Bollettino della Società Paleontologica Italiana, 49 (1), 2010, 13-33. Modena, 15 maggio 201013
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] state.edu 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 time- constrained 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 (Jeppsson & Aldridge, 2000). Urbanek (1993) described crisis C3 in the graptolite fauna at this level, which has The mid-Ludfordian (late Ludlow) Lau Primo- the second highest extinction rate of any event (70%) in Secundo Event of Jeppsson (1998) is one of three major the Silurian (Neocucullograptus kozlowskii Event; oceanic events that occurred during the Silurian (Calner, Melchin et al., 1998). Many other faunal groups were 2008). The Lau Event was characterized by a major also affected, as summarized by Calner (2008), and Calner turnover in conodont faunas where the diverse association (2005) reported that the appearance of microbialites and of the Polygnathoides siluricus Zone disappeared anachronistic facies on Gotland in the aftermath of the
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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 Fig. 1 - Paleogeographic reconstruction for the Silurian. Study area Jeppsson et al., 2007; Podolia, Kaljo et al., 2007; Czech indicated by cross in southern Laurentia. Star marks Gotland. Map Republic, Lehnert et al., 2007). Published information from Scotese (2002). 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 faunas, and stable isotopes across the Lau Event interval Gotland. in three areas in midcontinent North America, the region In this paper we present preliminary results of our that stretched across the southern margin of Laurentia investigation of the lithologic succession, conodont 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).
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THE LAU EVENT Calner, 2008). Just as the more gradual decline in δ13C is more poorly constrained relative to the conodont Calner & Eriksson (2006), Jeppsson et al. (2007), and zonation, it is also poorly constrained in the graptolite Eriksson & Calner (2008) provide a thorough zonation. The end of the excursion is generally shown as documentation of the lithologic, geochemical, and faunal ending in the late Ludfordian Monograptus formosus changes through the Lau Event of Gotland, as well as a Biozone (Calner, 2008), but questions remain about its comparison to these changes in northeastern Australia. precise placement (Kaljo et al., 2007). Lehnert et al. (2007) give a thorough description of the In each region where the Lau Event has been identified, complexity of the sedimentologic, isotopic, and it is associated with lithofacies shifts indicative of a paleontological change through the Lau Event in the regression, followed by a transgression. Eriksson and Prague Basin. Additional information available from Calner (2008) have documented that on Gotland the Lau Lithuania comes from Martma et al. (2005) and Kaljo & Event encompasses three stratigraphic sequences Martma (2007) and from Podolia (Kaljo et al., 2007). separated by two periods of forced regression. The CIE Jeppsson (2005) and Jeppsson et al. (2007) developed started at the onset of the first sequence. Values increased a detailed zonation across the Lau Event for conodont through the first sequence and the first regression before faunas and the boundaries of zones correspond to step- reaching the peak in the highland systems tract of the like changes through the Lau Event. The Polygnathoides second sequence. The second forced regression took siluricus Zone characterizes a major interval of pre-Lau place in the lower Ozarkodina snajdri Zone. The δ13C time during which conodont faunas attained high diversity. values do not decrease until the early transgressive The first effects of the Lau Event are recorded in the systems tract of the third sequence and return to Upper Po. siluricus Subzone. In this subzone, many background values in the highland systems tract. Calner species typical of the Po. siluricus Zone disappear in rapid (2005) interpreted the widespread appearance of stepwise extinctions and conodont diversity decreases. stromatolites, oncolites and microbial wrinkle structure The final extinction of rare Po. siluricus marks the top during and shortly after the Lau Event to represent a short- of this subzone and the base of the overlying Icriodontid lived collapse of the marine ecosystem on Gotland, and Zone. The Icriodontid Zone is characterized by strongly potentially globally. impoverished faunas in which icriodontid conodonts, Jeppsson et al. (2007) noted that the sequence of especially elements of Coryssognathus occur. The lithologies on Gotland were similar to those in the Coral Icriodontid Zone is divided into three subzones based on Gardens Formation in northeastern Australia. A slow rise the relative abundance of different species. The Lower in δ13C occurred in lower argillaceous strata and a more Icriodontid Subzone is the basal interval of the zone with rapid rise in the overlying weathering-resistant limestones. strongly impoverished faunas. The appearance of a slender A sharp increase in values (>1‰) occurred near the form of Panderodus equicostatus marks the base of the transition and continued within the overlying argillaceous Middle Icriodontid Subzone. In the Upper Icriodontid oncolitic crinoidal limestones as stepwise disappearances Subzone, a single species shows extreme dominance of of conodonts occurred. Peak values appeared in the the faunas. On Gotland, Pa. equicostatus rises to a overlying