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JOBE, KELLY, E. B. LEDGER, PATRICIA SHARP, and M. C. CROCKER, Stephen F. Austin State University, Nacogdoches, TX

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

The Weches Formation of eastern Texas is a mudstone containing green sand-size clay aggregates. A vertical sequence of 43 samples was analyzed along an excavated cliff to determine radionuclide content. Samples average 5 ppm uranium and range from 2 to about 7 ppm. Thorium averages 20 ppm and ranges from 13 to almost 36 ppm. The gamma-ray spectrometer employed exhibits a certainty of about +/-10 % (one standard deviation) at the levels measured.

Heavy mineral studies of mudstones are few, so 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%), monatite (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 reflect different water-flow regimes at the time of deposition.< P>

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.

AAPG Search and Discovery Article #90989©1993 GCAGS and Gulf Coast SEPM 43rd Annual Meeting, Shreveport, Louisiana, October 20-22, 1993.