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Bollettino della Società Paleontologica Italiana, 49 (1), 2010, 13-33. Modena, 15 maggio 2010
The mid-Ludfordian Lau Event and Carbon Isotope Excursion (Ludlow,
Silurian) in southern Laurentia – Preliminary Results
James E. BARRICK, Mark A. KLEFFNER, Michael A. GIBSON, F. Nicole PEAVEY & Haraldur R. KARLSSON
J.E. Barrick, Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, U.S.A.; [email protected]
M.A. Kleffner, School of Earth Sciences, Division of Earth History, The Ohio State University at Lima, Lima, Ohio 45804, U.S.A.; [email protected]
M.A. Gibson, Department of Agriculture, Geosciences & Natural Resources, University of Tennessee at Martin, Martin, TN 38238, U.S.A.;
[email protected]
F.N. Peavey, Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, U.S.A.; [email protected]
H.R. Karlsson, Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, U.S.A.; [email protected]
KEY WORDS - Lau Event, Isotopes, Conodonts, Ludlow, Laurentia, Silurian.
ABSTRACT - The mid-Ludfordian Lau Event can be recognized in three areas along the southern margin of Silurian Laurentia in association
with a major positive carbon isotope excursion (CIE) and an abrupt turnover in conodont faunas: southern Oklahoma, southeastern Missouri,
and western Tennessee. Although the major features of the Lau Event and CIE in southern Laurentia are similar to those described from the
Baltic region, each site in southern Laurentia displays a different view of the effects associated with the Lau Event and a possible marine
flooding episode coincident with the start of the Lau Event. The Lau Event lies at a disconformity between the lower and upper members of the
Henryhouse Formation in southern Oklahoma at which the greater part of the CIE is missing. Diverse, but different offshore conodont faunas
occur below (Polygnathoides siluricus fauna) and above (Ozarkodina snajdri fauna) the disconformity. In the Moccasin Springs Member of the
Bainbridge Formation in southeastern Missouri, the CIE and the Lau Event occupy an offshore condensed section of argillaceous strata in
which Pseudooneotodus is the dominant conodont taxon. A less diverse Po. siluricus fauna occurs below the Pseudooneotodus interval and a
diverse O. snajdri fauna above it. In western Tennessee, the CIE and Lau Event lie within a grainstone unit assigned to the Bob Member of the
Brownsport Formation. The Po. siluricus conodont fauna of the underlying Beech River Member disappears within the base of the Bob
Member, but very few conodonts occur in the shallow water facies of the upper Bob and overlying Lobelville Member. No evidence of an
associated turnover in the diverse macrofauna of western Tennessee has been recognized. Identification of the Lau Event and the CIE in these
areas provides an important line of time-effective correlation across southern Laurentia that will allow better placement of poorly timeconstrained stratigraphic units and faunal assemblages in this region.
RIASSUNTO - [L’evento Lau e la variazione isotopica del carbonio durante il Ludfordiano medio (Ludlow, Siluriano) nella Laurentia
meridionale – Risultati preliminari] - Nel Siluriano sono stati individuati numerosi eventi, caratterizzati da estinzioni di faune e
variazioni nei rapporti isotopici del carbonio. L’evento Lau è documentato a scala mondiale in numerose aree in sedimenti del
Ludfordiano medio. Lungo il margine meridionale del paleocontinente di Laurentia viene riconosciuto in tre aree (Oklahoma meridionale,
Missouri sudorientale e Tennessee occidentale), sempre associato a una escursione positiva degli isotopi del carbonio (CIE) e a un
improvviso cambiamento nelle faune a conodonti. Le caratteristiche generali dell’evento Lau nella Laurentia meridionale sono simili a
quelle della regione baltica, dove l’evento è stato documentato per la prima volta, ma ognuna delle aree studiate in questo lavoro mostra
differenze negli effetti e una probabile trasgressione marina coincidente con l’inizio dell’evento.
Nell’Oklahoma meridionale l’evento Lau coincide con una discordanza tra i membri inferiore e superiore della Henryhouse Formation, in
cui gran parte della CIE è assente. Sopra e sotto la discordanza si trovano differenti associazioni di conodonti di mare aperto, rispettivamente
la Fauna a Polygnathoides siluricus e la fauna a Ozarkodina snajdri.
Nel Moccasin Springs Member della Bainbridge Formation nel Missouri sudorientale, la CIE e l’evento Lau sono documentati in una
sezione condensata di strati argillosi di mare aperto in cui la fauna è dominata dal genere Pseudooneotodus. Una fauna a Po. siluricus poco
differenziata è presente sotto all’intervallo a Pseudooneotodus, mentre sopra è documentata una ricca e varia fauna a O. snajdri.
Nel Tennessee occidentale il CIE e l’evento Lau si trovano all’interno di un grainstone appartenente al Bob Member. La fauna a Po.
siluricus presente nel sottostante Beech River Member scompare alla base del Bob Member, ma solo pochi conodonti sono documentati nelle
facies di acqua bassa della parte alta del Bob Member e del successivo Lobelville Member. Nessuna evidenza di estinzione è documentata nelle
ben diversificate macrofaune del Tennessee occidentale.
La documentazione dell’evento Lau e del CIE in queste aree costituisce una ottima modalità di correlazione temporale nella Laurentia
meridionale, che consentirà una migliore collocazione stratigrafica delle unità litostratigrafiche e delle associazioni faunistiche in questa
regione, fino ad ora poco calibrate.
INTRODUCTION
The mid-Ludfordian (late Ludlow) Lau PrimoSecundo Event of Jeppsson (1998) is one of three major
oceanic events that occurred during the Silurian (Calner,
2008). The Lau Event was characterized by a major
turnover in conodont faunas where the diverse association
of the Polygnathoides siluricus Zone disappeared
ISSN 0375-7633
(Jeppsson & Aldridge, 2000). Urbanek (1993) described
crisis C3 in the graptolite fauna at this level, which has
the second highest extinction rate of any event (70%) in
the Silurian (Neocucullograptus kozlowskii Event;
Melchin et al., 1998). Many other faunal groups were
also affected, as summarized by Calner (2008), and Calner
(2005) reported that the appearance of microbialites and
anachronistic facies on Gotland in the aftermath of the
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Bollettino della Società Paleontologica Italiana, 49 (1), 2010
extinctions was the result of a collapse of marine
ecosystems. A significant positive δ13C excursion (CIE)
that commonly attains values of +8‰ to +12‰ occurs at
the level of the Lau Event. Munnecke et al. (2003) stated
that this excursion is the strongest δ13C excursion during
the entire Paleozoic and that its maximum values are
exceeded only by values from the Proterozoic.
The Lau Event was originally recognized on Gotland
(Fig. 1), where the details of the lithologic succession,
the conodont succession, and δ13C values have been
documented in the greatest detail (Calner, 2005; Calner
& Eriksson, 2006; Eriksson & Calner, 2008). The level
of the Lau Event has been identified at numerous sites
around the globe, generally on the basis of the major
positive δ13C excursion (CIE) (e.g., Carnic Alps, Austria,
Wenzel, 1997; United States, Saltzman, 2001; Lithuania,
Martma et al., 2005; Australia, Talent et al., 1993 and
Jeppsson et al., 2007; Podolia, Kaljo et al., 2007; Czech
Republic, Lehnert et al., 2007). Published information
on lithofacies associations and faunal ranges across the
Lau Event and the CIE vary from publication to
publication, but few papers present combined data
(lithofacies, conodont faunas, and stable isotopes) of
sufficient detail for comparison with the Lau Event on
Gotland.
