Marine Facies in Upper Pottsville: New Data from Black Warrior Basin in Northern Alabama

Robert Pody, Chris Dewey

Five Morrowan (Late Pennsylvanian) faunas are described from the upper Pottsville Formation in the Black Warrior basin of northern Alabama. The fossil assemblages occur within predominantly fine-grained clastic sequences between major coal groups in the basin. Three assemblages are indigenous, two are exogenous.

The Linoproductus community is a diverse, abundant, indigenous fauna consisting of trilobites, disarticulated crinoids, gastropods, bivalves, and brachiopods. The fauna is associated with extensive Helminthopsis and rarer Scalarituba ichnogenera. Sedimentologic evidence indicates a pro-delta silty shale environment for this community. The Linoproductus community grades vertically through a coarsening-upward sequence into the Eolissochonetes community, which inhabited the delta-front silts. A much less abundant ichnofauna is associated with this sparse, low-diversity brachiopod community.

The Orbiculoidea community occurs in extensively bioturbated fine silty sands, succeeded by overclays. Inarticulate brachiopods form the main macrofaunal elements and chonetids are absent. The community is thought to have occupied a brackish, restricted, interdistributary-bay environment.

Two similar, fragmented but unabraded, locally derived, exogenous assemblages include brachiopods, bivalves, crinoids, gastropods, and wood. They differ in their mode of occurrence. An exogenic assemblage occurs as a fossil lag in discontinuous units below major sands, interpreted as bar fingers. The other exogenic assemblage occurs in continuous tabular sands, interbedded with interdistributary bay silts, the latter sands contain locally abundant Zoophycos.

All the described associations occur within a major deltaic setting. The sedimentologic and faunal evidence together indicate a shallowing-upward trend from distal to proximal deltaic environments.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.