Preservation of late Quaternary sandy fluvial-deltaic deposits in the Texas Shelf
A study of the Late Quaternary Colorado and Brazos systems reveals backstepping fluvial-deltaic complexes on the Texas shelf. Sequence stratigraphy and radiometric dates place their formation within the most recent transgression. Along the shelf break these discrete deposits are muddy in nature while they amalgamate to form an extensive, sandy plain on the middle shelf. High-resolution 2D seismic data, samples from 4 platform borings, and over 40 platform boring descriptions were used to define the architecture of the updip, sandy, fluvial-deltaic plain.
The sandy fluvial-deltaic deposit is strike-aligned, running over 50 km along the coast. It is approximately 30 km wide and on average 7 m in thickness. An undulatory, erosive basal surface and small scale cut and fill internal geometry characterize this extensive unit. Lateral accretion within individual channels is common. Core and platform boring descriptions record a fining upward sequence of sand, from medium to very fine, with marine fossils, mica, and some organic debris.
Rate of sea-level rise is an important control for preserving sandy fluvial-deltaic deposits. During the initial sea-level rise, fluvial-deltaic systems formed along the outer shelf. These were transgressed with continued rise. The sandy facies in the deposits were ravined, leaving only the muddy distal deltaic toes. An extensive fluvial deltaic deposit formed on the middle shelf, showing signs of strong wave influence. Increased rate of sea-level rise resulted in the abandonment of the system and preservation of the extensive, sandy facies.