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West Virginia University (2019)

Beneath the Gypsum Dunes : Cenozoic History of Wind and Water from a Core Drilled at White Sands, New Mexico

Jakeway, Jackson Bentley

Titre : Beneath the Gypsum Dunes : Cenozoic History of Wind and Water from a Core Drilled at White Sands, New Mexico

Auteur : Jakeway, Jackson Bentley,

Université de soutenance : West Virginia University

Grade : Master of Science (MS) in Geology 2019

Résumé
White Sands, New Mexico is the largest gypsum dune field on planet Earth, the result of reworking of gypsum deposits. The dunes have been well studied, but the Cenozoic history preceding the formation of the dune field has been poorly studied. A core drilled to a depth of 192 ft (58.5 m) beneath the modern dune field contains deposits from saline lakes, sandflats, perennial freshwater lakes, perennial brackish to saline lakes, and saline mudflats. The core is composed of bottom-growth bedded gypsum, gypsum sandstone and siltstone, mixed siliciclastic-gypsum sandstones and siltstones, laminated siliciclastic mudstones, gypsum mudstones, and carbonate mudstones. Bottom-growth bedded gypsum was precipitated from saline lakes. Gypsum sandstones and siltstones were deposited by eolian processes. Mixed siliciclastic-gypsum sandstones and siltstones were deposited during periods of increased surface water inputs. Laminated siliciclastic mudstones were deposited in perennial freshwater lakes. Calcite mudstone containing charophytes and ostracods were deposited in shallow, perennial brackish to saline lakes. Gypsum mudstones indicate either shallow saline lakes or saline mudflats. Wavy lamina, climbing ripple cross-bedding, and dewatering structures in gypsum mudstones are evidence for rapid deposition of sediment by shallow, decelerating surface waters such as sheetfloods. Mudcracks and eolian reworked gypsum grains are evidence for subaerial exposure. Black beds, possibly manganese oxides, were present at two depths in the core. Abundant displacive gypsum is interpreted as evidence for extensive saline groundwaters. Gypsum grain size and shape provide insight into production and subsequent transport of grains. Gypsum sandstones in the upper 80 ft (24.4 m) were commonly composed of very lightly reworked gypsum grains displacive in origin and are evidence for saline mudflats and subsequent subaerial exposure. The high porosity of these units indicates that a large quantity of fine-grained sediment was deflated. Seeds and other organic material were found throughout the core but were most abundant in the upper 80 ft (24.4 m) of core. A radiocarbon age date of 22 ka was determined from a seed at 35.4’ ft depth. Although the timespan of deposition of the core sediments is not known, it is estimated that the sediments at the base of the core may be as old as 200 ka, due to the diverse assemblage of megafauna fossils found throughout White Sands and Bull Lake Glaciation. Sediment in the White Sands Core was deposited from perennial and ephemeral saline lakes, sandflats, perennial freshwater lakes, perennial brackish to saline lakes, and saline mudflats. There was abundant evidence for eolian processes. Two periods of perennial freshwater lake deposition and several saline lake deposits, as well as common eolian deposits strongly suggest fluctuations in climatic humidity and aridity during the Pleistocene.

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Page publiée le 19 janvier 2020