Journalof volcanoIogy and geothermal research

ELSEVIER Journal of Volcanology and Geothermal Research 73 (I 996) 28.5-30 I

Large-magnitude Middle Ordovician volcanic ash falls in North America and Europe: dimensions, emplacement and

post-emplacement characteristics

W.D. Huff a, * , D.R. Kolata ‘, S.M. Bergstrijm ‘, Y-S. Zhang d

” Department of Grolog?, Uniwrsir?, of Cincinnati, Cincinnati, OH 45221, USA

h Illinois State Geological Suwey 615 E. Peabod? Dr.. Champuign, IL 61820, USA

’ Deprrrtmrnt oj’Geological Sciences. The Ohio Stnte lJni[.ersi~. 155 S. Owl Mall. Columbus, OH 43210. USA

* I~epnrtment o#Geolog~, (Itkw-sity of Cincinntlti, Cincintrati, OH 45221, USA

Received I1 July 1994; accepted I I February 1996

Abstract

Middle Ordovician K-bentonites represent some of the largest known fallout ash deposits in the Phanerozoic Era. They

cover minimally 2.2 x IOh km’ in eastern North America and 6.9 X IO’ km’ in northwestern Europe, and represents the coeval accumulation of plinian and co-ignimbrite ash on both Laurentia and Baltica during the closure of the Iapetus Ocean.

The three most widespread beds are the Deicke and Millbrig K-bentonites in North America and the Kinnekulle K-bentonite

in northwestern Europe. The vents were located near the Laurentian margin of Iapetus on an arc or microplate undergoing

collision with Laurentia. The volume of ash preserved in the stratigraphic record converted to dense rock equivalent (DRE) of silicic magma is minimally estimated to be 943 km3 for the Deicke, 1509 km’ for the Millbrig and 972 km3 for the

Kinnekulle. The Millbrig and Kinnekulle beds are coeval and possibly equivalent, yielding a combined DRE volume of nearly 2500 km”. Some unknown but probably large amount of additional ash fell into oceanic regions of the Iapetus, but these areas became subducted and the ash is not preserved in the geologic record. The symmetry of the thickness contours is

suggestive that one or more ash clouds interacting with equatorial stratospheric and tropospheric wind patterns dispersed

pyroclastic material to both the northwest and southeast in terms of Ordovician paleogeography. Based on grain size measurements and thickness/area’/” plots we conclude the three beds were each formed from co-ignimbrite or possibly

phreatoplinian eruption columns.

Analyses of melt inclusions in primary quartz crystals indicate the parental magma contained approximately 4% dissolved water at the time of the eruption. This water provided the explosive energy during the initial gas thrust phase. The implied fragmentation pressure on the magma would have reduced much of the ejecta to small particles, forming a deposit

composed largely of single crystals and glassy dust. Conversion of the ash to K-bentonite resulted in a mass loss of approximately 35%. mostly in the form of Si with lesser amounts of Na and K.

Kuwordst Ordovician: co-ignimbrite; volcanic ash: K-bentonite; lapetus

’ Corresponding author. Phone: (5 13) 556-373 1. Fax: (5 13) 556-698 1. E-mail: warren.huff@uc.edu.

0377.0273/96/$15.00 Copyright 0 1996 Elsevier Science B.V. All rights reserved

PIf SO377-0273(96)00025-X

1. Introduction

Evidence of explosive silicic volcanism is pre-

served in the stratigraphic record from the Protero-

zoic to the Holocene in the form of ignimbrites,

ash-flow tuffs and fallout ash beds. Ash beds range from a few millimeters to hundreds of meters in

thickness and cover tens to hundreds of thousands of

square kilometers in area (Sarna-Wojcicki et al.,

198 1; Walker, 198 la). In addition to the chronos-

tratigraphic value of widespread time-parallel layers,

explosively formed ash can also provide important information about parental magma composition, tec-

tonic setting and the dynamic modes of central vent

eruptions associated with island arc and plate margin

collisional tectonics. The most powerful explosive

events are plinian and ultraplinian central vent erup-

tions, which may be accompanied by equal-scale or

larger co-ignimbrite ash clouds (Woods and Wohletz, 1991). Large-magnitude events tend to occur with

less frequency than smaller magnitude eruptions

(Walker, 198 1 b), but are more likely to result in ash

beds which are preserved in the stratigraphic record

because of their widespread distribution and ten-

dency to produce large volumes of material, particu-

larly where co-ignimbrite phases also occur (Sparks

and Walker, 1977). The extent of preservation also

depends upon the nature of regional sedimentary environments. For example, fallout ash beds are

more likely to be preserved in subtidal to peritidal

environments as opposed to terrestrial or supratidal

conditions. Also, in the case of Paleozoic and older

fallout ash beds, there is a preference for preserva-

tion on continents because most pre-Mesozoic ocean

crust has been consumed by subduction processes. Recent work by Wiesner et al. (1995) has shown that

atmospheric tephra dispersal patterns are recorded on

the deep-sea floor within days of the tephra eruption, thus not only corroborating the value of ash beds as

chronostratigraphic markers but also demonstrating the nearly complete absence of contamination or modification of the ash bed by other sedimentologi- cal or biological processes. In the more stable cra- tonic marine environments some beds may be suffi- ciently well preserved so as to make it possible to determine their original area1 extent and thickness, and thus permit an analysis of their dynamic condi- tions of formation. However, geologically old beds

Fig. I. Global paleogeographic reconstruction for the Middle

Ordovician modified from Scotese and McKerrow (1991). The

shaded area represents the distribution of K-bentonite beds in

eastern North America and northern Europe. and the varying

intensities represent regional thicknes< trend\ of individual bed\.

