New Mexico Geological Society Annual Spring Meeting — Abstracts

The limited distribution of outcrops and paucity of petrographic analyses of Upper Mississippian rocks in the southwest has made detailed paleogeographic reconstructions of the Late Mississippian tenuous at best. New data from the Paradise Formation provides information about the paleogeography and paleoclimates of the region.

Detailed petrographic analysis of the Paradise Fonnation revealed a surprising volume of detrital quartz. The quartz is found both as individual sandstone beds and as disseminated grains throughout the carbonate portion of the section. The average grainsize of the quartz is in the very fine sand to coarse silt (0.06 mm) range. The quartz is monocrystalline and angular to subrounded in shape, some grains are tabular. Sandstones and siltstones are compositionally and texturally mature. A lack of both a clay and finer silt fraction in conjunction with the degree of sorting and rounding of the very fine-grained to silt-size quartz suggests an eolian origin for the clastics.

The association with marine carbonates and the presence of marine fossils within the sandstones reflects the reworking of sands in a shallow marine setting. Delivery of these clastics to the marine environment is possible by either direct or indirect means. Finer grained sand and silt was delivered directly to the Paradise sea by eolian activity, where the material was incorporated into the carbonate sediments. Indirectly, fine sand and silt was delivered to the marine environment along the strandline by marine marginal dunes and fluvial systems meandering through the dunes. Sands and silts delivered to the sea by coastal dunes and fluvial systems were reworked by storms and marine currents and distributed as thin sand and silt bodies along the shelf. The distribution of sand and silt along the shelf was further governed by variations in relative sea level. During times of relative sea-level rise (highstands) the clastics were trapped closer to shore on the shelf and during times of relative sea-level fall (lowstands) the clastics would by-pass the inner shelf and be deposited in a more basinward position.

Paleogeographic reconstructions of the Late Mississippian indicate that the Pedregosa basin would have been in a geographic position consistent with global wind patterns and weather conditions conducive to the evolution of arid to semiarid lands. This would create a landscape not unlike the modem North African and the Persian Gulf region where fine clastics are delivered to offshore settings by eolian activity. We favor this to be a reasonable modern analog to the Late Mississippian landscape in southwestern New Mexico and southeastern Arizona.