadalquivir ; 2, Postorogdnique et roches volcaniques ; 3, Nevado- Fildbride ; Alpujdrride ; Maldguide et Rondaide ; 4, Flysch du Champ de Gibraltar ; 5, Mass i f hercynien ; 6, Couverture tabulaire 7, Prdbdtique ; 8, Unitds Intermgdiaires ; 9, SubbOtique externe ; 10, Subbdtique rnoyen ; 11, Subbgtique interne ; 12, Penibgtique.

I N T R O D U C T I O N

T h e r e h a v e been m a n y s tudies abou t the U p p e r Toa rc i an b i o s t r a t i g r a p h y and a m m o n i t e f a u n a in the M e d i t e r r a n e a n and S u b m e d i t e r r a n e a n pro- v inces a n d specif ical ly in the Betic Cordi l lera . A m o n g the m o s t r ecen t it is w o r t h m e n t i o n i n g those by Rivas 1972 ; J i m 6nez & Rivas 1981 ; Ga rc i a Gdmez & Rivas 1981a,b ; E lmi et a l . 1989 ; Goy & M a r t i n e z 1990 ; Goy e t al . 1988, etc. The- re a re also seve ra l notes on the Bet ic Aalenian , such as L ina r e s 1972 ; L ina re s e t a l . 1987, etc. L ikewise r ecen t s tud ies of the Toarc i an -Aa len ian b o u n d a r y h a v e been ca r r i ed out by O h m e r t 1985 ;

Henr iques 1989 ; E lmi & Ru l l eau 1991 ; C re s t a 1991 ; Goy & U r e t a 1991 ; O h m e r t e t a l . 1991. However , a p a r t f rom a shor t and l i t t le k n o w n pa- pe r by L inares and Rivas 1973 on the Cer ro M6ndez 1 section, (in which dupl ica t ions of the ser ies by fau l t ing was not noticed), o the r d a t a on the Toarc i an -Aa len ian b o u n d a r y n e i t h e r exis t in tlie Betic Cordi l lera nor in o the r typ ica l ly Medi- t e r r a n e a n areas . There fore a s t u d y a b o u t this li- m i t s eems ve ry necessary .

In th is p a p e r we s tudy for f i rs t t i m e the Toar- c ian-Aalen ian b o u n d a r y in the Betic Cord i l l e ra b y m e a n s of careful sampl ing , bed by bed, of four

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Figu re 2 - Li thological success ion and a m m o n i t e s ver t ica l r a n g e in the Upper Toarc ian - Lower A a l e n i a n of the S i e r r a de Ricote section, Middle Subbe t ic province of Murcia . Succession lithologique et rgpartition stratigraphique des ammonites du Toarcien supdrieur - Aaldnien infdrieur dans la section de la Sierra de Ricote, province de Murcie (Subbdtique moyen).

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stratigraphic sections and the detailed analysis of their successive ammonite assemblages. This in turn will also allow us to determine more clearly the behaviour of the Lower-Middle Jurassic boundary in the Mediterranan cordilleras.

GEOGRAPHICAL AND GEOLOGICAL SETTING

After a general view of different localities where the Toarcian and Aalenian materials outcrop, we have selected four stratigraphic sections (Fig. 1) in three areas of the Subbetic domain (Southern Spain) on the base of their stratigraphic continui- ty, bedding, exposition and faunal content. In this way they display the most adequate condi- tions to carry out a fine stratigraphical s tudy ba- sed on ammonite fauna vertical range. These are : Northern Zegri (Province of Granada), two sec- tions in Cerro M~ndez (Alamedilla, province of Granada) and the Sierra de Ricote (province of Murcia).

From the paleogeographic point of view (Fig. 1), all the sections belong to the Middle Subbetic (the central part of the external zones of the Be- tic Cordillera), this being the only Betic domain with a more or less continuous pelagic sedimen- tation during the Upper Toarcian-Lower Aale- nian. Data from other sectors of the Middle and the External Subbetic have been added. In this last domain and in the Internal Subbetic the Up- per Lias and Lower Dogger sections are very con- densed : the Upper Toarcian-Lower Aalenian may even coincide with a sedimentary gap.

