ARTICLE IN PRESS
Quaternary International 178 (2008) 276–287
Ordovician K-bentonites: Issues in interpreting and
correlating ancient tephras
Warren D. Huff
Department of Geology, University of Cincinnati, 2100 Clifton Avenue, Cincinnati, OH 45221, USA
Available online 23 May 2007
Abstract
Prominent Upper Ordovician K-bentonites include the Deicke and Millbrig beds in the Late Ordovician of eastern North America and
the Kinnekulle K-bentonite in the uppermost Sandbian of Baltoscandia. These beds are thought to represent some of the largest ash fall
deposits known in the Phanerozoic record. This report presents a comprehensive study of biotite compositions from the three beds
covering a far more extensive geographic range than has previously been described. These data show that the Kinnekulle and Millbrig are
multiple event ash beds, some parts of which are indistinguishable from one another. Tectonomagmatic discrimination diagrams
combined with Mg number data indicate that the Deicke–Millbrig–Kinnekulle sequence represents the progressive transformation from
calc-alkaline to peraluminous magmatic sources, consistent with a model of progressively evolving magmatism during the closure of the
Iapetus Ocean. It is concluded that the Millbrig and Kinnekulle beds are coeval and represent primarily simultaneous episodes of
explosive volcanism although it cannot be excluded that some portions of both beds were produced by the same event.
r 2007 Elsevier Ltd and INQUA. All rights reserved.
1. Introduction
Explosive volcanism plays a fundamental role in the
exchange of material and energy from the Earth’s interior
to the hydrosphere and atmosphere as well as the Earth’s
surface, and major explosive events are thought to be
capable of perturbing the Earth’s climate on time scales of
1–5 years, generally resulting in net global surface cooling
(Rampino et al., 1988). Historically documented eruptions
do not, however, represent the full range of intensity and
magnitude of all explosive eruptions in geologic history.
Deposits in the geologic record provide compelling
evidence for eruptions that were orders of magnitude
larger than ones witnessed by mankind. Contemporary
explosive eruptions may have lithofacies associations of
near-vent subaerial phenomena that typically include
pyroclastic surge deposits, thin welded tuff beds, various
lava flow morphologies, abundant erosional unconformities, and fluviatile and laharic facies. While volcanotectonic subsidence might aid in the preservation of such
deposits, they generally are not well known in the geologic
Tel.: +1 513 556 3731; fax: +1 513 556 6931.
E-mail address: warren.huff@uc.edu
record. Ash layers in marine sediments, on the other hand,
are the best geologic record of explosive volcanism, and
many of the Phanerozoic volcanic ash layers represent
enormous explosive eruptions that dispersed fine ash and
aerosols through the atmosphere over tens of thousands of
km2. While evidence of very large magnitude events can be
found throughout the Phanerozoic, this report will focus
on Ordovician events recorded in both North America and
Europe. The Ordovician record of explosive volcanism
consists of examples of both near-vent pyroclastic flows
and ignimbrites and distal sequences of altered fallout
tephras known as K-bentonites.
Questions frequently arise as to whether a particular
clay-rich bed might be an altered volcanic ash fall in the
form of a bentonite or K-bentonite. These beds are often
datable using fission track and U/Pb dating of zircons,
K/Ar, and Ar/Ar of amphibole, biotite and/or sanidine.
Due to their unique composition, they provide an
indispensable tool when correlating sections. The criteria
for recognizing such beds are varied, but fall into two
broad categories, field criteria and laboratory criteria.
Ideally, one would want both, but often that is not
possible. However, there are key features to look for in
each case that can aid in reliable identification.
1040-6182/$ - see front matter r 2007 Elsevier Ltd and INQUA. All rights reserved.
doi:10.1016/j.quaint.2007.04.007
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W.D. Huff / Quaternary International 178 (2008) 276–287
1.1. Field criteria
K-bentonites can be different colors when wet (blue,
green, red, yellow), but are characteristically yellow when
weathered. Due to their clay-rich nature, they will feel
slippery and waxy when wet. Some K-bentonites contain
euhedral to anhedral volcanogenic biotite, quartz, feldspar,
amphibole, zircon and apatite, and less commonly,
clinopyroxene, magnetite and garnet. The typical appearance of a K-bentonite bed in outcrop (Fig. 1) is that of a
fine-grained, clay-rich band ranging between 1 mm and 2 m
277
in thickness that has been deformed by static loads from
the enclosing siliciclastic or carbonate sequence. Accelerated weathering of K-bentonites causes them to be recessed
into the outcrop face. For thicker K-bentonites, there is
sometimes a zone of nodular or bedded chert in the
adjacent strata at both the base and the top of the bed.
