Print this pageLandslides above the Walnut and Paluxy Formation’s Contact
By
Mahipal Jadeja (UTA, Geology Department), Dr. John Wickham (UTA, Geology Department) & Dr. Laureano Hoyos, Jr. (UTA, Civil & Env. Engineering Department)
Recent landsliding occurs in Lower Cretaceous sedimentary rocks in north Texas. The failure plane lies near the contact between the overlying Walnut Formation of the Lower Fredricksburg Group and the underlying Paluxy Formation of the Trinity Group within the Comanchean Series. The Walnut is predominantly an indurated fossiliferous (oyster rich) limestone with interbedded clay seams, and is an aquitard. The lower 5-6 feet of the Walnut consists of less indurated expansive clay seams. The Paluxy consists of silty and clayey sands and sandstone. The Paluxy has higher permeability, and is an economic aquifer in north Texas. The less indurated expansive clay seams at the base of the Walnut clay could behave like cohesive soils. With a high Plasticity Index, those layers would loose significant strength if the water table in the underlying Paluxy sand rises and saturates them. Another possible failure mechanism is a pore pressure increase in the Paluxy reducing effective stress.
Laboratory investigations to determine the mechanism(s) that induce the landsliding include Direct Shear and Plasticity tests on undisturbed samples from the slide zones. Grain size distribution was determined by sieve analysis, and X-ray diffraction was used to confirm the presence of expansive clays. Data from various lab tests were used to generate computer models of possible natural conditions during the sliding to test the various failure mechanisms.AAPG Search and Discovery Article #90010©2003 AAPG Southwest Section Meeting, Fort Worth, Texas, March 1-4, 2003