ABSTRACT: Characteristics of Transgressive Systems Tracts: Examples from the Brazos River Valley, Eastern Texas

DAVIDOFF, ANDREW J., and THOMAS E. YANCEY, Texas A&M University, College Station, TX

Sequence stratigraphy of the Paleocene and Eocene strata in the Brazos River Valley, eastern Texas, was studied using seismic, well-log, and outcrop data. These strata are composed of clastic sediments, with major sedimentation pulses associated with distant tectonism. Depositional environments range from alluvial plain to midshelf. Sequences consist of transgressive and highstand systems tracts; lowstand deposits occur basinward of the study area.

Transgressive systems tracts range from tens to hundreds of feet thick, and comprise 10 to 40% of the sequences. Thickness and lithology depend upon sediment type available, sedimentation rate, and rate of sea level rise. Parasequences that comprise the tracts thicken landward and show an increase in sand content. These parasequences tend to coarsen upward in open marine strata and fine upward in marginal marine strata. These systems tracts thin landward by onlap of basal strata, and basinward by depositional thinning. Depositional environments were primarily marginal and open marine; coastal and alluvial plain sediments accumulated in the highstand systems tract.

Seismic data and cross sections show that transgressive systems tracts directly onlap sequence boundaries. On well-logs, sequence boundaries display a change from progradational to retrogradational parasequence stacking patterns. Incised valleys were not observed; smaller channels appear filled with highstand sediments. Transgressive and overlying highstand systems tracts are separated by a maximum flooding surface. This appears on seismic and cross sections as a downlap surface. Across this surface parasequence stacking patterns change from retrogradational to progradational, and in marginal marine strata from fining to coarsening upward. At the outcrop this surface is a shale rich section tens of feet thick.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)