1.2. Laboratory criteria
Most bentonites and K-bentonites are smectite- or illite/
smectite-rich, although some may contain a considerable
Fig. 1. (a) Core showing a K-bentonite (arrow) in a carbonate section, (b) Roadcut at Gladeville, TN, with Deicke (D) and Millbrig (M) separated by
about 4 m of Eggleston Fm. limestone, (c) Deicke 3.2 m below the Millbrig in the Decorah Fm. at Minke Hollow, MO, and (d) Deicke in the Eggleston
Fm. at Carthage, TN.
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W.D. Huff / Quaternary International 178 (2008) 276–287
amount of kaolinite, and those that have undergone lowgrade metamorphism may be dominated by R3 I/S and/or
sericite plus chlorite/smectite (corrensite) and/or chlorite.
Initial steps should begin with separation and XRD
analysis of the clay fraction as a preliminary means of
identification. Bentonites often contain volcanic phenocrysts and, for younger beds, even remnants of unaltered
volcanic glass, and their presence provides confirming
evidence of a volcanogenic origin. Wet sieving followed by
conventional heavy mineral separation techniques is
usually sufficient to isolate these different components.
Study of the non-clay fraction using high-quality optical
microscopy or SEM is satisfactory to determine the type
and characteristics of crystals in the sample. Thin section
study is also useful when more quantitative data are
desired.
2. Ordovician K-bentonites
K-bentonites have been recognized since the early
decades of the 1900s as very useful stratigraphic tools
(Nelson, 1921, 1922; Kay, 1935). Since they were deposited over large areas in a short time period, they provide precise event-stratigraphic correlation besides the
biogeographical, paleoecological and sedimentological
studies on both local and regional scales. They also have
tectonomagmatic and paleogeographic significance because
they preserve mineralogical and geochemical evidence
of their origin. As with every Phanerozoic system, Ordovician successions contain a number of K-bentonites
representing episodes of explosive volcanism, most commonly associated with large caldera-forming tectonic
events. Fig. 2 shows the general stratigraphic and
geographic distribution of beds. Numerous beds have been
reported from North and South America, Asia and
Europe.
The Ordovician successions of North America are
known to contain nearly 100 K-bentonite beds, one or
more of which are distributed over 1.5 106 km2 (Kolata
et al., 1996) and around 150 beds are recorded in
Baltoscandia (Bergström et al., 1995). The first report of
an Ordovician K-bentonite in North America was made by
Ulrich (1888), who described a thick bed of clay in the
upper part of what is now known as the Tyrone Limestone,
near High Bridge, Kentucky. Subsequent work by Nelson
(1921, 1922) showed that the bed was volcanogenic in
origin and that it could be correlated into Tennessee and
Alabama. After the 1930s, K-bentonite beds of Ordovician
age began to be reported from localities throughout eastern
North America (Kay, 1935; Weaver, 1953; Brun and
Chagnon, 1979; Huff and Kolata, 1990; Kolata et al.,
1996). Two prominent K-bentonites occur throughout
the eastern Midcontinent, the early Chatfieldian Millbrig
K-bentonite or T-4 bed and the late Turinian Deicke
K-bentonite or T-3 bed (Wilson, 1949).
The Deicke and Millbrig K-bentonites are the two most
widespread and prominent of the many ash beds in the
Upper Mohawkian/Champlainian ( ¼ uppermost Sandbian) strata of eastern North America. In their type areas
in the Upper Mississippi Valley, the Deicke, Millbrig and
other Mohawkian K-bentonites are only a few centimeters
thick and are noticeably lacking in non-clay minerals
(Willman and Kolata, 1978; Kolata et al., 1986). Using
geophysical logs of wells, Huff and Kolata (1990)
correlated the Deicke and Millbrig from that region with
the T-3 (Pencil Cave) and T-4 (Mud Cave) K-bentonites of
the Cincinnati Arch and the western Valley and Ridge. The
T-3 and T-4 beds are well known (Wilson, 1949; Milici,
Fig. 2. Global stratigraphic distribution of Ordovician K-bentonites.