have generally undergone some degree of post-em-

placement chemical alteration or erosion, which makes interpretation of the precise conditions of

their formation difficult. Pre-Mesozoic ash beds have

been completely devitrified to K-bentonite, or ton- stein in the case of those preserved in coal se-

quences, and consist mainly of clay minerals with

accompanying phenocrysts of feldspar, quartz, zir-

con, apatite, biotite and monazite. Geologically

younger beds typically have retained some of their original vitric character, but are likely to be at least

partially altered to bentonitic clays. We describe here

some features of several large-magnitude explosive eruptions of silicic magma that occurred on the

Laurentian margin of the Iapetus Ocean during the

Middle Ordovician, and formed the Deicke and Mill- brig K-bentonite beds in North America (Huff and Kolata, 1990; Kolata et al., in press) and the Kin- nekulle K-bentonite bed in Baltoscandia (BergstrGm

et al., 199.5). The closing of the Iapetus Ocean separating

Baltica, Avalonia and Laurentia (Fig. 1) occurred by means of the subduction of oceanic crust beneath, and the consequent collision of, volcanically active

W.D. Huff et al. / Journal of Volcanology and Geothermal Research 73 (1996) 285-301 287

island arcs or microplates against the southeastern

margin of Laurentia (Scotese and McKerrow, 1991). These collisions were associated with the Taconic orogeny, which began during the Middle Ordovician

and produced a complex deformational and sedimen-

tological record that has been extensively docu-

mented (Rowley and Kidd, 1981; Stanley and Rat-

cliffe, 1985; Tucker and Robinson, 1990) and which includes numerous K-bentonite beds in the eastern North American, British and Baltoscandian succes-

sions. Baltica was surrounded by a passive margin

during the Middle Ordovician, but it apparently was in close proximity to Laurentia (McKerrow et al.,

1991; Huff et al., 1992), consequently we attribute the origin of the approximately 150 Middle Ordovi-

cian ash beds in southern Sweden, including the Kinnekulle K-bentonite, to the explosive volcanic activity in the magmatic arcs assciated with the

Taconian Orogeny. The Deicke and Millbrig have been correlated by

chemical fingerprinting from southeastern Minnesota

to southeastern Missouri (Kolata et al., 1987) and by wireline logs from Missouri to the southern Ap-

palachians and into the Michigan Basin and southern

Ontario (Huff and Kolata, 1990; Kolata et al., in

press). A transect from the southern Appalachians to

the most westerly known Deicke occurrence mea-

sures approximately 1500 km. Both beds range from 1.5 m or more in thickness in the southern Ap-

palachians to 3 cm or less in western Iowa. Unpub- lished data from wireline logs in central Oklahoma

suggest both beds extend farther to the southwest than has previously been mapped. Known occur- rences of the Kinnekulle bed extend from the Oslo,

Norway, area approximately 900 km across southern Sweden and northern Denmark to eastern Estonia,

and most likely into western Russia (Bergstrom et

al., 199.5). Its thickness ranges from more than 2 m in the Fylla Mosse core in Gstergotland, Sweden, to less than 5 cm in the subsurface near Slancy in

western Russia. The Millbrig and Kinnekulle beds appear to be coeval and have similar mineral and

bulk chemical compositions, which led Huff et al.

(1992) to propose they were derived from the same eruption. Recently Haynes and Melson (1995) have

suggested on the basis of selective biotite composi- tions that the Kinnekulle and Millbrig beds may have had separate source magmas. While the resolution of

that question requires further investigation we will

treat the Deicke, Millbrig and Kinnekulle as separate

deposits, although the analysis we use here can apply

equally to either interpretation. Using thickness data, distribution patterns. grain

size analyses and deduced values of dense rock

equivalent (DRE) ash volume, we apply contempo-

rary models of explosive eruptions to these ash-for- ming events in order to develop a more quantitative

picture of their behavior. This is the first effort to

evaluate a Paleozoic ash-fall deposit in the context of

current models of explosive volcanism and should be

seen as a preliminary attempt to reconcile field ob-

servations of the deposits from an ancient catas-

trophic event with dynamic models for modern erup-

tions. An assessment of the intensities and magni-

tudes of explosive eruptions depends upon reliable estimates of the regional variation in thickness, vol-

ume and grain size in tephra fall deposits. In the case

of both modem and ancient ash falls, the deposits immediately proximal to the vent and the most distal

deposits are frequently not preserved and require

estimation. For ancient deposits, particularly those in

the lower Paleozoic, the problem is further com- pounded by the mass balance effects that have ac-

companied the diagenetic transformation of the ash. Thus, primary pumice and vitric ash particles have

altered to clay minerals, and some recrystallization

and neomineralization may have modified the char- acter of the non-clay fraction. On the other hand, the

best preserved Paleozoic ash beds were deposited

during times of widespread epicontinental flooding and their stratigraphic record may be superior to

those of Quaternary and Holocene ashes that have been widely reported in the literature. In this study

we have chosen to measure and map the stratigraphic

thickness of the Deicke, Millbrig and Kinnekulle

K-bentonite beds. and to calculate the volumes of each. On the basis of melt inclusion chemistry, which

identifies parental magma composition, we then cal- culate relative post-depositional elemental gains and

losses and thereby arrive at an estimate of original

ash volume, expressed as dense rock equivalent

(DRE). A similar approach is used for grain size determination in which the size distribution of pre-

served primary volcanogenic crystals can be shown to serve as a reasonable approximation of original ash particle size.

