DEFORMATION OF MOLLUSK SHELL CASTS IN THE GLEN ROSE FORMATION IN MILLS COUNTY, TEXAS

The Late Cretaceous Glen Rose Formation, exposed in river-cut outcrops in Mills County, TX, is characterized by interbedded chalks and marls, which often contains deformed mollusk shell casts. This formation has been interpreted as a shallow marine carbonate deposited nearshore, and diagenesis studies have estimated shallow burial depth. In order to assess depth of burial for this unit, which has previously only been applied to shales, collections of bivalve casts were made and utilized for this study. Measurements of the deformed shell casts were taken, including total length and width. These measurements are used to estimate average warping of the bivalve post-burial. Clay spheres were then made, measured, and then put under pressure to mimic compaction by burial of the shell casts. Results of the clay sphere dimensions after compaction were similar to that of the shell casts. The amount of cross sectional area and total compression were calculated for the spheres and estimated for the shell casts. The results for cross sectional area ranged from 18.3 square centimeters to 25.5 square centimeters for both casts and spheres. The compression ranged from to . For this study, shales and clays are assumed to be more likely to compact under pressure than other types of sediment. Porosity affects the rate at which platelets in the shale or clay can compact or compress. The shortening of the fossil casts in this experiment can aid in the understanding of how they may have compressed, as well as the volume. In this experiment the constant volume model was used to estimate the compressibility of the shell casts. This experiment could be further investigated, using calculations for porosity of the shale and clay used for the spheres, and by using Athy’s equation. It is important to understand the compression and deformation of these shell casts, as burial depth can be then calculated and used to understand the rate of deformation, how shales are likely to rearrange platelets to allow such compressibility, unlike carbonates.