New Mexico Geological Society Annual Spring Meeting — Abstracts

In the Plio-Pleistocene Palomas basin of southern New Mexico, groundwater and pedogenic carbonate are commonly found in both the footwall-derived and hanging wallI derived facies of the Palomas half-graben. Site investigation and sample collection on a basin-wide scale has resulted in the identification of features by which nonpedogenic and pedogenic carbonate may be differentiated. For both the hanging wall-derived and the footwall-derived facies carbonates of pedogenic and nonpedogenic origin is developed within gravel, sand, silt, and clay parent material. The thickness of the carbonate beds varies but, in general, groundwater carbonate beds are thicker than the corresponding pedogenic carbonate horizons. Subtle but discernible changes in the vertical fabric help differentiate between both forms of carbonate. In the case of nonpedogenic carbonates the base of the indurated carbonate bedding is sharply defined and parallel to the overall bedding, resulting in a laterally extensive sheet-like deposit. This is a reflection of an origin associated with either a groundwater table close to the surface or springs that issue near fan toes. In contrast, pedogenic carbonate exists as nodular or massive horizons with gradational bases. Unless eroded before burial, pedogenic carbonate is overlain by the argillic B horizon characterized petrographically by clay coats (argillans) around grains andlor peds. The nature of the sediment exerts a subtle influence on the presence or absence of carbonates and their type. In the Palomas basin the grain size of the sediment is an important factor such that groundwater carbonates are typically associated with the coarsest sediment. Capillary rise exerts a greater influence on finer grained soils and was an important process in the formation of pedogenic carbonates in the Palomas basin, although it may also have influenced groundwater carbonates if the groundwater table was close to the surface.