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1969; Milici and Smith, 1969; Drahovzal and Neathery,
1971; Chowns and McKinney, 1980), and are relatively
thick and coarse grained.
In Mohawkian strata of the southern Appalachian basin,
of which the Rocklandian K-bentonites are a part,
carbonate rocks predominate in most of the area, with
clastic rocks confined to the easternmost outcrop belts of
the Valley and Ridge (Rodgers, 1953; Rader, 1982; Chowns
and Carter, 1983; Benson, 1986). The Deicke and Millbrig
occur in the Tyrone Limestone of the uppermost high
Bridge Group in central Kentucky; the Carters Limestone
of the uppermost Stones River Group in central and
southeast Tennessee and northwest Georgia; the upper
Moccasin and lower Eggleston Formations in southwest
Virginia, southeast West Virginia, and northeast Tennessee; and the upper Stones River Group (undivided) and
Chickamauga Limestone in Alabama (Kolata et al., 1996)
(Fig. 3). The Stones River and Nashville Groups are
defined and subdivided in the type area of the Chickamauga limestone Group (Milici and Smith, 1969).
In Kentucky, the Tyrone Limestone, the youngest
formation of the High Bridge Group is 18–36 m thick
(Wolcott et al., 1972) and is sublithographic, dense, creamcolored limestone, breaking with conchoidal fracture and
with small facets of coarsely crystalline calcite. Several
K-bentonites occur in the Tyrone Limestone. The Deicke
279
K-bentonite occurs 4.5–7.5 m below the top of the Tyrone,
and is the principal and the most consistently developed
bed. It ranges in thickness from 10 to 60 cm (McFarlan,
1943). The second most persistent and widespread
K-bentonite horizon is the Millbrig K-bentonite, which
occurs at the top of the Tyrone at the contact with the
overlying Lexington Limestone and is more coarsely
textured than the Deicke. A third, unnamed K-bentonite
has been observed in several localities at about 24 m below
the top of the Tyrone, and is generally about 3 cm or less
thick (Cressman, 1973).
In central Tennessee, the Carters Limestone is the
uppermost unit of the Stones River Group, and is divided
into upper thin-bedded and lower massive-bedded units
separated by the Deicke. The Upper member of the Carters
Limestone occurs between the Deicke and the overlying
Curdsville member of the Hermitage Formation. It is
characterized by dense, fine-grained, dove-colored lithology and thin bedding, which contains partings of silt and
clay. Throughout a large part of the central basin its
average thickness is 4 m. At numerous localities within the
eastern part of the central basin the Millbrig occurs near
the top of this member (Wilson, 1949).
In Alabama, the western facies of the Middle and Upper
Ordovician lies west and north of the Helena thrust fault
(Drahovzal and Neathery, 1971). The Stones River Group
Fig. 3. Stratigraphic distribution of the Deicke and Millbrig K-bentonites in the southern Appalachian basin (after Ryder et al., 1997). Note: Pencil Cave
and Mud Cave are drillers’ names previously applied to the Deicke and Millbrig K-bentonites, respectively.
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W.D. Huff / Quaternary International 178 (2008) 276–287
and the Nashville Group constitute the Chickamauga
Limestone Group of the Middle Ordovician. Several
K-bentonite horizons have been reported (Drahovzal and
Neathery, 1971) which can be correlated with the
Tennessee K-bentonites. In this area, the Deicke and
Millbrig K-bentonites have been traced from Tennessee
(Haynes, 1994). The Deicke K-bentonite is often 1–1.2 m
thick in Alabama (Drahovzal and Neathery, 1971) and its
color is yellow–gray to light yellow–green. It is characteristically underlain by a chert layer. The upper Millbrig
K-bentonite may be as much as 1 m thick, and is
characterized by abundant biotite flakes and chert layers
both above and below.
The clastic rocks along the eastern Valley and Ridge in
which the K-bentonites occur are assigned to the Blount
Group (Rodgers, 1953), a great clastic wedge that extends
along strike from Alabama to Virginia and persists
laterally across only two or three thrust sheets in the
southern Valley and Ridge. Both the Deicke and Millbrig
occur in the Colvin Mountain Sandstone in Alabama, but
only the Deicke has been found in the Bays Formation in
southeast Tennessee. In northeast Tennessee and southwest
Virginia, only the Millbrig is present in the Bays Formation
(Haynes, 1994).
