Sea Level Control on Facies Architecture of Incised Valley-Fill Sequences: Case Study of Sabine and Trinity Valleys, Texas Continental Shelf

Mark A. Thomas, John B. Anderson, Wendy Smyth

The rate of sea level rise likely has varied during the late Wisconsinan-Holocene transgression. We are investigating the sea level history of the northwest Gulf of Mexico by examining the sequence stratigraphy of incised valleys on the Texas continental shelf. Glaciologists argue that mass wasting of marine ice sheets can cause rapid and episodic relative sea level (RSL) rises on the order of 5 m/100 years. Such an event would produce a large (^approx25 km) landward translation of the shoreline on low-gradient shelves like that of north Texas. RSL rise events are expected to be manifested as discontinuities in valley-fill sedimentation and as changes in valley shape.

Nearly 1,000 km of high-resolution seismic profiles collected in an area extending from Sabine Pass to Galveston and to 60 km offshore were integrated with engineering borings, vibracores, and piston cores; these data allow mapping of incised valleys and valley-fill facies associated with the ancestral Sabine and Trinity rivers.

An RSL rise event is characterized by a change from slow rate of rise to rapid and back to slow. The period of rapid rise produces a major flooding surface; bayhead delta development is suppressed, and lower estuarine or marine deposits lie directly on fluvial deposits. Because accommodation increases as rapidly as RSL rises, valley-fill deposition is limited to the original, deeply incised valley. In map view, the valley appears relatively straight and narrow. The valley just offshore Galveston Island represents this situation.

During a period of slow rise, bayhead deltas prograde and downlap onto estuarine deposits. In this case, sediments may completely fill the original incised valley, and the river supplying this valley may meander beyond previous valley edges. In map view, such a valley will be broad and irregular in shape, much like modern Galveston Bay.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.