argillaceous oncolites associated with dominance of 90%, but in Scania (southern Sweden), “unbalanced” conodont faunas. The decline in δ13C values Ozarkodina scanica obtains a similar dominance. The occurred in the more typical suite of lithologies above and base of the overlying Ozarkodina snajdri Zone is the as conodont faunas increased in diversity. level at which a diverse, reasonably balanced fauna, The three sections in the Prague Basin studied by including O. snajdri, appears. Lehnert et al. (2007) demonstrate how features of the In both Gotland (Calner & Eriksson, 2006; Eriksson local sedimentary environments and the presence of & Calner, 2008) and Australia (Jeppsson et al., 2007), disconformities can obscure the record of the Lau Event. the δ13C record begins to increase during the Upper Poly- Their data indicate that the full record of the Lau Event gnathoides siluricus Zone. δ13C continues to increase and the CIE may be missing or truncated in the shallow to maximum values in the Middle Icriodontid Subzone to water sections because a significant fall in sea level Upper Icriodontid Zone (Calner, 2008). The highest value produced stratigraphic gaps by subaerial erosion, as of δ13C is given in Jeppsson et al. (2007, p. 133) as shown by the presence of karst features and diagenetic 10.54‰ at Glasskär 1 from the O. snajdri Zone. The alteration of the carbonate strata. In a deeper water timing of the end of the CIE is less clear. Calner (2005, section, reduced sedimentation occurred during the sea 2008, fig. 10) shows that values decline through the Ozar- level fall, but the carbon isotope excursion of about kodina snajdri Zone into the lower part of succeeding +8.0‰ is preserved. In the Prague Basin there was a O. crispa Zone. In Bohemia (Lehnert et al., 2007) and complete overturn in macrofaunal assemblages during the Lithuania (Kaljo & Martma, 2006), however, the δ13C Lau Event and completely new faunas invaded the values fall to background levels somewhat before the first “biologically devastated” basin (Lehnert et al., 2007). In occurrence of O. crispa. the Muslovka section, the faunal overturn coincides with Precise correlation of the Lau Event conodont zones the maximum values of the apparently truncated δ13C peak, and the CIE to the graptolite zonation is uncertain (Kaljo +4.2‰, just above a disconformity surface. Although & Martma, 2006; Kaljo et al., 2007). It is generally agreed FADs and LADs of a few zonally significant conodonts that the initial steep increase of δ13C lies in the are shown on the diagrams, the overall changes in the Neocucullograptus kozlowskii Biozone. The peak values conodont faunas across the Lau Event are not described. may lie within the N. kozlowskii Biozone (Azmy et al., In Lithuania, the CIE is confined to the Mituva 1998; Saltzman, 2005), or higher (Lehnert et al., 2007; Formation, which comprises nodular and laminated
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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 fine- grained 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 Fig. 3 - Correlation of Wenlock, Ludlow, and Pridoli units in southern Oklahoma, southeastern Missouri, and western Tennessee. Position from these rocks were drilled in the same manner, even of the Lau Event indicated with a dashed line. 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 southern Oklahoma, eastern Missouri, and western may have affected the packstone and grainstone strata did Tennessee (Fig. 2). The stratigraphic nomenclature and not alter the original carbon isotope values significantly. correlation of units discussed here is shown in Figure 3. Why this was the case deserves more investigation. Details of the procedures used in the isotopic analyses Henryhouse Formation, southern Oklahoma can be found in Barrick et al. (2009) or Jacobi et al. The Henryhouse Formation comprises the lower (2009). Results (Tables 6-10) are reported relative to V- portion of the late Silurian to Early Devonian Hunton PDB (‰). marlstone section that crops out in southern Oklahoma and extends into the adjacent Anadarko and Arkoma basins. The distinction between the late Silurian Henryhouse SOUTHERN LAURENTIA Formation and the overlying Early Devonian Haragan Formation was based largely on significant differences Reconstructions of continental positions during the in the shelly faunas as discussed by Amsden (1960, 1988). Silurian (Fig. 1) place the southwestern margin of Although the entire Henryhouse-Haragan marlstone Laurentia approximately 20º to 30º latitude south of the interval is characterized by calcareous shale and equator, facing to the south to southwest (Cocks & argillaceous to silty carbonate mudstone and wackestone, Scotese, 1991; Golonka et al., 1994; Cocks & Torsvik, the greatest paleontological and lithological contrast 2002). Other work on continental positions during the occurs within the Ludlow portion of the Henryhouse, Ordovician and early Silurian (e.g. Dalziel, 1997; Niocaill across the interval that includes the Lau Event. The et al., 1997) shows Laurentia rotated in such a way that at Henryhouse Formation in outcrop represents deposition the beginning of the Silurian the modern southern margin on the deeper part of a carbonate ramp that shallowed of North America faced west to southwest and Gondwana northward into central Oklahoma (Stanley, 2001). lay a greater distance away from Laurentia than in the The basal portion of the Henryhouse Formation, which older reconstructions. During the Silurian, a series of we here informally designate as the “lower member” of carbonate platforms and ramps extended across the the Henryhouse (Fig. 3), includes a relatively thin southern margin of Laurentia from New Mexico eastward succession of extremely argillaceous and silty carbonate into central Tennessee. Two major reentrants, the mudstone and silty shale that includes graptolite-bearing Southern Oklahoma Rift and the Reelfoot Rift, both shale in the upper portion. A basal unit of brown Proterozoic rift basins, intersected the margin. Most of argillaceous, silty carbonate mudstone of the lower the Silurian strata in this region are now buried in the Henryhouse rests with apparent disconformity on the subsurface, but crop out in the three areas studied here: slightly argillaceous carbonate mudstone of the upper
