Monday, 6 October 2008: 2:15 PM
George R. Brown Convention Center, 320F
The Wilkins Peak Member contains nine discrete, composite siliciclastic bedsets, interpreted as fluvial to lacustrine deltaic deposits. One of these (the D marker) contains evidence for pedogenic modification of its original depositional fabric. In order to objectively describe variable pedogenic effects, a rating system was used to rank the intensity of pedogenic modification on a scale of 1-4, from least developed to most developed. Pedofacies 1 is characterized by minimal development, and consists of a thin sandstone bed overlain by a thin, pedogenically-altered horizon that has formed above it. Together these form a sequence of A-over-C horizons. These are common in the overbank environment. Pedofacies 2 tends to have thicker profiles than Pedofacies 1, and also shows few pedogenic features, including root traces and faint horizons. Pedofacies 3 is characterized by a greater degree of pedogenic alteration but these do not have thick B horizons. They can contain roots and bioturbation, as well as filaments. Pedofacies 4 shows the greatest amount of pedogenic alteration. These contain root traces, plant material, pseudo-anticlines, siderite nodules, large shrink-swell cracks filled with finer material, and abundant smectite, the presence of which has been confirmed by XRD analysis. Horizons are not clearly evident, possibly due to the churning that is common among pedogenically-altered strata experiencing repeated swelling and shrinking. These pedofacies resemble Vertic paleosols. Pedofacies in the D marker vary both laterally and vertically, in relation to channel proximity and temporal facies changes, and the intensity of pedogenesis (higher pedofacies rating) represents periods of relative stability of the land surface.