Morphology and Sedimentology of a Central Brazos River Point Bar, Boxley Bend, Brazos County, Texas: ABSTRACT

Abstract

A reconnaissance of Brazos River point bars reveals great variety in their morphology and sedimentology, owing to the complex interaction of climate, local hydrography, and local sediment sources. This paper presents the first in a series of studies of point bars of the Brazos River and concentrates on the Boxley Bend point bar near Snook, Texas. The summer morphology of the point bar is complex, consisting of an upper and lower tier separated by a scarp but connected by a central ramp. The surfaces of the lower tier and ramp display numerous large gravel bars and shallow scour pools as well as low-amplitude sand waves, ripples, and current lineations. In cross section, the lower tier and ramp are characterized by (1) trough cross-bedded, medium to fine sand produced by megaripple migration during floods; (2) massive gravel beds, the product of formation and migration of gravel bars during floods; and (3) fine rippled sand and clay drapes formed during falling flood. The surface of the upper tier displays ripple-laminated eolian sand and deflation deposits of mud clasts. In cross section, the upper tier is characterized by thick beds of horizontally stratified fine sand. During the winter, the entire surface of themore » point bar is covered by large (2 m high) transverse bars separated by deep scour troughs. Transverse bars migrate into the troughs to produce a sequence of fine sand with backflow ripple cross-stratification overlain by thick beds of tabular cross-bedded medium sand. The transverse bars appear to be transitory features with little net effect on sedimentation, because they are removed from the surface of the point bar by summertime.« less