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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 well- known 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 Fig. 4 - Map of Murray County and northern Carter County in Henryhouse Formation is completely exposed below a southern Oklahoma showing locations of Dougherty West section small natural dam formed by the underlying Clarita (34º24’41.71”N, 97º05’03.92”W) and Highway 77 section Formation. The lower member of the Henryhouse is (34º26’46.53”N, 97º05’08.88”W). 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. Clarita Formation. Conodonts of the Ludlow Kockelella The variation in thickness of the basal and upper units crassa Zone appear in the uppermost few centimeters of of the lower member of the Henryhouse and the pattern the Clarita Formation and extend into the basal unit of of conodont first and last occurrences suggests the the lower member. Elements of the Ancoradella presence of multiple depositional disconformities in the ploeckensis Zone occur in the upper part of the basal unit. lower member. Although conodont elements are relatively Intervals of brown calcareous, silty shale and greenish common in the lower Henryhouse, the number of clay shale that are interbedded with argillaceous, silty elements per kilogram varies greatly. Most elements are carbonate mudstone characterize the upper unit of the broken, which apparently occurred during the depositional lower member. Many of the greenish clay shale beds process, making element counts difficult. contain poorly preserved graptolites. Conodonts of the Polygnathoides siluricus Zone range through to the top Dougherty West section (DW) - The upper few meters of the upper shaly unit of the lower member. The upper of the lower member of the Henryhouse Formation shaly unit of the lower member comprises the pre-Lau comprises interbedded soft marly limestones stratigraphic interval in the Henryhouse Formation. (mudstones) and greenish shales that contain poorly Because the lower member has a restricted geographic preserved graptolites (Fig. 5). The diverse conodont fauna distribution, occurring only in the central Arbuckle is strongly dominated by elements of Dapsilodus Mountain region, the transition through the Lau Event is obliquicostatus. Elements of Panderodus unicostatus, present at only a few localities. P. recurvatus and Decoriconus fragilis are common, The “upper member” of the Henryhouse Formation those of Belodella sp. are uncommon, and comprises the greater thickness of the formation, which Pseudooneotodus elements are rare. Elements of attains a thickness of up to 100 meters on the Lawrence Wurmiella excavata, Ozarkodina confluens, Oulodus uplift. In the central Arbuckle Mountain region the upper siluricus, Kockelella absidata, and Polygnathoides member rests on the lower member and the lower-upper siluricus occur in moderate numbers in most samples. member contact coincides with the level of the Lau Event. The top of the lower member is marked by a 20 cm Outside of the central Arbuckle Mountain region, the bed of dark brown to gray calcareous shale that is overlain upper member of the Henryhouse Formation rests by light brown slightly argillaceous carbonate mudstone unconformably on the lower, Sheinwoodian part of the of the upper member. The ranges of species characteristic Clarita Formation. The upper member is characterized of the siluricus Zone terminate within or at the top of by an overall lower proportion of argillaceous matter in the brown shale bed. There is no indication of steps of the carbonate mudstones and packstones, and silty beds extinction nor steps in changes of relative abundance of are uncommon. The basal part of the upper member of taxa. the Henryhouse Formation contains conodont elements At the base of the upper member, Ozarkodina snajdri of the late Ludlow Ozarkodina snajdri Zone, the post- appears and ranges higher. The conodont fauna is Lau interval. The upper member ranges through the Pridoli dominated by elements of Dapsilodus obliquicostatus, to near the base of the Devonian (Barrick & Klapper, as below, and W. excavata and D. fragilis persist from 1992; Jacobi et al., 2009) with little vertical change in below. However, Pseudooneotodus elements now form lithofacies. The abundant shelly fauna of the Henryhouse an important part of the fauna, and no Panderodus
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elements occur in the lower 2 m of the upper member. The δ13C record in the upper part of the lower member About 2 m above the base of the upper member, where varies only slightly, from 0.0 to +1.0‰ through this relatively clay-free limestones occur, P. equicostatus interval, but falls to -0.6‰ in the upper 20 cm of the lower appears with Ozarkodina auriformis. 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.
