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Memoir 18—Geomorphic surfaces and surficial deposits in southern New Mexico

By R. V. Ruhe, 1967, 66 pp., 15 tables, 31 figs., 2 sheets, 1 index.

Geomorphic surfaces ranging from post late Kansan-Illinoian to historic time, and the related surficial deposits are described for an area near Las Cruces that stretches westward from the Organ Mountains, crosses the Rio Grande fault-controlled trench, to the edge to the Robledo Mountains horst. Geomorphic surfaces are grouped as alluvial fans, piedmonts, aprons, and valley-border surfaces. This complete study of landscapes and soils of a desertic area is unique, and is applicable to other arid and semiarid regions of the world.

  Bedrock types, which served as sources for the surficial sediments, are Paleozoic limestone, dolomite, sandstone, and shale, Tertiary volcanic rocks, and monzonitic intrusives. The area is block-faulted with major faults along the mountain fronts but that in places strike across country disrupting old geomorphic surfaces.

The Doña Ana Mountains are an ellipsoidal structural dome whose geographic parameters can be described by an ellipse equation. Their major axis parallels the trend of the fault-controlled Rio Grande trench and the strike of the Robledo fault. Morphometric features conform to structural components of the dome. Displacements of as much as 200 ft along faults and axes of the dome took place in late Pleistocene time. Minor tectonic adjustments have continued into the Recent.

Geomorphic surfaces are grouped as alluvial fans, piedmonts, and aprons that adjoin the mountain fronts, basins and scarplet surfaces, and valley-border surfaces that are adjacent to the Rio Grande. Profiles of alluvial fans are described in terms of elevation and distance. Basins and adjoining areas are also described. Radii of curvature of contours and centers of radii of these land forms have specific geometry. Sediment distribution from source areas is related to the spatial geometry.

The geomorphic surfaces range in age from historic time to post late Kansan-Illinoian. The oldest surface is dated relative to a subsurface vertebrate fauna and younger surfaces by radiocarbon. A valley-border surface and its mountain front analogue are multicycle in origin and are 1,100-4,900 years old. The late Wisconsin Picacho surface is more than 9,550 years old. Distribution of the surficial deposits' rock constituents relates to the geometry of land forms and in places is independent of the present drainage net. Clay-mineral assemblages are similar; consequently, cation-exchange capacities of fine-earth fractions are related to amount of clay in the sediment. Surficial deposits commonly are layered and separated by paleosols that are usable stratigraphically in delineating and relating bodies of sediment.     On the fans and piedmont, organic carbon and carbonate content and depth to carbonate horizon are related to the orographic-climatic regimen controlled by the rise up the mountain front. In the valley-border surface sequence, increase in redness of the soil B horizon, amount of clay in and thickness of the B horizon, and amount of carbonate in and thickness of the carbonate horizon are related to age of geomorphic surface. Complications are introduced by possible climatic changes of the pluvials and interpluvials.

The origin of carbonate horizons is in part pedologic, in part the result of ground-water deposition, and in part the result of surface-water deposition. In low-calcium-content sediments, the source of carbonate is eolian dust that falls on the ground surface, is dissolved and transferred downward, and precipitated in a subsurface zone. Detailed analysis points out the discrepancies that occur and that are to be expected in the radiocarbon dates of the inorganic carbon of the carbonate in caliche.

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