Southwest Section AAPG Convention. March 1-4,
2003.
ABSTRACT: Landslides Above the Walnut and Paluxy Formation?s Contact
By
Mahipal Jadeja1, John Wickham1,
and
Laureano Hoyos, Jr.2
1UTA,
Geology Department
2UTA, 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.
Copyright © 2004 by Fort Worth Geological Society. All rights reserved.
