Evaporitic paleosols in continental strata of the Carroza Formation, La Popa Basin, Mexico: Record of Paleogene climate and salt tectonics

A succession of continental red beds in the Paleogene Carroza Formation, northeastern Mexico, contains an assemblage of evaporite paleosols previously unknown in pre-Neogene strata that record the syndepositional exposure of nearby diapiric evaporite and a climatic shift to increasing aridity. Carroza red beds were deposited in an ephemeral braided-fluvial system in a high-accommodation setting. Paleosols developed in nearly all depositional settings, including channels, crevasse splays, and floodplains, and contain salic/natric, gypsic, baritic, and calcic horizons. Calcic paleosols are limited stratigraphically to the lowermost part of the formation in oyster-bearing estuarine strata and yield upsection to evaporitic paleosols, thus providing a record of increasingly arid conditions as the Paleogene marine shoreline shifted eastward, toward the Gulf of Mexico Basin. The increase in aridity reduced vegetation and residuum thickness on the exposed diapiric salt, consequently increasing the influx of evaporitic minerals into the basin, and driving the development of salic/natric, gypsic, and baritic horizons in all depositional environments. Evaporitic paleosols of the Carroza Formation have characteristics similar to soils forming today in climates with annual precipitation ranging from <80 mm/yr to as much as 450 mm/yr, in apparent conflict with estimates of subhumid to subtropical conditions from Carroza fossil leaf data. Because evaporitic paleosols are persistent throughout the Carroza section, we infer that a combination of spring-fed, high water tables, augmented by flood-basin inundation from high-discharge seasonal fluvial flood events sustained perennial woodlands, and sodium-caused clay dispersion created poor drainage in topographically low parts of a rapidly subsiding salt-withdrawal basin.