Polygenetic development of the Mormon Mesa, NV petrocalcic horizons: geomorphic and paleoenvironmental interpretations.

Abstract Mormon Mesa, Nevada, USA is of one of Earth's rare and unique landforms. Regionally, Mormon Mesa is important because of its relationship to the lower Colorado River and its connection to stream incision. Petrocalcic horizons at Mormon Mesa developed from multiple exposure, erosion, and sedimentation events driven by variations in paleoclimate since the early Pliocene. This study utilizes pedogenic features at Mormon Mesa to construct a sequence of development for the soil profile. We sampled and described seven profiles at Mormon Mesa to determine the soil and geomorphic processes that have influenced the development of this geomorphically important soil. The Mormon Mesa soil profile generally consists of four petrocalcic soil horizons from the base upward (transitional, laminar, massive, and brecciated) and an additional set of horizons that formed in unconsolidated surficial eolian sediments. The transitional horizon formed after cessation of Muddy Creek Fm. deposition (~ 5 to 4 Ma) and accumulated calcium carbonate to a stage II–III morphology (maybe IV in some locations). After a period of landform stability, the upper portion of the transitional horizon was exposed and eroded (~ 2.8 Ma). Petrocalcic fragments and stage II pendants that formed in the transitional horizon were exposed on the surface and later mixed with aggrading eolian sediments. After the surface re-stabilized, calcium carbonate accumulated along the underlying discontinuity with the transitional horizon and formed a laminar horizon. Shallower accumulations of calcium carbonate formed pendants on the undersides of the rotated petrocalcic fragments and pendants that had previously been eroded from the transitional horizon. Continued rooting and bioturbation caused rotation and multiple directions of pendant formation. Today, in plan view, these pendants appear concentric and are the pisoliths described by Bachman and Machette [Bachman, G.O., Machette, M.N., 1977. Calcic Soils and Calcretes in the Southwestern United States., USGS: Open-File Report 77–797]. Continued calcium carbonate precipitation eventually plugged the horizon (massive horizon). After the massive horizon became indurated, the upper portion was exposed, fractured, eroded, and mixed with aggrading eolian sediments. These brecciated fragments experienced erosion/brecciation and soil stabilization/cementation with pedogenic calcite multiple times, probably as a result of the Pleistocene glacial/interglacial climate cycles. All of the features described in this study are interpreted to have formed in response to climatic changes that caused either soil erosion with concurrent eolian deposition, or soil development and pedogenic carbonate accumulation. It is important to understand the development of the Mormon Mesa soil because it can provide an understanding of the pedogenic and geologic processes that effect the development of stages V and VI soil horizons. This study provides a relative sequence of events for the formation of pedogenic features at Mormon Mesa that can be used for future dating applications. More precise dates for the development of the Mormon Mesa soil will contribute to our understanding of the geomorphic, tectonic, and climatic processes that have affected this region.