The Kinnekulle K-bentonite is the thickest and most
widespread among the many K-bentonites in the Ordovician of Baltoscandia. It is generally present in stratigraphically continuous Johvian–Keilan sections from
southeastern Norway to Scandia and Bornholm and the
East Baltic (Fig. 4). It can be traced for about 1000 km
across Baltoscandia. The thickness ranges from more than
2 m in the Fylla Mosse drill core in Östergotland, Sweden
to o5 cm in southeastern Estonia, central Latvia and
western Russia. Bergström and Nilsson (1974) reported
more than 150 K-bentonite beds in the Middle Ordovician
shale succession of the Koangen drill core in south-central
Scania, but the majority of these are thin and apparently
not represented in the shallow-water carbonate-dominated
successions in much of south-central Baltoscandia.
The type locality of the Kinnekulle bed is the Mossen
section on the eastern slope of Kinnekulle. This abandoned
quarry is situated 2.0 km north of Osterplana church and
0.2 km west of Mossen farm (Thorslund, 1948; Jaanusson,
1964). At Mossen, it has a total thickness of 180 cm, but
Fig. 4. Stratigraphic position of the known Ordovician K-bentonites in North American and northern Europe. M, Millbrig, D, Deicke and K, Kinnekulle.
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due to the slumping of the overlying rocks of the Skagen
Formation only the uppermost part of the bed is exposed
(Bergström et al., 1995).
The possibility of a common source for these North
American and Baltoscandian ash beds was suggested by
Huff et al. (1992). They presented biostratigraphical,
geochemical, isotopic and paleogeographic data that
indicated that the Millbrig K-bentonite, a widespread
isochron and marker bed in North America, and the
Kinnekulle bed in Baltoscandia are coeval and probably
derived from the same eruptive event. Saltzman et al.
(2003) summarized the correlation of the Guttenberg 13C
Isotope Excursion (GICE) between eastern North America
and northern Europe, with a closely similar stratigraphic
position to both the Millbrig and Kinnekulle K-bentonites.
This excursion occurs in the same conodont and graptolite
zones as both the Millbrig and Kinnekulle beds and shows
clearly that these beds are coeval deposits.
Haynes et al. (1995) suggested that the proposed transAtlantic correlation of the Millbrig and Kinnekulle beds is,
at best, permissive. They argued that the utility of the
discriminant function analysis of whole rock compositions,
while perhaps appropriate for more localized regional
correlations, is not valid for large-scale regional or global
correlations. They maintained that uncertainty results from
the variable mobility of several major and certain trace
elements during diagenesis that might result in regional
shifts in bulk composition. Haynes et al. (1995) studied the
compositions of primary biotite phenocrysts and concluded
that they are more reliable as specific bed indicators than
bulk rock composition even though long-distance correlation based on phenocryst compositions can be still suspect
for various reasons. They reported a compositional
difference between Kinnekulle and Millbrig biotites with
respect to their FeO, MgO, Al2O3, MnO and TiO2 content,
and further suggested that these variations represent
separate magmatic sources. However, Haynes et al.
(1995) used data from only one Millbrig site in North
America and one Kinnekulle site in Baltoscandia, and they
did not evaluate lateral variation as well as within-bed
variation in biotite compositions. Furthermore, they culled
some of the analyses, leaving a data set representative
of both the modal and average composition of biotite in
each sample set. Hence, the consanguinity of these ash
beds, the largest recorded in the Paleozoic, has remained
uncertain.
3. Isotopic ages
One method of trying to resolve the question of the
equivalence of the Millbrig and Kinnekulle beds might be
to compare isotopic ages. Fig. 5 is a graphic compilation of
published ages for the Deicke, Millbrig and Kinnekulle
beds since 1960 (Table 1). The methods include fission
track, K–Ar, Ar–Ar, Rb–Sr and both batch and single
grain U–Pb isotopic dating. Stratigraphically, the Deicke
lies below the Millbrig, but this cannot be demonstrated on
281
Fig. 5. Published isotopic ages for the Deicke, Millbrig and Kinnekulle
K-bentonite beds since 1960. Computed standard deviations are not
shown. Methods include fission track, K–Ar, Ar–Ar, Rb–Sr and U–Pb.