In this paper we present preliminary results of our
investigation of the lithologic succession, conodont
Fig. 1 - Paleogeographic reconstruction for the Silurian. Study area
indicated by cross in southern Laurentia. Star marks Gotland. Map
from Scotese (2002).
faunas, and stable isotopes across the Lau Event interval
in three areas in midcontinent North America, the region
that stretched across the southern margin of Laurentia
during the Silurian (Fig. 2).
Fig. 2 - Map of the southern United States (southern Laurentia) showing distribution of Silurian strata in outcrop (black) and in the
subsurface (lined). SOK: southern Oklahoma outcrop area (Dougherty West and Highway 77 sections); SEM: southeastern Missouri
(Greither Hill section); TN: western Tennessee (Linden and Eagle Creek sections). Map modified from Berry & Boucot (1970).
J.E. Barrick et al. - Mid-Ludfordian events in southern Laurentia
THE LAU EVENT
Calner & Eriksson (2006), Jeppsson et al. (2007), and
Eriksson & Calner (2008) provide a thorough
documentation of the lithologic, geochemical, and faunal
changes through the Lau Event of Gotland, as well as a
comparison to these changes in northeastern Australia.
Lehnert et al. (2007) give a thorough description of the
complexity of the sedimentologic, isotopic, and
paleontological change through the Lau Event in the
Prague Basin. Additional information available from
Lithuania comes from Martma et al. (2005) and Kaljo &
Martma (2007) and from Podolia (Kaljo et al., 2007).
Jeppsson (2005) and Jeppsson et al. (2007) developed
a detailed zonation across the Lau Event for conodont
faunas and the boundaries of zones correspond to steplike changes through the Lau Event. The Polygnathoides
siluricus Zone characterizes a major interval of pre-Lau
time during which conodont faunas attained high diversity.
The first effects of the Lau Event are recorded in the
Upper Po. siluricus Subzone. In this subzone, many
species typical of the Po. siluricus Zone disappear in rapid
stepwise extinctions and conodont diversity decreases.
The final extinction of rare Po. siluricus marks the top
of this subzone and the base of the overlying Icriodontid
Zone. The Icriodontid Zone is characterized by strongly
impoverished faunas in which icriodontid conodonts,
especially elements of Coryssognathus occur. The
Icriodontid Zone is divided into three subzones based on
the relative abundance of different species. The Lower
Icriodontid Subzone is the basal interval of the zone with
strongly impoverished faunas. The appearance of a slender
form of Panderodus equicostatus marks the base of the
Middle Icriodontid Subzone. In the Upper Icriodontid
Subzone, a single species shows extreme dominance of
the faunas. On Gotland, Pa. equicostatus rises to a
dominance of 90%, but in Scania (southern Sweden),
Ozarkodina scanica obtains a similar dominance. The
base of the overlying Ozarkodina snajdri Zone is the
level at which a diverse, reasonably balanced fauna,
including O. snajdri, appears.
In both Gotland (Calner & Eriksson, 2006; Eriksson
& Calner, 2008) and Australia (Jeppsson et al., 2007),
the δ13C record begins to increase during the Upper Polygnathoides siluricus Zone. δ13C continues to increase
to maximum values in the Middle Icriodontid Subzone to
Upper Icriodontid Zone (Calner, 2008). The highest value
of δ13C is given in Jeppsson et al. (2007, p. 133) as
10.54‰ at Glasskär 1 from the O. snajdri Zone. The
timing of the end of the CIE is less clear. Calner (2005,
2008, fig. 10) shows that values decline through the Ozarkodina snajdri Zone into the lower part of succeeding
O. crispa Zone. In Bohemia (Lehnert et al., 2007) and
Lithuania (Kaljo & Martma, 2006), however, the δ13C
values fall to background levels somewhat before the first
occurrence of O. crispa.
Precise correlation of the Lau Event conodont zones
and the CIE to the graptolite zonation is uncertain (Kaljo
& Martma, 2006; Kaljo et al., 2007). It is generally agreed
that the initial steep increase of δ 13 C lies in the
Neocucullograptus kozlowskii Biozone. The peak values
may lie within the N. kozlowskii Biozone (Azmy et al.,
1998; Saltzman, 2005), or higher (Lehnert et al., 2007;
15
Calner, 2008). Just as the more gradual decline in δ13C is
more poorly constrained relative to the conodont
zonation, it is also poorly constrained in the graptolite
zonation. The end of the excursion is generally shown as
ending in the late Ludfordian Monograptus formosus
Biozone (Calner, 2008), but questions remain about its
precise placement (Kaljo et al., 2007).
In each region where the Lau Event has been identified,
it is associated with lithofacies shifts indicative of a
regression, followed by a transgression. Eriksson and
Calner (2008) have documented that on Gotland the Lau
Event encompasses three stratigraphic sequences
separated by two periods of forced regression. The CIE
started at the onset of the first sequence. Values increased
through the first sequence and the first regression before
reaching the peak in the highland systems tract of the
second sequence. The second forced regression took
place in the lower Ozarkodina snajdri Zone. The δ13C
values do not decrease until the early transgressive
systems tract of the third sequence and return to
background values in the highland systems tract. Calner
(2005) interpreted the widespread appearance of
stromatolites, oncolites and microbial wrinkle structure
during and shortly after the Lau Event to represent a shortlived collapse of the marine ecosystem on Gotland, and
potentially globally.
Jeppsson et al. (2007) noted that the sequence of
lithologies on Gotland were similar to those in the Coral
Gardens Formation in northeastern Australia. A slow rise
in δ13C occurred in lower argillaceous strata and a more
rapid rise in the overlying weathering-resistant limestones.
A sharp increase in values (>1‰) occurred near the
transition and continued within the overlying argillaceous
oncolitic crinoidal limestones as stepwise disappearances
of conodonts occurred. Peak values appeared in the
overlying argillaceous oncolites associated with
“unbalanced” conodont faunas. The decline in δ13C values
occurred in the more typical suite of lithologies above and
as conodont faunas increased in diversity.
The three sections in the Prague Basin studied by
Lehnert et al. (2007) demonstrate how features of the
local sedimentary environments and the presence of
disconformities can obscure the record of the Lau Event.
Their data indicate that the full record of the Lau Event
and the CIE may be missing or truncated in the shallow
water sections because a significant fall in sea level
produced stratigraphic gaps by subaerial erosion, as
shown by the presence of karst features and diagenetic
alteration of the carbonate strata. In a deeper water
section, reduced sedimentation occurred during the sea
level fall, but the carbon isotope excursion of about
+8.0‰ is preserved. In the Prague Basin there was a
complete overturn in macrofaunal assemblages during the
Lau Event and completely new faunas invaded the
“biologically devastated” basin (Lehnert et al., 2007). In
the Muslovka section, the faunal overturn coincides with
the maximum values of the apparently truncated δ13C peak,
+4.2‰, just above a disconformity surface. Although
FADs and LADs of a few zonally significant conodonts
are shown on the diagrams, the overall changes in the
conodont faunas across the Lau Event are not described.
