A New Model of Succession of Middle and Late Pennsylvanian Fossil Communities in North Texas, Mid-Continent, and Appalachians with Implications on Black Shale Controversy: ABSTRACT

Abstract

A new model for the succession of Pennsylvanian fossil communities, preserved in cyclothems, is proposed on the basis of more than 200 fossil localities in the Mid-Continent, Appalachians, and north Texas. Early models for Mid-Continent cyclothems placed the black shales in shallow water, with maximum transgression at the fusulinid-bearing zone in the overlying limestone. The most recent model proposed that the black phosphatic shales, which commonly occur between two subtidal carbonates, are widespread and laterally continuous over great distances and represent maximum transgression. The black phosphatic shales contain: ammonoids; inarticulate brachiopods; radiolarians; conularids; shark material and abundant and diverse conodonts. This assemblage represents a pelagic or epipelagic community developed in a stratified water column over an anoxic bottom. The black shales grade vertically and laterally into dark gray-black shales which contain many of the same pelagic and epipelagic forms found in the phosphatic black shales, plus the following: low diversity of articulate brachiopods; large numbers and diversity of ammonoids together with other cephalopods; hyolithids; blastoids; trilobites; corals; and moderate diversity and numbers of bivalves and gastropods. This facies contains the deepest water benthic community. Most of these forms are immature, pyritized, and generally are preserved as molds. The ammonoids include both nepionic and late juvenile-early mature forms with the body chambers. These ammonoids, along with the other immature invertebrates, suggest mass mortality due to fluctuating low bottom oxygen as the deeper water tratification was breaking up. The dark gray-black facies grades into a medium gray shale facies which contains a mature molluscan fauna. This assemblage contains many of the same benthics as the dark gray facies, but with greater diversity. The pelagics and epipelagics, including plants, are rare to absent, except for the conodonts, which are diverse and abundant. The medium gray shale grades into a lighter gray facies, which is dominated by brachiopods, crinoids, and corals, with occasional bivalves and gastropods. Fusulinid and coral communities may also occur in the slightly shallower depths. (These facies are interpreted as being a moderate to shallow depth shelf community.) The brachiopid-crinoid community is succeeded by shallow water communities which may have occupied shoreline, lagoonal, bay, interdeltaic, or shallow prodeltaic environments. These communities are low to high diversity molluscan assemblages, generally lacking ammonoids, and have a very low diversity conodont assemblage. These shallow water assemblages are discontinuous and occur commonly interbedded with sandstone, in the regressive and early transgressive portions of each cycle. In addition, coals are sometimes present that grade vertically into black carbonaceous shales that are non-phosphatic, lack benthic and pelagic forms, and contain plant compressions. These black shales are interpreted as being marsh deposits. This model is consistent with the findings of Yancey and Stevens with the Lower Permian fossil communities in the western United States. In addition, this model agrees with Calver's work on the succession of communities associated with the cyclothems in the Westphalian of England. End_of_Article - Last_Page 426------------