LITHOLOGY A N D F A C I E S

SIERRA DE RICOTE (Fig. 2)

In the Sierra de Ricote section (Fig. 1,2) the ma- terials belonging to the Reynesi Zone are 8 to 10 m thick ; where grey marls predominate over marly-limestones. The beds vary in thickness from 10 to 50 cm. The Aalensis Zone is composed of al ternat ing beds of marls and grey marly-lime- stones. At the beginning of the Zone marly-lime- stones predominate over marls, while the reverse is true for the upper part of the zone.

CERRO Mt~NDEZ 1. (Fig. 3)

A preliminary note about the biostratigraphy of this section has been published by Linares & Ri- vas 1973, which located the Toarcian-Aalenian boundary between beds 50-58. In Cerro M~ndez 1, the materials belonging to the Upper Toarcian are rather uniform. The Reynesi Zone is approxi- mately 5 m thick with regularly al ternating grey marls and marly-limestones in beds varying in thickness from 20 to 70 cm. The Aalensis Zone is about 15 m thick and calcareous beds may predo- minate locally over detritic ones.

The transition from the Toarcian to the Aalenian comes about in facies similar those of lower beds; if we examine them more closely, we find how- ever that they contain a great amount of Zoophy- cos. Calcareous beds are more de tritic than at lo- wer levels.

In the upper part of the Opalinum Zone (Comp- tum Subzone) we note a sharp transit ion to "am- monitico rosso" facies where a subsequent change in the faunal content can be seen.

CERRO MI~NDEZ 2 (Fig. 4)

This section is located approximatively 1,5 km to the SW of the previous one, within the same pa- leogeographical context. However, both thickness and facies of the Upper Toarcian-Lower Aalenian materials are completely different in the two sec- tions. The materials from the Reynesi Zone (Pseudoradiosa Zone from the Sub-Mediterranean zonations) reveal marly "ammonitico rosso" fa- cies, with a total thickness of 2,5 m, which is much less than the thickness of the materials of the same age in Cerro M~ndez 1. The Aalensis

• Zone is made up of little more than 1 m of par- tially nodulose limestone with resedimented fau- na, where the Aalensis and Buckmani subzones, at least, are condensed. At the base of the Opali- num Zone (which is thicker than in Cerro M~ndez 1) we find beds with aboundant Zoophy- cos. The upper nodular limestones (Figs. 3,4) oc- curs in the Murchisonae Zone while in Cerro M~ndez 1 they are in the Opalinmn Zone (Comp- turn Subzone).

The Aalenian begins with similar facies to those of the uppermost Toarcian, but with a great amount of Zoophycos. In the lower part of the Comptum Subzone the sequence becomes more Calcareous with laminated limestones, becoming more detritic again in the upper part of this subzone and at the base of the Middle Aalenian.

These sharp differences, both in thickness and fa- cies between two paleogeographic locations lying so close together may be explained by the exist- ence of an important distensive phase within the Betic basin, together with the effects tha t vulca- nism (very frequent at this age) could have had on the sea floor marine sedimentation.

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Figure 3 - Lithological succession and an~monltes vertical range in the Upper Toarcian - Lower Aalenian of Cerro M~ndez 1 section, Middle SubbetJc, province of Granada. Succession lithologique et rgpartition stratigraphique des ammonites dans la section du Toarcien sup~rieur - Aaldnien infdrieur de Cerro Mdndez 1, province de Grenade (Subb~tique re,•yen)+

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F i g u r e 4 - L i t h o l o g i c a l s u c c e s s i o n a n d a m m o n i t e s v e r t i c a l r a n g e in t he U p p e r T o a r c i a n - L o w e r A a l e n i a n of C e r r o M ~ n d e z 2, s ec t i on . Succession lithologique et rgpartition stratigraphique des ammonites dans la section du Toarcien supgrieur - Aalgnien infdrieur de Cerro Mgndez 2, province de Grenade (Subb~tique moyen).

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TOARCIAN i =o [ m O ' . , g l , I -< I t - i r" I I ~ i rn~ I I~I C I

AALENIAN r

M U R C H I S O N A E

S T A G E

Z O N E

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i j - I I _ I _ . L E V E L

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0 0 - . . . . . . . . . - - - - O . . . . . . . . . . . .

