A Model for Transgressive Black Shales in a Sequence Stratigraphic Framework

Abstract

Commonly, at times of sea-level rise, basinal organic-rich facies extend their distribution into areas characterized by shallow water deposition. This occurrence of black shales in transgressive systems tracts is a widespread but little understood phenomenon. It has been suggested that such black shales are themselves formed in shallow water, although they are separated from underlying shallow marine facies by a transgressive surface often associated with a coarse lag. Several formative mechanisms have been proposed to explain this distribution, including either restricted circulation in a sea of limited extent or a zone of nearshore, high productivity. However, none of these models predicts the asymmetric distribution of the black shales for they are not found in highstand systems tracts when the shallow marine area was of a similar extent to that of the transgression. A new (expanding puddle) model is proposed in which the water depths of early transgression are considered to be the deepest achieved at any point on the sea-level curve. Sea-level rise is characterized by the formation of sediment-trapping estuaries and infilling of the preceding lowstand drainage system that leads to sediment starvation in the marine realm. The combination of sea-level rise, sediment starvation, and subsidence causes a rapidmore » deepening of the water column to a depth at which organic-rich shales can accumulate (assuming oxygen renewable to the bottom waters is by vertical advection). Black shale deposition will be initiated in the most rapidly subsiding areas and the anoxic puddle of bottom water will progressively expand outwards into basin margin settings.« less