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sharply to +3.5‰, declines to +1.0‰, over the next 0.5 Oulodus siluricus disappear at the base of the bed. m and then falls to between 0.0 and +1.0‰ higher in the Panderodus unicostatus and P. recurvatus range into the section. The δ18O record is more variable in the upper base of the overlying upper member of the Henryhouse part of the lower member, mostly between -2.0 and Formation, disappear and then recur about 1.0 m above -5.0‰, then drops to near -5.5‰ at the top of the lower the base of the upper member. Elements of Ozarkodina member. Values in the base of the upper member are snajdri appear just below the top of the lower member higher, between -3.0 and -5.0‰, before rising just above and range into the upper member. Except for the -4.0‰ higher in the section. recurrence of a few Panderodus elements in sample 309B, Panderodus elements do not occur in the lower Highway 77 section (H77) - Like the DW section, part of the upper member. About three meters above the the upper few meters of the lower member of the base of the upper member, where relatively clay-free Henryhouse Formation comprise interbedded soft marly limestones occur, P. equicostatus and O. auriformis limestones (mudstones) and greenish shales that contain appear. Fragments of Pedavis latialatus have been poorly preserved graptolites (Fig. 6). The diverse recovered 3 m above the extinction level and Ozarkodina conodont fauna is strongly dominated by elements of crispa appears 6 m above the extinction level. Dapsilodus obliquicostatus. All species of the Po. Values of δ13C range from about 0.0 to +1.0‰ through siluricus Zone found at the DW section are present, but the upper 2.5 m of the lower member. Just above the occur in more equal abundance than at DW. Unlike at DW, contact between the lower and upper member, values dip species ranges terminate at slightly different stratigraphic from +1.0 to -0.5‰. At 20 cm above the contact, values levels. Ozarkodina confluens and Kockelella absidata of δ13C rise to +2.8‰ and then decline gradually to disappear just below the base of a distinct 10 cm bed at around +1.5‰ a meter higher. Values of δ18O display a the top of the lower member, and Po. siluricus and 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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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‰ completely determined because the unit is generally just below the top of the lower member. In the base of poorly exposed and biostratigraphic information is sparse. the upper member, values of δ18O rise to near -2.0‰ and Based on graptolite and conodont faunas, the member then decline gradually to -3.50‰ two meters higher. ranges in age from the Ludlow to near the top of the Silurian (Thompson, 1993). The outcrop area of the Southeastern Missouri Bainbridge Formation sits on the western margin of the Wenlock through Pridoli strata that crop out along the Illinois Basin, which lies at the northern end of the early western edge of the Illinois basin are assigned to the Paleozoic Reelfoot Rift (Fig. 7). The Reelfoot Rift is Bainbridge Formation (Thompson, 1993). The lower part of the New Madrid rift complex (Braile et al., 1982) portion of the Bainbridge is generally assigned to the St. that initially formed in latest Proterozoic time (600 Ma) Clair Member, which is mostly Wenlock in age and the as the Proterozoic supercontinent that included Laurentia upper more argillaceous portion is assigned to the began to break up. Subsurface studies in the Illinois Basin Moccasin Springs Member (Fig. 3). The stratigraphy of to the east of the outcrops show a westward sloping steep the Moccasin Springs Member, which appears to be carbonate ramp on which deeper water lithofacies of the approximately 30 to 40 meters thick, has not been Bainbridge lie near the southwestern edge of Illinois.
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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.
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. Fig. 7 - Map of Reelfoot Rift area showing locations of southeastern Although abundant shelly fossils have been described Missouri and western Tennessee outcrops relative to regional from the Moccasin Springs Member in the Greither Hill structural features. area and other Moccasin Springs outcrops, no systematic
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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 Fig. 8 - Map of southeastern corner of Ste. Genevieve County, elements are absent from this section, although they Missouri, showing location of the Greither Hill section (37º50’20.19”N, 90º00’54.39”W).
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.
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occur in the same stratigraphic position at the nearby obliquicostatus dominate the conodont fauna of the upper Lithium section (Branson & Mehl, 1933; Rexroad & tan limestone unit, and Decoriconus fragilis, Pseudo- Craig, 1971). oneotodus, and Belodella sp. elements are common. Overlying the reddish carbonates lie 1.2 meters of Within the upper limestone unit occurs a small resistant argillaceous green shale and limestone (Fig. 9). The basal ridge-forming interval, which may correspond to the 0.3 m of these beds contains the same abundant and “Merista bed” of Ball and Dunn (1931) and Ball (1942). diverse faunas found in the underlying unit, with the Values of δ13C are about +1.5‰ in the upper part of exception of the absence of Ozarkodina confluens. At the lower reddish limestone units, then fall to a low of 0.3 m, Panderodus unicostatus, P. recurvatus, and -3.5‰ at the extinction level near the base of the overlying Walliserodus elements disappear. Through the greater greenish unit. They rise as high as +4.1‰ near the top of thickness of the greenish interval, an impoverished fauna the greenish unit and rise higher, up to +5.1‰ in the lower characterized by Pseudooneotodus, was recovered. The part of the overlying interbedded reddish shale and tan inpoverished Pseudooneotodus fauna continues into the limestone. Values of δ13C then fall in an irregular pattern overlying interbedded reddish shale and thin tan-colored to around +1.0‰ with the lower meter of the upper tan carbonate mudstone. As the reddish shales are replaced limestone unit. Unlike the δ13C pattern, values of δ18O by a thick interval of tan-colored carbonate mudstone, remain near -5.0‰ through the entire interval. Values rise faunal diversity and abundance increases. In successive slightly, as high as -4.0‰ during the δ13C peak and fall as samples appear Ozarkodina auriformis, Panderodus low as -6.0‰ in the lower part of the tan limestone unit, equicostatus, and O. snajdri. Elements of Dapsilodus 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.