288 W.D. Huff et al. /Journal of Volcanology and Geothermal Research 73 (1996) 285-301

2. Ash volume

The stratigraphic thicknesses of the Deicke, Mill-

brig and Kinnekulle K-bentonites are shown by max- imum thickness contour maps based mainly on field

observations and supplemented by core measure-

ments (Figs. 2-4). Solid lines are drawn where

reliable data exist and dashed lines show the inferred

thickness where no data are available. The data

points shown are representative of a larger set of

measurements taken from the same general areas.

The Deicke and Millbrig beds have been identified

in outcrop, on wireline logs, or in cores at approxi-

mately 400 localities in North America (Kolata et al.,

1987; Huff and Kolata, 1990; Haynes, 1994) and the

Kinnekulle bed at approximately 50 localities in Baltoscandia (Bergstrom et al., 1995) using biostrati-

graphic, lithostratigraphic, mineralogic and geochem-

ical information. The data used to construct the thickness contour

maps reflect actual stratigraphic thickness of the

K-bentonite beds. Contour lines were drawn without

regard to localized post-depositional erosional effects

resulting from tectonic events and other causes. For

example, Kolata et al. (1995) have shown that coin-

cident with the Blountian tectophase of the Taconic

orogeny, regional warping of the eastern Midconti-

nent and the consequent formation of several basins and arches resulted in the partial or complete erosion

of the K-bentonite beds at some localities. Moreover,

Fig. 2. Maximum thickness contour map for the Deicke K-bet&mite in eastern North America. Contour values are in cm. The selected data

are compiled from Kolata et al. (1987), Haynes (1992, 1994), Huff and Kolata (1990) and Huff (unpubl. field notes).

W.D. Huff et al. /Journal of Volcanology and Geothermal Research 73 (19961285-301 289

the regional marine transgression that occurred at or

near the base of the Chatfieldian (formerly Trento-

man) succession is, in many places, coincident with the Millbrig K-bentonite (Leslie and Bergstrom,

1995) and erosion prior to, or during, the initial

phase of the transgression is responsible for its ab- sence. Our intention here is to reconstruct a regional

stratigraphic view of the Deicke, Millbrig and Kin-

nekulle beds which is in as complete agreement as

possible with existing field evidence. The abrupt

termination of the contour lines indicates that these

beds were initially more persistent laterally than the preserved record reveals, hence our volume estimates

should be taken as minimal values. Finally, our

interpretation of the regional variation in thickness is

supported by 6th order trend surface analyses which

show a very similar pattern to the fitted contours. Many characteristics of tephra fall deposits de-

crease in a linear manner when the logarithm of the

characteristic is plotted against distance from the vent. This implies that variations in features such as

grain size and thickness follow simple exponential

decay laws, and most tephra layers can be defined by one or two straight-line segments on such plots.

Difficulties can arise, however, when data are ex-

trapolated to determine thickness at the vent and

distal volume beyond the last contour. Pyle (1989) proposed an improved treatment of the data by plot-

ting the logarithm of the thickness versus the square

root of the area between each contour. This method

Fig. 3. Maximum thickness contour map for the Millbrig K-bentonite in eastern North America. Contour values are in cm. The selected data

are compiled from Kolata et al. (1987), Haynes (1992. 1994). Huff and Kolata (1990) and Huff (unpubl. field notes).

W.D. Huff et (11. ,I Journal of Volcanology and Geothermal Reaearch 73 (19961 285-301

Fig. 4. Maximum thickness contour map for the Kinnekulle K-bentonite in Baltoscandia. The selected data points come prtmarily from cores

plus outcrops in the Oslo, Norway, area and in Sweden and Denmark. Contour values are in cm. For additional information see Bergstrom et

al. ( 1995).

assumes that the isopachs are elliptical or circular in importing a scanned image of the map into Canvas ” shape, however, and a modified version, which we on a Macintosh desktop computer, and then fitting use here, was proposed by Fierstein and Nathenson polygons to segments defined by successive contour (1992) in which the area between each contour is lines. By setting the polygon tool to the map scale measured and integrated directly as an independent the area of each segment can be accurately deter- variable. mined. The results are given in Table 1.

The area between each contour was measured by On a plot of the log of thickness versus the square

Table 1

Cumulative area measurements and ash volume calculations for the Deicke. Millbrig and Kinnekulle K-bentonite beds

Deicke Millbrig Kinnekulle

Thickness (cm) Area (km’ ) Thickness (cm) Area (km’) Thickness (cm) Area (km’)

150 5.5.704 140 144,480 200 70.636 100 107,837 100 252,362 100 165.525 60 230.200 60 485,418 80 242. I29 40 406,223 40 665,100 60 303,860 20 633,584 20 1,024,296 40 406,224 IO I ,047,560 IO 1.304,911 20 5.57.279 5 1577.094 3 I,928087 10 690,293

2 2152,367

Total volume ’ (km’) 658 1000 695

” Following the method of Fierstein and Nathenson (1992).

W.D. HyfSet al. /Journal of Volcunolog~ and Geothermal Research 73 (1996) 285-301 291

100

E - 10

s yc 2 I

I-

1000

100

400 600 800 1000

Deicke Area”* (Km)

I

400 600 800

Millbrig Area”’ (Km)

J

1000 1200 ,400

300 400 500 600 700 800 900

Kinnekulle Area”’ (Km)

Fig. 5. Log of thickness versus the square root of area for the Deicke, Millbrig and Kinnekulle K-bentonites with best-fit

calculating volume. straight lines for

292 W.D. Huff et al. /Journal of Volcanology and Geothermal Research 73 C 1996) 285-301

root of the area of each bed we have taken eq. 7 of

Fierstein and Nathenson (1992), the simplest case of

a single straight line in which the equation for

thickness as an exponential function of area”’ is:

T= T, exp( -kA’/‘)

where T,, is the extrapolated thickness at A = 0 and

-k is the slope of the line on a log T versus A”*

plot. The slope may be calculated knowing any two

points on the line, A, and A,, by their eq. 8:

In Th - In T, -kc

A’/* _ A’/’ h ,i

To calculate volume, the thickness as described by

eq. 7, is directly integrated using area as the indepen-

dent variable. This avoids the assumption made by

Pyle (1989) that each contour is necessarily circular

or elliptical in shape, and which would constitute a

source of error for any non-ideally distributed ash.

