Abstract: Bed Forms and Stratification Types of Modern Gravel Point Bars, Nueces River, Texas

Thomas C. Gustavson

All major streams draining the southwestern flank of the Edwards Plateau in south-central Texas transport large volumes of gravel and sandy muddy gravel, and are developing predominantly coarse-gravel meander-belt sequences. The largest of these streams, the Nueces River, has a sinuosity index of 1.3 and a water-surface slope of 1.9 m/km in the studied reaches. Stream discharge is variable and has ranged from no flow to more than 17,000 cu m/sec.

Mean clast b-axis length for the 25 largest clasts at 13 sample sites ranged from 4 to 10 cm. Velocities of 1.8 to 3 m/sec 1 m above the stream bed are required to transport these clasts. Stream velocities of these magnitudes occur about once every 2 to 3 years when discharge of the Nueces River exceeds 500 cu m/sec. Under these conditions flow is subcritical with critical shear stresses on bar surfaces ranging from 3.0 to 10.2 kg/sq m.

Gravel clasts are imbricated and channel bed forms are predominantly transverse bars with slip faces as much as 2 m high and wavelengths as much as 30 m. Stratification includes graded planar cross-beds and horizontal beds. Lower point-bar sediments also are predominantly transverse bars; upper point-bar deposits occur as longitudinal gravel ridges deposited in the lee of vegetation and less commonly as chute bars. Near the upper limit of point bars, where vegetation is heavy, mud and muddy sand are present as overbank deposits; in these deposits sedimentary structures other than desiccation cracks are rare.

Sedimentary sequences in gravel meander-belt systems are graded or non-graded horizontal beds and planar cross-beds overlain by mud and muddy sand interbedded with horizontally bedded gravels. These deposits, in turn, are overlain by overbank deposits of mud and muddy sand. Similar sedimentary sequences are present in the extensive Quaternary terraces that parallel the Nueces, Frio, and Guadalupe Rivers.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA