LOESS- AND CALICHE-BEARING BIOMANTLES AND COLD-DRY DESICCATION FEATURES IN TEHUELCHE GRAVELS, SANTA CRUZ PROVINCE, PATAGONIA

During a recent (Fall 2006) 3000 km trip through Santa Cruz Province we observed in a wind-eroded environment well expressed biomantles and desiccation-induced wedges in desert soils and sediments of ventifacted, gravel-covered (Tehuelche) surfaces. These structures were recorded in roadcuts, borrow pits, and quarries across the region. Our trip included the segments Rio Gallegos north to Jaramillo, west to Pico Truncada and Perito Moreno, south to El Chaltén-Calafate, and southeast back to Rio Gallegos. Evidence for Late Cenozoic glaciations is extensive, ranging from complex Andean-frontal moraines and associated proglacial sediment to widespread, variably aged, relict outwash gravels. These “Tehuelche gravels” are differentially uplifted and deposited widely across windswept arid eastern Patagonia. Strikingly, loess was not observed within or on the gravels beyond that temporarily stored as fine fraction in numerous wind-scoured depressions (ephemeral lakes) and in the ubiquitous Tehuelche biomantle. In fact, a thin (10-30 cm) to moderately thick (? 1 m) destratified biomantle was observed to cap these gravels in every excavation we investigated. Variable amounts of caliche had precipitated in the lower parts of most biomantles, and in some cases into the subjacent stratified fluvial gravels.

Desiccation casts and sand wedges were not-uncommonly observed in the exposures – always in the uppermost part of the stratified gravels, but below the biomantle. They were observed across Patagonia from the Atlantic to the Andean foothills and steppe. In eastern Patagonia, some casts and wedges were partially or largely overprinted by caliche. Notably, the stonelayer of two-layered biomantles, where present, was developed across the top of the casts and wedges.

Pin-cushion plants (Bolax sp.), short grasses (Festuca sp.), low shrubs (Junellia, sp.) and other dry-adapted plants were observed sparingly in the biomantle. These and other dwarf species function as variably effective, fine sediment (loess) traps. Episodic bioturbations in the biomantle by small mammals act to re-release trapped sediment for transport downwind. The persistently strong Patagonian Westerlies appear to have stripped most fine sediment from this part of Patagonia, no doubt depositing it offshore into the Atlantic and beyond.