Data are taken from various literature sources (Adams et al., 1960;
Byström-Asklund et al., 1961; Harland et al., 1964; Ghosh, 1972; Ross
et al., 1981, 1982; Williams et al., 1982; Baadsgaard and Lerbekmo, 1983;
Kunk and Sutter, 1984; Kunk et al., 1985, 1986, 1988; Samson et al., 1989;
Tucker et al., 1990; Compston and Williams, 1992; Tucker, 1992; Tucker
and McKerrow, 1995; Toulkeridis et al., 1998; Min et al., 2001).
the basis of published isotopic age dates alone. Similarly
the Kinnekulle and Millbrig beds occur in the same
biostratigraphic zones, which makes them broadly coeval
at the very least, however, isotopic ages also do not resolve
the age relations any further. Part of the problem surely lies
in the dating methodologies themselves and their inherent
constraints on precision and accuracy (see Renne et al.,
2000; Schmitz et al., 2003) notwithstanding substantial
improvements in equipment and procedures over the past
four decades. But the available data do not show any
progressive or systematic change in measured ages over
that time span that would suggest convergence upon a
‘‘true’’ value for each bed. Certainly another factor
contributing to the apparent age variance is the complex
nature of the Millbrig and Kinnekulle beds themselves
(Huff et al., 1999). Close examination in the field shows
that both beds consist of several internally graded units,
suggesting that each bed represents the cumulative deposition of multiple ash falls in environments characterized by
low background sedimentation rates. This aspect was
examined in some detail by Kolata et al. (1998) and
Haynes (1994) for the Millbrig and by Huff et al. (1999) for
the Kinnekulle, all of whom showed systematic mineralogical and grain size variation within individual subunits.
A careful examination of apatite compositions from the
Deicke and Millbrig (Fig. 6) reveals two distinct populations in the Millbrig from the lower and middle portions of
the bed. In the Deicke, apatite occurs only in the basal
portion. Given higher rates of sediment accumulation, it is
conceivable that these units would be preserved as a series
of closely spaced but separate beds (Bergström et al., 1997;
Huff et al., 1999).
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W.D. Huff / Quaternary International 178 (2008) 276–287
Table 1
Isotopic ages for the Deicke, Millbrig and Kinnekulle K-bentonites published since 1960
Bed
Formation
Location
Age (Ma)
Method
Reference
Deicke
Deicke
Deicke
Deicke
Deicke
Deicke
Deicke
Deicke
Deicke
Deicke
Deicke
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekullle
Millbrig
Millbrig
Millbrig
Millbrig
Millbrig
Millbrig
Millbrig
Millbrig
Millbrig
Millbrig
Millbrig
Millbrig
Millbrig
Tyrone
–
–
Plattin
Stones River
Lexington
–
Carters
Tyrone
–
Carters
–
Dalby
Dalby
Dalby
Dalby
Dalby
–
–
–
–
–
–
–
Dalby
Carters
Little Oak
Decorah
–
Stones River
Shakertown
Decorah
–
Tyrone
–
Stones River
–
–
Kentucky
–
–
Missouri
Alabama
Kentucky
–
Tennessee
Kentucky
–
Tennessee
Hullegard
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
Kinnekulle
–
–
–
–
–
–
–
Kinnekulle
Tennessee
Alabama
Missouri
–
Alabama
Kentucky
Missouri
–
Kentucky
–
Alabama
–
–
447715
462715
438715
450710
454.273
449.872.3
454.570.5
453.873.6
457.773
454.570.5
457.171
455.873.3
45474
444720
44673
455.673.7
45573
45778
456.871
44573
45076
454
45274
456.971.8
44771
453710
453
454712
456712
453.373
448.072.0
453.771.8
453.171.3
453.373
453.171.3
455.779.8
454.172.1
453.171.3
Fission track
Fission track
Fission track
Fission track
Ar–Ar
Ar–Ar
U–Pb
Ar–Ar
Ar–Ar
U–Pb
U–Pb
Ar–Ar
Ar–Ar
K–Ar
K–Ar
Ar–Ar
Ar–Ar
K–Ar
U–Pb
Rb–Sr
K–Ar
K–Ar
SHRIMP
U–Pb
Rb–Sr
U–Pb
K–Ar
Fission track
Fission track
Ar–Ar
Ar–Ar
U–Pb
U–Pb
Ar–Ar
U–Pb
K–Ar
U–Pb
U–Pb
Ross et al. (1982)
Ross et al. (1982)
Ross et al. (1982)
Ross et al. (1982)
Kunk and Sutter (1984)
Min et al. (2001)
Tucker (1992)
Kunk and Sutter (1984)
Kunk and Sutter (1984)
Tucker and McKerrow (1995)