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Pseudogrammoceras sp Calliphylloceras sp Phylloceras sp Calliphylloceras beatricis Dumortieria latiumbilicata Catulloceras meneghmii Alocolytoceras sp Lytoceras sp Alocolytoceras ophioneum Catulloceras sp Pleydellia subcompta P. aalensis Dumortieria rhodanica Tmetoceras sp Erycites rotundi formis E. fallifax Leioceras sp Leioceras comptum Spinammatoceras sp Sp. tenax Sp. shindewolfi Ludwigia haugi Abbasitoides modestum Pianammatoceras planifo,me Pseudographoeeras umbilicatum Brasilia b r a d f o r d e n s i s

Brasilia similis Malladaites sp Graphoeeras sp

F igu re 5 - Litholog~cal success ion and a m m o n i t e s ver t ica l r a n g e of Zegr~ Nor te section, Middle Subbetic. province of G r a n a d a . Succession lithologique et rdpartition stratigraphique des ammonites dans la section du Toarcielz supdrieur - AaIdnien infdrieur de Zegri Norte, province de Grenade (Subbdtique moyen).

ZEGRI NORTE (Fig. 5)

In the Zegr /Nor te section, both the Upper Toar- cian (Reynesi and Aalensis Zones) and the Lower and Middle Aalenian are of "ammonitico rosso" facies, which are typical of a talus edge. The ma-

terials from this age, in beds ranging from 10 to 40 cm, are, in all, barely 6 m thick. Resedimented and reworked fauna is common in the Aalenian. From the Middle Aalenian onwards sedimenta- tion reestablished with alternating marls and marly-limestones.

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TI - IE A M M O N I T E A S S O C I A T I O N S A N D T H E I R C O N N E X I O N WITH F A C I E S T Y P E (Fig. 2-4 ; P1. 1)

In m a r l y and g rey mar ly- l imes tone facies the ma- ter ials of the Reynesi Zone (the base of which is de te rmined by the first record of Osperle ioceras gr. reynesi (MONESTIER) contains p redominan t ly s e m i o x y c o n e s l ight ly o r n a m e n t e d forms, belon- ging to the genus Osperle ioceras : O. reynesi (Mo- NESTIER), O. carezi (MONESTIER), O. w u n s t o r f i (MONESTIER) and O. s u b b e t i c u m RIVAS. Together wi th t h e s e , we find serpent icones belonging to the D u m o r t i e r i a : D. rhodan ica (HAUG), D. la- t iurnbi l ica ta GECZY ; Ly tocera t ina (Alocolytoceras oph ioneurn (VACEK) and a few Phyl locera t ina and H a m m a t o c e r a s s. 1.

In those sections (e. g. Cerro M6ndez 2) where the mate r i a l s from this age contain "ammonit ico rosso" facies, the faunal associat ions are r a the r

218

different from those which exist in mar l and grey marly-l imestones. It is, sometimes, difficult to es- tabl ish chronos t ra t igraphic correlat ions be tween both types of sections even wi th in the same sedi- m e n t a r y basin. In this facies type, the faunal as- semblages are dominated by Phyl locera t ina , and evohite platycones belonging to the genus H a m - matoceras : H . m e n e g h i n i i BONARELLI, H. victorii BONARELLI, H. porcare l lense MERLA, Eryc i tes e l a p h u s MERLA ; serpent icones [Durnortiera leves- que i (D'ORBIGNY)] are somet imes locally f requent and there are very few oxycones. We should point out the presence of forms such as Pseudo l i l l i a (see Garcia G6mes & Rivas 1980b), a genus which appears ha rd ly in grey facies. The Reynesi Zone is corresponding wi th the Pseudorad iosa Zone sensu Goy & Mart fnez (1990) and wi th XXII & XXIII horizons of Elmi & Rul leau (1991).

On the sections with grey mar ls and m a r l y lime- stones facies there is a relat ively s h a r p f auna

P L A T E 1

Fig. 1 - Osper l e ioc e ras s u b b e t i c u m RIVAS. CM1 - 42-2 ; Upper Toarcian, Aalensis Zone, Mactra Subzone ; CM1. section, Cerro M6ndez area, Granada prov. Toarcien supgrieul, zone it Aalensis, sous-zone & Mactra ; section CM1, secteur de Cerro M6ndez, prov. de Grenade.

Fig. 2 - P l e y d e l l i a m a c t r a BUCKMAN. CM1 - 39-5 ; Upper Toarcian, Aalensis Zone, Mactra Subzone ; CM1 section, Cerro M6ndez area, Granada prov. Toarcian supdrieur, zone h Aalensis, sous-zone & Mactra ; section CM1, secteur de Cerro Mdndez, prov. de Grenade.

