Origin and Sedimentary Characteristics of Discrete Sand Beds in Modern Sediments of the Central Texas Continental Shelf (Storm, Hurricane, Geostrophic, Waves, Turbidities).

Thin (1-9 cm) discrete beds of sand and silt are present in the clayey modern sediments of the Texas shelf. Observation of seaward size-grading within the beds indicates that the source of the clastics is the adjacent coastal zone. Consideration of the nature of wave and current processes during storms and the dimensions of possible coastal sub-environment sources suggests that the shoreface supplied the sand found in the discrete sand beds. The seaward grading and along-shelf continuity of a correlative horizon dated as having been deposited during the passage of Hurricane Carla in 1961 imply that transport was along-shelf but also obliquely offshore. The internal stratigraphy, subtle upward grading, and sequence of sedimentary structures within the beds all point to deposition during the waning phase of an impulsive event such as a turbidity current, a simple wind-forced current motion, or the near-bottom interaction of a geostrophically balanced steady current and oscillatory wave orbital motion. The along-shelf continuity of the Carla bed precludes the turbidity current hypothesis. Analysis of records of fluid motion and boundary shear stress during storm events leads to the conclusion that bottom currents are not just a simple result of wind-forcing. Storm flows represent a complex interaction between the wind stress (both orientation and magnitude), regional pressure gradients induced by wind set-up, and wave orbital motion. Forcing occurs in different dynamic zones, each with an individual response, all of which eventually converge to produce net sand transport. Absolute dating of selected cores reveals that in the mid-shelf (20-50 m water depths) area of the central Texas continental shelf, the sand bed deposited during Hurricane Carla in 1961 has survived over twenty years of physical and biological reworking. Here the Carla bed, and other discrete sand layers like it, stand an excellent chance of becoming a permanent part of the sedimentary record of the Texas shelf. For this reason, the discrete sand beds represent viable modern analogs to thin bedded sandstones present in ancient shelf sequences