Facies Characteristics of Modern Size-Graded Shelf Deposits, Northwestern Gulf of Mexico: ABSTRACT

Abstract

General decrease in grain size of modern surficial sediments with increasing water depth across the continental shelf off south-central Texas suggests that the sediments are in equilibrium with the hydraulic regime during fair weather conditions. The stratigraphic record, however, indicates storm-dominated shelf sedimentation resulting in zonation of sedimentary structures, bed types, and bed sequences. Three facies are defined. Lower shoreface (water depth: 10 to 30 m): sediment has a significant fine sand component and occasional thin shell beds. Bioturbation is generally high with a diverse trace assemblage. Sand beds exhibit parallel to subparallel lamination with erosional basal contacts. Bedding relations define two major sequences: (1) thin, clean, laminated sand ^rarr thick, sandless, nonbioturbated mud ^rarr heavily bioturbated muddy sand with mottled tecture, and (2) thick, clean, laminated sand ^rarr interlaminated mud and sand ^rarr muddy sand with mottled texture. Both sequences are cyclic and result from variation in hydraulic energy related to storm events. Midshelf (30 to 120 m): clayey silt sediment containing little sand and no shell beds. Sediments are moderately bioturbated; trace diversity is intermediate. The only physical structures are occasional parallel-laminated sand beds. Bed-type diversity is intermediate. Moderately thick mud beds are separated by thinly laminated or bioturbated, storm-related sand beds. Outer shelf (120 to 200 m): clayey silt sediment (clay content greater than on the midshelf). Bioturbation and trace diversity are low. Bed-type diversity is low, and bedding relations simple. Thick, very thinly but faintly laminated, slightly bioturbated mud beds are separated by thin, heavily bioturbated, relatively clean sand beds (distal storm layers). End_of_Article - Last_Page 938------------