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Climatically Controlled Redox Cycles of the Upper Cretaceous Mooreville Formation in Mississippi
The Upper Cretaceous Mooreville Formation in Noxubee County, Mississippi, conformably overlies the sandstones of the Eutaw Formation and is separated from the overlying Demopolis Chalk by the Arcola Limestone. The Mooreville Formation is 268 feet (82 m) thick in this area and consists of 53 redox cycles. The cycles range in thickness from 1 to 11 feet (0.3 to 3.6 m) and display three distinct intervals: a basal laminated, a middle partly bioturbated, and an upper burrowed interval. The basal laminated section indicates sedimentation under anoxic bottom water conditions, hostile to burrowing organisms. The laminated layer grades upward into a gray, partly burrowed interval, suggesting that bottom waters became slightly oxygenated and organisms tolerant of low oxygen conditions were sparsely present. The cycles are capped by thoroughly bioturbated layers indicative of deposition under fully oxygenated conditions.
Cycles were initiated during wet climatic periods with the introduction of siliciclastics and terrestrial organic matter into the basin. Oxidation of organic matter at the sediment-water interface or in the uppermost portion of the sediment column resulted in oxygen depletion and bottom water anoxia, leading to the deposition of laminated strata. Riverine input decreased as climatic conditions became arid, limiting the input of terrestrial organic matter. As a result, the amount of oxygen in the sediments increased to the point that deposition of the dysoxic section of the cycle began. Eventually, organic matter input decreased, leading to full oxygenation of bottom water when heavily bioturbated sedimentation resumed.
The lower and upper contacts of the Mooreville Formation are approximately marked by the base and top of the Globotruncanita elevata biozone, suggesting 3.5 My of sedimentation at a rate of 2.34 cm/ky. At this sedimentation rate, cycle durations range from 13 ky to 143 ky (average = 64 ky), with major peak frequencies at 30-40 ky, 50-60 ky, and 60-70 ky, none of which correspond to Milankovitch orbital forcing mechanisms of climate change.
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