High-density bioturbated sandstones in the Mississippian Mauch Chunk Formation, eastern Pennsylvania, USA: Implications for continental ecospace exploitation

Abstract

Continental communities represented by complex, rapidly evolving invertebrate ecosystems first appeared during the Early Palaeozoic Era and are recorded in surface and subsurface bioturbation. The Mississippian-age Mauch Chunk Formation contains several high-density bioturbated, fine-grained sandstones that are preserved as erosively based fluvial channel fills. Several of these channel sandstone bodies are almost completely homogenized by subsequent bioturbation throughout their entire thickness, up to a maximum depth of 1.6 m.

The Mauch Chunk ichnofossil record demonstrates that high-density bioturbation occurred at least 30 my prior to the Permian and well before its widespread distribution in the Triassic. Hence, the Late Mississippian high-density bioturbated sandstones demonstrate a much earlier high-density exploitation of the continental subsurface ecospace than previously known.

Introduction

Marine strata of various ages have yielded important insights into the evolutionary history of bioturbation depth and the paleoenvironmental distribution of invertebrate burrowing (Sepkoski and Miller, 1985, Droser and Bottjer, 1987, Droser and Bottjer, 1988, Droser and Bottjer, 1989, Sepkoski, 1991, Droser and Li, 2001). Successful discoveries in the marine record spurred the examination of bioturbation in continental settings in order to glean similar insights into how substrate exploitation varied though time (Buatois and Mángano, 1993, Buatois et al., 1998, Miller and Labandeira, 2002, Miller et al., 2002). Applying semiquantitative indices to fluvial and lacustrine Permian and younger strata, high-density bioturbation was shown to occur on bedding planes before the vertical exploitation of substrate ecospace took place (Miller et al., 2002), supporting earlier observations that penetrative burrowing was rare, if not absent in Palaeozoic continental strata (Miller, 1984, Maples and Archer, 1989). As an example, the ichnogenus Beaconites has been reported to burrow to 20 cm depth in the Devonian to Lower Carboniferous strata (Graham and Pollard, 1982, Morrissey and Braddy, 2004).

The Mississippian Mauch Chunk Formation of eastern Pennsylvania has recently proven to be a rich source for understanding vertebrate and invertebrate ichnofossils yielding important insights into continental ichnodiversity during the Mississippian (Fillmore et al., 2010, Fillmore et al., 2011, Fillmore et al., 2012, Lucas et al., 2010a, Lucas et al., 2010b, Storm et al., 2010). This paper reports on recently discovered, high-density, bioturbated fluvial channel deposits, discusses the conditions of their formation, and explains the implications for ecosystem development in the Mississippian-age Indian Run Member of the Mauch Chunk Formation of eastern Pennsylvania (Fig. 1).