In Lithuania, the CIE is confined to the Mituva
Formation, which comprises nodular and laminated
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Bollettino della Società Paleontologica Italiana, 49 (1), 2010
limestones overlain by skeletal limestones with oncolitic
intercalations (Martma et al., 2005). Brachiopod
communities show a shallowing upward trend through the
Mituva Formation. The record of conodont faunas is
limited to FADs and LADs of a few zonally significant
species.
METHODS
Samples for conodont processing and isotope analysis
were collected at the same time. Conodont samples were
on the order of 2 to 6 kg and the typical sample interval
for conodont and isotope samples was 10 to 30 cm. All
conodont samples were processed with buffered formic
acid using the procedure of Jeppsson & Anehus (1995).
Faunal counts (Tables 1-5) are based on elements
collected on a 125-μm sieve (120 mesh). See Barrick et
al. (2009) for additional comments on the effects of
sample spacing and size. Isotope samples were obtained
by drilling of powder from sawed slabs of the carbonates.
The report by Cramer et al. (2006) that micrites and finegrained carbonates are reliable for stable isotope
chemostratigraphy has been well demonstrated. Most
samples were either carbonate mudstones or sparse
skeletal wackestones bearing few skeletal grains that were
avoided as much as possible. In the two Tennessee
sections, coarse-grained skeletal packstones and
grainstones occupy the position of the Lau Event. Samples
from these rocks were drilled in the same manner, even
though a high proportion of skeletal grains (largely
echinoderm and bryozoan grains) and calcite cement
comprised the resulting powder. As can be seen in the
resulting analyses, the peak of the CIE was still clearly
recorded. It appears that later diagenetic processes that
may have affected the packstone and grainstone strata did
not alter the original carbon isotope values significantly.
Why this was the case deserves more investigation.
Details of the procedures used in the isotopic analyses
can be found in Barrick et al. (2009) or Jacobi et al.
(2009). Results (Tables 6-10) are reported relative to VPDB (‰).
SOUTHERN LAURENTIA
Reconstructions of continental positions during the
Silurian (Fig. 1) place the southwestern margin of
Laurentia approximately 20º to 30º latitude south of the
equator, facing to the south to southwest (Cocks &
Scotese, 1991; Golonka et al., 1994; Cocks & Torsvik,
2002). Other work on continental positions during the
Ordovician and early Silurian (e.g. Dalziel, 1997; Niocaill
et al., 1997) shows Laurentia rotated in such a way that at
the beginning of the Silurian the modern southern margin
of North America faced west to southwest and Gondwana
lay a greater distance away from Laurentia than in the
older reconstructions. During the Silurian, a series of
carbonate platforms and ramps extended across the
southern margin of Laurentia from New Mexico eastward
into central Tennessee. Two major reentrants, the
Southern Oklahoma Rift and the Reelfoot Rift, both
Proterozoic rift basins, intersected the margin. Most of
the Silurian strata in this region are now buried in the
subsurface, but crop out in the three areas studied here:
Fig. 3 - Correlation of Wenlock, Ludlow, and Pridoli units in southern
Oklahoma, southeastern Missouri, and western Tennessee. Position
of the Lau Event indicated with a dashed line.
southern Oklahoma, eastern Missouri, and western
Tennessee (Fig. 2). The stratigraphic nomenclature and
correlation of units discussed here is shown in Figure 3.
Henryhouse Formation, southern Oklahoma
The Henryhouse Formation comprises the lower
portion of the late Silurian to Early Devonian Hunton
marlstone section that crops out in southern Oklahoma
and extends into the adjacent Anadarko and Arkoma basins.
The distinction between the late Silurian Henryhouse
Formation and the overlying Early Devonian Haragan
Formation was based largely on significant differences
in the shelly faunas as discussed by Amsden (1960, 1988).
Although the entire Henryhouse-Haragan marlstone
interval is characterized by calcareous shale and
argillaceous to silty carbonate mudstone and wackestone,
the greatest paleontological and lithological contrast
occurs within the Ludlow portion of the Henryhouse,
across the interval that includes the Lau Event. The
Henryhouse Formation in outcrop represents deposition
on the deeper part of a carbonate ramp that shallowed
northward into central Oklahoma (Stanley, 2001).
The basal portion of the Henryhouse Formation, which
we here informally designate as the “lower member” of
the Henryhouse (Fig. 3), includes a relatively thin
succession of extremely argillaceous and silty carbonate
mudstone and silty shale that includes graptolite-bearing
shale in the upper portion. A basal unit of brown
argillaceous, silty carbonate mudstone of the lower
Henryhouse rests with apparent disconformity on the
slightly argillaceous carbonate mudstone of the upper
J.E. Barrick et al. - Mid-Ludfordian events in southern Laurentia
Fig. 4 - Map of Murray County and northern Carter County in
southern Oklahoma showing locations of Dougherty West section
(34º24’41.71”N, 97º05’03.92”W) and Highway 77 section
(34º26’46.53”N, 97º05’08.88”W).
Clarita Formation. Conodonts of the Ludlow Kockelella
crassa Zone appear in the uppermost few centimeters of
the Clarita Formation and extend into the basal unit of
the lower member. Elements of the Ancoradella
ploeckensis Zone occur in the upper part of the basal unit.
Intervals of brown calcareous, silty shale and greenish
clay shale that are interbedded with argillaceous, silty
carbonate mudstone characterize the upper unit of the
lower member. Many of the greenish clay shale beds
contain poorly preserved graptolites. Conodonts of the
Polygnathoides siluricus Zone range through to the top
of the upper shaly unit of the lower member. The upper
shaly unit of the lower member comprises the pre-Lau
stratigraphic interval in the Henryhouse Formation.
Because the lower member has a restricted geographic
distribution, occurring only in the central Arbuckle
Mountain region, the transition through the Lau Event is
present at only a few localities.
The “upper member” of the Henryhouse Formation
comprises the greater thickness of the formation, which
attains a thickness of up to 100 meters on the Lawrence
uplift. In the central Arbuckle Mountain region the upper
member rests on the lower member and the lower-upper
member contact coincides with the level of the Lau Event.
Outside of the central Arbuckle Mountain region, the
upper member of the Henryhouse Formation rests
unconformably on the lower, Sheinwoodian part of the
Clarita Formation. The upper member is characterized
by an overall lower proportion of argillaceous matter in
the carbonate mudstones and packstones, and silty beds
are uncommon. The basal part of the upper member of
the Henryhouse Formation contains conodont elements
of the late Ludlow Ozarkodina snajdri Zone, the postLau interval. The upper member ranges through the Pridoli
to near the base of the Devonian (Barrick & Klapper,
1992; Jacobi et al., 2009) with little vertical change in
lithofacies. The abundant shelly fauna of the Henryhouse
17
Formation (see Amsden, 1988) comes almost entirely
from the upper member.
Because the lower member of the Henryhouse
Formation has a restricted geographic distribution and is
generally poorly exposed, only two sections were
identified where the changes through the Lau Event could
be studied in detail (Fig. 4). The first section is the wellknown and well-exposed Hunton section at the Highway
77 road cut at the north end of the Arbuckle Mountains.