Eq. 12 of Fierstein and Nathenson (1992) is thus:

V = 2TJK,

The calculated volumes for the Deicke, Millbrig and Kinnekulle beds are shown in Table 1, and the

thickness versus area’/’ data are shown on three

plots in Fig. 5. The calculated respective volumes for

the Deicke, Millbrig and Kinnekulle K-bentonite beds

are 658, 1000 and 695 km’.

Eight contour points are used for the Deicke and seven each for the Millbrig and Kinnekulle. For all

three beds the data are fitted remarkably well by a

single straight line. Rose et al. (1973) suggested using two straight-line segments to more closely fit

the distribution data and to calculate volume, since

many Holocene and Recent ash falls appear to fol- low a different logarithmic curve in the proximal

region. The May 1980 eruption of Mt. St. Helens

provides an illustration of the two segment plot

(Fierstein and Nathenson, 1992) where a break in slope for small values of A appears due to near-vent phenomena such as localized surges and fallback ejects. Slope breaks at larger values of A may indicate a transition from direct fallout ash to co- ignimbrite ash, or a similar fundamental change in eruption dynamics. For Middle Ordovician K-be- ntonites, the lack of any observable slope break can most likely be attributed to the missing proximal

facies. Grain size distribution measurements suggest

that the largest particles preserved in the Millbrig or

Kinnekulle K-bentonite beds represent fallout accu- mulation at least 200 km from the source vent

(Zhang and Huff, 1995). Considering both grain size

data and the comparatively gentle negative slopes of

the curves in Fig. 5 we suspect that the preserved

portions of these three K-bentonites represent co- ignimbrite or possibly phreatoplinian accumulations,

and that there is no contiguous record of their near-

vent equivalents.

Use of the log T versus A’/’ plot permits extrap-

olation to both the proximal and distal portions of

the ash falls and to avoid doing so would result in a

significant underestimate of the total ash volume. This is particularly true in view of the fact none of

the three beds has been traced beyond the 1 cm

contour where the extrapolation to infinity could add

between 10 and 15% of the measured volume (Fier- stein and Nathenson, 1992). A simple linear extrapo-

lation of the fitted curve to the vent, assuming no

break in slope, would give the Deicke a maximum stratigraphic thickness of 3.6 m, the Millbrig 8 m

and the Kinnekulle 10 m. These are minimum fig-

ures since the magnitude indicated for these erup-

tions by their distribution would most likely have

been characterized by two straight-line segments (Rose et al., 1973) and would therefore have greater

near-vent thickness.

3. Rock equivalent volume

Conversion of measured ash volumes to dense

rock equivalent (DRE) values is useful for compara-

tive purposes and can normally be done using mea- sured densities of compacted ash if the bulk compo-

sition of the tephra is known. Ordovician K-be-

ntonites are altered ash beds, however, and essen- tially all original pumice and vitric components have been devitrified to clay minerals and associated au- thigenic phases. The beds are composed dominantly of clay minerals which indicates that most of the original ash was glass. Accompanying volcanogenic phenocrysts include quartz, feldspar, biotite, Fe-Ti oxides, zircon and apatite. Primary beta-form quartz phenocrysts contain glass melt inclusions which have

W.D. Huff et al. /Journal of Volcanology and Geothermal Research 73 (1996) 285-301 293

not been altered and which can be used to calculate

the mass balance changes that occurred during devit-

rification and burial metamorphism. From the rela-

tionship between the chemical composition and den- sity of the unaltered glass and the K-bentonite, it can

be inferred that substantial losses of mass took place

during the alteration process. The inclusions repre- sent magma composition at the time of quartz crys-

tallization and can be presumed to be indicative of

the glass composition at the time of eruption (Mc-

Vey and Huff, 1995).

Analysis of whole-rock K-bentonite samples was

made by instrumental neutron activation analysis (INAA). Oxide totals sum near 100% when included

with loss on ignition (LOI) measurements of the

volatile components (Table 2). The stoichiometric oxide totals average near 90.5% with the balance

accounted for by volatiles. The volatiles are thought

to consist mainly of water as structural OH with

minor amounts of CO, plus Cl, F and S species. Microprobe analysis of glass melt inclusions yielded

oxide totals near 96.6%. Care was taken to minimize

the loss of Na by reducing the beam current, enlarg-

ing the spot size and averaging repeated analyses (Spray and Rae, 1995; H anson et al., in press), so it

is believed the major part of the remaining 3.4%

consists of H,O. This conclusion is supported by

previous studies of glass melt inclusions in which

total 0 was measured and the major deficit in the oxide total was found to be H 2O (Nash, 1992).

Mass balance calculations were made by normal-

izing the glass and rock (K-bentonite) chemical data to Al which was presumed to have remained con-

stant during devitrification. This is a common as-

sumption in low-temperature rock alteration calcula-

tions since the surface and pore water content of Al species is usually quite low. Inasmuch as Al is not

completely absent from natural waters, however, the

assumption cannot be strictly accurate. Both glass and rock analyses were recalculated to 100% on an

anhydrous basis in order to directly compare gains and losses of individual oxides.

