The Paleoecology of a Late Ordovician Shale Unit from Southwest Ohio and Southeastern Indiana Author(S): Robert C

Paleontological Society The Paleoecology of a Late Ordovician Shale Unit from Southwest Ohio and Southeastern Indiana Author(s): Robert C. Frey Source: Journal of Paleontology, Vol. 61, No. 2 (Mar., 1987), pp. 242-267 Published by: Paleontological Society Stable URL: http://www.jstor.org/stable/1305320 Accessed: 14/03/2010 21:30 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. 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Paleontological Society and SEPM Society for Sedimentary Geology are collaborating with JSTOR to digitize, preserve and extend access to Journal of Paleontology. http://www.jstor.org JOURNALOF PALEONTOLOGY,V. 61, NO. 2, P. 242-267, 7 FIGS.,MARCH 1987 THE PALEOECOLOGY OF A LATE ORDOVICIAN SHALE UNIT FROM SOUTHWEST OHIO AND SOUTHEASTERN INDIANA ROBERTC. FREY Department of Geography and Geology, Centenary College, Shreveport,Louisiana 71134-0188 ABSTRACT- The Treptocerasduseri shale unit withinthe WaynesvilleFormation of LateOrdovician (early Richmondian)age in southwestOhio and the equivalentTrilobite shale unit in the same formationexposed in adjacentportions of Indianarepresent an Ordovicianshallow marinemud- bottom epeiricsea facies. These fine-grainedclastics contain a moderatelydiverse mollusk-trilobite assemblagedominated by vagrantepifaunal detritus-feeding calymenid and asaphidtrilobites, large endobyssateand infaunalfilter-feeding pelecypods, and nektonicnautiloids. Articulate brachiopods, ectoprocts,and pelmatozoanechinoderms form only minor elements of this fauna. This mollusk-trilobiteassemblage was common in Late Ordovicianshallow marineclastic en- vironmentswhere mobility was an asset and therewas an abundanceof oxygenand food resources. Such assemblagesare characteristicof the LorraineFauna of Late Ordovician(Edenian to Rich- mondian)age that occurs from the Ohio Valley north and east into New York, Ontario,Quebec, and Ireland.These early Paleozoic mud-bottom assemblageswere considerablymodified by the Late Ordovicianextinction event and were replacedin the Silurianand Devonian by distinctly differentassemblages dominated by largeepifaunal strophomenid and spiriferidbrachiopods, crinoids, and phacopidtrilobites. INTRODUCTION from mollusks in these Ordovician marine PALEOZOICsubtidal mud-bottom environ- environments. ments have been describedas supportingas- Shale units in the CincinnatianSeries (Up- semblages dominated by relatively immo- per Ordovician)in the Ohio Valley make up bile, epifaunal suspension-feeding benthos at least 60% of the exposed Ordovician sec- (Ausich and Bottjer, 1982; Jablonski and tion. Very little has been published concern- Bottjer, 1983; Thayer, 1983) or, less-com- ing the biota of these shales and their paleo- monly, mobile infaunaldeposit-feeders (Lev- ecology. Many of the shales represent the inton and Bambach, 1975). Sepkoski and deposits of muddy epeiric sea floors that sup- Sheehan(1983), citing primarilythe work of ported moderately diverse assemblages dom- Bretsky (1969, 1970a, 1970b), have de- inated not by articulate brachiopods, but by scribedUpper Ordovicianshelf communities filter-feeding pelecypods, trilobites, nauti- in easternNorth Americaas being dominated loids, and, occasionally, echinoderms. The by a diverse assemblage of orthid, stropho- Treptoceras duseri shale unit in the Waynes- menid, and atrypidbrachiopods, trepostome ville Formation is one such unit. and cryptostome ectoprocts, phacopine trilobites, and crinoids.Most of this assemblage STRATIGRAPHY persists beyond the Ordovician to constitute The Treptoceras duseri shale unit is a blue the brachiopod-ectoproct-pelmatozoanechi- claystone unit about 1.5 m thick located with- noderm assemblage so characteristic of Pa- in the Waynesville Formation of Late Or- leozoic shelf environments.These authorslist dovician (early Richmondian) age exposed in pelecypods as part of their "Modem Fauna" southwest Ohio and adjacent portions of and indicate that members of this group were southeastern Indiana (Figure 1). Shideler locally important in the Late Ordovician in (unpublished notes) and Flower (1946) both nearshore, inner shelf environments, spa- noted that the Treptoceras duseri shale, as tially distinct from the