Hydrogeology
The surface geology of the Solomon Basin consists of unconsolidated and consolidated rocks. The unconsolidated surface deposits consist of Quaternary alluvium, loess and the Tertiary Ogallala Formation
Unconsolidated surface deposits consist of Quaternary fluvial and eolian sediments and the Tertiary Ogallala Formation. The alluvium consists mainly of gravel, sand, silt and clay that can be found in the channels and flood plains of all the major streams. The terrace deposits are reworked sediments of older alluvium and the Ogallala Formation. The eolian or loess deposits occur mainly in the uplands and on valley slopes. The thickness of the fluvial and eolian deposits varies from approximately 10 to 60 feet.
The Ogallala Formation, found in the western part of the Solomon Basin, was formed during the Pliocene by eastward flowing streams that filled pre-existing valleys with alluvial sediments. Continued deposition of alluvial sediments formed a broad alluvial plain. The Ogallala consists mainly of silt, sand, gravel and mortarbeds formed by cementation of sediments with calcium carbonate. Lenticular beds of well-sorted sand, gravel, bentonite and volcanic ash also occur in the Ogallala. The top of the Ogallala consists of a few feet of dense, sandy, algal limestone.
The Ogallala ranges in thickness from approximately 60 feet in northern Phillips County to approximately 260 feet in central Thomas County. The Ogallala lies unconformably on the Pierre Shale in the western part of the basin and on the Niobrara Formation in the central part of the basin. The surface of the Ogallala dips to the east-northeast and the average gradient is 11 feet per mile.
The North Fork Solomon and South Fork Solomon are underlain by or incised into progressively old Cretaceous beds in a west to east direction across the Solomon Basin. The Upper Cretaceous is composed of the younger Pierre Shale, Niobrara Formation, Carlile Shale, Greenhorn Limestone and Graneros Shale. The Lower Cretaceous is composed of the younger Dakota Formation and the Kiowa Shale. The Ogallala and the alluvium like unconformably on the Cretaceous Pierre Shale in the western counties of Thomas and Sheridan. The Pierre Shale is a black to dark gray-brown fissile, noncalcareous shale with thin beds of bentonite, chalky shale, limonite and gypsum in localized areas.
The Cretaceous beds generally dip to the west while the erosional surface generally slopes to the east. The Pierre Shale lies conformably on the Niobrara Chalk. The Niobrara Chalk is exposed in Graham County in areas where streams have cut down through the alluvium and loess. The Niobrara Chalk is a gray, shaly, fossiliferous chalk with weather surfaces of white, yellow and orange. The chalk contains bentonite beds and limonite concretions.
The Carlile Shale is exposed in stream valleys in Phillips County. The Carlile Shale is a blue-gray to black, thin bedded shale with limonite nodules and selenite crystals. A thin, fine to medium grained, silty, limonitic sandstone caps the shale.
The Greenhorn Limestone, Graneros Shale and Dakota Sandstone outcrop as far east as western Clay County. Permian beds outcrop in counties farther east. The Greenhorn Limestone consists of alternating beds of calcareous shale and chalky limestone. The Graneros Shale is noncalcareous, fissile shale with sandstone lenses. The Dakota Formation consists of Lenticular sandstone bodies that are embedded in mudstone. Generally, the sandstones are fine to medium grained, well sorted and exhibit cross-bedding.
The Ogallala component of the High Plains is the main aquifer in the western part of the Solomon Basin. The Ogallala varies in saturated thickness from less than 50 feet to greater than 200 feet. Water levels vary from approximately 150 feet in central Thomas County to less than 50 feet in Norton and Graham counties.
The alluvial deposits also represent a major aquifer in the Solomon Basin. Much of the groundwater pumped in the Lower Forks and Mainstem come from this aquifer.
The Dakota Formation forms an aquifer in the eastern part of the basin although water quality is generally poor. The Dakota aquifer exists under both confined and unconfined conditions in the basin. The Dakota is confined by the Graneros Shale in the upper part of the basin. The Dakota is unconfined as it outcrops in the eastern part of the basin in Mitchell, Lincoln, Ottawa and Cloud counties. The Permian Cedar Hills Sandstone underlies the Dakota in the central counties of Phillips, Smith, Rooks and Osborne. Upward flowing brines from the Cedar Hills Sandstone cause higher total dissolved solids in the Dakota in these counties.
