About This Item
Share This Item
Salt-flat playas of West Texas-New Mexico are represented by a series of north-south aligned Holocene-Pleistocene dried salt lakes. Shallow (up to 100 cm) cores on the alkali flats of these lakes reveal a well-defined sequence of alternating dark and light laminae in the evaporitic sediments. These laminites bear a close resemblance to the laminated calcite-, dolomite-, and anhydrite-bearing sequences of the Middle Devonian Winnipegosis Formation of western Canada, Permian Castile Formation of west Texas and New Mexico, and recent sediments from hypersaline pools of the Gulf of Aqaba and other locations. XRD studies, augmented by petrographic evidence, show that laminites from the salt-flat playas are comprised of layers of differing mineral composition. The darker layers are dominated by gypsum (average 36.89%), halite (average 27.71%), calcite (average 17.74%), and dolomite (average 15.81%), whereas lighter layers are impoverished in calcite and dolomite and consist mainly of gypsum (average 72.86%) and halite (average 27.14%). Beside the mineralogic variations, the laminae also differ significantly in the abundance of total organic matter (TOM) content, with darker laminae being invariably richer (average 51% TOM in contrast to average 21% in lighter laminae). Micritic sediments that are relatively less altered by later recrystallization have retained a higher concentration of organic matter. In the gypsiferous sediments, which seem to owe their origin to a micritic mass, the organic matter is widely dispersed. In addition to the interspersed organic m tter in evaporitic laminae, laminae comprised exclusively of algal mats also are common in these sequences. Liquids chromatographic studies reveal that these algal mats are remarkably similar in their hydrocarbon content to that of the algal mats that are now growing sporadically on the moist playa surfaces. These observations suggest that the laminite sequence of the salt-flat playa sediments have originated in a restricted shallow basin characterized by alternating periods of desiccation (precipitation of gypsum and halite) and freshening (algal growth and deposition of micritic sediments).
End_of_Article - Last_Page 268------------