ABSTRACT

One-hundred eight clay-sized samples from the Arlington, Lewisville, and Dexter members of the Cretaceous Woodbine Formation in eastern Tarrant County, Texas were found to have an average clay mineral assemblage of approximately 28 percent illite, 36 percent kaolinite, and 36 percent expandable-lattice clays.

Floral and faunal assemblages and sedimentary structures demonstrate that the Lewisville Member in the Arlington area is part of a deltaic interdistributary complex where admixing of fresh and saline water creates a transition zone. It appears that for the Lewisville Member specifically, and probably for the Woodbine Formation in general, vertical changes in clay mineral distributions correlate well with changes in depositional environments.

The predominant expandable-lattice clay in the Lewisville Member is a Ca²⁺ - Mg²⁺ montmorillonite. Kaolinite is the most abundant clay mineral in samples from the Dexter Member and the lower part of the Lewisville Member.

Lenticular channels, extensive sands, and interbedded silts and mudstones indicate that the Dexter Member is part of a meander-belt facies. The basal Lewisville marks the transition between a fluvial environment and swamp and bay environments.

Randomly interstratified illite-montmorillonite occurs throughout the sampled intervals and no clear relationship between this clay mineral and depositional environments is apparent.

An increase in abundance of kaolinite and expandable lattice clays in the upper Lewisville and lower Arlington members reflects an increase in transport energy and a westward shift in the shoreline.