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When did the James River breach the Blue Ridge: Using the Pliocene Coastal Plain sediment to determine the timing of the breaching
Cole, Julia
Cole, Julia
Abstract
One model for the evolution of the James River in Virginia suggests the James River eroded headward through the Blue Ridge Province and Valley and Ridge Province and captured Valley and Ridge streams until it created its present drainage basin. Changes in composition of Coastal Plain units should reflect this change in the James River’s headwaters and in source rock. Since chert, fossiliferous chert, and silicified oolitic carbonates only crop out west of the Blue Ridge, they are sedimentary indicators in the Coastal Plain for geomorphic changes in the Valley and Ridge. The presence of these indicators in Pliocene Coastal Plain units suggested that the James River was draining Valley and Ridge units at the time of deposition of these units. I chose three Coastal Plain units to compare their compositions: Early Pliocene sand and gravel deposits, the Sunken Meadow Member of the Early Pliocene Yorktown Formation, and the Varina Grove Member of the late Pliocene Bacons Castle Formation. (Pliocene sand and gravel is the fluvial equivalent of the marine Yorktown Formation.) Chert was present in the gravel fraction of the Bacons Castle Formation and Yorktown Formation and in the sand fraction of all three units. Fossiliferous chert—containing silicified Devonian rachiopods of the Order Spiriferida—was also present in the gravel fraction of the Bacons Castle Formation. Gravel samples from the Pliocene sand and gravel contained one silicified oolitic limestone clast.
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2000-01-01
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Geology