Fig. ~ - P l e y d e l l i a aft. a r c u a t a (BuCKMAN). CM1 - 45-3 ; Upper Toarcian, Aalensis Zone and Subzone ; CM1 section, Cerro M6ndez area, Granada prov. Toarcien supdrieur, zone et sous-zone it Mactra ; section CM1, secteur de Cerro Mdndez, prov. de Grenade.

Fig. 4 - P l e y d e l l i a s u b c o m p t a (BRANCO). CM1 - 43-1 ; Upper Toarcian, Aalensis zone and Subzone ; CM 1 section, Cerro M6ndez area, Granada prov. Toarcien supdrieur, zone et sous-zone h Aalensis ; section CM1, secteur Cerro M~ndez, prov. de Grenade.

Fig. 5 - P l e y d e l l i a a a l e n s i s (ZIETEN). CM1 - 46-2 ; Upper Toarcian, Aalensis Zone and Subzone ; CM1 section, Cerro M6ndez area, Granada prov. Toarcien supdrieur, zone et sous-zone it Aalensis ; section CM1, secteur Cerro M~ndez, prov. de Grenade.

Fig. 6 - P I e y d e l l i a f a l c i f e r MAUBEUGE. CM2 - 29-2 ; Upper Toarcian, Aalensis Zone ; CM2 section, Cerro M6ndez area, Granada prov. Toarcien supgrieur, zone it Aalensis ; section CM2, secteur Cerro Mdndez, prov. de Grenade.

Fig. 7 - L e i o c e r a s c o s t o s u m (QUENSTEDT). SRil - 61-1 ; Lower Aalenian, Opalinum Zone, Comptum Subzone ; SRil section, Sierra de Ricote area, Murcia prov. Aaldnien infdrieur, zone & Opalinum, sous-zone it Cornpturn ; section SRi, secteur Sierra de Ricote, prov. de Murcie.

Fig. 8 - P l e y d e l l i a f a l c i f e r MAUBEUOE. CM 1 - 56-3 ; Upper Toarcian, Aalensis Zone, Buckmani Subzone ; CM1 section, Cerro M6ndez area, Granada prov. Toarcien sup~rieur, zone it Aalensis, sous-zone it Buckmani ; section CM1, secteur Cerro Mendez, prov. de Grenade.

Fig. 9 - L e i o c e r a s o p a l i n u m (REYNECKE). CM2 - 33-1 ; Lower Aalenian, Opalinum Zone and Subzone ; CM2 section, Cerro M6ndez area, Granada prov. Aal6nien infdrieur, zone et sous-zone it Opalinum ; section CM2, secteur Cerro Mdndez, prov. de Grenade.

Fig. 10 - L e i o c e r a s o p a l i n u m (REINECKE). SRil - 56-1. Lower Aalenian, Opalinum Zone and Subzone ; SRil section, Cerro M6ndez area, Granada prov. Aaldnien infdrieu~, zone et sous-zone it Opalinum ; section SRi l , secteur Sierra de Ricote, prov. de Murcie.

Fig. 11 - A n c o l i o c e r a s o p a l i n o i d e s (MAYER). SRil - 72-1 ; Middle Aalenian, Murchisonae Zone, Haugi Subzone ; SRil section, Sierra de Ricote area, Murcia prov. Aaldnien moyen, zone it Murchisonae, sous-zone h Haugi ; section SRi l , secteur Sierra de Ricote, prov. de Murcie.

Fig. 12 - L e i o c e r a s c o m p t u m (REINECKE). SRi -64-14 ; Lower Aalenian, Opalinum Zone, Comptum subzone ; SRil section, Sierra de Ricote area, Mnrcia prov. Aaldnien inf~rieur, zone it Opalinum, sous-zone h Comptum ; section SRi l , secteur Sierra de Ricote, prov. de Murcie.

Fig. 13 - Eryc i t e s f a l l i f a x ARKELL. CM1 -75-1 ; Lower Aalenian, Opalinum Zone, Comptum Subzone ; CM1 section, Cerro M6ndez area, Granada prov. Aaldnien infdrieul, zone it OpaIinu m, sons-zone & Cornptum ; section CM1, secteur Cerro Mdndez, prov. de Grenade.

Page 9: The Toarcian Aalenian boundary in the Betic Cordillera (Southern Spain)

G e o b i o s P1. 1 M.S . n ° 17 R . G a r c i a - G o m e z et al.

