2001-12-11_PERMIT FILE - C1980005 (4)

Home Browse Search

State Reg. Late Mesozoic sediment was primarly continental in origin with clastics being ;>upplied by the erosion of the central Craton area to the east. Throughout the Cretaceous period, the interior basin was covered by a vast, transgressive and regressive, epicontinental seaway. During this time, the area vvas in constant change. The basin was continually subsid- ing and sedimentation rotes varied with uplift and subsequent denudation. Formations of the early Cretaceous period such as the Dakota Sandstone, Mowry, Frontier, Niobarra, and Mancos Shale are products of marine or near marine environments. During the late Cretaceous, the sediment became more terrestrial in nature. Swamps were formed near the sea coast which, due to the fluctuation of the sea levels, became interfingered with marine and coastal marine sediment. Toward the later part of the Cretaceous period, the continental Lance Formation was deposited (Gaffke, 1979). The. early Tertiary period was also marked by swamps and continental deposition. Throughout this time period, there was' intermittent volcanic • activity in some areas and continual deformation caused by the Laramide Orogeny. Present deposition occurs in the form of stream deposited alluvium and gravity induced colluvium. 2.04.5(1) Figure 6-I is a generalized geologic map showing the relationship of sedimentary and volcanic rocks to the general area of the Seneca II Mine. The same map, showing contours drawn on top of the Trout Creek Sandstone, is shown in Exhibit 6-12. This map illustrates, in greater detail, the complex geologic structure of the mine plan and adjacent areas. A generalized columnar section giving a brief stratigraphic and lithologic description of strata found in the region is presented in Figure 6-2. This information shows the variety of lithologies possible in the vicinity of the Seneca II Mine. At the Seneca II Mine site, the Seneca Coal, Ltd. is primarily concerned with that portion of the strotigraphic column between the Trout Creek Sandstone member of the Cretaceous Iles Formation and the Twenty Mile • Sandstone member of the Williams Fork Formation. This part of the ~,.; column is known as the Middle Coal Group (Fenneman and Gale, 1906). x. .. -~~ 6-2 Revised 5/27/1981 ,,