New Mexico Geological Society Annual Spring Meeting — Abstracts

Field data gathered from measured sections of the upper 40 m of the Camp Rice Formation (late Pliocene-early Pleistocene) in the Hatch-Rincon basin of the Southern Rio Grande rift, south-central New Mexico, reveal significant lateral variability of axial-fluvial lithofacies and paleosols. Five sections of the Camp Rice Formation, spaced 1 to 1.5 km apart, were measured along the easten edge of Rincon Arroyo. The line of sections is oriented perpendicular to the basin axis, extending from basin center to edge, and correlation between sections is possible using three marker horizons; the constructional top of the formation (La Mesa Surface), a 1.6 Ma pumice conglomerate, and a meter-thick micrite bed deposited by shallow geothermal ground-water flow. The Camp Rice interval is dated by reversal magnetostratigraphy between about 2.22 and 0.8 Ma.

Lithofacies and paleosols exhibit variability relative to their positions within the basin. The section closest to the basin center consists primarily of multistory channel pebbly sands, while the relative abundances of floodplain fine sands and mudstones increase toward the basin edge. Paleosols, which display Bt or Bw horizons overlying Bk or K horizons, increase in number and maturity toward the basin edge, as well as showing evidence of gley conditions associated with a shallow, fluctuating water table. Also more common along the edge of the basin are thin (<0.5 m) beds of micrite precipitated near the water table. Near the basin center, channels arrived and subsequently avulsed away four times between 2.2 and 1.0 Ma and once in the final 200 ky of deposition, while only three channels occupied the basin edge between 2.2 and 1.0 Ma and only floodplain deposition and paleosol formation occurred during the final 200 ky of deposition.