A Late Triassic soil catena: landscape and climate controls on paleosol morphology and chemistry across the Carnian-age Ischigualasto-Villa Union basin, northwestern Argentina

TABOR, NEIL; MONTAÑEZ, ISABEL; KELSO, KELLY; CURRIE, BRIAN; SHIPMAN, TODD; COLOMBI, CARINA

GEOLOGICAL SOCIETY OF AMERICA BULLETIN

Geological Society of America

Año: 2006 vol. 416 p. 17 - 41

0016-7606

The stratigraphy of the Late Triassic (Carnian) Ischigualasto Formation undergoes a significant thickness change, from ~300 to 700 m, across a 15 km transect within the Ischigualasto Provincial Park, San Juan, Northwest Argentina. Channel sandstone deposits dominate in the thickest stratigraphic section, whereas pedogenically altered layers dominate in the thinnest stratigraphic section. Eight different paleosol types are recognized within the study area, and they are unevenly distributed across the basin. In particular, paleosol B-horizons are thinnest and redoximorphic soil morphologies dominate in the thickest stratigraphic section, whereas B-horizons are thickest and argillic and calcic morphologies dominate in the thinnest stratigraphic section. Collectively, the distribution of channel sandstone deposits and paleosol morphologies across the Ischigualasto Fm indicate that the geomorphic evolution of the Ischigualasto basin exerted the primary control on sediment distribution and depositional rate, soil drainage, and depth of the groundwater table throughout most of Late Triassic time in the Ischigualasto basin. Carbonate nodules associated with paleosol profiles exhibit three major petrographic textures: (1) micritic calcite nodules that exclude detrital siliciclastics, (2) fibrous radiaxial calcite nodules and (3) “sandy” nodules composed of siliclastic silt- and sandsized grains that float within a micritic calcite cement. Based upon the textural composition of modern soil calcites that form via pedogenic processes, type (1) calcites provide the most conservative data set for evaluation of Late Triassic pedogenic conditions during Ischigualsto Fm deposition. Although the importance of the various factors that are responsible for their measured oxygen isotope composition cannot be known with absolute certainty, their δ18O values are similar to modern soil calcites that form in frigid to cool climates between ~0° and 10°C.Considering both lateral and stratigraphic distribution of paleosol morphological variability, there appears to be three different general modes of climate recorded throughout deposition of the Ischigualasto Fm: (1) Humid conditions recorded by Argillisols, Gleysols and Vertisols in the lower ~1/4 of the formation; (2) Relatively dry, perhaps arid conditions, recorded by Calcisols, calcic Argillisols and calcic Vertisols in the middle ~1/2 of the formation; (3) return to generally more humid conditions in the upper ~1/4 of the formation recorded by Argillisols, Gleysols and Vertisols although the occurrence of a single, thick Calcisol within the thinnest section suggests that portions of the landscape remained relatively well-drained with long-term soil moisture deficiency.