Development of soils on terraces associated with the Brazos River in Young and Throckmorton Counties, Texas

Abstract

The terrace system and associated soils of the Brazos River in north-central Texas were studied to determine (1) the stratigraphy and origin of the terrace deposits; (2) the landscape distribution and classification of soils associated with the terrace system; and (3) the probable mode of pedogenesis for these soils. Three terraces, both paired and unpaired, were identified. All three terraces are bedrock-cut, alluvial fill terraces. The upper, older terrace does not fit into the terrace "stair-step" landscape setting as in most major rivers. This terrace is an erosional remnant adjacent to the contemporaneous floodplain with a dissected level between the terrace and the bedrock upland. The lower two terraces fit into the typical terrace "stair-step" landscape setting. The stratigraphy and differences in erodibility of the underlying bedrock during the erosion and deposition cycles of terrace formation gave rise to this unusual terrace sequence. The upper terrace is elevated 23 to 30 m above the river. It's alluvial composition is primarily siliceous sands and gravels. Upper terrace units on the east side of the river are mantled with loamy and silty eolian sediments. Severe dissection is prominent on these upper surfaces. The middle terrace at 12 to 23 m above the river contains two distinct stratigraphic units; an upper moderately-fine textured surface and a very gravelly moderately-coarse lower unit. Carbonates are dominant in the gravels of the lower unit. The shallow thickness and slope gradient with the surrounding upland suggested possible truncation of the middle level surface. The lower terrace (3 to 12 m above the river) was the most stable surface. Its upper parent alluvium was moderately fine-textured underlain by various thin strata dominated by siliceous components.