Radiostratigraphy and Heavy Mineral Content of the Weches Formation (Eocene), Nacogdoches County, Texas

Abstract

ABSTRACT The Weches Formation of eastern Texas is a mudstone containing green sand-size clay aggregates. A vertical sequence of 23 samples were analyzed along an excavated cliff to determine radionuclide content. Samples average 5 ppm uranium and range from 2 ppm to about 7 ppm. Thorium averages 20 ppm and ranges from 13 ppm to almost 36 ppm. The gamma ray spectrometer employed exhibits an uncertainty of about ±10 percent (one standard deviation) at the levels measured. As there are few heavy mineral studies of mudstones, one sample of the Weches mudstone was selected for heavy mineral separation and point counting of grain mounts. A total of 2606 grains were counted giving mineral percentages of zircon (28.8), garnet (17.9), tourmaline (10.5), titanite (8.7), apatite (7.6), staurolite (6.4), green hornblende (5.2), epidote (5.1), sillimanite (2.8), monazite (2.2), kyanite (1.9), basaltic hornblende (1.5), and biotite (1.3). In addition actinolite, spinel, rutile, and collophane were observed but not counted. Previous studies found a similar heavy mineral suite in the underlying Queen City Formation, but in different proportions. Differences in heavy mineral percentages probably reflected different water flow regimes at the time of deposition. Heavy minerals in the Weches and Queen City formations are from the same general source area. Measured radionuclide ratios are similar to granitic ratios and suggest that detrital heavy minerals,particularly zircon and monazite, are the main site of uranium and thorium and their decay products in the Weches Formation.