New Mexico Geological Society Annual Spring Meeting — Abstracts

THe Upper Cretaceous rocks of the southeastern San Juan Basin, New Mexico resulted from a series of transgressions and regressions. Seventy eight samples were chosen as representative of the clay-bearing Upper Cretaceous sequence, Mulatto. Tongue of the Mancos Shale through the Fruitland Formation. and analyzed for clay mineralogy, whole rock mineralogy,and whole rock geochemistry. Whole rock mineralogy and geochemistry show only slight variations in the stratigraphic interval. However, the Fruitland Formation shows a relative decreases in quartz and Al₂O₃, and an increase in sodic plagioclase and Na₂O. The principal clay mineral species is smectite. This may suggest a new sediment source in Fruitland times.

Vertical and lateral variations in clay mineralogy in the other shale units were observed. Kaolinite generally was observed increasing from marine to nonmarine facies (Menefee Formation); and, illite, chlorite. and random interstratified illite-smectite increased towards marine depositional sites (Mancos Shale, La Ventana Tongue, and Lewis Shale). Clay mineral assemblages follow the prescribed pattern set forth in lateral distributions study by Parham (1966).

Lateral variations in clay mineralogy within stratigraphic units can also be ascribed to particle size sorting, e.g. upper Menefee Formation lithologies. Finer-grained humic shales (distal from channel sands) contain less kaolinite and more platy clay minerals (interstratified illite-smectite) than coarser-grained shales and mudstones (proximal to channel sands). This is the result of particle size sorting in the fluvial-paludal depositional envirorunent. Kaolinite in the upper Menefee shales is probably related to provenance, diagenesis or both.