Principal Stress Ratios in Cretaceous Limestones from Texas Gulf Coast ¹

Study of calcite twin lamellae abundance in cores of low-porosity limestones indicates that the average number of lamellae per millimeter increases by 10 for every 5,500 ft of depth through the 20,800-ft interval sampled. The conditions of lamellae formation have been determined by creep tests on a representative limestone at several differential stress levels and loading durations at appropriate confining pressures and temperatures. Results show that (1) twin lamellae do not form when the specimens are subjected to a uniform confining pressure alone, (2) the lamellae increase in abundance with increasing differential stress, and (3) the abundance of lamellae increases with the duration of loading. Creep tests at differential stresses of 1,450-5,800 psi over durations of 5,000 minutes indicate that the average number of lamellae per millimeter increases by 10/510 psi differential stress. From these results, we infer that the differential stress gradient in situ could not have exceeded about 510 psi/5,500 ft of burial and, because of the much longer time intervals in nature, this gradient probably was much less. Assuming a total vertical stress gradient of 1 psi/ft and a pore-pressure gradient of 0.5 psi/ft, we conclude that the probable maximum ratio of vertical-to-horizontal effective stresses (σ_v/σ_h = σ₁/σ₃) in the Texas Gulf Coast is about 1.2.