Melt inclusion and K-bentonite samples were both

selected from the southern Appalachians in order to minimize any regional variations in bulk or glass chemical composition that might be inherent in the beds. Microprobe analyses of glass melt inclusions were made for Deicke and Millbrig samples from the

Big Ridge, Alabama, section (Haynes, 1994) and the

data compared with whole-rock analyses of Deicke and Millbrig samples from seven localities in central

and southern Virginia (McVey and Huff, 1995). Data

for the Kinnekulle samples were taken from

Bergstrom et al. (1995) and supplemented with addi- tional new measurements. Melt inclusion SiO, val- ues were in the range 73-75% (Table 2) indicating

the glass was rhyolitic in composition. Average

Al,O, for twelve Deicke melt inclusions was 11.92%

with a standard deviation of 0.17, for 26 Millbrig

melt inclusions it was 12.11% with a standard devia-

tion of 0.29, and for nineteen Kinnekulle melt inclu-

sions it was 12.32% with a standard deviation of

0.18. The dissolved water content was calculated

from the probe data and converted to corresponding

glass density based on the curve of Mueller and Saxena (1977). The glass density was thus deter-

mined to be 2.32 g/cm”. K-bentonite density measurements were made on

oven-dried samples by repeated weighing of pre-

cisely cut 1 cm cubes. For both the Deicke and

Millbrig, density measurements averaged 1.93 g/cm3

and for the Kinnekulle the density was 1.92 g/cm3.

Repeated measurements showed very little variation

in these values. Converting on the basis of total aluminum oxide the glass:bentonite ratio for the

Deicke is 1.43, for the Millbrig 1.5 1 and for the Kinnekulle 1.40. Table 2 gives the calculated DRE

values based on the total volume measurements as

943 km’ for the Deicke, 1509 km’ for the Millbrig

and 972 km” for the Kinnekulle. If the Millbrig and

Kinnekulle beds are equivalent as suggested (Huff et

al., 1992) then the total rock represented by the

present day stratigraphic record is approximately 2500 km3. Even if the two beds are not equivalent, it

can be seen that about one third of the mass of the

original ash (expressed as rock) is lost during subse-

quent burial and diagenesis. When viewed from the

standpoint of individual oxides, the principal contrib- utor to the loss of mass is SiO, (Fig. 6). Of the total loss by conversion of rhyolite glass 55% of the

original silica was removed during devitrification and subsequent diagenesis. This is not surprising in view of the well-known association of chert beds with K-bentonites in carbonate sequences (Haynes,

1994; Kolata et al., in press) and it has also been documented in other studies of bentonite formation

Tab

le

2

Ana

lytic

al

data

an

d m

ass

bala

nce

calc

ulat

ions

fo

r th

e D

eick

e,

Mill

brig

an

d K

inek

ulle

K

-ben

toni

tes

Dei

cke

Mill

brig

K

inne

kulle

Gla

ss

Gla

ss

Roc

k R

ock

Gla

ss-r

ock

Gla

ss

Gla

ss

Roc

k R

ock

Gla

ss-r

ock

Gla

ss

Gla

ss

Roc

k R

ock

Gla

ss-r

ock

(n =

12)

(a

nhyd

l (?

I=

71

(anh

ydl

(g)

a (n

=26)

(a

nhyd

) (n

=

8)

(anh

ydl

(g)”

(n

=

12)

(anh

yd)

(n

= 31

1 (a

nhyd

) (g

) ’

SiO

l

Al2

03

CaO

MgG

N

a,O

K,O

Fe

0

MnO

T

iO

p&l:

LO

1

Tot

als

74.1

5 76

.7 I

56

.87

62.5

6

11.9

2 12

.33

20.5

3 22

.58

1.21

1.

25

3.02

3.

33

0.09

0.

09

2.59

2.

85

2.64

2.

73

0.66

0.

73

5.27

5.

45

4.59

5.

05

1.16

1.

20

2.19

2.

41

0.02

0.

02

0.02

0.

02

0.21

0.

21

0.41

0.

45

0.12

0.

13

8.66

96.6

7 10

0.00

99

.66

100.

10

- 42

.54

0.00

0.57

I .46

- 2.

33

- 2.

70

0.12

-0.0

1 0.03

0.05

74.9

0 77

.39

12.1

1 12

.52

0.87

0.

89

0.02

0.

02

2.75

2.

84

5.19

5.

36

0.84

0.

87

0.04

0.

04

0.07

0.

07

96.7

8 10

0.00

53.9

8

21.9

6

2.31

2.64

0.72

5.08

3.51

0.02

0.49

0.13

8.21

99.0

5

59.3

7

24.1

6

2.54

2.9

1 0.

79

5.59

3.86

0.03

0.54

0.14

99.9

2

- 46

.63

74.8

1

78.3

5 54

.4 I

0.

00

12.4

0 12

.99

20.4

5

0.42

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74

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46

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06

0.07

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W.D. Huff et al. / Journal of Volcanology and Geothermal Research 7.3 f 1996) 285-301 295

Deicke Mtllbrlg Kinnekulle

-10

-20

-30

-40

-50

Fig. 6. Plots of oxide gain loss as a result of the conversion of primary vitric ash to K-hentonite. Data are based on Al-normalized

compnrisiom of whole-rock K-bemonite composition and glass melt inclusions in quartz crystals.

