SEDIMENTOLOGY OF AN ARCHANODON-BEARING CHANNEL BODY IN THE CATSKILL FORMATION (UPPER DEVONIAN) NEAR STEAM VALLEY, PA

A >10-m-thick multistorey channel body exposed near Steam Valley, PA contains a distinctive fossil assemblage that includes the freshwater bivalve Archanodon sp., the placoderm fish Bothriolepis sp., the large sarcopterygian fish Holoptychius sp., as well as a biostratigraphically-informative palynomorph assemblage.

The incompletely exposed channel body has a strongly erosional base that incises through at least 10 m of heterolithic, redbed-dominated overbank and fluvial deposits.  The lower storey contains sandy, downstream-accretion macroforms separated by numerous internal erosion surfaces.  In situ Archanodon fossils are present within bar deposits on a wing of the channel just above the lower storey boundary.  The upper storey consists almost entirely of heterolithic lateral-accretion deposits.  Minor internal erosional surfaces within the lateral accretion deposits are lined with intraformational conglomerates with abundant, well preserved fish scales and plates that were not reworked from older deposits.  Mudcracks and carbonized roots are present throughout this interval.  The channel body is capped by an intensely bioturbated micaceous siltstone with a low diversity trace fossil assemblage. 

The incised, multistorey architecture of this channel body may indicate that it is actually an incised paleovalley.  The initial channel fill records deposition by a perennially-flowing sandbed river; Archanodon colonized the shallow water areas near the banks.   The upper story records deposition within a smaller, ephemeral meandering stream that was colonized by detritus-feeding placoderms and predatory fish.  Plant roots record either shallow-water vegetation or colonization of the streambed during periods of drought.  Trace fossils within the overlying micaceous siltstone is interpreted to record transgression of the floodplain by brackish waters.