GEOMORPHIC AND PALEOENVIRONMENTAL INTERPRETATIONS OF THE STAGE VI PETROCALCIC HORIZONS OF MORMON MESA, NEVADA, USA

Stage VI petrocalcic horizons contain evidence for past climates and geomorphic processes. To test this, we sampled and described seven profiles at Mormon Mesa (MM) near Overton, Nevada and constructed a relative sequence of events that can be used for future isotopic and dating applications. Four petrocalcic horizons may be present from the base of the profile upward (transitional, laminar, massive, and brecciated). These are usually overlain by several soil horizons formed in unconsolidated eolian sediments. After deposition of closed-basin Muddy Creek Fm. (~5 to 4 Ma), the transitional petrocalcic horizon formed with a stage II-III morphology, and stage IV in some locations. Following that, erosion of the upper portion of the transitional horizon (~2.8 Ma) exposed and concentrated petrocalcic fragments and stage II pendants at the surface. These fragments and pendants were later mixed with aggrading eolian sediments and pedogenic carbonate accumulation resumed, eventually forming the massive horizon. During this time, pendants formed on the undersides of the rotated petrocalcic fragments and the older pendants from the transitional horizon. In some places on the mesa, a laminar horizon formed where deeply-penetrating meteoric water carrying calcium carbonate reached the underlying discontinuity with the transitional horizon. Rotation and multiple directions of pendant formation were caused by continued rooting and bioturbation. These pendants appear concentric in plan view and are the pisoliths described by Bachman and Machette (1977). The massive horizon became indurated from continued calcium carbonate precipitation and the upper portion was exposed multiple times, fractured, eroded, and mixed with aggrading eolian sediments to form the brecciated horizon. These final events occurred during the Pleistocene glacial/interglacial climate cycles. This study provides a relative sequence of events that showcases how important geomorphic and paleoclimatic records can be preserved in late-stage petrocalcic horizons and provides a framework for future studies using isotopic techniques.