(Caballero et al., 1992). However, it is somewhat

surprising to see that 50% of the original K,O was also lost during the process. It is commonly ihought

that the process of illitization which resulted in

conversion of much of the original smectite to

mixed-layer illite/smectite was the result of K meta-

somatism, principally during episodes of brine mi-

gration associated with regional tectonism (Altaner

et al., 1984; Hay et al., 1988; Elliott and Aronson,

1993). Under this concept, the K-bentonite layer is a geochemically open system with K coming mainly

from external sources. Our data indicate that more than enough K is present in the parent glass to

account for the composition of illite/smectite and

that a substantial portion of original K is in fact

removed during post-emplacement processes. There- fore the illitization reaction need involve only short-

range diffusion of K rather than a regional migration, although the timing of this reaction may in fact be

related to tectonic or other regional thermal events.

An exception occurs in the case of the localized formation of massive K feldspar as a replacement of

K-bentonite that has been documented in some parts of the eastern Midcontinent (Hay et al., 1988). With a whole-rock K,O content of 1 l-12% and few accessory minerals the evidence indicates there was an external potassic source.

4. Grain size

Eruptive models based on direct measurement of

fall deposit characteristics frequently use grain size

distribution of the initial populations as a source of

information relating to intensity and column height (Walker, 1980; Sigurdsson and Carey, 1989). Grain

size is an important factor in convective eruptive

columns since it determines the rate of energy trans-

fer from magma particles to in-drawn air (Wilson et

al., 1978), and thus strongly influences the height of

the convecting column. For Paleozoic K-bentonites initial population size characteristics cannot be ob-

tained by direct measurement since all original vitric

material has been altered to clay minerals. Some

indication of shard dimension can be obtained from

rarely preserved relict shards. Haynes (1992) has

described an example from the Millbrig at Catawba,

Virginia, in the southern Appalachians. Measurement

of these on a micrograph gives an average long

dimension of 0.101 mm. By comparison, the median diameter of phenocrysts in a Millbrig sample col-

lected near Catawba is 0.074 mm. Because of the density difference between the phenocrysts and glass shards, the average size of the phenocrysts might be expected to be smaller than that of the latter at the same locality. Hence the two sources of size data are

296 W.D. Huff et al. / Journal of Volcanology and Geothermal Research 73 (19961 285-301

not inconsistent and suggest that grain size studies of

primary phenocrysts in the Millbrig and Kinnekulle K-bentonites could yield reasonable and reliable in-

formation about particle diameters in the initial pop-

ulation.

The principal phenocrysts are feldspar, quartz and biotite, with lesser amounts of zircon, apatite and

Fe-Ti oxides. Petrographical study demonstrates that

some of the feldspar phenocrysts in the Millbrig-

Kinnekulle K-bentonite samples have changed com-

positionally, but not morphologically. The secondary

feldspars are pseudomorphs of primary crystals in

most cases and have retained their original dimen-

sions. Hence, the size distribution of these phe-

nocrysts may well represent the distribution of a

large majority of original volcanic ash particles.

Primary phenocrysts do remain, however, and are the

source of important information about age and mag-

matic history (Samson et al., 1989; Delano et al.,

1990; Tucker et al., 1990). The question of whether their size distribution could proxy for original grain

size characterization depends upon the probability of

single crystals having acted as pyroclastic particles.

Biotite flakes, for example, may travel in airborne

ash as a discrete particles as far as 4000 km from the

source vent (Nusbaum et al., 1988). Moreover, re-

cent studies have shown that in highly explosive eruptions the degree of fragmentation is remarkably

high. For the Taupo eruption, 85% of the total ejecta were of sub-millimeter size and the estimated fine

ash ( < 63 km> and dust ( < 4 p.m) content was 60%

(Walker, 198 1 a). Most individual phenocrysts are of

sub-millimeter size and from a purely physical stand-

point could have traveled as discrete particles.

Grain size data were acquired by sieve analyses

using calibrated screens. Particles > 3.75 0 (< 0.074 mm) were beyond the range of reliable sieve

analysis and were discarded. Most of these particles

were clay minerals and other secondary minerals and

are not considered to have constituted a significant component of the primary grains. Clay particles were first separated by wet sieving, then carbonates and sulfates were removed by acid treatment, and finally the remaining materials were dried, weighed and separated by dry sieving with sieves calibrated at both 0.25 0 and 0.5 0 intervals depending upon the specific sample. The details of error analysis are discussed by Zhang (1993).

Originally, phenocrysts were transported in two

modes: (1) as single grains of whole or fractured

crystals; and (2) as aggregates cemented by glass

into pumice fragments. Once the glass has been

destroyed it is impossible to distinguish between the

two. The distribution of both pumice and free crys-

tals varies inversely with the distance from the source

and the total weight percent of each mode is a direct function of the magnitude, intensity and composition

of the parent magma. Redistribution of pyroclasts as

co-ignimbrite ash may be biased toward the finer sizes and thus constitute enrichment of the distal ash

in single crystals. Significant co-ignimbrite redistri-

bution would be expected to be reflected in an

increase in grain size of the distal ash with increas-

9999 r (A) 99.9

99

0 1 2 3 4 5 6 1.00 050 0.25 0125 00625 0.0313 0.0156 ,",

Grain size in o

99.99 r (W 99.9 t

99

.l .O, r-3

0 1 2 3 4 5 6 0 1.00 050 0.25 0125 0.0625 0.0313 0.0156 mm

Gram size in 0

Fig. 7. Representative grain size cumulative percent curves for the

Millbrig (A) and Kinnekulle (B) K-bentonite beds. Samples shown:

I = Ft. Payne, Alabama; 2 = Shelbyville, Tennessee; 8 =

Barnhart, Missouri; 9 = Warrenton, Missouri; IO = New London.

Missouri; 31= Oslo, Norway; 34 = Kinnekulle, Sweden: 38 = Saaremaa, Estonia: 43 = Valgu, Estonia.