~-~_- : " ~

i - - - -~:~. ! 1

i /:N~

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220

transit ion between the Reynesi and Aalensis zo- nes (the base of the Aalensis Zone is determined by the first record of the genus Pleydellia). We observe the massive appearance of forms belon- ging to the genera PleydeUia and Cotteswoldia while Osperleioceras, limited to the lower part of the Zone, becomes a minoritary genus. The va- rious forms of the genera Pleydellia and Cottes- woldia supersede each other over time, but they are always the dominant ammonite fauna, reaching their "acm~" in the Aalensis Subzone wi th heavely ornamented forms belonging to the species PleydeUia foUeata BUCKMAN and P. ar- cuata Buckman, together with P. fluens BUCK- MAN and P. aalensis (ZIETEN).

The different assemblages of Pleydellia allow us to identify the Mactra, Aalensis and Buckmani Subzones. The Mactra Subzone is characterized by the presence of Pleydellia mactra (DUMOR- TIER), P. subcompta BRANCO, Cotteswoldia bifax BUCKMAN, Paradumortieria tectiforme ELMI & CALOO-FONIER together with abundant Ospeleio- ceras subbeticum RIVAS, O. wunstorfi (MONES- TIER), Dumortieria rhodanica (HAUG), Phyllocera- t ina (Calliphylloceras y Holcophylloceras) and Ly- toceratina (Alocolytoceras ).

The Aalensis Subzone coincides with the height of Pleydellia and Cotteswoldia. Together with P. aalensis (ZIETEN) we find P. arcuata BUCKMAN, P. burtonensis (BUCKMAN), P. falcifer MAUBEUGE, P. fluens BUCKMAN, P. folleata BUCKMAN, P. sub- compta (BRANCO), Cotteswoldia bifax BUCKMAN, C. crinita BUCKMAN and C. limatula BUCKMAN ; Catulloceras dumortieri (THIOLLIERE), C. insigni- similis (BRAUNS) and C. perroudi (DUMORTIER), are likewise frequents.

Hammatoceras, Parammatoceras, Erycites, Calli- phylloceras, Holcophylloceras, Ptychophylloceras and Alocolytoceras are more scarce, but they oc- cur throughout the Aalensis Subzone. This faunal association is similar to those from Northern Spain by Goy & Martinez 1990, Goy & Ureta 1991.

The ammonite fauna is scarce in the Buckmani Subzone, and the involute and slightly oranmen- ted Pleydellia such as P. falcifer MAUBEUGE and P. buckmani MAUBEUGE are the more repre- sentative forms. In addition of PleydeUia, we find frequent CatuUoceras ans scarce Parammatoce- ras, Bredyia, Phylloceratina (Calliphylloceras, Holcophylloceras and PtychophyUoceras) and Ly- toceratina (Alocolytoceras). In materials belonging to the Aalensis Zone, the ammonite fauna is more scarce in facies "ammonitico rosso" (Zegr/ Norte) than in grey marls and marly-limestones

facies ; but diferences do not seem to exist be- tween the ammonite assemblages present in dif- ferent types of lithology.

In the Subbetic domain the Toarcian-Aalenian boundary is established at the first record of Leioceras and Tmetoceras which approximately coincides with the last records of Pleydellia and Catulloceras. It is not easy to distinguish, mor- phologically, the last Pleydellia (P. falcifer MAC- BEUGE, P. buckmani MAUBEUGE) from the first Leioceras [L. opalinum (REINECKE)]. Leioceras opalinum (REINECKE) is finely ribbed, and has a relatively large umbilicus, smooth sides and a rounded umbilical edge : these characteristics make these examples similar in appearance to Pleydellia buckmani MAUBEUGE, and some speci- mens of P. falcifer MAUBEUGE.

At the base of the Aalenian (Opalinum Subzone) the ammonite in general and the Leioceras, in particular, are usually scarce. In the Upper part of the Lower Aalenian (Comptum Subzone) the fauna is usually more aboundant and varied : we have identified grooved, fasciculated and pseudo- ribbed forms of Leioceras. The last Leioceras in this Subzone have thick ribbing and a larger um- bilicus. Associated with these Leioceras we find a few Plananmatoceras, Parammatoceras and Ery- cites fallifax ARKELL, especially in the "ammoniti- co rosso" facies (Cerro M~ndez 1 and Zegrl Norte). Tmetoceras scissum (BENEKE) and Spi- nammatoceras tenax (VACEK), S. pugnax (VACEK) are often found together with the previous ones, and are sometimes abundant.