This Hunton outcrop has been a stop on numerous
fieldtrips and often serves as a reference section for study
of Hunton stratigraphy. This is section M17 of Amsden
(1960), the Highway 77 (H77) section of Barrick &
Klapper (1976, 1992), field trip Stop 2 of Barrick et al.
(1990) and field trip Stop 9 of Stanley (2001). The second
section, the Dougherty West (DW) section, lies
approximately 6 km southeast of the Highway 77 section.
This is a natural exposure formed by stream erosion of
the nearly vertical beds of the Hunton Group. The
Henryhouse Formation is completely exposed below a
small natural dam formed by the underlying Clarita
Formation. The lower member of the Henryhouse is
considerably thicker at the DW section (12.5 m) than at
the Highway 77 section (4.8 m), and based on the
conodont succession, the lower member at the DW
section appears to more complete stratigraphically.
The variation in thickness of the basal and upper units
of the lower member of the Henryhouse and the pattern
of conodont first and last occurrences suggests the
presence of multiple depositional disconformities in the
lower member. Although conodont elements are relatively
common in the lower Henryhouse, the number of
elements per kilogram varies greatly. Most elements are
broken, which apparently occurred during the depositional
process, making element counts difficult.
Dougherty West section (DW) - The upper few meters
of the lower member of the Henryhouse Formation
comprises interbedded soft marly limestones
(mudstones) and greenish shales that contain poorly
preserved graptolites (Fig. 5). The diverse conodont fauna
is strongly dominated by elements of Dapsilodus
obliquicostatus. Elements of Panderodus unicostatus,
P. recurvatus and Decoriconus fragilis are common,
those of Belodella sp. are uncommon, and
Pseudooneotodus elements are rare. Elements of
Wurmiella excavata, Ozarkodina confluens, Oulodus
siluricus, Kockelella absidata, and Polygnathoides
siluricus occur in moderate numbers in most samples.
The top of the lower member is marked by a 20 cm
bed of dark brown to gray calcareous shale that is overlain
by light brown slightly argillaceous carbonate mudstone
of the upper member. The ranges of species characteristic
of the siluricus Zone terminate within or at the top of
the brown shale bed. There is no indication of steps of
extinction nor steps in changes of relative abundance of
taxa.
At the base of the upper member, Ozarkodina snajdri
appears and ranges higher. The conodont fauna is
dominated by elements of Dapsilodus obliquicostatus,
as below, and W. excavata and D. fragilis persist from
below. However, Pseudooneotodus elements now form
an important part of the fauna, and no Panderodus
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Bollettino della Società Paleontologica Italiana, 49 (1), 2010
elements occur in the lower 2 m of the upper member.
About 2 m above the base of the upper member, where
relatively clay-free limestones occur, P. equicostatus
appears with Ozarkodina auriformis.
The δ13C record in the upper part of the lower member
varies only slightly, from 0.0 to +1.0‰ through this
interval, but falls to -0.6‰ in the upper 20 cm of the lower
member. At the base of the upper member, δ13C increases
Fig. 5 - Detailed stratigraphic column of the Dougherty West section, southern Oklahoma. Conodont data given in Table 1. Stable isotope data
given in Table 6. Measurements in meters above base of Henryhouse Formation.
J.E. Barrick et al. - Mid-Ludfordian events in southern Laurentia
sharply to +3.5‰, declines to +1.0‰, over the next 0.5
m and then falls to between 0.0 and +1.0‰ higher in the
section. The δ18O record is more variable in the upper
part of the lower member, mostly between -2.0 and
-5.0‰, then drops to near -5.5‰ at the top of the lower
member. Values in the base of the upper member are
higher, between -3.0 and -5.0‰, before rising just above
-4.0‰ higher in the section.
Highway 77 section (H77) - Like the DW section,
the upper few meters of the lower member of the
Henryhouse Formation comprise interbedded soft marly
limestones (mudstones) and greenish shales that contain
poorly preserved graptolites (Fig. 6). The diverse
conodont fauna is strongly dominated by elements of
Dapsilodus obliquicostatus. All species of the Po.
siluricus Zone found at the DW section are present, but
occur in more equal abundance than at DW. Unlike at DW,
species ranges terminate at slightly different stratigraphic
levels. Ozarkodina confluens and Kockelella absidata
disappear just below the base of a distinct 10 cm bed at
the top of the lower member, and Po. siluricus and
19
Oulodus siluricus disappear at the base of the bed.
Panderodus unicostatus and P. recurvatus range into the
base of the overlying upper member of the Henryhouse
Formation, disappear and then recur about 1.0 m above
the base of the upper member. Elements of Ozarkodina
snajdri appear just below the top of the lower member
and range into the upper member. Except for the
recurrence of a few Panderodus elements in sample
309B, Panderodus elements do not occur in the lower
part of the upper member. About three meters above the
base of the upper member, where relatively clay-free
limestones occur, P. equicostatus and O. auriformis
appear. Fragments of Pedavis latialatus have been
recovered 3 m above the extinction level and Ozarkodina
crispa appears 6 m above the extinction level.
Values of δ13C range from about 0.0 to +1.0‰ through
the upper 2.5 m of the lower member. Just above the
contact between the lower and upper member, values dip
from +1.0 to -0.5‰. At 20 cm above the contact, values
of δ13C rise to +2.8‰ and then decline gradually to
around +1.5‰ a meter higher. Values of δ18O display a
similar pattern. In the upper part of the lower member,
Tab. 1 - Distribution of conodonts across the Lau Event in the Dougherty West section, Oklahoma. Measurements are in meters above the base
of the Henryhouse Formation.
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Bollettino della Società Paleontologica Italiana, 49 (1), 2010
Fig. 6 - Detailed stratigraphic column of the Highway 77 section, southern Oklahoma. Conodont data given in Table 2. Stable isotope data given
in Table 7. See legend to Figure 5 for explanation. Measurements in meters above base of Henryhouse Formation.
δ18O range between -1.8 and -3.0‰ and then fall to -4.5‰
just below the top of the lower member. In the base of
the upper member, values of δ18O rise to near -2.0‰ and
then decline gradually to -3.50‰ two meters higher.
Southeastern Missouri
Wenlock through Pridoli strata that crop out along the
western edge of the Illinois basin are assigned to the
Bainbridge Formation (Thompson, 1993). The lower
portion of the Bainbridge is generally assigned to the St.
Clair Member, which is mostly Wenlock in age and the
upper more argillaceous portion is assigned to the
Moccasin Springs Member (Fig. 3). The stratigraphy of
the Moccasin Springs Member, which appears to be
approximately 30 to 40 meters thick, has not been
completely determined because the unit is generally
poorly exposed and biostratigraphic information is sparse.
Based on graptolite and conodont faunas, the member
ranges in age from the Ludlow to near the top of the
Silurian (Thompson, 1993). The outcrop area of the
Bainbridge Formation sits on the western margin of the
Illinois Basin, which lies at the northern end of the early
Paleozoic Reelfoot Rift (Fig. 7). The Reelfoot Rift is
part of the New Madrid rift complex (Braile et al., 1982)
that initially formed in latest Proterozoic time (600 Ma)
as the Proterozoic supercontinent that included Laurentia
began to break up. Subsurface studies in the Illinois Basin
to the east of the outcrops show a westward sloping steep
carbonate ramp on which deeper water lithofacies of the
Bainbridge lie near the southwestern edge of Illinois.
