CONTROLS ON SAND DEPOSITION IN MUDDY SYSTEMS: THE BOSSIER SHALE (TITHONIAN) OF THE EAST TEXAS BASIN

Sandy environments ranging from fluvial to deep water occur in mud-dominated systems, including the Bossier Shale of the East Texas basin. Their occurrence is controlled by a combination of climate and sea level change.

Sequence/seismic stratigraphic analysis of well logs and 2-D seismic lines from the East Texas basin demonstrates that the Bossier Shale can be subdivided into two sequences separated by a major sequence boundary (SB-2). Bossier Shale is also bracketed by a basal (SB-1) and upper (SB-3) sequence boundary separating it from the Cotton Valley Lime below, and the Cotton Valley Sand above.

In seismic sections, the mid-Bossier (SB-2) boundary was identified by tracing mounded reflectors, and sigmoid signatures representing basin floor and slope fans. SB-2 correlated onto the shelf below stacked deltas. In well log sections, basin floor fans were traced laterally into slope fans and stacked deltas. These basin floor and slope fans represent a lowstand systems tract, whereas the Lower Bossier represents a transgressive systems tract and the upper Bossier is a prograding complex.

Burial history analysis suggests the Lower Bossier accumulated during rapid mechanical subsidence when the East Texas basin was underfilled. Sea level drop associated with the SB-2 represents a major climate shift from tropical to cooler conditions favoring rapid influx of sands from the ancestral Mississippi, Ouachita and Red River Systems. These sands developed inner shelf prograding deltas, outer shelf and incised valley fill stacked deltas, and basin submarine fan systems. Their occurrence is due to the combination of global cooling and sea level change.