Playa-lake Environments Research Articles

Geochemistry and origin of albite gneisses, northeastern Adirondack Mountains, New York

Albite gneisses containing up to 8.7 percent Na2O and as little as 0.1% K2O comprise a significant part of the Proterozoic Lyon Mountain Gneiss in the Ausable Forks Quadrangle of the northeastern Adirondacks, New York State. Two distinct types of albite gneisses are present. One is a trondhjemitic leucogneiss (LAG) consisting principally of albite (Ab95–Ab98) and quartz with minor magnetite and, locally, minor amounts of amphibole or acmiterich pyroxene. LAG probably originated by metamorphism of a rhyolitie or rhyodacitic ash-flow tuff with A-type geochemical affinities, following post-depositional analcitization in a saline or saline-alkaline environment. The other type is a mafic albite gneiss (MAG) containing albite and pyroxene along with 0–45 percent quartz, minor amphibole, and titanite. MAG locally displays pinstripe banding and contains albite (Ab98) megacrysts up to 5 cm across. Its precursor may have been a sediment composed of diagenetic analcite or albite, dolomite, and quartz. Both types of albite gneiss are interlayered with granitic gneisses (LMG) of variable composition derived from less altered tuffs. A potassium-rich (up to 9.7% K2O) microcline gneiss facies may have had a protolith rich in diagenetic K feldspar. We propose that the albite gneisses and associated granitic gneisses are the granulite-facies metamorphic equivalent of a bimodal, dominantly felsic, volcanic suite with minor intercalated sediments, probably including evaporites. The volcanics were erupted in an anorogenic setting, such as an incipient or failed intracontinental rift. Deposition took place in a closed-basin, playa lake environment, where diagenetic alteration resulted in redistribution of the alkalis and strong oxidation.

Neogene avian and mammalian tracks from Death Valley National Monument, California: Their context, classification and preservation

The Copper Canyon Formation consists of over 3000 m of basalt and alluvial fan and lacustrine strata deposited in a small Neogene basin. Lacustrine strata are of two typs: fine-grained dolomitic carbonates deposited in a playa-lake environment and calcitic bioclastic carbonates deposited in a freshwater lake environment. Bedding surfaces of the playa-lake carbonates deposited in shoreline environments commonly contain avian and mammalian footprints. Tracks of artiodactyls are most abundant; those of birds, horses, carnivores and proboscideans are less common. Using the binomial avian and mammalian footprint classification scheme of Vialov (1966), four different mammalian ichnogenera containing thirteeen ichnospecies, as well as six ichnospecies of avian tracks are recognized. Environmental factors of the ancient Copper Canyon playa-lake which contributed to good footprint preservation included: (1) periods of flooding and intense evaporation which resulted in receding shorelines that presented wet, fine-grained sediment surfaces for impression and subsequent subaerial exposure; and (2) periodically high sedimentation rates which insured quick burial of the tracks. While vertebrate footprints also occur in sandstones and conglomerates deposited in alluvial fan environments, the morphological distinctness of tracks is best preserved in fine-grained dolomitic carbonate and siltstone strata deposited in the playa-lake environment. Distinctness of preserved tracks also increases with apparent decrease, within a certain range, of sediment water content at the time of impression. An examination of preserved morphologies has allowed several sediment water content states important to footprint preservation to be deduced. Changes in sediment water content at the time of impression acted to produce a wide range of track morphologies for each individual trackmaker. An examination of track abundance and occurrence within the formation suggests increasing visitation by larger vertebrates to the playa-lake as lake water freshened. The rare ocurrence of tracks of small mammals and low amount of vertebrate bioturbation within the playa deposits also suggests that the ancient Copper Canyon playa-lake lacked a resident vertebrate population of its own, probably receiving only migrating visitors seeking food and water. The footprints in the Copper Canyon Formation reflect part of the wide diversity of vertebrates present in southern California during the Hemphillian.

Geochemical dispersion patterns associated with the Lake Yindarlgooda sulphide mineralization, Western Australia

Geochemical investigations have been carried out in the Lake Yindarlgooda area, 40 km east of Kalgoorlie, Western Australia, where sulphide deposits and black shales occur in a sequence of metavolcanics and metasediments of the Eastern Goldfields greenstone belt. The studies were made in order to understand the processes of element migration and the formation of dispersion patterns in semi-arid regions having partly lateritic weathering conditions. Outcrop exposure in the vicinity of the mineralized zones is limited to the siliceous caprock and gossans. Mineralogical data, and geochemical data on the distribution of Cu, Ni, Co, Zn, Pb and other elements, have been obtained from unweathered and weathered rocks, and different horizons of the overlying soil in the area surrounding the mineralized zones of Lake Yindarlgooda, Scotia and Ringlock. The results indicate, in principle, two different kinds of weathering environment: (1) playa lakes, with the development of solonchaks; and (2) Tertiary peneplains outside the playa lakes, with lateritic soils, solonized brown soils or desert loams. The trace element distribution patterns in the solonchaks are determined only by the initial element content of the alluvial sediments. Therefore, sampling of solonchaks generally cannot be recommended for geochemical exploration surveys, but sampling of weathered bedrock below the solonchaks is highly recommended in the playa lake environment. By sampling of the bedrock, the non-outcropping continuation of the Lake Yindarlgooda sulphide body has been precisely delineated. In areas of the Tertiary peneplains outside the playa lakes, lateritic soils, solonized brown soils or desert loams cover the weathered bedrock. Comparative geochemical investigations in these areas in the vicinity of the nickel sulphide deposits at Scotia and Ringlock have shown that the migration of elements in solution is restricted, a result of the very low water table. In this area the main process of dispersion is mechanical. Therefore, the secondary dispersion haloes of Scotia and Ringlock are narrow, restricted to a few tens of metres, and sampling at closer intervals is needed to locate the orebodies.