J.E. Barrick et al. - Mid-Ludfordian events in southern Laurentia
21
Tab. 2 - Distribution of conodonts across the Lau Event in the Highway 77 section, Oklahoma. Measurements are in meters above the base of
the Henryhouse Formation.
Fig. 7 - Map of Reelfoot Rift area showing locations of southeastern
Missouri and western Tennessee outcrops relative to regional
structural features.
Although discontinuous in geographic distribution owing
to subsequent faulting and erosion, the southeastern
Missouri and southwestern Illinois outcrops also appear
to represent deeper water shelf to basinal facies.
Studies of conodonts at Lithium, Missouri by Branson
& Mehl (1933) and Rexroad & Craig (1971) show that
the exposures of the Moccasin Springs Member there
range from the Po. siluricus Zone up through the Pridoli.
However, the stratigraphic interval that crosses the Lau
Event is not exposed at Lithium. About 11.5 km northwest
of Lithium, west of St. Mary, Missouri, on the south side
of Greither Hill (Fig. 8), excavation of a drainage ditch
for residential construction in spring of 2007 fortuitously
exposed a section of the Moccasin Springs that did
transect the interval of the Lau Event (Fig. 9). Greither
Hill is the area from which Ball (1939) and Lowenstam
(1949) described sections of the Moccasin Springs.
Although abundant shelly fossils have been described
from the Moccasin Springs Member in the Greither Hill
area and other Moccasin Springs outcrops, no systematic
22
Bollettino della Società Paleontologica Italiana, 49 (1), 2010
Fig. 8 - Map of southeastern corner of Ste. Genevieve County,
Missouri, showing location of the Greither Hill section
(37º50’20.19”N, 90º00’54.39”W).
study of these faunas has been accomplished since the
work of Ball (1939, 1942).
The lower 6.2 meters of the Moccasin Springs
Member at Greither Hill includes dominantly reddish
argillaceous skeletal wackestone in which echinoderm
debris is the most common constituent. The lower part
of the section that is partially exposed along County Road
Z appears to be the locality from which crinoids, including
pisocrinoids have been described (Ausich, 1987).
Conodont faunas from the lower reddish wackestones are
dominated by elements of Panderodus unicostatus, with
lesser numbers of P. recurvatus, Dapsilodus
obliquicostatus, Decoriconus fragilis, and Wurmiella
excavata. Ozarkodina confluens and Oulodus siluricus
are uncommon. Polygnathoides siluricus and Kockelella
elements are absent from this section, although they
Fig. 9 - Detailed stratigraphic column of the Greither Hill section, southeastern Missouri. Conodont data given in Table 3. Stable isotope data
given in Table 8. See legend to Figure 5 for explanation. Measurements in meters above base of exposed section.
J.E. Barrick et al. - Mid-Ludfordian events in southern Laurentia
occur in the same stratigraphic position at the nearby
Lithium section (Branson & Mehl, 1933; Rexroad &
Craig, 1971).
Overlying the reddish carbonates lie 1.2 meters of
argillaceous green shale and limestone (Fig. 9). The basal
0.3 m of these beds contains the same abundant and
diverse faunas found in the underlying unit, with the
exception of the absence of Ozarkodina confluens. At
0.3 m, Panderodus unicostatus, P. recurvatus, and
Walliserodus elements disappear. Through the greater
thickness of the greenish interval, an impoverished fauna
characterized by Pseudooneotodus, was recovered. The
inpoverished Pseudooneotodus fauna continues into the
overlying interbedded reddish shale and thin tan-colored
carbonate mudstone. As the reddish shales are replaced
by a thick interval of tan-colored carbonate mudstone,
faunal diversity and abundance increases. In successive
samples appear Ozarkodina auriformis, Panderodus
equicostatus, and O. snajdri. Elements of Dapsilodus
23
obliquicostatus dominate the conodont fauna of the upper
tan limestone unit, and Decoriconus fragilis, Pseudooneotodus, and Belodella sp. elements are common.
Within the upper limestone unit occurs a small resistant
ridge-forming interval, which may correspond to the
“Merista bed” of Ball and Dunn (1931) and Ball (1942).
Values of δ13C are about +1.5‰ in the upper part of
the lower reddish limestone units, then fall to a low of
-3.5‰ at the extinction level near the base of the overlying
greenish unit. They rise as high as +4.1‰ near the top of
the greenish unit and rise higher, up to +5.1‰ in the lower
part of the overlying interbedded reddish shale and tan
limestone. Values of δ13C then fall in an irregular pattern
to around +1.0‰ with the lower meter of the upper tan
limestone unit. Unlike the δ13C pattern, values of δ18O
remain near -5.0‰ through the entire interval. Values rise
slightly, as high as -4.0‰ during the δ13C peak and fall as
low as -6.0‰ in the lower part of the tan limestone unit,
before rising gradually back to -4.5‰.
Tab. 3 - Distribution of conodonts across the Lau Event in the Greither Hill section, southeastern Missouri. Measurements are in meters above
the base of the exposed section.
24
Bollettino della Società Paleontologica Italiana, 49 (1), 2010
Western Tennessee
Silurian sedimentary rocks in central and western
Tennessee were deposited on a gently sloping ramp
locally referred to as the Western Tennessee Shelf
(Broadhead & Gibson, 1996), which was structurally
bounded by the Illinois Basin to the north, the Reelfoot
Rift embayment to the west, the Laurentian continental
edge to the south and the Nashville Dome to the east (Fig.
7). The main band of Silurian outcrop, which extends from
the Central Basin into the Western Valley of the Tennessee
River, forms a transect from a shallower to deeper
depositional position on a carbonate ramp. However, this
transect does not run directly toward the axis of the
Reelfoot embayment, but rather more toward the modern
southwest and the open continental margin of Laurentia,
and possibly along the same line of latitude, about 25ºS,
during the Silurian (Cocks & Torsvik, 2002). The effects
of this orientation on depositional patterns are not clear,
but the configuration of the shelf relative to paleowind
and paleocurrent directions is believed to have influenced
the influx of clastics from the Appalachian Basin region
(Stearns & Reesman, 1986; Broadhead & Gibson, 1996).
Ludlow through Pridoli strata on the Western
Tennessee Shelf have been assigned to three formations:
in ascending order, the Dixon Formation, Brownsport
Formation, and Decatur Limestone (Fig. 3). Although
numerous shelly fossils have been described from these
units over the years, especially from the Brownsport
Formation, the exact age relations remain unclear. Barrick
(1983) showed that the base of the Ludlow, which
corresponds to the base of the Kockelella crassa Zone,
lies near the base of the Dixon Formation, based on the
appearance of Kockelella variabilis. The age of the upper
Dixon was unknown. Rexroad and Nicoll (1971) cited a
personal communication from Robert Lundin, who
reported that Polygnathoides siluricus ranged from the
base of the Brownsport Formation into the lower beds of
the overlying Decatur Limestone at one section,
restricting the age of the Brownsport to the Po. siluricus
Zone. Lundin also stated that specimens of “Spathognathodus” remscheidensis had been recovered from higher
in the Decatur Limestone. Conodont faunas indicated that
the top of the Decatur is latest Silurian to earliest
Devonian, based on the occurrence of Icriodus
woschmidti (Harris et al., 1995).
