ABSTRACT: Use of Organic Petrology in Sequence Stratigraphic Interpretations: Example from the Eocene-Oligocene Boundary Section, St. Stephens Quarry, Washington County, Alabama

Mark A. Pasley, Joseph E. Hazel

The type and preservation of organic matter is related to the depositional systems tract in which the sediments were deposited. Shelf sediments in the transgressive systems tract contain sparse, highly degraded phytoclasts whereas organic matter in the highstand systems tract is dominated by well-preserved phytoclasts introduced to the shelf during progradation. Because of this relationship, integration of data from organic petrology with sedimentologic and biostratigraphic results provides greater resolution in locating critical surfaces (sequence boundaries, transgressive surfaces, and surfaces of maximum starvation) that bound depositional systems tracts within the depositional sequence.

The Eocene-Oligocene shelf sediments exposed in St. Stephens Quarry provide an excellent example of the relationship between depositional systems tract and organic matter deposition. Deposition of the Shubuta Clay in the transgressive systems tract terminated with the surface of maximum starvation. This surface is marked by a thin (<2 cm), laterally extensive, phosphate-rich shell lag that contains only minor amounts of highly degraded phytoclasts. Graphic correlation of biostratigraphic data reveals a marine hiatus (120,000 years) within the shell lag. Deposition in the subsequent highstand systems tract resulted in an increase in well-preserved phytoclasts in the overlying Bumpnose Limestone and Red Bluff Clay. A transgressive surface forms the contact between the Red Bluff Clay nd the Mint Springs Marl. Phytoclasts are less common and more degraded in the Mint Springs above the transgressive surface than in the Red Bluff below. No hiatus is observed at this surface, suggesting that the type 2 sequence boundary is conformable at this section and may exist in the Red Bluff below the transgressive surface. This integrated approach confirms and refines previous sequence stratigraphic work performed on this important Gulf Coast section.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990