A GEOCHEMICAL ANALYSIS OF THIN BEDS OF KAOLINITE IN THE MID- TO UPPER-CONIACIAN, SAN VICENTE MEMBER, BOQUILLAS FORMATION, BIG BEND REGION, TRANS-PECOS, TEXAS

The upper, SanVicente Member, of the Boquillas limestone is a variously interpreted platform environment along the western margin of the Cretaceous Western Interior Seaway. Thick shale units alternate with thinly bedded marl to limestone couplets. Just below and within the Platyceramus platinus in the San Vicente Member there are occasional thin to very thin, internally cross-laminated beds of kaolinite. Utilizing XRD and SEM, samples from a wide range of localities have been confirmed as virtually pure kaolinite with minor detrital quartz. These beds range in thickness from 1 to 2 centimeters up to 8 to 10 centimeters. Limonite precipitate occurs below, above, and along joints. The thicker beds occur in the western portion of the study area, thinning to the east and absent to the north. The western portion of the study area is closest to the presumed Cretaceous shoreline. Kaolinite is unstable in marine environments, precluding in situ formation. In addition, there is a suggestion of very small scale cross bedding and other sedimentary features in some localities. These two facts and the thinning-eastward nature of the bedding seem to suggest a terrigenous influx of allochemical debris. A warm, humid, Gulf Coast-like environment beset by possible repeated catastrophic flooding events is postulated. The preservation of the clay could be explained by stagnant anoxic bottom water and rapid resumption of normal marine deposition.