When we started work on the Tennessee sections, we
projected that the Lau Event and associated CIE should
lie near the base of the Decatur Limestone. However,
because no carbon isotope event could be located in
sections spanning the Brownsport-Decatur contact and
the sparse conodont faunas from the upper beds of the
Brownsport appeared to be a post-Lau Event fauna, we
shifted our work downward into the Brownsport
Formation. Our preliminary stratigraphic work, combined
with conodont biostratigraphy and stable isotope
stratigraphy, suggest that the internal stratigraphy of the
Brownsport Formation is far more complicated than
previously thought.
The Brownsport Formation has been divided into three
fossiliferous members (Fig. 3), in ascending order, the
Beech River, Bob, and Lobelville members (Pate &
Bassler, 1908; Amsden, 1949). The Beech River Member
comprises 16 to 20 m of interbedded, thin-bedded shale
to medium-bedded echinoderm-bryozoan wackestone and
packstone (Amsden, 1949; Broadhead & Gibson, 1996).
The glades that form during weathering of the Beech River
are abundantly fossiliferous. The sponge Astraeospongia
meniscus serves as a guide to the member, which is
typified by a diverse echinoderm fauna (e.g.,
Troostocrinus and Eucalyptocrinites; Pate & Bassler,
1908; Springer, 1917, 1926). Sampling of the upper
Dixon Formation and Brownsport Formation at several
localities shows that Polygnathoides siluricus does not
occur in the Dixon, but that it appears at or near the base
of the Beech River Member and ranges through this
member.
Medium- to thick-bedded coarse-grained echinodermbryozoan packstone and grainstone that lie above the
Beech River Member have been assigned to the Bob
Member. Brachiopods are the most abundant
macrofossils followed by lesser amounts of ostracodes,
and corals. The brachiopod fauna of the Bob is
characterized by the conspicuous Rhipidium, but outcrops
generally contain a wide variety of brachiopod taxa
compared to other fossil groups and relative to overlying
and underlying strata. Although the name “Bob Member”
has had widespread use, there has always been some doubt
whether it forms a contiguous stratigraphic unit, or
whether the coarse-grained beds are discontinuous lenses
that are a facies of the upper part of the Beech River
Member (Amsden, 1949). Our work, although
preliminary, shows that in the western sections near the
Tennessee River, the type area of the Bob Member, the
coarse-grained beds are difficult to distinguish from the
associated Beech River beds and appear to lie below the
Lau Event and CIE. However, in sections well to the east
of the Tennessee River, a well-defined, coarse-grained
packstone to grainstone unit overlies typical Beech River
beds and contains the Lau Event. For this reason we use
the term “Bob Member” in quotes to designate this
Fig. 10 - Map of Decatur, Perry, Hardin, and Wayne counties in
western Tennessee showing locations of the Linden section
(35º36’12.67”N, 87º57’54.05”W) and the Eagle Creek section
(35º17’06.93”N, 87º59’08.20”W).
J.E. Barrick et al. - Mid-Ludfordian events in southern Laurentia
grainstone unit in the eastern sections that are discussed
in this paper.
The Lobelville Member superficially resembles the
Beech River Member, also comprising interbedded, thinbedded shale to medium-bedded echinoderm-bryozoan
wackestone and packstone. However, its bedding
characteristics allow it to be easily recognized, for it
consists of thin 1 to 4 cm thick graded beds of echinoderm
packstone or grainstone that grade upward to wackestone
or mudstone. Lobelville biodiversity becomes great in
some horizons with a wide array of pentamerid
brachiopods, rugose corals and large tabulate corals.
Disarticulated stalked echinoderms (89% of the
accumulating skeletal sediment) with relatively common
calyxes and articulated stem pieces are abundant
especially near the tops of limestone beds and within
shale beds of the lower Lobelville. Bryozoans increase
in diversity and abundance up-section (up to 40% of
allochems). Tabulate corals are a major component of
Lobelville coral assemblages, with as many as twelve
different species of tabulates, including auloporid,
halysitid, favositid, heliolitid, alveolitid and theciid
tabulates, co-occurring (Amsden, 1949; Olson &
Broadhead, 1995) and locally rugose corals occur as
monospecific patches of Cyathophylum sp.
25
Thus far, we have sampled in detail two sections of
the Brownsport Formation that include the CIE and the
Lau Event, as represented by the δ13C peak and the
associated shift in the conodont fauna. Both of these
sections lie east of the Tennessee River, in the area where
the coarse grainstone unit we call the “Bob Member” is
well developed (Fig. 10). The section near Linden (LD)
is the more complete, with the upper Beech River, the
“Bob” Member, the Lobelville Member and the base of
the overlying Decatur Limestone completely exposed.
At the Eagle Creek (EC) section farther to the southwest,
the upper Beech River and “Bob” are exposed, but the
Lobelville is mostly covered.
Linden section (LD) - At the Linden section (Fig. 11),
an older road cut 1.6 km south of the center of Linden on
the west side of Tennessee Highway 13, the lower portion
of the section has typical Beech River lithology, greenishgray interbedded shale, carbonate mudstone and
echinoderm-bryozoan wackestone that grades into
packstone. Near the top of the member, beds of coarser
grained packstone appear and chert nodules occur. A
distinctive grainstone bed with abundant bluish-white
chert nodules appears 1.5 m below the top of the Beech
River Member. The uppermost 1.5 m of the Beech River
Tab. 4 - Distribution of conodonts across the Lau Event in the Linden section, western Tennessee. Measurements are in meters above the base
of the exposed section.
26
Bollettino della Società Paleontologica Italiana, 49 (1), 2010
Fig. 11 - Detailed stratigraphic column of the Linden section, western Tennessee. Conodont data given in Table 4. Stable isotope data given in
Table 9. See legend to Figure 5 for explanation. Measurements in meters above base of exposed section.
is a coarsening upward cherty interval that begins with
argillaceous wackestone at the base and slightly
argillaceous echinoderm-bryozoan grainstone at the top.
The base of the “Bob” Member is a 1.35 m bed of
coarse-grained echinoderm-bryozoan grainstone that
forms the lower part of a small cliff. Bluish-white chert
occurs through the lower 2 meters and sparse glauconite
grains appear 2.5 meters above the base of the “Bob” and
occur higher into the Lobelville Member. The top of the
“Bob,” which is about 4.5 m thick, is placed where shale
beds reappear in the section. Above a basal unit of
interbedded grainstone and shale, the Lobelville Member
is dominated by shale and argillaceous wackestone with
thin (10 cm) packstone to grainstone beds.
The lower part of the Beech River at the Linden
section contains the typical conodont fauna of the
Polygnathoides siluricus Zone. Conodont abundance is
low to moderate, about 10 to 50 elements per kilogram.
