New Mexico Geological Society Annual Spring Meeting — Abstracts

The San Juan Basin is a Laramide-age broken foreland basin in northwestern New Mexico. The basin contains sediment deposits which record paleodrainage patterns and source areas located in mountains to the north and northwest, from late Cretaceous through early Eocene time. However, while diverse Paleocene vertebrate fossils found in the Nacimiento Formation have been extensively studied, the corresponding petrology has not been documented. This petrologic study encompasses four localized collection areas in the southwestern and southern perimeter of the San Juan Basin based on the stratigraphic work of Williamson and Lucas (1992), and Williamson (1994).

The depositional environment of the formation has been interpreted as a poorly drained alluvial floodplain (Williamson, 1994). The forty-three samples studied range from strongly-cemented arkose and lithic arkose channel sandstones interbedded with mudstones, siltstones, and thin layers of lignite. The sandstones are the focus of petrographic study, while fine-grained samples supply additional information. Lithics include unaltered plutonic and volcanic fragments, volcanic chert, clay-cemented sedimentary Ethics, and grains interpreted as highly altered lithics.

The Kutz Canyon section, representing the most complete vertical section sampled, ranges from bottom to top from zeolite (analcime) and gypsum cements in the"main body" member, to clays and discontinuous calcite in the "unnamed" member, to pedogenic silcretes and
related silica-cemented units in the Escavada Member. In the Arrollo Chijuillita Member of the Mesa de Cuba section calcite and clays are the dominant cements, which is distinct from the temporally equivalent "main body" member to the west. X-ray diffraction (XRD) results indicate that smectite and kaolinite are the dominant clays.

The results are consistent with provenance identified by Smith (1988, 1992) and Lucas and Williamson (1992) as drainages extending from surrounding uplifted Precambrian granite and associated strata. These areas include the Four Comers Platform, Brazos-Sangre de Cristo
Uplift, Nacimiento Uplift, and the San Juan Uplift and Tertiary volcanics.