W.D. Huff et al. /Journal of Volcanology and Geothermal Research 73 (1996) 285-301 291

ing distance, or at the least a stabilization of grain

size beyond some point of progressive decrease.

The median particle size ranges from a maximum of 0.8 mm in the southern Appalachians (proximal)

to 0.06 mm in the Mississippi Valley (distal), and

likewise from southern Sweden to Estonia (Fig. 7).

The data are essentially log normal and the moment mean, the average size of all measured particles

(Folk, 1980) exhibits a very slow decay rate with

distance from the presumed vent (Fig. 8). The de-

crease in median size over approximately 1000 km

in eastern North American is 0.13 mm. Zhang and

Huff (1995) have interpreted these data as indicative of a glass-rich tephra of probable co-ignimbrite ori-

gin for the ash deposited beyond 200 km from the

computed vent location. Post-Taconian events have

obliterated evidence of the location of the vent and

associated facies and consequently no sample locali-

ties closer than approximately 200 km are preserved in the stratigraphic record. Nevertheless, sample lo-

calities that are nearest the vent in the southern

Appalachians frequently display two to three graded units with coarse biotite and quartz concentrated near

the base of each (Haynes, 1994). These fining up-

ward units resemble those described by Sparks and

Huang (1980) for the Minoan ash layer of Santorini and attributed to co-ignimbrite ash deposition, Mi-

noan ash beds exhibit bimodal grain size in proximal samples and unimodal in more distal ones. The two

size groups are attributed to mixed plinian and co-

ignimbrite fallout close to the source and a co-

1 1

North America

ignimbrite origin for the distal ash (Sparks and

Huang, 1980; Cornell et al., 1983). A similar inter-

pretation for the Tambora eruption of 1815 was proposed by Self et al. (1984). Mixed plinian and

co-ignimbrite eruptions are difficult to model due to

the uncertainties concerning magnitude, intensity and column height. Co-ignimbrite ash clouds are en-

trained from hot, dense pyroclastic flows resulting

from collapsed plinian columns, and may originate either close to or at some distance from the vent.

They tend to be enriched in fine vitric ash and rise

by convection to as much as 30 km (Woods and Wohletz, 1991). Sustained co-ignimbrite plumes can

be responsible for the distribution of ash at distances

in excess of 1000 km compared to plinian eruption

columns which tend to distribute clasts on the order

of hundreds of km maximum from the vent (Sparks

and Walker, 1977; Moore, 199 1; Self, 1992; Woods and Self, 1992; Koyaguchi and Tokuno, 1993).

However, if the co-ignimbrite (or Phoenix) plume

originates above or close to the vent (Dobran et al., 1994) theoretical plinian column models might be

used to estimate minimum mass eruption rate and

column height. The co-ignimbrite column will

achieve the shape of a buoyant plume as it acts to

conserve mass during upward acceleration. Estimates

of the amount of pyroclastic material entrained in

this process are on the order of 35% of the total

erupted mass (Woods and Wohletz, 1991) so the

missing proximal facies may amount to as much as twice the volume of the preserved ash.

Baltoscandia

500 1000 500 0 0 500 1000 1500 Distance from Fort Payne. Alabama (km) Distance from Oslo, Norway (km)

Fig. 8. Variation of the moment mean of both the Kinnekulle K-bentonite in Baltoscandia and the Millbrig K-bentonite in North America as

a function of distance away from the location of maximum grain size. For the Kinnekulle bed, this position is at Oslo, Norway (Bergstrom et

al., 1995). For the Millbrig it is the Fort Payne, Alabama, site described by Haynes (1994). For plinian eruptions the average grain size

diminishes rapidly with distance from the source. The slow decay rate of these curves strongly suggests these beds represent co-ignimbrite

ash accumulations.

298 W.D. Huf et 01. /Journal of Volcanology and Geothermal Research 73 f I9961 285-301

5. Discussion and conclusions

Magnitude, intensity and dispersive power can be

deduced for ancient ash falls provided reasonable

estimates of area1 distribution, clast dispersal and

thickness can be made. Intensity and dispersive power

are closely related because the height of an eruption

column controls the dispersal of ash and is a direct

result of the magma discharge rate, and both together

are key factors in the convective rise and collapse of an eruption column (Sparks and Wilson, 1976).

Data from numerous Pleistocene and Holocene

plinian deposits were compiled by Walker (198 lb),

Carey and Sigurdsson (1989) and Rose and Chesner

(19901, and have been used to model the intensity and magnitude characteristics of explosive eruptions.

These data show that values range over three orders of magnitude, from lo6 to lo9 kg s- ’ (roughly

105-IO* m” s-‘) for intensities and 10” to 10” kg

for magnitudes, and that the two parameters are

positively related. Most of these eruptions produced

both fallout (plinian and co-ignimbrite) and ash flow

deposits and have DRE volumes that range from less

than 1 km’ to more than 800 km”. Walker (19801 proposed the class of ultraplinian eruptions for events

of exceptional magnitude, intensity or dispersive

power, and pointed out that high values in one

category do not necessarily correspond to high val-

ues in the other two. Thus the Taupo pumice in New Zealand had an exceptionally high eruption column

and rate of discharge but was rather modest in terms

of total volume of magma ejected.

The Deicke, Millbrig and Kinnekulle eruptions

produced fallout ash beds ranging from more than 900 to 1500 km’ DRE. It is difficult to compare

these events directly with the younger eruptions since

many of the literature descriptions combine fallout

and pyroclastic flow measurements to calculate total

magnitude. We have no information concerning ei- ther the near-vent equivalent facies of the K-be- ntonites, the volume of distal ash in the stratigraphic record, or the quantity of ash lost in the Iapetus Ocean during its closure. Nevertheless, on the basis of the incomplete record it would seem each of these

eruptions ranks as one of the largest fallout ash-pro- ducing events in the geologic record.

