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 Plioccne-Pleistocene alluvial-fan and axial-fluvial sediment of the Palomas half graben in the s outhern Rio Grande rift. Pedogenic and other vadose carbonate includes (1) pedogenic carbonate of stage II and stage III morphology underlying Bm 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) pedog enic calcic nodules and tubules in eolian sand, and (4) gully-bed cement of proximal-fan conglomerates formed by infiltration and evaporation of surfa ce runoff Shallow groundwater carbonate exists as (5) thin (30-50 cm), mass ive beds with an upper fringe of nodules and tubules precipitated at the wa ter table and in the capillary fringe, (6) thick (1.5-3 m), massive beds de posited 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 conglomerat es and sandstones and locally exist as (9) oriented concretions. Groundwater carbonates are best developed near the toes of the large, hangi ng-wall-derived alluvial fans, whereas phreatic cement preferentially exist s in footwall-derived, alluvial-fan conglomerates. Pedogenic carbonate is d istinguished from groundwater carbonate by the association with other diagn ostic paleosol horizons, a predominantly vertical arrangement of root trace s, peds, desiccation cracks, and calcic tubules, and by gradational contact s, The delta(13)C and delta(18)O values are similar among pedogenic and sha llow groundwater carbonate, although locally nodules in eolian sand and gul ly-bed cement have higher delta(18)O values, perhaps due to the effects of evaporation. Some phreatic cements mag be distinguished from pedogenic and shallow groundwater carbonate by lon er values of delta(13)C and delta(18)O . Authigenic carbonate in footwall-derived, alluvial-fan sediment has consi stently higher values of delta(13)C than that in hanging-wall-derived sedim ent, which may reflect differences in vegetative type and/or density on eit her side of the basin.