Partitioning of Longshore Versus River-derived Mud on the Inner Shelf of the Modern Asymmetric Wave-Influenced Brazos Delta, Texas Gulf Coast, USA

Abstract

Prodelta muds characterize the inner shelf of many modern systems. Inner shelf mud may be derived directly from onshore river systems, but may also migrate along the shelf for long distances. These modern mudbelts provide a potential analog for understanding facies variability within ancient mudstones and shales, which are of ongoing economic importance as unconventional hydrocarbon reservoirs. The Brazos Delta is a multi-sourced system, receiving sediment directly from the Brazos River, on the Texas Gulf Coast, as well as indirectly from the longshore transport of Mississippi River-derived mud. The sediments derived from these two sources are quite distinct, and differ in color, grain size, and mineralogy. Bed and lamina-scale analysis (<1 cm resolution) was conducted on 19 legacy offshore soft-sediment cores previously used to define the delta, and significant differences of mineralogy, grain size, sand distribution, and mud facies distribution were identified between the updrift and downdrift offshore regions. The downdrift region has a larger proportion of red Brazos-derived mud, which is mixed with minor amounts of gray longshore transport-derived mud. The updrift region is composed of flood-related red Brazos mud interbedded with gray longshore transport-derived mud. The stratigraphic differences between the two regions indicate that post-flood oceanic conditions (i.e., wave reworking) play a primary role in the stratigraphic successions preserved in core within the downdrift area. Unlike the nearshore portion of the Brazos Delta, we propose that within the offshore region, the downdrift area is more susceptible to wave-reworking than updrift.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017