brachiopod-dominat- typified by the occurrenceof large numbers ed "Paleozoic Fauna" of offshore shelf en- of the nautiloid Treptoceras duseri (Hall and vironments. Trilobites, an important Whitfield),is restrictedto portions of Warren component of this offshorePaleozoic Fauna, and Clinton counties in Ohio. Studies by the are inferredto have been spatiallysegregated writer have confirmed this distribution. Copyright@ 1987, The PaleontologicalSociety 242 0022-3360/87/0061-0242$03.00 ORDOVICIAN PALEOECOLOGY 243 However, a temporally-equivalent shale unit, lithologically identical to the Treptoceras du- mo seri shale but minus the distinctive nautiloid INDIANA OHIO fauna, has been found in Butler and Preble . * counties in western Ohio and in adjacent por- 5A&4 A tions of southeastern Indiana. This shale will 2 3. be differentiated from the Treptoceras duseri shale and is referred to as the Trilobite shale 6A unit from the abundance of the calymenid trilobite, Flexicalymene meeki (Foerste). The Treptoceras duseri shale unit is located 11-13 m above the contact between the underlying rubbly limestones of the Arnheim KENTUCKY Formation and the even-bedded shales and thin limestones of the Waynesville Forma- tion (Figure 2). At most localities in Warren A* TREPTOCERASDUSERI SHALE OUTCROPS SCALE TRILOBITESHALE OUTCROPS I": 2Omi. and Clinton counties in Ohio, the basal con- f ORDOVICIAN-SILURIANBOUNDARY DAI tact of the Treptoceras duseri shale is well- defined, with the massive claystone of the FIGURE1 -Map of the Indiana-Ohioarea showing shale resting on a ripple-marked, irregularly the location of the exposuresof the Treptoceras bedded packstone composed of jumbled, duseriand Trilobiteshale units thatwere studied whole, disarticulated brachiopod valves. The and collectedas partof this investigation.Num- upper contact of the shale unit is gradational beredlocalities correspondto localities listed in with the occurrence of the localityregister. These localities include those in most places, limy where these shale units are more in the 10 cm of the extensively lenses increasing upper exposed and those exposures more intensively shale. However, at most localities the shale studied. is capped by a well-bedded, 10 cm thick packstone composed almost entirely of the disarticulated valves of the entelacean brachio- The monotonous massive claystone li- pod Onniella meeki (Miller) in a matrix of thology that makes up much of the Treptoc- clay and sparry calcite. eras duseri shale unit is interrupted locally by The predominant lithology of the Treptoc- the development of shelly beds less than 1 eras duseri shale is best termed a claystone, cm thick and by thin limestone beds from 1- using the terminology of Potter et al. (1980). 6 cm thick (Figure 3, column 1). The shelly These massive claystones, when fresh, are beds consist of calcareous indurated clays with blue-gray, hard, dense, and well-indurated. concentrations of pelecypod and nautiloid They show no evidence of bedding, with the shells and trilobite carapaces and exuviae. rock splitting conchoidally when struck with These shelly beds are usually not continuous a hammer. These rocks are remarkably uni- and rarely traceable for more than 100 m in form in texture, fine-grained, with this mo- any direction. The thin limestone beds con- notonous lithology relieved only by occa- sist of well-bedded packstones with sharp sional layers of shells and thin limestone beds basal contacts with the underlying claystones. (Figure 3). The claystones are predominantly These limestones are composed of poorly- clay in composition (illite, 70-80%) with mi- sorted biogenic allochems ranging from fine nor amounts of silt (20-24%) and carbonate shell detritus to whole, unbroken shells in a (less than 8%). The carbonate content of the matrix of clay, micrite, and sparry calcite. claystone increases adjacent to the localized The basal portions of the limestone beds are accumulations of shell debris. Biogenic struc- commonly a jumbled mass of large, random- tures are uncommon in the massive claystone ly-oriented shells. These limestones exhibit lithology, being represented by branching graded bedding, fining upwards with shells tube-like structures referable to the ichno- becoming aligned parallel to bedding

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