LATERAL VARIABILITY OF AXIAL-FLUVIAL LITHOFACIES AND AUTHIGENIC CARBONATE IN THE PLIO-PLEISTOCENE CAMP RICE FORMATION, HATCH-RINCON GRABEN, SOUTHERN RIO GRANDE RIFT

Lateral variability of pebbly sand channels, floodplain fine sands, silts, and mudstones, and authigenic carbonates is evident in axial-fluvial strata of the late Pliocene-early Pleistocene Camp Rice Formation in the Hatch-Rincon graben, southern Rio Grande rift, New Mexico. Five stratigraphic sections, averaging 35 m thick and spaced approximately 1.5 km apart, were measured along the northern flank of Rincon Arroyo, which bisects the northern half of the graben. The sections were correlated with (1) a lower carbonate bed of probable geothermal origin (white bed), that is traceable between the middle three sections, (2) a 1.6 Ma pumice-clast conglomerate that is present at all five sections, and (3) the constructional top of the formation, the La Mesa Surface, which connects all five sections. Reversal magnetostratigraphy at one of the sections establishes an age range from the Reunion subchrons (2.15 Ma) to near the Matuyama-Brunhes chron boundary (0.78 Ma).

The relative abundance of channel sands and their multistory character increase toward basin center, while near the basin edge channel sands are restricted to isolated ribbons. Channel recurrence intervals, calculated as the number of channels divided by the age of the stratigraphic interval, is 6.5 in the basin center and 2.2 near the edge. The ratio of floodplain mudstone to fine sand and silt increases toward the basin edge, as do the number of calcic paleosols, the frequency of soil welding, and the maturity of the paleosols expressed as a greater number of K horizons. Assuming an average time of development of 10 kyrs for stage II and 100 kyrs for stage III calcic horizons, approximately half of the stratigraphic history near the edge of the basin was taken up in soil formation. The number of groundwater carbonates and paleosols with gleyed horizons also increase toward the basin edge.

The ancestral Rio Grande spent more time near the center than along the northern edge of the basin. This can be explained by greater overall subsidence along the basin axis or by a see-saw motion of the basin related to independent histories of movement of the border faults. A high water table is responsible for more groundwater carbonates and gleyed horizons near the basin edge, and may reflect lateral, southward flow of groundwater into the basin from northern recharge areas.