Sequence stratigraphy and depositional dynamics, basal Atoka Formation (Orr-Patterson members), Middle Pennsylvanian, Arkoma Basin, central Arkansas.

摘要

The Orr and overlying Patterson sandstones comprise the basal Atokan sandstones in the Arkoma basin. The Off sandstone has been an important exploration target since the 1950's, when commercial quantities of dry gas were discovered. Determining the sequence stratigraphic framework and establishing a depositional model for these units will improve the geologic understanding and enhance the exploration potential of those reservoirs within the Arkoma basin.; The Atoka Formation represents one third-order eustatic sequence with superimposed fourth and fifth-order sequences of coupled sand and shale intervals. These fourth and fifth-order sequences can be attributed to climatically-driven variations in sediment supply and accommodation space reflecting transgressing or regressing, upper shelf depositional environments and complexes. The Morrowan/Atokan chronostratigraphic boundary is clearly a Type 1 sequence boundary being characterized by subaerial exposure and concurrent subaerial erosion associated with stream rejuvenation and fluvial incision. The contact between the sub-Orr shale and the first Atoka sandstone likely represents this sequence boundary and establishes the sub-Orr shale as Morrowan in age.; Basal Atoka through Lower Off deposition represent the lowstand systems tract. The upper Orr sandstone likely marks the onset of the transgressive systems tract, but is interrupted by a fourth-order regressive event. The Trace Creek Shale is a tectonic event in which the basin is lowered forcing an abrupt transgression, creating an overstepping of Orr-Patterson units which could lead to up-dip stratigraphic traps in addition to possible Orr lowstand wedges to the south.