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Western Tennessee to medium-bedded echinoderm-bryozoan wackestone and Silurian sedimentary rocks in central and western packstone (Amsden, 1949; Broadhead & Gibson, 1996). Tennessee were deposited on a gently sloping ramp The glades that form during weathering of the Beech River locally referred to as the Western Tennessee Shelf are abundantly fossiliferous. The sponge Astraeospongia (Broadhead & Gibson, 1996), which was structurally meniscus serves as a guide to the member, which is bounded by the Illinois Basin to the north, the Reelfoot typified by a diverse echinoderm fauna (e.g., Rift embayment to the west, the Laurentian continental Troostocrinus and Eucalyptocrinites; Pate & Bassler, edge to the south and the Nashville Dome to the east (Fig. 1908; Springer, 1917, 1926). Sampling of the upper 7). The main band of Silurian outcrop, which extends from Dixon Formation and Brownsport Formation at several the Central Basin into the Western Valley of the Tennessee localities shows that Polygnathoides siluricus does not River, forms a transect from a shallower to deeper occur in the Dixon, but that it appears at or near the base depositional position on a carbonate ramp. However, this of the Beech River Member and ranges through this transect does not run directly toward the axis of the member. Reelfoot embayment, but rather more toward the modern Medium- to thick-bedded coarse-grained echinoderm- southwest and the open continental margin of Laurentia, bryozoan packstone and grainstone that lie above the and possibly along the same line of latitude, about 25ºS, Beech River Member have been assigned to the Bob during the Silurian (Cocks & Torsvik, 2002). The effects Member. Brachiopods are the most abundant of this orientation on depositional patterns are not clear, macrofossils followed by lesser amounts of ostracodes, but the configuration of the shelf relative to paleowind and corals. The brachiopod fauna of the Bob is and paleocurrent directions is believed to have influenced characterized by the conspicuous Rhipidium, but outcrops the influx of clastics from the Appalachian Basin region generally contain a wide variety of brachiopod taxa (Stearns & Reesman, 1986; Broadhead & Gibson, 1996). compared to other fossil groups and relative to overlying Ludlow through Pridoli strata on the Western and underlying strata. Although the name “Bob Member” Tennessee Shelf have been assigned to three formations: has had widespread use, there has always been some doubt in ascending order, the Dixon Formation, Brownsport whether it forms a contiguous stratigraphic unit, or Formation, and Decatur Limestone (Fig. 3). Although whether the coarse-grained beds are discontinuous lenses numerous shelly fossils have been described from these that are a facies of the upper part of the Beech River units over the years, especially from the Brownsport Member (Amsden, 1949). Our work, although Formation, the exact age relations remain unclear. Barrick preliminary, shows that in the western sections near the (1983) showed that the base of the Ludlow, which Tennessee River, the type area of the Bob Member, the corresponds to the base of the Kockelella crassa Zone, coarse-grained beds are difficult to distinguish from the lies near the base of the Dixon Formation, based on the associated Beech River beds and appear to lie below the appearance of Kockelella variabilis. The age of the upper Lau Event and CIE. However, in sections well to the east Dixon was unknown. Rexroad and Nicoll (1971) cited a of the Tennessee River, a well-defined, coarse-grained personal communication from Robert Lundin, who packstone to grainstone unit overlies typical Beech River reported that Polygnathoides siluricus ranged from the beds and contains the Lau Event. For this reason we use base of the Brownsport Formation into the lower beds of the term “Bob Member” in quotes to designate this the overlying Decatur Limestone at one section, restricting the age of the Brownsport to the Po. siluricus Zone. Lundin also stated that specimens of “Spathogna- thodus” 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 & Fig. 10 - Map of Decatur, Perry, Hardin, and Wayne counties in western Tennessee showing locations of the Linden section Bassler, 1908; Amsden, 1949). The Beech River Member (35º36’12.67”N, 87º57’54.05”W) and the Eagle Creek section comprises 16 to 20 m of interbedded, thin-bedded shale (35º17’06.93”N, 87º59’08.20”W).
