The distribution and discrimination of shallow, authigenic carbonate in the Pliocene–Pleistocene Palomas Basin, southern Rio Grande rift
Nine types of authigenic carbonate are present in the Pliocene–Pleistocene alluvial-fan and axial-fluvial sediment of the Palomas half graben in the southern Rio Grande rift. Pedogenic and other vadose carbonate includes (1) pedogenic carbonate of stage II and stage III morphology underlying Bw and Bt horizons, (2) mudstones in which the carbonate nodules may be pedogenic or the result of shallow groundwater invasion of the vadose zone, (3) pedogenic calcic nodules and tubules in eolian sand, and (4) gully-bed cement of proximal-fan conglomerates formed by infiltration and evaporation of surface runoff. Shallow groundwater carbonate exists as (5) thin (30–50 cm), massive beds with an upper fringe of nodules and tubules precipitated at the water table and in the capillary fringe, (6) thick (1.5–3 m), massive beds deposited by lateral flow of groundwater or at springs, and (7) thin (30 cm), calcified root mats associated with near-surface, water-saturated sediment or springs. Phreatic spar cements (8) occupy the interstices of conglomerates and sandstones and locally exist as (9) oriented concretions. Groundwater carbonates are best developed near the toes of the large, hanging-wall–derived alluvial fans, whereas phreatic cement preferentially exists in footwall-derived, alluvial-fan conglomerates. Pedogenic carbonate is distinguished from groundwater carbonate by the association with other diagnostic paleosol horizons, a predominantly vertical arrangement of root traces, peds, desiccation cracks, and calcic tubules, and by gradational contacts. The δ 13 C and δ 18 O values are similar among pedogenic and shallow groundwater carbonate, although locally nodules in eolian sand and gully-bed cement have higher δ 18 O values, perhaps due to the effects of evaporation. Some phreatic cements may be distinguished from pedogenic and shallow groundwater carbonate by lower values of δ 13 C and δ 18 O. Authigenic carbonate in footwall-derived, alluvial-fan sediment has consistently higher values of δ 13 C than that in hanging-wall–derived sediment, which may reflect differences in vegetative type and/or density on either side of the basin.