Nonmarine Depositional Patterns in from Arid Piggyback Basin: Sedimentology, Stratigraphy and Paleoenvironments from Monumento Natural Laguna De Los Pozuelos, Argentina
Shallow, expansive lakes housed within piggyback basins are important and conspicuous components of many retroarc foreland basin systems. Over geologic timescales, sediments deposited in these lacustrine systems can possibly develop into viable hydrocarbon source rocks. The Cuenca de Pozuelos, a thrust-fault-bounded piggyback basin in northwestern Argentina, provides an opportunity to assess lacustrine and other nonmarine depositional patterns in the arid Andean Puna, and may serve as a modern analogue for some ancient intracontinental foreland basin source rocks. The locus of modern deposition is along the basin axis, where an endorheic lake, Laguna de los Pozuelos, is fed longitudinally by small-catchment rivers and laterally by ephemeral alluvial fan complexes. The lake is large (~ 105 km2), shallow (< 2 m deep), and slightly alkaline (pH ~ 8.65). Surface sediment samples collected from the lake indicate that total organic carbon (TOC) values increase toward the lake center, reaching a maximum of ~2.6 wt %. In contrast, total inorganic carbon values increase toward the lake margins, where values commonly exceed ~1.5 wt %. Hydrogen index values range from 55 to 190 mg HC/g TOC, indicating that sedimentary organic matter is dominated by contributions from altered or oxidized terrestrial plants, which may eventually lead to the formation of gas-prone source rocks. Pb-210 data indicate that recent sedimentation rates in the basin are relatively rapid, approaching 0.15 cm/yr. Short (3-6 m) hammer cores collected from lake-margin, delta, and axis environments exhibit a wide range of lithofacies, including sheet-flood and deltaic sands, finely-laminated organic-rich sapropels, massive ostracode-rich or pyrite-rich silty clays, and mud-cracked sandy clays. Facies transitions in many cases are abrupt, and are interpreted to be controlled by water-level fluctuations associated with late Quaternary climatic and tectonic changes.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009