Relating Sequence Stratigraphy to Lithostratigraphy in Paleogene Siliciclastic-Dominated Shelf Settings, Central-East Texas

Abstract

The western Gulf of Mexico is a passive margin with high rates of sedimentation where lithostratigraphy is controlled by sedimentation pulses related to distant tectonism. These pulses exert much control on long-term transgressions and regressions. Superimposed on these long-term cycles are higher orders of cyclicity related to eustatic sea level change (sequences, parasequences), producing a complex history of shoreline change in which sequence stratigraphy is superimposed upon, but unrelated to, lithostratigraphy. lithostratigraphic and sequences stratigraphic relationships of Paleogene deposits in the Brazos River Valley section were studied using seismic, well, logs, outcrop, and core. Paleocene Midway deposits contain midshelf highstand shales with minor lowstand and transgressive sands, whereas Wilcox and Eocene deposits contain shoaling-upward highstand deposits with minor deepening-upward transgressive deposits. Formations commonly contain multiple sequences. The Yegua and Jackson intervals are characterized by patchy development (in time and space) of sand units from outcrop to mid-outer shelf settings. Formation-scale depositional trends are generalized and fragmented due to abrupt shifts in depositional environments at sequence/parasequence boundaries. Fragmentation of depositional gradient within formations occurs laterally and vertically. Thus, regional correlations of lithostratigraphic units represent matching of unconnected depositional episodes. Unlike formations, sequences and parasequences display orderly and predictable progression of depositionalmore » change. Shoaling-upward sand units and flooding surfaces within parasequences are relatively continuous. Parasequences of the Claiborne can be correlated between outcrop and subsurface from shore zone to the mid-outer shelf, crossing formation-scale facies boundaries.« less