Evidence of Syndepositional Subsidence and the Evolution of Multiple Coal Splits in the Hartshorne Formation, Western Arkoma Basin, Oklahoma

Abstract

The Pennsylvanian (Desmoinesian) Hartshorne Formation of the Krebs Group was studied in Haskell, Latimer, McIntosh and Pittsburg Counties, Oklahoma. The purpose of this study was to examine evidence obtained from gas exploration wells to develop an interpretation of the depositional environments and stratigraphy of the Hartshorne Formation. Data from over 2800 well logs, cross sections, isomaps, and a seismic survey were integrated to complete this work. The Hartshorne Formation is divided into two coal-bearing members: the upper and lower. Previous depositional models interpreted the Hartshorne as a fluvial-deltaic complex that contains a single coal-split line. Multiple coal splits were recognized in this study. Both the upper and lower coals are present in many parts of the study area, not just south of the previously identified single coal split line. Coal splits are associated with thick valley fill deposits. Thicker, undifferentiated single coals found north and south of the incised valley fill appear to contain the upper and lower Hartshorne coals. Coal splits converge toward the thicker, single undifferentiated coals. "Bony coal" or the shale zone in the middle of the undifferentiated coal may correlate to the split. The upper coal thins toward and is absent over the thickest valley fill sandstones. A modified depositional model for the Hartshorne was developed that incorporates the new findings. This model suggests that localized subsidence and deposition are responsible for the coal split. Fluvial drainage followed and eroded the subsided areas, establishing the association of coal split with the incised valley. The cause of subsidence was not established, but it could relate to deeper faulting or differential compaction in the underlying Atokan section.