One hundred clay-sized samples selected from two cores and two measured sections of the Lewisville Member, three samples from the overlying Arlington Member, and five from the underlying Dexter Member of the Cretaceous Woodbine Formation in eastern Tarrant County, Texas, were analyzed. The amounts of each clay mineral present were calculated from X-ray diffractograms of natural, ethylene glycol-saturated, and heated samples. The average clay mineral assemblage was 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 association where admixing of fresh and saline water creates a transition zone. Vertical changes in clay-mineral distributions correlate well with the changes in depositional environments from fluvial to swamps and bays and finally to marine by the end of Lewisville deposition. Apparently for the Lewisville Member specifically, and probably for the Woodbine Formation in general, clay-mineral distributions are environmentally significant.

The predominant expandable lattice clay in the Lewisville Member is Ca-Mg montmorillonite. In his studies of the Woodbine in east-central Texas, Beall found these exchangeable cations to be most abundant in nearshore sediments with Na + cation content increasing basinward, reflecting the more marine character of the sediments.

Kaolinite is the most abundant clay mineral in the five Dexter Member samples and in samples from the lower 5 to 10 ft of the Lewisville Member. Several lenticular channels, extensive sands, and interbedded silts and clays provide evidence that the Dexter Member is part of the meander-belt facies. The basal Lewisville marks the transition between a fluvial environment below and swamp and bay environments above.

Randomly interstratified montmorillonite/illite is present throughout the sampled intervals, but no clear relation between this type of clay and depositional environment is apparent, as the semiquantitative method used in this investigation is useful only in determining the relative amounts of the end members of the mixed-layer clay.

An increase in kaolinite and expandable-lattice clays with respect to the average composition in the upper Lewisville and in three Arlington Member samples reflects an increase in transport energy and a westward shift in the strandline.

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