Geologic History and Geomorphology
Mesozoic EraThe sea withdrew completely from the area by the close of Paleozoic time and the surface was eroded, uplifted, and warped. Erosion proceeded throughout much of Triassic time and at least part of the area again received deposits during part of Jurassic time. Landes and Keroher (1939, p. 25) have described the subsurface geology of Logan, Gove, and Trego counties to the south of Thomas County and stated concerning these Jurassic deposits:
The shale section 100 to 200 feet thick which immediately overlies the Permian, in some places, has been tentatively correlated with the Morrison formation. The age of this formation has not been exactly determined, but evidence based on paleontology strongly indicates that it is Jurassic. These rocks consist predominantly of green shale with an abundance of pink jasper-like chert. Translucent pink gypsum occurs near the base. An interesting zone of doubly terminated quartz crystals which was noted in a number of wells may prove to he useful in correlation. An unconformity at the base of the Jurassic (?) rocks marks the top of the Permian system. Due to the lithologic change in the rocks this unconformity is easily recognized and therefore is a valuable correlation point for geologists.An erosion interval followed the deposition of these supposed Jurassic deposits and this general area probably stood above sea level during part of Jurassic time and the early part of Cretaceous time. As the early Cretaceous sea inundated this area, clastic sediments accumulated at and near the shore line as beach deposits deltas, and offshore bars. The sea completely inundated this area and marine sediments were laid down. Nonmarine conditions probably again existed throughout part of the time of deposition of the Dakota formation, and then the sea completely transgressed the area for the last time and the Graneros shale and overlying marine formations of Upper Cretaceous age were deposited. That this area gradually subsided throughout Upper Cretaceous time is indicated by the presence of more than 2,000 feet of fine-textured shallow-water Upper Cretaceous deposits. These strata consist of chalk and chalky limestones (Greenhorn limestone and Niobrara chalk) alternating with shale and calcareous shale (Graneros shale, Carlile shale, and Pierre shale).
Kansas Geological Survey, Thomas County Geohydrology|
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Web version Nov. 2001. Original publication date Dec. 1945.