The boundary between ~.he Lower and Middle Aalenian can easily be traced. At almost all the points studied it coincides with a marked faunal transition. The graphoceratids replace most of the leioceratids, the genus Ludwigia (L. haugi DOUVILLE, L. crassa HORN) appears, and we find Ancoleioceras for the first time, which are typical of th first subzone of the Murchisonae Zone.

C O N C L U S I O N S

The s tudy of the Toarcian-Aalenian boundary has been made in three different areas of the Middle Subbetic : Sierra de Ricote, prov. of Murcia (1 section), Cerro M~ndez (2 sections), and Zegr/ Norte (1 section), prov. of Granada.

In two sections ; Sierra de Ricote and Cerro M~ndez 1, the beds corresponding to the mentio- ned boundary are made up of well stratified al- ternances of marls and marly limestones, whe- reas "ammonitico rosso" facies also occur in the

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221

other two sampled sections (Cerro M~ndez 2 and Zegr /Nor te ) .

The successive ammoni t e s assemblages clearly allow to d is t inguish the uppe rmos t Toarcian, Aalensis Zone (Mactra, Aalensis and Buckmani Subzones) and the Lower Aalenian, Opa l inum Zone (Opal inum and C o m p t u m subzones).

In the Aalensis Zone, the ammoni t e f auna is do- mina t ed by pla tycones belonging to the genus Pleydellia, espacial ly wi th regard to the Mact ra and Aalensis Subzones ; in the Buckmani Subzone the f auna is ve ry scarce as a rule, ser- penticones of the genus Catulloceras toge ther wi th Phyl locera t ina being dominant .

Hammatoce ra t i nae , Eryci t inae, Phyl locerat idae and Lytocera t idae are likewise presen t and they range t h roughou t the complete Aalensis Zone.

The ammoni t e f auna is very scarce in the initial beds of the Aalen ian (Opal inum Subzone) where only few Leioceras and Tmetoceras toge ther with Lytocera t inae , Phyl locera t idae and rare H a m m a - tocera t idae occur.

The f auna is ve ry abundant , bu t scarcely varied, in the C o m p t u m Subzone ; he re the ammoni tes assemblages are domina ted by oxycones belon- ging to the genus Leioceras, which coexist with platycones (Tmetoceras and Hammatocera t idae) , Phyl locera t idae and Lytocerat idae.

The r ep lacement of the last Pleydellia and Ca- tulloceras by the first Leioceras and Tmetoceras, clearly defines the Toarc ian-Aalenian boundary in the Subbetic domain. Except for Lytocera t ina and Phyl locerat ina , the ammoni te-associa t ions here recorded are s imilar to those from Nor the rn Spain, Sou th -Eas t e rn of France and Por tugal (Submed i t e r r anean province).

Zoophycos is very a b u n d a n t in the last beds of the Toarc ian and the first ones of the Aalenian, precisely where the ammoni t e are rare.

In the In t e rna l and Ex te rna l Subbetic, the Upper Lias and the Aa len ian sections are general ly very condensed and the Upper Toarc ian-Lower Aale- n ian m a y even coincide wi th a s ed imen ta ry gap.

R E F E R E N C E S

CRESTA S. 1991 - Lithostratigraphic and Biostratigra- phic features of the Middle Jurassic sequence in the Monte Nerone area (Apennines, Italy). In N. NORTON (ed.) : Conference on Aalenian and Bajo-

cian Stratigraphy Isle of Skye, Scotland. Birbeck College, Univ. of London: 7-31.

ELMI S., GoY A., MOUTERDE R., RIVAS P. & ROCHA R. 1989 - Correlaciones bioestratigr~ficas en el Toar- ciense de la Peninsula Ib~rica. Cuad. Geol. Ibdrica, 13 : 265-277.

ELMI S. • RULLEAU L. 1991 - Le Toarcien des carri~- res Lafarge (Bas-Beaujolais, France) : Cadre bios- tratigraphique de r~f~rence pour la R~gion Lyon- naise. Geobios, 24/3 : 315-331.

