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1 Institut für Paläontologie, Universität Wien, Althanstraße 14, A-1090 Vienna, Austria
2 Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843
The Main Glauconite Bed (MGB), near the top of the Eocene Stone City Member (Crocket Formation), Texas, has been considered to contain a typical local paleocommunity (parautochthonous assemblage formed within a stable habitat). Microstratigraphic analysis, however, reveals a complex sedimentologic and taphonomic history for the MGB, a unit that is 1.7–1.9 m thick and consists of three intercalated small-scale facies interpreted to represent differing modes of deposition. The primary autochthonous inner-shelf sediments are dark glauconitic clay-silts with a matrix-supported polytaxic fossil assemblage. Recurrent storms produced thin (few mm to cm) layers of mostly simple, bioclast-supported, polytaxic shell concentrations. These distal tempestites occur mainly as small-scale lenses and as a few beds and pods, associated with glauconite-pellets, terrigenous sands, and scarce sedimentary structures. Subsequent burrowing destroyed most skeletal concentrations and formed patches of fossils, glauconite-pellet sand, and terrigenous, very fine sand. The assemblages in the three facies are dominated by corbulids, naticids, turrids, noetiids, and the solitary coral Turbinolia sp., and are indistinguishable based on their taxonomic composition and most of the taphonomic features (disarticulation, fragmentation, incrustation, corrasion, shell repair, and predatory drill holes). Only drilled shells are significantly more abundant in the bioturbated patches than in the two other facies. The only strong evidence for the presence of allochthonous faunal elements is the lack of right valves of anomiid bivalves. The scarcity of significant differences between facies indicates the presence of one basic paleocommunity that was modified by small-scale and short-term depositional events and bioturbation, but which can still be recognized in spite of having been preserved by three different suites of depositional processes. Microstratigraphic analysis of bioclastic deposits can recognize small-scale sedimentologic and biostratinomic processes that otherwise frequently are overlooked in paleoecological studies. Such processes have only minor influence on taxonomic composition and taphonomic features, which are therefore robust characteristics of a fossil assemblage.
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