Panderodus unicostatus strongly dominates the fauna
and some elements of P. recurvatus and Ozarkodina
confluens occur in most samples. Other important taxa
occur at a frequency near 1-2 per kilogram: Po. siluricus,
Oulodus siluricus and Wurmiella excavata. The last
occurrences of Ou. siluricus, Po. siluricus and O.
confluens lie within the upper Beech River. The
uppermost 1.5 m of the Beech River produced a larger
and more diverse conodont fauna in which elements of
Dapsilodus obliquicostatus, Decoriconus fragilis, W.
excavata and Pseudooneotodus spp. are common, unlike
the lower beds. Ozarkodina snajdri appears just above
the LAD of Po. siluricus, a short interval below the base
of the massive grainstone unit of the “Bob.” The basal
J.E. Barrick et al. - Mid-Ludfordian events in southern Laurentia
27
Fig. 12 - Detailed stratigraphic column of the Eagle Creek section, western Tennessee. Conodont data given in Table 5. Stable isotope data
given in Table 10. See legend to Figure 5 for explanation. Measurements in meters above base of exposed section.
0.5 m of the “Bob” contains a small conodont fauna,
dominated by elements of Panderodus, both P.
unicostatus and P. recurvatus. The last occurrence of
Walliserodus sp. is just 0.20 m above the base of the
“Bob.” Above the middle of the lower grainstone unit of
the “Bob,” few conodonts were obtained. From a
combined weight of about 20 kg collected through the
next 5 meters, only 12 elements were recovered of five
species, W. excavata, Decoriconus fragilis, Pseudooneotodus sp., Belodella sp., and Panderodus sp.
Values of δ13C are about +1.5 to +2.0‰ through most
of the upper part of the Beech River member. They rise
gradually through the uppermost 1.5 m of the member to
as high as +3.0‰ near the top of the unit, except for the
sample at the top of the Beech River, which has a value of
only +0.6‰. Values continue their gradual rise through
the lower 2.0 m of the “Bob,” and attain peak values
between +4.0 and +5.2‰ in the middle 1.5 m of the
member. In the upper “Bob”, starting near where
glauconite appears in the grainstone, δ13C values fall
steeply to less than +1.0‰ at the top of the member. In
the lower beds of the Lobelville, δ13C values remain near
+1.0‰. In contrast to the pronounced peak in δ13C, values
of δ18O change little through this interval. There are some
minor short term fluctuations, but generally the δ18O
values remain between -3.0 and -5.0‰.
Eagle Creek (EC) - The Eagle Creek section (Fig. 12)
is a recently exposed road cut on the north side of US
Highway 64, 4.4 km east of Olive Hill. The section lies
28
Bollettino della Società Paleontologica Italiana, 49 (1), 2010
Tab. 5 - Distribution of conodonts across the Lau Event in the Eagle Creek section, western Tennessee. Measurements are in meters above the
base of the exposed section.
37.5 km southeast of the Linden section and is the most
southern and western exposure where the coarse-grained
grainstone termed here as the “Bob” has been recognized
thus far. The lowest part of the Beech River Member
exposed here consists of about 5.8 m of purple and green
shale and argillaceous mudstone and echinoderm
wackestone and packstone that contain increasing
proportions of skeletal material up section, where
argillaceous packstone to grainstone layers become
common. At the top of the Beech River occur two
successive coarsening upward intervals, 1.30 and 1.40 m
thick. Each interval begins with a 20 cm shale that is
overlain by argillaceous wackestone that grades up to an
echinoderm-bryozoan grainstone at the top. The lower
3.55 m of the “Bob” Member consists of coarse-grained
echinoderm-bryozoan grainstone that forms a small cliff.
At the top of this cliff occurs a 0.7 m section of
interbedded shale and grainstone lenses, which is then
overlain by another 3.85 m of grainstone. The upper
grainstone interval is similar to the base of the “Bob,”
but tends to somewhat finer grained and contains common
brachiopods at some levels. A covered slope, which
should represent the shaly strata of the Lobelville
Member, lies above.
The δ13C values through most of the Beech River
section lie near +1.0‰. Near the top of the typical Beech
River and in the basal shale of the lower of the two
coarsening upward intervals at the top of the member, δ13C
values fall as low as -2.0‰, before rising back to +1.5‰
through the lower coarsening upward interval. In the upper
coarsening upward interval, δ13C value increases to more
than +2.0‰ and this increase continues through the lower
2.5 m of the “Bob” to a peak of +5.2‰. Values stay above
+4.0‰ for the next meter before showing a steady decline
in value to just less than +1.0‰ at the top of the section.
Values of δ18O fluctuate through the Beech River and
“Bob” section, but generally stay in the -5.0 to -3.0‰
range.
DISCUSSION
In each of the three areas of southern Laurentia
studied here, southern Oklahoma, southeastern Missouri,
and western Tennessee, the Lau Event is represented by
some combination of changes in conodont faunas, the
associated major positive CIE, and shifting lithofacies.
However, each area preserves its own unique record of
the Lau Event, little of which corresponds closely with
those records of the Lau Event reported from Baltica and
peri-Gondwana. In particular, no icriodontids nor any
indications of the subdivisions of the Icriodontid Zone
J.E. Barrick et al. - Mid-Ludfordian events in southern Laurentia
29
Tab. 6 - Stable isotope data across the Lau Event in the Dougherty
West section, southern Oklahoma. Levels were measured above the
base of the Henryhouse Formation.
Tab. 7 - Stable isotope data across the Lau Event in the Highway
77 section, southern Oklahoma. Levels were measured above
the base of the Henryhouse Formation.
can be identified. Investigations of additional sections
are needed to document more thoroughly the preliminary
results described here.
Lithofacies and biofacies in pre-Lau and post-Lau
strata in southern Oklahoma indicate that deeper water
conditions prevailed before and after the Lau Event. Finegrained skeletal wackestone and mudstone dominate both
below and above the event, but pre-Lau beds are distinctly
more argillaceous and silty. The conodont faunas are
strongly dominated by Dapsilodus elements below and
above the event. The high abundance of Dapsilodus
elements has been interpreted to be indicative of far
offshore marine settings during the Silurian (e.g. Barrick,
1983). Graptolites occur in shales below the event, but
not above it. The change in conodonts is relatively abrupt
and no step-wise extinction pattern is apparent. Conodont
diversity is reduced from several (11-12) to a few (5-6)
species across the event and Ozarkodina snajdri appears
just above the extinction level. Elements of Panderodus
spp. are notably absent in the post-event fauna and return
a couple of meters higher with one new conodont species,
O. auriformis. The carbon isotope excursion ends well
before the first appearance of Ozarkodina crispa.
The conodont extinction level coincides with a thin
dark shale (DW) or a bedding surface (H77) that probably
represents a disconformity surface. Below this surface
is a distinct drop in values of both δ13C and δ18O, a pattern
that may represent diagenetic alteration of beds just below
a submarine disconformity surface. Ludvigson et al.
(2004) proposed that similar negative δ13C excursions in
Ordovician strata were produced possibly by the local
Tab. 8 - Stable isotope data across the Lau Event in the Greither Hill
section, southeastern Missouri. Levels were measured above the
base of the exposed section.
30
Bollettino della Società Paleontologica Italiana, 49 (1), 2010
diagenetic effects of incursions of euxinic bottom waters
during marine flooding events (see also Dickinson et al.,
2008). Above the disconformity surface, the δ13C curve
appears truncated such that the greater part of CIE is
absent, and only the final decrease in values near the end
of the excursion are preserved. This appears to be similar
to the observations of Ludvigson et al. (2004), who
reported that the record of positive Middle Ordovician
carbon isotope excursions disappears because of
sediment starvation in deeper water settings during marine
flooding events.