GARCIA GOMEZ R. & RIVAS P. 1980a - Pseudogrammo- ceras (Hildoceratacea, Ammonoidea) del Toarciense medio y superior de las Cordilleras B~ticas (Anda- lucia, Espafia). Estudios Geol., 36 : 263-274.

GARCIA GOMEZ R. & RIVAS P. 1980b - E1 g@nero Pseu- dolillia en las Cordilleras B~ticas. Cuad. Geol. Univ. Granada, 11 : 195-207.

GOY A., JIMENEZ A.P., MARTINEZ G. & RIVAS P. 1989 - Difficulties in correlating the Toarcian ammonite succession of the Iberian and Betic Cordilleras. In R. ROCHA & A. SOARES (eds.) : 2nd International Symposium on Jurassic Stratigraphy, Lisboa, 1 : 155-178.

GoY A. & MARTINEZ G. 1990 - Biozonacidn del Toar- ciense en el ~rea de la Almunia de Dofia Godina. Ricla. Sector Central de la Cordillera Ib~rica, Cuad. Geol. Ibdrica, 14 : 11-53.

GOY A. & URETA M.S. 1991 - The lower boundary of the Aalenian in the Fuentelsaz section (Iberian Range, Spain). In N. NORTON (ed.) : Conference on Aalenian and Bajocian stratigraphy Isle of Skye, Scotland. Birbeck College, Univ. of London : 33-47.

HENRIQUES M.H. 1989 - O limite Lias - Dogger no Baixo Mondego (Portugal) : Ensaio de Caracteriq~o Biostratigr~fica. Publ. Lab. Mineral. Geol. Univ. Coimbra, 107 : 37-42.

JIMENEZ A. P. • RIRTAS P. 1981 - E1 Jur~sico de las Zonas Externas de las Cordilleras B~ticas. III. Zona Subb~tica. III. 4 E1 Toarciense. Cuad. Geol. Univ. Granada, 10 : 397-411.

LINARES A. 1979 - E1 Jur~sico de las Zonas Externas de las Cordilleras B~ticas III. Zona Subb~tica III. 5. El Aaleniense. Cuad. Geol. Univ. Granada, 10 : 107-119.

LINARE$ A. & RIVAS P. 1973 - La transiciSn Lias - Dogger en el sector de Alamedilla, Zona Subb~tica. Cuad. Geol. Univ. Granada, 4: 155-160.

LINARES A. & SANDOVAL J. 1986 - Malladaites nov. gen. et Spinammatoceras (Hammatoceratidae, Am- monitina) de l'Aal~nien de la Zone Subb~tique, Sud de l'Espagne. Geobios, 19, 2 : 207-224.

LINARES A. & SANDOVAL J. 1988 - Asthenoceras y Fon- tannesia (Grammoceratinae ?, Ammonitina) del tr~nsito Aaleniense - Bajociense de la Zona Subb~- tica (Sur de Espafia). Rev. Espafil. Paleontol., 3 : 3-11.

LINARES A., URETA M.S. & SANDOVAL J. 1987 - Compa- rison between the Aalenian ammonite associations from the Betic and Iberian Cordilleras : elements of correlations. In R. ROCHA & A. SOARES (eds.) : 2nd International Symposium on Jurassic Stratigraphy. Lisboa, 1 : 193-208.

OHMERT W. 1985 - Problems at the Toarcian/Aalenian boundary. In O. MICHELSEN & A. ZEISS (eds) : Inter- national Symposium on Jurassic Stratigraphy. Er-

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l angen 1984. Geol. Surv. Denmark. Copenhagen, I : 259-266.

OHMERT W., PRAUSS M. • WEISS M. 1991 - The Toar- c ian-Aalenian boundary at Wi t tnau (Oberrhein area, South West Germany), a possible boundary Stratotype for the Aalenian. In N. NORTON (ed.) : Conference on Aalenian and Bajocian stratigraphy Isle of skye, Scotland. Birbeck College, Univ. of London : 7-31.

RIVAS P. 1973 - Estudio Paleontol6gico - Estratigr4fico del Lias en el sector Central de las Cordilleras B6ti-

cas. Resdmenes Tesis Doct. Univ. Granada, 29 : 1 87.

R. GARCIA-GOMEZ, A.P. JIMENEZ, A. LINARES, P. RIVAS & J. SANDOVAL

Departamento de Estratig-rafia y Paleontologia Universidad de Granada

Avda Fuentenueva s/n. 18071


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