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Bed Forms and Stratification Types of Modern Gravel Meander Lobes, Nueces River, Texas: Abstract
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 meander lobe sequences consisting predominantly of coarse gravel. The largest of these streams, the Nueces River, has a sinuosity index of 1.3 and a water surface slope of 1.8 m/km in the study area. Stream discharge is variable and has ranged from no flow to more than 17,000 m3/s.
Mean clast b-axis length for the 10 largest clasts at 13 sample sites ranged from 2.5 to 10.8 cm. Velocities of 2.7 to 4.4 m/s 1 meter above the streambed are required to transport these clasts. Stream velocities of these magnitudes occur about once in 8 years when discharge of the Nueces River exceeds 3,000 m3/s. Mean grain size of Nueces River alluvium ranges from 1.2 to 3.4 cm. At a flow depth of 1 m sediment of this size has a critical erosion velocity of 1.8 to 3 m/s. Velocities of this magnitude occur about once in 2 years when discharge exceeds 340 m3/s. Under these conditions flow is subcritical, with critical shear stresses on depositional surfaces ranging from 6.4 to 12.7 kg/m2.
Gravel clasts are imbricated and channel bed forms are predominantly transverse gravel bars with slip faces ranging up to 2 m high and wavelengths in excess of 100 m. Stratification includes graded planar crossbeds and horizontal beds. Lower lateral accretion face sediments are also predominantly transverse bars; upper lateral accretion face deposits occur as longitudinal gravel ridges deposited in the lee of vegetation and, less commonly, as chute bars. Near the upper limit of meander lobes where vegetation is heavy, mud and muddy sand occur as overbank deposits; in these deposits sedimentary structures other than desiccation cracks are rare.
Sedimentary sequences in gravel meander lobe systems deposited by low sinuosity streams are graded or nongraded horizontal beds and planar crossbeds overlain by mud and muddy sand interbedded with horizontally bedded gravels. These deposits in turn are overbain by overbank deposits of mud and muddy sand. Similar sedimentary sequences occur in the extensive Quaternary terraces that parallel the Nueces River.
Acknowledgments and Associated Footnotes
1 Bureau of Economic Geology, The University of Texas at Austin, Texas 78712
Copyright © 2009 by the Canadian Society of Petroleum Geologists