THE INFLUENCE OF SEA LEVEL CHANGES AND POSSIBLE PYCNOCLINE SHIFTS ON BENTHIC COMMUNITIES IN THE FINIS SHALE (VIRGILIAN) NEAR JACKSBORO, NORTH- CENTRAL TEXAS LOBZA, Vadec, SCHIEBER, Jürgen, and NESTELL, Merlynd, Dept. of Geology, UT Arlington, Arlington, Texas 76019 Abstract number of studies to work out regressive-transgressive The Finis Shale (Upper Pennsylvanian, Virgilian, facies patterns in sedimentary sequences (e.g. Heckel, Cisco Group, Graham Formation; Barnes, 1987) in the 1991; Ettensohn, 1985). Jacksboro-Graham area represents a well preserved However, especially in shale dominated transgressive-regressive cycle above lenticular cycles, the commonly poor outcrop quality does not sandstone and mudcracked greenish shale. allow a critical assessment of the relevance of the Exceptionally well preserved in situ benthic stratified basin model to a given succession. A new invertebrate fossil assemblages allow for a detailed exposure of the Finis Shale near Jacksboro (Fig. 1) paleoecological investigation, providing information allowed the examination of a complete transgressive- about past environmental conditions of the water regressive cycle almost entirely in shale facies. column and the substrate. Benthic communities Although quite uniform from a petrographic and preserved within the Finis Shale cycle begin with a sedimentological perspective (almost entire sequence very shallow water Myalina community, followed by a consists of carbonaceous and heavily bioturbated shale, deeper water, relict mature Crurithyris-Paraconularia with almost no compositional variability), the sequence community, a Crurithyris-Glabrocingulum community, shows a surprising faunal variability that belies the and a relatively mature Hustedia-Phymatopleura environmental uniformity suggested by the shales. community which persists through the transgression- Although the stratified basin model suggests that water regression shift (and disappears a few decimeters depth is the major control on faunal development in above). A Rhipidomella-Dentalium community is these shales, faunal variability and diversity may as present at the very top within the regressive portion of well depend on combined effects of small scale bottom the cycle. The described community changes could be water turbidity caused by some benthic organisms, interpreted as primarily due to changes in sea level, storm winnowing and resuspension, and terrestrial sediment supply, and energy input. However, only 20 sediment input (e.g. deltas), rather than water depth miles to the west fissile, poorly fossiliferous, black and oxygen budget (McCollum, 1988). shales dominate the lower part of the Finis Shale, Because the study of fossil communities is at suggesting that lateral pycnocline shifts may also have the heart of this paper, the usage of the term will be influenced living conditions of benthic communities. briefly defined. The term "community" has been a subject of numerous discussions among ecologists and Introduction paleoecologists alike (e.g. Boucot, 1981; Springer and Cyclic Pennsylvanian successions in the Miller, 1990; Yu et al., 1987), with an impressive Midcontinent region have been a subject of intense number of proposed solutions as to its clear usage. In interest by geologists because they afford opportunities this paper it is applied strictly in reference to the to study interaction between sedimentation, sea level preserved fraction of initially present benthic changes, and tectonic regime in epicontinental invertebrate assemblages that were affected by specific deposits. For some time now, black shales within environmental conditions such as water depth, these cycles were considered the deepest portion of the turbulence, turbidity, substrate firmness, and oxygen cycle, due to maximum continental flooding and level. Changes in those conditions, brought about by extension of the pycnocline into epicontinental seas relatively long term processes and not necessarily by (Heckel, 1991). competition among the species, played the leading role Black shales as such have been studied in in influencing the described variations within the many settings, and in a number of cases a stratified investigated assemblages. Because such fluctuations basin model, as proposed by Byers (1977), has been of environmental factors usually occur gradually, the applied. In such a scheme a distinction was made communities presented here display a degree of between aerobic (high faunal diversity, shallow), coexistence resulting from decreasing environmental dysaerobic (intermediate depth, lower diversity), and tolerance of certain species and increased tolerance of anaerobic (deep, virtually barren) environments. The others. In this study two different localities of the implicit connection in this scheme between Finis Shale are compared with regard to differences in environment and relative depth has been used in a environmental conditions. with a regressive, phylloidal limestone, which contains occasional Composita brachiopods, Phylloid algae, and a variety of corals. Its was probably deposited in shallow and clear water. Methods The Finis Shale near Jacksboro was subdivided into 14 segments, A through M (Fig 3), and sampled accordingly. Surface samples of equal areas (where absolutely all fossils were collected) were complemented by bulk samples. Segments were sampled in vertical succession in the same part of the outcrop. The exceptional condition of the outcrop allowed for the majority of the fossils to be found in situ. Obtaining a surface sample of the Myalina bed was made impossible by slumping shales which covered it. Only shell fragments larger than one half of the entire shell were accounted for. Such an Fig.1 Map showing the localities under investigation. approach was permitted by the fact that, with the exception of crinoids and bryozoa, fossils were seldom present in a state of dismemberment. Abundance of The purpose of this paper is to document the epifaunal and infaunal taxons throughout the sequence, succession and changes within fossil benthic as well as their morphology, are basis for this study. invertebrate communities of the Finis Shale and to Three groups of fossils, brachiopods, gastropods, and reconstruct the environmental conditions in which they bivalves, have proven to be most useful in yielding existed. The data demonstrate that fossil communities information about substrate conditions. Occurrences make it possible to establish the transgressive- of selected members of these groups, presented regressive shift (Rollins et al., 1977) in this sequence. graphically in Fig. 5, display the trends discussed in later portions of this paper. A list of species OBSERVATIONS abundances within each sample is provided in Fig. 4. Jacksboro locality Cephalopods were omitted from the study, for only The transgressive portion of the Finis Shale is epifaunal and infaunal organisms were investigated. underlain by a unit of greenish mudstone Bryozoa and crinoids were not studied in as much (approximately 77cm thick), containing plant fossils detail as the fossil groups named above. However, (e.g. Calamites) and mudcracks. This mudstone their environmental significance was noted and overlies lenses of probably deltaic sandstone and is contributed to interpretation of environmental interpreted as a deltaic marsh deposit. Above the conditions. greenish mudstone unit, the base of the Finis Shale cycle consists of lenses of bioturbated ferruginous Basis for Defining Communities sandstone and a reddish calcareous mudstone with Trends in occurrence and abundance of abundant Myalina (10-30cm thick). The Myalina bed individual species within each sampled interval were is overlain by an approximately 13 cm thick bed of graphed and compared, resulting in grouping selected sandy mudstone containing abundant fusulinids, vertical intervals into larger segments. Assemblages of representing a shallow water marine environment those segments were then subjected to a statistical (Boardman et al., 1989). Its thin development analysis by means of Morisita's index of similarity probably reflects fast transgression. This bed is a (Krebs, 1989), where a similarity index of 1.0 indicates focus of a separate study by M. Nestell. The fusulinid complete similarity and 0 no similarity. The results are zone is overlain by some 9.1 m of shale, the main presented in Table 1. A statistical comparison of the focus of this study. These shales contain occasional Crurithyris-Paraconularia with the Crurithyris- horizons of siderite nodules and calcified burrows. Glabrocingulum (B and C in Table 1) community Both types of carbonate deposits represent early yields a similarity index of 0.958. This large value is diagenetic concretionary growth, probably reflecting caused by the high abundance of the brachiopod episodes of very small rates of sediment accumulation Crurithyris in both communities. Also, values of (Fchtbauer, 1988). The upper portion of the Finis similarity indices in excess of 0.5 for communities C Shale contains horizons of lenses of shell hash and fine and D (0.545), and D and E (0.788) are probably due sand and silt, probably a result od storm winnowing to overlap (Fig. 3). (discussed in more detail below). The uppermost 10 This latter assumption is probably justified because cm of the Finis Shale cycle are marked by another these communities were established under fusulinid horizon (also subject of a detailed environmental parameters that did not change abruptly. investigation by M. Nestell). The preserved