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and Assoclafes, LC <br />• 3.3.1 Stratigraphy <br />The bedrock formations in the West Elk Mine area were deposited during transgressions and <br />regressions of the shoreline of the Western Cretaceous Interior Seaway during the Late <br />Cretaceous and Eazly Tertiary. This ancient shoreline was located along the eastern edge of <br />the tectonically uplifted mountains of the Sevier Orogenic Belt. Sediments eroded from the <br />uplifted mountains were carved towaFd the seaway by fluvial systems and deposited as <br />terrestrial, shoreline, marine, and interfingered non-marine and marine sedimentary <br />sequences. <br />On the terrestrial side of the shoreline, sediment deposition occurred in lacustrine (lake <br />carbonates, marls, and sands), alluvial plain (sands and clays), fluvial (stream sands and <br />• overbank muds), and carbonaceous backshore (coal swamp) environments. Along the <br />shoreline, marine foreshore deposits (beach sands) accumulated. Offshore, sands swept from <br />the beaches were laid down as bars and blankets of sand in the near-shore shallow marine <br />water. These blankets of sand are known as shoreface deposits. The clay fraction of stream- <br />transported sediments which reached the shoreline was deposited as thick marine muds <br />(shale) in the deeper and more quiescent portions of the seaway. Because the transgression <br />and regression of the shoreline was accompanied by the continued deposition of sediments, a <br />variety of horizontally and vertically discontinuous sediment types occur throughout the <br />basin. This depositional history has resulted in a heterogeneous rock record that has had a <br />profound effect on the water-bearing characteristics of [he stratigraphic sequence. <br /> <br />Characterization ofGroundwater Systems in [he V icinity of the West Elk Mine, Somerset, Colorado <br />29 January 1999 <br />Page 13 <br />