Sedimentology and possible paleoceanographic significance of mudstone turbidites and associated deposits of the Pen Argyl Member, Martinsburg Formation (Upper Ordovician), eastern Pennsylvania

Abstract

The Pen Argyl Member, the stratigraphically highest unit of the Upper Ordovician Martinsburg Formation of eastern Pennsylvania, consists of the three mudstone facies: laminated mudstone, uniform mudstone, and black shale. These deposits conformably overlie a coarsening-upward sequence of fine-grained and classic sandstone turbidites of the underlying Bushkill and Ramseyburg Members, respectively. Petrographic and X-radiographic analysis suggests that laminated and uniform mudstone facies beds accumulated from muddy turbidity currents. Interbedded black shale facies beds, however, are interpreted to reflect slow, gravitational settling of sediment in a density stratified, anoxic basin. The marked transition from sandstone and siltstone turbidite deposits of the Ramseyburg Member to mudstone turbidite and black shale deposits of the Pen Argyl Member may reflect the combined effects of a Caradocian rise in sea level (increased organic-carbon production; lack of sand-size sediment) and a tropical to subtropical location of the basin (density stratification). Such a model, however, does not explain adequately the very thick laminated and uniform mudstone facies beds common only to the Pen Argyl Member. An alternative hypothesis involves amplification of global eustatic- and climatic-related oceanographic effects by local or basinal influences. The very thick beds of the laminated and uniform mudstone facies, by comparison with lithologically similar deposits of the eastern Mediterranean, are interpreted to have been ponded in a topographically restricted basin. Accordingly, deposition of the Pen Argyl Member may record a significant change in basin morphology from a wide extensive basin to a topographically restricted or enclosed anoxic basin.