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2.0 GENERAL FEATURES--Continued
<br />2.4 Geology
<br />2.4.7 Stratigraphy
<br />Precambrian To Quaternary Rocks Underlie Area
<br />Sedimentary rocks in the area record the presence of Pennsylvanian
<br />and Cretaceous seas and intervening episodes of land emergence.
<br />The oldest rocks in the area are Precambrian granite,
<br />peematite, gneiss, schist, and quartzite (Johnson, 19b9).
<br />These crystalline rocks form the core of the Sangre de
<br />Cristo Range and the Wet Mountains ((ig. 2.4.I-I).
<br />Paleozoic and Mesozoic formations Hank the moun-
<br />tains and extend beneath the rest of the area (Griggs and
<br />Northrop, 1956; Oriel and Mudge, 1956; Shaw, 1956). The
<br />rocks consist of marine limestone, shale, and sandstone of
<br />the Pennsylvanian h1inturn and Belden Formations in
<br />Colorado and the Afagdalena Group in New Mexico over-
<br />lain by terrestrial sediments of the Pennsylvanian to Juras-
<br />sic Sangre de Cristo Formation, Yeso Formation, Lyons
<br />(Glorietta) Sandstone, Dockum Group, Entrada Sand-
<br />stone, Ralston Creek (Wanakah) Formations, and Morri-
<br />son Formation. The Sangre de Cristo Formation and
<br />Dockum Group wedge out northeastward across the area
<br />as the Yeso Formation and Lyons Sandstone thicken. The
<br />aggregate thickness of Paleozoic and Mesozoic rocks de-
<br />creases from more than 10,000 feet (ft) in the Sangre de
<br />Cristo 0.ange to a tew hundred feet in the northeastern
<br />cornero(the area.
<br />Deposits o(a Cretaceous sea border the west side of
<br />the Park Plateau and extend beneath it to the eastern edge
<br />of the area (Griggs, 1448, p. 22-34; Cobban, 1956; ~VOOd
<br />and others, 1957, p. 17-22). The Purgatoi re Formation
<br />(Cheyenne Sandstone and Kiowa Shale (vlembers) and
<br />Dakota Sandstone, which together average about 200 ft
<br />thick, mark the advance of the sea. They are overlain by
<br />the Graneros Shale, Greenhorn Limestone, Carlile Shale,
<br />Niobrara Formation (Fort Hays Limestone and Smoky Hill
<br />Marl Members), and Pierre Shale. These formations com
<br />rain more than 3,000 (t of shale and interbedded sandstone
<br />and limestone. A shoreline deposit intertonguing with the
<br />top of the Pierre Shale, the Trinidad Sandstone (0 to 310 ft
<br />thick), marks the retreat of the sea near the end of the
<br />period.
<br />coastal plain to terrestrial sediments records the rise of the
<br />Rocky lstountains (Johnson and \VOOd, 1956). At the base
<br />of this sequence, the Vermejo Formation, a Cretaceous
<br />coastal plain deposit as much as 550 ft thick, intertongues
<br />with the Trinidad Sandstone. The Vermejo Formation
<br />consists of carbonaceous and silty shale with lenses of coal
<br />and sandstone. Nis unconformably overlain by the Raton
<br />Formation, a Cretaceous and Paleocene coastal plain
<br />deposit as much as 2,000 ft thick. The middle of the Raton
<br />Formation resembles the Vermejo, but the upper and lower
<br />several hundred (eet of the Raton Formation are predomi-
<br />nantly sandstone, siltstone, and noncarbonaceous shale.
<br />On the northern and southern edges of the plateau, the
<br />Poison Canyon Formation intertongues with and overlaps
<br />the Raton Formation. The Poison Canyon Formation,
<br />which consists of 2,500 ft of sandstone, conglomerate, and
<br />shale, is mostly Paleocene. Where the lower part of the
<br />formation intertongues with the Raton Formation in the
<br />western part of the area, it is Cretaceous. The Cuchara,
<br />Huerfano, Farasi[a, Devils Hole, and Santa Fe Formations
<br />of Eocene to Pliocene age overlie the Poison Canyon
<br />Formation and consist of conglomerate, sandstone, shale,
<br />and volcanic tuff as much as 10,000 ft thick. These
<br />formations, and the Poison Canyon Formation, were
<br />formed mostly by erosion o(the rising mountains.
<br />Recently deposited clay to boulder-sized alluvium as
<br />much as 50 ft thick occurs in stream channels, terraces, and
<br />pediments. Landslides, eolian deposits, and talus also are
<br />present (Griggs, 1948, p. 65-70; Powell, 1952).
<br />Stocks, laccoliths, plugs, dikes, and sills ranging in
<br />composition from granite to diabase and lamprophyre
<br />intrude the sedimentary rocks and form prominent land-
<br />marks such as the Spanish Peaks, Black Hills, Mount
<br />Mesias, and Cimarron Range (Johnson, 1468). Along the
<br />southern and southeastern edges of the basin, basalt flows
<br />cap uplands such as Raton Mesa and Eagle Tail Mountain
<br />(Griggs, 1448, p. 38-40).
<br />In the Park Plateau, a transition from marine to
<br />
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