58 Stratigraphy (Posters)

FIRST RECORD OF PALEOSOLS FROM THE CENOMANIAN OF TEXAS; WOODBINE FORMATION AT LAKE GRAPEVINE, DENTON, CO

Paper number: 58-3
Program: Technical Sessions
Time: 1:30 PM – 5:30 PM

Authors

Abstract

We report the first record of paleosols from the Cenomanian (Mid – Cretaceous) of Texas. The paleosols were mapped while conducting a facies analysis of Woodbine Formation outcrops along the north shore of Lake Grapevine, Denton Co. The paleosols occur in the lower and upper delta plain deposits of the Lewisville Member of the Woodbine Formation. The lower delta plain paleosol is a poorly developed soil horizon; a Protosol. The Protosol has a silt matrix with a Munsell Chart hue of 10YR 7/1 – 7/2; light gray to brownish gray. Sulphur stains in the Protosol have a hue of 10YR 6/8 – 2.5YR 8/6; brownish yellow to olive yellow. Roots occur sporadically and vary in size from 0.5 cm to 1.5 cm in length. Roots are exposed as both oblong circles and as cylindrical features extending down vertically. The Protosol is poorly developed, as the sedimentation rate was likely great compared to plant growth. The root traces are relatively small and the beds lack sufficient organic matter to be considered a well developed “A” soil horizon.

The upper delta plain paleosol is a well developed Histic Gleysol that grades laterally in section into a Histic Vertisol. The Histic Gleysol is composed of silt and fine sand with a mottle Munsell Chart hue of 2.5YR 6/2; light brownish gray. Unique to this paleosol bed is a heavy concentration of siderite. The siderite has a hue of 5YR 4/6 to 2.5YR 3/6; yellowish red to dark red. The Histic Gleysol is heavily rooted, rich with organic matter such as plant fragments and coal. The root traces are vertical, branching and longer than those of the Protosol, 2 - 4 cm in length. The Histic Gleysol is more massive and bedded with large siderite concretions. Laterally in section the Histic Gleysol grades into a Histic Vertisol. The bed becomes much thicker and sulfuric at the surface, siderite concretions are not seen and the bed is much less rooted. The Vertisol is cross thatched with slickenslides and swale structures, a morphology common in soils that experience fluctuating rates of either precipitation or flooding. Both paleosols occur in the delta plain of a prograding Mississippi style delta system. These paleosols record fluctuating environmental conditions that left stress structures recorded in the soil development.