EVAPORITE DEPOSITION AND SOURCE ROCK EVALUATION OF A HOLOCENE-PLEISTOCENE CONTINENTAL SABKHA (SALT FLAT PLAYA) IN WEST TEXAS

摘要

The Salt Flat sabkha (dry playa) in West Texas - New Mexico is located within Salt Basin Graben, an intermontane basin at the foothills of the Guadalupe-Delaware Mountains. Major facies in this sabkha comples are: bajada, sand flat and mud flat sabkaha. The Bajada is a wedge of poorly-sorted, coarse grained alluvial sediment at the base of the Guadalupe Mountains and adjoining foothills. The sand flat is a dune complex of cross-bedded quartz and gypsum sands underlain by a Pleistocene lacustrine sequence. Sabkha flat sediments are mainly laminated varve-like sequences of alternating millimeter-scale dark and light laminae. In addition to the millimeter-scale laminated texture, the entire sequence is characterized by a superimposed coloration of light and dark layers some centimeters in thickness.;The sabkha flat sediments are a suite of evaporite and carbonate minerals of both primary and diagenetic origin. Dolomite and gypsum comprise over 70 percent of the sediment; the rest is halite, cacite, aragonite, and minor amounts of quartz. The nature and distribution of minerals in different units, coupled with radiocarbon dates: suggest that lighter gypsum-rich couplets were originally deposited during more saline evaporative phases of the brine lake water body. Freshening during more humid times in the Late Pleistocene diluted the surficial brine, enhanced density stratification of the water body and created anoxic bottom conditions favoring carbonate deposition.;Lake sediments were originally deposited under subaqueous conditions; a change in climate created more arid depositional setting reflected in Holocene deposition in the lake. The change in climate to more arid conditions had two effects on the playa. First, it converted the playa from a large perennial water body to a deflation lake. Second, it moved the water surface down into the sediment column where it became a water table. Lowering of the water table moved the laminated evaporite from a subaqueous setting into the capillary and vadose zones and converted the Holocene playa flats into regional discharge zones for basinward-flowing subsurface water. Pore brines evaporating from the capillary zone and flowing through the phreatic zone of the mud flat play an important role in the diagenetic alteration of the sediment matrix.;The nature, and abundance of the organic matter in the sabkha flat sediments, as well as overall depositional setting, indicate that shallow-water continental sabkha sediments, if preserved, may be a potential hydrocarbon source rock.