Samson et al. (1989)
Min et al. (2001)
Min et al. (2001)
Byström-Asklund et al. (1961)
Williams et al. (1982)
Kunk et al. (1985)
Kunk and Sutter (1984)
Harland et al. (1964)
Tucker (1992)
Williams et al. (1982)
Williams et al. (1982)
Kunk et al. (1985)
Compston and Williams (1992)
Tucker and McKerrow (1995)
Baadsgaard and Lerbekmo (1983)
Adams et al. (1960)
Ghosh (1972)
Ross et al. (1981)
Ross et al. (1981)
Kunk and Sutter (1984)
Min et al. (2001)
Tucker et al. (1990)
Tucker (1992)
Kunk and Sutter (1984)
Tucker and McKerrow (1995)
Toulkeridis et al. (1998)
Kunk et al. (1985)
Tucker (1992)
The Middle Ordovician section at Röstånga in Scania
(southern Sweden) contains 18 K-bentonite beds ranging
from 1 to 67 cm in thickness, and all occur within the
D. foliaceus graptolite biozone. At Kinnekulle, 290 km to
the north, this interval includes the type section of the
Kinnekulle K-bentonite, which is very widespread and has
been correlated throughout northern Europe (Bergström
et al., 1995). In most sections, the Kinnekulle K-bentonite
can be recognized by distinctive geochemical fingerprints,
its prominent thickness, and by its biostratigraphic and
lithostratigraphic position. However, at Röstånga, whole
rock chemistry is inconclusive at identifying which of the
18 beds is the Kinnekulle K-bentonite. Several beds at
Röstånga correlate equally well with the Kinnekulle
bed (Bergström et al., 1997) and thus argue strongly
for the composite nature of what is called the Kinnekulle
K-bentonite. The Deicke, on the other hand, appears to be
a single event deposit. The regional aspects of ash
accumulation on submarine surfaces has been discussed
by Kolata et al. (1998) and Ver Straeten (2004).
4. Biotite geochemistry
Any comparative chemostratigraphic study of the
Deicke, Millbrig and Kinnekulle beds must clearly take
into account both vertical and lateral within-bed variations
in phenocryst content and phenocryst composition. Single
samples cannot be considered fair representations of bed
chemistry. This report presents a comprehensive microprobe study of K-bentonite biotite composition covering a
far more extensive geographic range for these three
Ordovician beds than has previously been reported. The
Deicke and Millbrig beds were sampled at 16 localities in
five US states and the Kinnekulle bed was sampled at 15
localities in Norway, Sweden, Denmark and Estonia
(Fig. 7). Separated biotite grains were mounted in epoxy,
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polished and analyzed by electron beam microprobe
operating at 15 kV and 20 nA with a 1 mm beam size.
After removing several hundred analyses due to mineral
grain alteration, a total of 666 Kinnekulle biotite analyses, 97 Millbrig biotite analyses and 39 Deicke biotite
analyses representing 31 separate geographic localities
provide the most comprehensive view to date of the
nature and extent of internal compositional homogeneity of these widespread Laurentian and Baltoscandian
ash beds.
283
Bivariate plots of some major element oxides are
compared in Fig. 8, much in the manner used by Haynes
et al. (1995). In each case, the Millbrig and Kinnekulle beds
are characterized by a central cluster of data points but
with significant scatter and overlap between the two and
also with the Deicke. Secondary clusters of data points
suggest multiple episodes of biotite crystallization, consistent with field evidence pointing to a multiple event history
for both the Millbrig and Kinnekulle. The Deicke
continues to appear as a single event deposit. Total iron
as FeO is an effective discriminator of Deicke biotite, but
not of the other two beds. These show partial overlap using
FeO and MgO (Fig. 8a and c), and nearly complete overlap
using MnO (Fig. 8b).
5. Tectonomagmatic setting
Fig. 6. Compositional variation in phenocrystic apatite grains from the
Millbrig and Deicke K-bentonites at Gadsden, Alabama.
