New Mexico Geological Society Annual Spring Meeting — Abstracts

The Estancia Basin Playa Complex, located about 40 km east of the Manzano Mountains in central New Mexico, consists of ~87 ephemeral salt lakes (playas) and associated lunette dunes, which together encompass an area of approximately 400 km². The playas occupy ~12% of this area (50 km²). The playa basins are mid-Holocene eolian deflation features cut into the latest Pleistocene lake-beds which were deposited on the floor of pluvial Lake Estancia in the center of the Estancia basin. The lunette dunes, adjacent to and downwind from the playas, are composed of material deflated during playa basin formation, mostly gypsum and clay. These dunes are up to one km wide, with elevations ofup to 50 meters above the adjacent playa surface. Depth of post deflation playa-fill sediment in different playas ranges from 0.4 -2.4 meters. Radiocarbon dates from gastropod shells taken from the base of one of the dunes, and from cysts of the brine shrimp Artemia salina, taken from the bottom-most sediment of one of the playas, both indicate ~7000 years BP as the time of playa basin deflation.

The playas are zones of groundwater discharge for the hydrologically closed Estancia basin (approximately 5,000 km²). The Estancia basin (central valley floor, town of Estancia) receives 30 cm ( 12 inches) of precipitation and experiences 152 cm (60 inches) of potential evaporation in an average year. Groundwater discharging into the playas is rich in the dissolved ions Na⁺, Ca⁺⁺, Mg⁺⁺, SO₄, CI^-, HCO₃, evaporative concentration of these ions results in precipitation of gypsum, calcite, halite, dolomite, and to a lesser extent bloedite, in and on the playa sediment. Playa sediment consists of 30-50 % gypsum, 2-10 % carbonate (calcite and dolomite), and the remainder silt and clay. Efflorescent crusts on playa surfaces is mainly halite and thenardite. Crystal morphology of gypsum crystals which grow in the sediment at a depth of 30-40 cm is controlled by groundwater flow (delivery of ions), temperature, salinity, and concentration of dissolved organic carbon. Growth of these sand sized gypsum crystals results in increased hydraulic conductivity of this zone.

A few of the wettest playas are colonized at their surface by microbial communities. At the top of the sediment column, just below the halite crust, is an algal mat composed of filamentous cyanobacteria (blue-green algae). This mat excludes oxygen from the zone below where Desulfovibrium , an anaerobic sulfate reducer, performs dissimilatory sulfate reduction (the reduction takes place outside the cell), producing hydrogen sulfide gas. Some of the reduced sulfur combines with aqueous ferrous ions and results in precipitation of botryoidal pyrite both free and on the surfaces of growing gypsum crystals.