The combination of lithofacies, biofacies, and
geochemical evidence suggests that the position of the
Tab. 9 - Stable isotope data across the Lau Event in the Linden
section, western Tennessee. Levels were measured above the base
of the exposed section.
Tab. 10 - Stable isotope data across the Lau Event in the Eagle
Creek section, western Tennessee. Levels were measured above
the base exposed section.
J.E. Barrick et al. - Mid-Ludfordian events in southern Laurentia
Lau Event coincides with a submarine hiatus in the
southern Oklahoma sections. The greater part of the
record of the Lau Event and the CIE is absent because
strata that might have recorded the event were never
deposited in starved offshore setting during a major
flooding event. The reappearance of offshore lithofacies
and conodont biofacies immediately after the Lau Event
represents the end of sediment starvation shortly after
the marine flooding event.
The one section in southeastern Missouri, Greither
Hill, provides a slightly different version of the Lau Event.
Argillaceous wackestones below the position of the Lau
Event contain a Panderodus-dominated conodont fauna,
in which Dapsilodus is only a minor constituent.
Conodont diversity is moderate, only nine species occur
just below the extinction event. Polygnathoides
siluricus, Kockelella absidata and Oulodus siluricus are
absent, but the outcrop exposure and our sampling do not
permit us to resolve a step-wise pattern of extinction for
these species. The extinction level of conodonts coincides
with a strongly negative excursion (-3.6‰) in δ13C, above
which δ13C reaches values typical of the CIE, in the +3.0
to +5.0‰ range. A strongly impoverished conodont fauna,
only a few specimens of Pseudooneotodus, is present.
As the values of δ13C decline to background values, a
moderate diversity conodont fauna reappears (4 to 7
species), including O. snajdri and O. auriformis. The
fauna is strongly dominated by elements of Dapsilodus
and contains only rare elements of Panderodus.
The position the Lau Event in Missouri can also be
interpreted as coinciding with a major marine flooding
event across a somewhat shallower position on a ramp
than that found in southern Oklahoma. The basal flooding
surface may be marked by the conodont extinction level
and the negative carbon isotope excursion. The
argillaceous greenish limestone that yielded mostly
Pseudooneotodus elements may be a condensed,
offshore record of the Lau Event. Environmental
interpretation of the sparse Pseudooneotodus-only fauna
is problematic, but this may be a situation similar to lower
Wenlock conodont faunas from the Clarita Formation in
southern Oklahoma. There, a low abundance and low
diversity conodont fauna with abundant Pseudooneotodus
lies just above the major flooding surface at the base of
the formation (Barrick, 1977, 1997).
In Tennessee, the CIE associated with the Lau Event
is well preserved in shallow water carbonates. A welldeveloped negative carbon excursion is present in the
upper part of the Beech River, at the lower of the two
coarsening-upward intervals, just below the start of the
excursion, at the Eagle Creek section. This negative
excursion may be represented by a single point that lies
just below the base of the single coarsening upward
interval at the Linden section. The negative excursions
here may also represent diagenetic effects below a
discontinuity surface. The steady rise of δ13C to a peak in
the grainstone of the “Bob” Member, followed by the
steady decline suggests that the grainstone unit was
aggrading with little reworking or admixture of older
skeletal grains. Also, initial cementation must have
occurred near the time of deposition and later diagenetic
effects were minimal for the excursion to be so well
preserved in this facies. Although the data are not
31
definitive, a marine flooding event across the shallow
water Western Tennessee Shelf, followed by progradation
and aggradation of skeletal grainstone during the
maximum of the CIE may be the best description of the
sedimentary history at these sections during the Lau
Event.
Conodont faunas in the Beech River Member below
the excursion are diverse (10-12 species), but not
especially abundant. Elements of Panderodus dominate
the faunas. Some species disappear in sequence as the
values of δ13C begin to increase in the top of the Beech
River, Oulodus siluricus, Ozarkodina confluens, and
then Polygnathoides siluricus, but this may be an artifact
of the low numbers of elements we recovered.
Ozarkodina snajdri appears at both the Linden and Eagle
Creek sections near the base of the “Bob” Member, just
after the LAD of Po. siluricus, but is associated with other
species characteristic of the pre-Lau fauna. Just above
this and a short interval below the preserved peak in δ13C
values, conodonts essentially disappear from the section,
and only a few conodont elements were recovered from
the upper “Bob” and basal beds of the Lobelville Member.
In the Brownsport Formation, the abundant and diverse
shelly fauna displays a paleocommunity restructuring with
the shift to each successive lithofacies member. Many
taxa are common to all three members and there is not
distinct pre-Lau and post-Lau macrofaunal composition
(Amsden, 1949). Although sparse oolitic layers and algal
coated grains have been reported from the “Bob” Member,
we have not found any evidence of a major benthic
ecosystem collapse or pervasive microbialitic
“anachronistic” faunas. The Lau extinction event appears
to have a weak signature in benthic faunas in the western
Tennessee strata, or at least one that can be equally
explained by ecological shifts in response to normal
environmental changes associated with a rapid rise in sea
level.
The Henryhouse Formation, Moccasin Springs
Member of the Bainbridge Formation, and the Brownsport
Formation have been considered to be generally
correlative units of Ludlow age based on the similarity
of the macrofaunal assemblages by numerous workers
for over one hundred years (e.g., Ball, 1942, with a
summary of previous work; Amsden, 1949; Berry &
Boucot, 1970; Amsden 1988). However, no consensus
existed on the detailed correlation of individual units from
southern Oklahoma to southeastern Missouri to western
Tennessee. The recognition of the abrupt change in
conodont faunas and large positive CIE associated with
the short-lived Lau Event in southern Laurentia now
provides us with an easily recognizable and reliable time
horizon for the region. In the future, studies of the
distribution of Ludlow lithofacies patterns and faunal
assemblages at well-known and new sections across
southern Laurentia can be easily situated in time as being
either before or after the Lau Event.
SUMMARY
Our preliminary work shows that the Lau Event is
represented on the southern margin of Laurentia by an
extinction event in conodont faunas and the associated
32
Bollettino della Società Paleontologica Italiana, 49 (1), 2010
major positive CIE. In each of the three areas, the Lau
Event and CIE appear to be associated with a marine
flooding event and sequence boundary, but the resulting
lithofacies patterns are different in each area, depending
on their position on the local carbonate ramp. Limited
data on macrofaunal associations, mostly from
Tennessee, do not display evidence of a major extinction
or collapse of benthic ecosystems across the Lau Event,
but a normal ecological adjustment to environmental
changes caused by marine flooding event. Recognition
of the Lau Event and the associated CIE provides a new
time horizon for southern Laurentia that can be used to
better constrain the ages of Ludlow lithofacies and faunal
assemblages in the region.
ACKNOWLEDGEMENTS
Research on the conodont faunas and stable isotope
chemostratigraphy in Ludlow strata in southern Laurentia was
supported by the National Science Foundation Grant No. EAR0517976 to J. Barrick and M. Kleffner. Robert Lundin shared samples
and locality information for sections in Tennessee. James Browning
and Neyda Cordero-Rodriquez assisted with sample preparation for
stable isotope analyses at Texas Tech University. L. Jeppsson and
O. Lehnert provided useful suggestions that improved the quality of
this paper.
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Manuscript received 13 October 2009
Revised manuscript accepted 11 February 2010