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grainstone unit in the eastern sections that are discussed Thus far, we have sampled in detail two sections of in this paper. the Brownsport Formation that include the CIE and the The Lobelville Member superficially resembles the Lau Event, as represented by the δ13C peak and the Beech River Member, also comprising interbedded, thin- associated shift in the conodont fauna. Both of these bedded shale to medium-bedded echinoderm-bryozoan sections lie east of the Tennessee River, in the area where wackestone and packstone. However, its bedding the coarse grainstone unit we call the “Bob Member” is characteristics allow it to be easily recognized, for it well developed (Fig. 10). The section near Linden (LD) consists of thin 1 to 4 cm thick graded beds of echinoderm is the more complete, with the upper Beech River, the packstone or grainstone that grade upward to wackestone “Bob” Member, the Lobelville Member and the base of or mudstone. Lobelville biodiversity becomes great in the overlying Decatur Limestone completely exposed. some horizons with a wide array of pentamerid At the Eagle Creek (EC) section farther to the southwest, brachiopods, rugose corals and large tabulate corals. the upper Beech River and “Bob” are exposed, but the Disarticulated stalked echinoderms (89% of the Lobelville is mostly covered. accumulating skeletal sediment) with relatively common calyxes and articulated stem pieces are abundant Linden section (LD) - At the Linden section (Fig. 11), especially near the tops of limestone beds and within an older road cut 1.6 km south of the center of Linden on shale beds of the lower Lobelville. Bryozoans increase the west side of Tennessee Highway 13, the lower portion in diversity and abundance up-section (up to 40% of of the section has typical Beech River lithology, greenish- allochems). Tabulate corals are a major component of gray interbedded shale, carbonate mudstone and Lobelville coral assemblages, with as many as twelve echinoderm-bryozoan wackestone that grades into different species of tabulates, including auloporid, packstone. Near the top of the member, beds of coarser halysitid, favositid, heliolitid, alveolitid and theciid grained packstone appear and chert nodules occur. A tabulates, co-occurring (Amsden, 1949; Olson & distinctive grainstone bed with abundant bluish-white Broadhead, 1995) and locally rugose corals occur as chert nodules appears 1.5 m below the top of the Beech monospecific patches of Cyathophylum sp. 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.
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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 Polygnathoides siluricus Zone. Conodont abundance is argillaceous wackestone at the base and slightly low to moderate, about 10 to 50 elements per kilogram. argillaceous echinoderm-bryozoan grainstone at the top. Panderodus unicostatus strongly dominates the fauna The base of the “Bob” Member is a 1.35 m bed of and some elements of P. recurvatus and Ozarkodina coarse-grained echinoderm-bryozoan grainstone that confluens occur in most samples. Other important taxa forms the lower part of a small cliff. Bluish-white chert occur at a frequency near 1-2 per kilogram: Po. siluricus, occurs through the lower 2 meters and sparse glauconite Oulodus siluricus and Wurmiella excavata. The last grains appear 2.5 meters above the base of the “Bob” and occurrences of Ou. siluricus, Po. siluricus and O. occur higher into the Lobelville Member. The top of the confluens lie within the upper Beech River. The “Bob,” which is about 4.5 m thick, is placed where shale uppermost 1.5 m of the Beech River produced a larger beds reappear in the section. Above a basal unit of and more diverse conodont fauna in which elements of interbedded grainstone and shale, the Lobelville Member Dapsilodus obliquicostatus, Decoriconus fragilis, W. is dominated by shale and argillaceous wackestone with excavata and Pseudooneotodus spp. are common, unlike thin (10 cm) packstone to grainstone beds. the lower beds. Ozarkodina snajdri appears just above The lower part of the Beech River at the Linden the LAD of Po. siluricus, a short interval below the base section contains the typical conodont fauna of the of the massive grainstone unit of the “Bob.” The basal
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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, only +0.6‰. Values continue their gradual rise through dominated by elements of Panderodus, both P. the lower 2.0 m of the “Bob,” and attain peak values unicostatus and P. recurvatus. The last occurrence of between +4.0 and +5.2‰ in the middle 1.5 m of the Walliserodus sp. is just 0.20 m above the base of the member. In the upper “Bob”, starting near where “Bob.” Above the middle of the lower grainstone unit of glauconite appears in the grainstone, δ13C values fall the “Bob,” few conodonts were obtained. From a steeply to less than +1.0‰ at the top of the member. In combined weight of about 20 kg collected through the the lower beds of the Lobelville, δ13C values remain near next 5 meters, only 12 elements were recovered of five +1.0‰. In contrast to the pronounced peak in δ13C, values species, W. excavata, Decoriconus fragilis, Pseudo- of δ18O change little through this interval. There are some oneotodus sp., Belodella sp., and Panderodus sp. minor short term fluctuations, but generally the δ18O Values of δ13C are about +1.5 to +2.0‰ through most values remain between -3.0 and -5.0‰. of the upper part of the Beech River member. They rise gradually through the uppermost 1.5 m of the member to Eagle Creek (EC) - The Eagle Creek section (Fig. 12) as high as +3.0‰ near the top of the unit, except for the is a recently exposed road cut on the north side of US sample at the top of the Beech River, which has a value of Highway 64, 4.4 km east of Olive Hill. The section lies