Igneous biotite composition is broadly indicative of the
magma from which it crystallized and thus it serves as a
general petrogenetic indicator that can provide insight
into its tectonomagmatic setting. The method of AbdelRahman (1994) was used, which employs the ratios
between MgO, FeO as total iron, and Al2O3 in biotite to
discriminate between anorogenic extensional-related alkaline rocks, calc-alkaline I-type suites and peraluminous
suites including S-type rocks. Anorogenic rocks tend to be
high in Fe and low in Mg, a trend that reverses itself in
calc-alkaline suites. Peraluminous rocks are characterized
by an increase in Al compared with both Fe and Mg with
the substitution occurring primarily in octahedrally coordinated sites. When these biotite data are plotted on the
FeO–Al2O3 discrimination diagram of Abdel-Rahman
(1994), the Deicke samples plot clearly in the calc-alkaline
field (Fig. 9). Both the Millbrig and Kinnekulle beds show
Fig. 7. Location map for analyzed samples. Location details are given in Bergström et al. (1995) and Kolata et al. (1996).
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W.D. Huff / Quaternary International 178 (2008) 276–287
Fig. 9. Magmatic discrimination diagram based on compositional
variation in biotite, after Abdel-Rahman (1994). The symbols are the
same as in Fig. 8.
Fig. 8. Bivariate plots of biotite major elements.
a bimodal distribution. Millbrig samples plot primarily in
the calc-alkaline field with a subset overlapping with
Deicke samples. The majority of Millbrig samples are
more enriched in Fe than the Deicke samples and fall at the
extreme edge of the calc-alkaline field. The Kinnekulle
samples cluster primarily in the peraluminous field with a
subset apparently less Fe-rich than the Deicke samples and
plotting to the left of them in the center of the calc-alkaline
field. Mg concentrations in these samples show just the
opposite trend, suggesting that there is a petrogenetic trend
from relatively high-Mg, low-Fe Deicke samples forming in
a subduction-related environment to Fe-enrichment in
most of the Millbrig to Al enrichment in most of the
Kinnekulle samples. This scenario is generally consistent
with a model of the tectonic closure of the Iapetus Ocean,
which includes the involvement of increasing amounts of
Al-rich continental crust in the formation of volcanic
magmas. The fact that some Kinnekulle and Millbrig
samples have low Fe/Mg ratios suggests those portions
may represent an earlier stage in this process, or perhaps
even separate source volcanoes. The compelling evidence,
however, points to distinct compositional differences
between many of the Kinnekulle and Millbrig biotites
indicating that most were not sourced from the same
eruptive event as proposed by Huff et al. (1992), but rather
represent successive events, closely spaced in time, triggered by the progressive closure of the Iapetus Ocean and
its associated volcanic arc magmatism. The data also
reinforce the concept that the Millbrig and Kinnekulle beds
represent multiple eruptive events, some of which are
indistinguishable from one another.
Fig. 10 shows the bivariate distribution of Al2O3 vs. the
Mg number. The Mg number (Mg#) is defined as Mg2+/
(Mg2++Fe2+) and is widely used as an index of fractional
crystallization in evolving magmas, with values decreasing
from 1.0 as magmas become more highly evolved and thus
more Si-rich (Ragland, 1989). Here, the Mg# from biotite
data alone is computed and consider it a reasonable proxy
for the whole rock value. A plot of Mg# against Al2O3
shows a transition from high-Mg to low-Mg biotites and a
corresponding transition from dominantly Deicke through
Millbrig to Kinnekulle samples. Some overlap between
Deicke and Millbrig and between Millbrig and Kinnekulle
samples supports the concept that the three ash beds
represent a broad evolutionary pattern of magmatic
evolution. As with the distribution pattern in Fig. 8, a
subset of Kinnekulle samples has the highest Mg# in the
entire collection and thus reinforces the notion that the
Kinnekulle represents a multiple event deposit with some
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W.D. Huff / Quaternary International 178 (2008) 276–287
285
Acknowledgments
I am indebted to T. Gerke for assistance in generating a
large portion of the microprobe data, and to J. Haynes for
sharing additional microprobe data he collected during his
post-doctoral tenure at the Smithsonian Institution. This
research was funded in part by National Science Foundation Grant INT-9513128.
References
Fig. 10. Mg number versus Al2O3 in biotite. The symbols are the same as
in Fig. 8.
portions accumulating earlier than others, and possibly
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earlier evolutionary stages than others and, at least
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the base of each layer (Haynes, 1994).
6. Conclusions
The Deicke K-bentonite appears to be a single event
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