Lithofacies and depositional processes from a Carboniferous lake, Sierra de Narváez, Northwest Argentina

Abstract

Thick and widespread Late Carboniferous lacustrine successions occur in the Sierra de Narváez area, northwest Argentina. A detailed lithofacies analysis forms the basis for the definition of fourteen lithofacies clustered into three assemblages. The delta assemblage consists of channelized cobble to pebble conglomerates, quartz-rich sandstones, and carbonaceous mudstones. This assemblage is present at the top of the lake succession and records the progradation of a short-headed stream delta. Wave- and current-rippled sandstones, and a heterolithic facies make up the shallow-lake assemblage. The nearshore area was probably composed of small bars, shoreline sand flats and low-energy sand-mud flats. The deep-lake assemblage is represented by two types of coarsening- and thickening-upward lithofacies sequences. The first type consists, from bottom to top, of laminated mudstones, thin-bedded turbidites, classic turbidites and massive sandstones. Each sequence is thought to represent turbidite lobe progradation in a basinal setting. The second type of sequence consists, from base to top, of underflow current sandstones and mudstones, thin-bedded and classic turbidites, deformed sandstones and hummocky cross-stratified sandstones. Each sequence records progradation of turbidite lobes on the delta front. The analyzed lithofacies record deposition in a hydrologically open lake. Final lake infill was related to the progradation of a deltaic system. It is suggested that four distinct processes controlled sediment transport and deposition in the Carboniferous lake: (1) delta progradation, (2) episodic sediment density surges, (3) river plume dispersion, and (4) wave action. During periods of high discharge, fine to very fine sand and silt bypassed mouth-bar zones and were deposited farther into the lake by underflow currents. Sands supplied by the delta system accumulated in the upper delta front in unstable conditions and were later redeposited by episodic turbidity currents. Wave action was able to rework shallow-water sands supplied by the delta system or emplaced by turbidity currents.