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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 River and in the basal shale of the lower of the two southern and western exposure where the coarse-grained coarsening upward intervals at the top of the member, δ13C grainstone termed here as the “Bob” has been recognized values fall as low as -2.0‰, before rising back to +1.5‰ thus far. The lowest part of the Beech River Member through the lower coarsening upward interval. In the upper exposed here consists of about 5.8 m of purple and green coarsening upward interval, δ13C value increases to more shale and argillaceous mudstone and echinoderm than +2.0‰ and this increase continues through the lower wackestone and packstone that contain increasing 2.5 m of the “Bob” to a peak of +5.2‰. Values stay above proportions of skeletal material up section, where +4.0‰ for the next meter before showing a steady decline argillaceous packstone to grainstone layers become in value to just less than +1.0‰ at the top of the section. common. At the top of the Beech River occur two Values of δ18O fluctuate through the Beech River and successive coarsening upward intervals, 1.30 and 1.40 m “Bob” section, but generally stay in the -5.0 to -3.0‰ thick. Each interval begins with a 20 cm shale that is range. 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 DISCUSSION echinoderm-bryozoan grainstone that forms a small cliff. At the top of this cliff occurs a 0.7 m section of In each of the three areas of southern Laurentia interbedded shale and grainstone lenses, which is then studied here, southern Oklahoma, southeastern Missouri, overlain by another 3.85 m of grainstone. The upper and western Tennessee, the Lau Event is represented by grainstone interval is similar to the base of the “Bob,” some combination of changes in conodont faunas, the but tends to somewhat finer grained and contains common associated major positive CIE, and shifting lithofacies. brachiopods at some levels. A covered slope, which However, each area preserves its own unique record of should represent the shaly strata of the Lobelville the Lau Event, little of which corresponds closely with Member, lies above. those records of the Lau Event reported from Baltica and The δ13C values through most of the Beech River peri-Gondwana. In particular, no icriodontids nor any section lie near +1.0‰. Near the top of the typical Beech indications of the subdivisions of the Icriodontid Zone
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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. Fine- grained 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. Tab. 8 - Stable isotope data across the Lau Event in the Greither Hill 13 (2004) proposed that similar negative δ C excursions in section, southeastern Missouri. Levels were measured above the Ordovician strata were produced possibly by the local base of the exposed section.
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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 Tab. 10 - Stable isotope data across the Lau Event in the Eagle section, western Tennessee. Levels were measured above the base Creek section, western Tennessee. Levels were measured above of the exposed section. the base exposed section.
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Lau Event coincides with a submarine hiatus in the definitive, a marine flooding event across the shallow southern Oklahoma sections. The greater part of the water Western Tennessee Shelf, followed by progradation record of the Lau Event and the CIE is absent because and aggradation of skeletal grainstone during the strata that might have recorded the event were never maximum of the CIE may be the best description of the deposited in starved offshore setting during a major sedimentary history at these sections during the Lau flooding event. The reappearance of offshore lithofacies Event. and conodont biofacies immediately after the Lau Event Conodont faunas in the Beech River Member below represents the end of sediment starvation shortly after the excursion are diverse (10-12 species), but not the marine flooding event. especially abundant. Elements of Panderodus dominate The one section in southeastern Missouri, Greither the faunas. Some species disappear in sequence as the Hill, provides a slightly different version of the Lau Event. values of δ13C begin to increase in the top of the Beech Argillaceous wackestones below the position of the Lau River, Oulodus siluricus, Ozarkodina confluens, and Event contain a Panderodus-dominated conodont fauna, then Polygnathoides siluricus, but this may be an artifact in which Dapsilodus is only a minor constituent. of the low numbers of elements we recovered. Conodont diversity is moderate, only nine species occur Ozarkodina snajdri appears at both the Linden and Eagle just below the extinction event. Polygnathoides Creek sections near the base of the “Bob” Member, just siluricus, Kockelella absidata and Oulodus siluricus are after the LAD of Po. siluricus, but is associated with other absent, but the outcrop exposure and our sampling do not species characteristic of the pre-Lau fauna. Just above permit us to resolve a step-wise pattern of extinction for this and a short interval below the preserved peak in δ13C these species. The extinction level of conodonts coincides values, conodonts essentially disappear from the section, with a strongly negative excursion (-3.6‰) in δ13C, above and only a few conodont elements were recovered from which δ13C reaches values typical of the CIE, in the +3.0 the upper “Bob” and basal beds of the Lobelville Member. to +5.0‰ range. A strongly impoverished conodont fauna, In the Brownsport Formation, the abundant and diverse only a few specimens of Pseudooneotodus, is present. shelly fauna displays a paleocommunity restructuring with As the values of δ13C decline to background values, a the shift to each successive lithofacies member. Many moderate diversity conodont fauna reappears (4 to 7 taxa are common to all three members and there is not species), including O. snajdri and O. auriformis. The distinct pre-Lau and post-Lau macrofaunal composition fauna is strongly dominated by elements of Dapsilodus (Amsden, 1949). Although sparse oolitic layers and algal and contains only rare elements of Panderodus. coated grains have been reported from the “Bob” Member, The position the Lau Event in Missouri can also be we have not found any evidence of a major benthic interpreted as coinciding with a major marine