Petrology of the Lower Arroyo Penasco (Mississippian), Taos County, New Mexico

The Lower Arroyo Penasco, which is probably of Mississippian age, is a relatively thin elastic unit composed of medium-and coarse-grained sand and gravel. It rests on a topographically irregular Precambrian crystalline basement or its regolith and grades conformably upward into a complex carbonate sequence. Nowhere in the studied field area does the lower part of the formation exceed fifteen meters in thickness. The dominant mineral is quartz which comprises over 95% of the framework material, thus making the sandstone categorically a quartzarenite. The unit was deposited within 10° or 15° of the Mississippian equator in a marginal system of high-energy braided streams, beaches, and tidal flats. This system alternately underwent periods of monsoonal flooding and aridity. An authigenic suite of minerals began forming soon after deposition and an episode or episodes of grain embaying and solution fracturing. Large amounts of quartz first precipitated as overgrowths. A complex assemblage of minerals including illite, chlorite, calcite, dolomite chert, chalcedony, pyrite, and apatite then followed, not necessarily in the preceding order. Microcline, which in some areas may have comprised 5% of the sandstone was replaced by illite, chlorite, or calcite; some may have been weathered and leached out of the system. There is practically no porosity. This sandstone with very few fossils grades into a calcitic, dolomitic, and dedolomitic carbonate unit, the Upper Arroyo Penasco.