Estimates of the column height of the co- ignimbrite column of these are difficult to calculate

as opposed to plinian columns since few theoretical

models exist in the literature. Any successful model

would have to account for the considerations neces- sary to produce at least 1500 km3 of dense rock

distributed over more than 2 X 10’ km’ as fallout

pyroclastic debris. Wilson et al. (1978) predicted a

maximum height limit of 5.5 km for plinian eruptions

and an eruption rate of approximately lOh m3 s ’ . The model of Woods and Wohletz (199 1) indicates that pyroclastic flows expanding several tens of

square kilometers can produce a co-ignimbrite buoy-

ant ash cloud up to 30 km high. Comparison of the two plume models indicates that for a given mass

eruption rate a plinian column will rise higher than a

co-ignimbrite column. At a cloud height probably somewhat less than 55 km the Deicke, Millbrig and

Kinnekulle events would have required no more than

lo-20 days of sustained plume support in order to produce the preserved record of rhyolitic material. If

the bulk of the distal ash lifted off the surface of

pyroclastic flows is also taken into consideration the dispersal time could have been longer by 50%. This

is a reasonable order of magnitude when compared

with other large ash-producing eruptions whose dura-

tions range from hours to days (Walker, 198 1 b;

Koyaguchi and Tokuno, 1993; Rampino and Self,

1993). Four of the five largest Quatemary eruptions rep-

resent island arc or plate margin events associated

with active subduction zones. The Huckleberry Ridge

ash was associated with the collapse of an intraplate caldera (Izett and Wilcox, 1982) and was not directly

related to a plate collision margin. Parental magma

trace-element composition indicates the Ordovician volcanic vents were associated with active subduc-

tion zones (Huff et al., 1992, 1993; Bergstrom et al..

1995) and the closure pattern of the isopachs suggest they were most likely situated on an island arc or

microplate that collided with Laurentia during the

late Middle Ordovician. Ignimbrite or ash-flow de- posits are commonly associated with large-magni- tude eruptions. but none has been identified with either the three K-bentonite beds, although they may well have been present initially and then destroyed

during later tectonism. Potential near-vent equiva- lents on the eastern margin of Iapetus, based on reported age and composition, include the Snowdo- nia volcanics of North Wales (Campbell et al., 1988)

W. D. Huff et al. /Journal of’ Volcanology and Geothermal Research 73 (I 9%) 285-301 299

and the Borrowdale volcanics of the English Lake

District (Branney and Sparks, 1990). On the Lauren- tian margin the felsic lavas of the Bronson Hill

anticlinorium, an abducted Ordovician island arc in

eastern Massachusetts, have produced U-Pb zircon ages very similar to those of the Millbrig and Kin-

nekulle beds (Tucker et al., 1990). Other potential vents lay to the east in southwestern Norway (Still-

man, 1986) and to the southeast in Belgium (AndrC

et al., 1986), where Caradoc to Wenlock calc-al-

kaline ignimbrites and ash falls accumulated in sig- nificant proportions. However, their positions with

respect to paleowind motion reconstructions (Parrish,

1982) do not favor these as probable source areas. The Oliverian terrane (McKerrow et al., 1991) is

thought to have originated as a continental splinter

from Laurentia during the early Ordovician, and

recollided with it during the Taconic orogeny later in

the Ordovician. Bordered by an active subduction

zone such a terrane would provide a logical setting, both magmatically and geographically, for the De-

icke, Millbrig, or Kinnekulle vent. The felsic nature

of the primary ash implies that the magmas were at least in part derived by partial melting of continental

crustal rocks (Delano et al., 1990). and the inferred

latitude of the Oliverian terrane during the Middle Ordovician corresponds to the location of the K-be-

ntonite ash vents implied by contour geometry. Zir-

con ages from metarhyolites in the Bronson Hill

anticlinorium of southern New England are the same as those reported for the Millbrig (Tucker and

Robinson, 1990) and these rocks could represent a

near vent extrusive equivalent. In Great Britain felsic

pyroclastics of equivalent age occur in the ensialic

plate margin Borrowdale Volcanics of the Lake Dis- trict and in the Snowdon volcanics of northern Wales.

The area1 distribution pattern and grainsize of the

Millbrig and Kinnekulle K-bentonites place certain constraints and requirements on the dynamic charac- teristics of the eruption column, including its height,

velocity, density, duration and upper atmosphere conditions. Fluid dynamics models of explosive

eruption columns based on both empirical and theo-

retical evidence have been proposed by Sparks (1986) and Wilson et al. (1978), among others. These mod- els may be used for estimating the plinian column portion of the K-bentonite ash eruption assuming a rhyolitic magma at about 800°C with a 3-5s water

content. A comparative model of co-ignimbrite

columns (Woods and Wohletz, 1991) permits addi- tional estimates of conditions responsible for the

widespread dispersal of fine ash.

Acknowledgements

We thank W.C. Melson, S. Self, J. Luhr and A.

Sarna-Wojcicki for discussion and advice on an early

version of this manuscript. We are also indebted to F. Lu and D. McVey for assistance with the micro-

probe analyses, and to A.T. Anderson, J. Delano, A.

Kilinc and J.B. Maynard for discussion and advice

regarding the melt inclusion analytical data. This

research was supported by NSF grants EAR-9005333

and EAR-9204893 to Huff and Kolata, and EAR-

9004559 and EAR-9205981 to Bergstriim.

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