flooding ecosystem collapse or pervasive microbialitic event across a somewhat shallower position on a ramp “anachronistic” faunas. The Lau extinction event appears than that found in southern Oklahoma. The basal flooding to have a weak signature in benthic faunas in the western surface may be marked by the conodont extinction level Tennessee strata, or at least one that can be equally and the negative carbon isotope excursion. The explained by ecological shifts in response to normal argillaceous greenish limestone that yielded mostly environmental changes associated with a rapid rise in sea Pseudooneotodus elements may be a condensed, level. offshore record of the Lau Event. Environmental The Henryhouse Formation, Moccasin Springs interpretation of the sparse Pseudooneotodus-only fauna Member of the Bainbridge Formation, and the Brownsport is problematic, but this may be a situation similar to lower Formation have been considered to be generally Wenlock conodont faunas from the Clarita Formation in correlative units of Ludlow age based on the similarity southern Oklahoma. There, a low abundance and low of the macrofaunal assemblages by numerous workers diversity conodont fauna with abundant Pseudooneotodus for over one hundred years (e.g., Ball, 1942, with a lies just above the major flooding surface at the base of summary of previous work; Amsden, 1949; Berry & the formation (Barrick, 1977, 1997). Boucot, 1970; Amsden 1988). However, no consensus In Tennessee, the CIE associated with the Lau Event existed on the detailed correlation of individual units from is well preserved in shallow water carbonates. A well- southern Oklahoma to southeastern Missouri to western developed negative carbon excursion is present in the Tennessee. The recognition of the abrupt change in upper part of the Beech River, at the lower of the two conodont faunas and large positive CIE associated with coarsening-upward intervals, just below the start of the the short-lived Lau Event in southern Laurentia now excursion, at the Eagle Creek section. This negative provides us with an easily recognizable and reliable time excursion may be represented by a single point that lies horizon for the region. In the future, studies of the just below the base of the single coarsening upward distribution of Ludlow lithofacies patterns and faunal interval at the Linden section. The negative excursions assemblages at well-known and new sections across here may also represent diagenetic effects below a southern Laurentia can be easily situated in time as being discontinuity surface. The steady rise of δ13C to a peak in either before or after the Lau Event. 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 SUMMARY skeletal grains. Also, initial cementation must have occurred near the time of deposition and later diagenetic Our preliminary work shows that the Lau Event is effects were minimal for the excursion to be so well represented on the southern margin of Laurentia by an preserved in this facies. Although the data are not extinction event in conodont faunas and the associated
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major positive CIE. In each of the three areas, the Lau Barrick J.E. & Klapper G. (1976). Multielement Silurian (late Event and CIE appear to be associated with a marine Llandoverian-Wenlockian) conodonts of the Clarita Formation, flooding event and sequence boundary, but the resulting Arbuckle Mountains, Oklahoma, and phylogeny of Kockelella. Geologica et Palaeontologica, 10: 59-100. lithofacies patterns are different in each area, depending Barrick J.E. & Klapper G. (1992). Late Silurian-Early Devonian on their position on the local carbonate ramp. Limited conodonts from the Hunton Group (Upper Henryhouse, Haragan, data on macrofaunal associations, mostly from and Bois d’Arc Formations), south-central Oklahoma. Oklahoma Tennessee, do not display evidence of a major extinction Geological Survey Bulletin, 145: 19-65. or collapse of benthic ecosystems across the Lau Event, Barrick J.E., Klapper G. & Amsden T.W. (1990). Late Ordovician- but a normal ecological adjustment to environmental Early Devonian conodont succession in the Hunton Group, Arbuckle Mountains and Anadarko Basin, Oklahoma. Oklahoma changes caused by marine flooding event. Recognition Geological Survey Guidebook, 27: 55-92. of the Lau Event and the associated CIE provides a new Barrick J.E., Kleffner M.A. & Karlsson H.R. (2009). Conodont time horizon for southern Laurentia that can be used to faunas and stable isotopes across the Mulde Event (late Wenlock, better constrain the ages of Ludlow lithofacies and faunal Silurian) in southwestern Laurentia (south-central Oklahoma and assemblages in the region. subsurface west Texas). Paleontographica Americana, 63: 41-56. Berry W.B.N. & Boucot A.J. (1970). Correlation of the North American Silurian rocks. Geological Society of America Special Paper, 102. 289 pp. Braile L.W., Hinze W.J., Sexton J.L., Keller G.R. & Lidiak E.G. ACKNOWLEDGEMENTS (1984). Tectonic development of the New Madrid seismic zone. U.S. Geological Survey Open File Report, 84-770: 204-233. Research on the conodont faunas and stable isotope Branson E.B. & Mehl M.G. (1933). Conodonts from the Bainbridge chemostratigraphy in Ludlow strata in southern Laurentia was (Silurian) of Missouri. University of Missouri Studies, 8: 39-53. supported by the National Science Foundation Grant No. EAR- Broadhead T.W. & Gibson M.A. (1996). Late Silurian sedimentary 0517976 to J. Barrick and M. Kleffner. Robert Lundin shared samples environments and biotas of west-central Tennessee. In and locality information for sections in Tennessee. James Browning Broadhead T.E. (ed.), Sedimentary Environments of Silurian and Neyda Cordero-Rodriquez assisted with sample preparation for Taconia. University of Tennessee Department of Geological stable isotope analyses at Texas Tech University. L. Jeppsson and Sciences Studies in Geology, 26: 1-30. O. 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The Ireviken Event Manuscript received 13 October 2009 in the lower Silurian of Gotland, Sweden - relation to similar Revised manuscript accepted 11 February 2010
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