Bighorn Basin hydrostratigraphy

The Bighorn Basin hydrogeologic units defined for this study are described below, youngest to oldest (Plate III, far right column). Bracketed […] classifications are from the Statewide Framework Water Plan (WWC Engineering, 2007). Below the major alluvial aquifer at the land surface and above the thick, widespread Upper Cretaceous Cody [Major] confining unit are three Tertiary or Upper Cretaceous major (mostly sandstone) aquifers (Fort Union, Lance, Mesaverde) and one major confining unit (Meeteetse), all components of the lower Tertiary/Upper Cretaceous aquifer system; the aquifers are generally accessible in the central basins. Below the Cody confining unit and above the Precambrian basal [Major] confining unit, lower Mesozoic and Paleozoic hydrogeologic units include two major sandstone aquifers (Cloverly, Nugget), five major limestone aquifers (Tensleep, Madison, Darby, Bighorn, Flathead) and two major confining units (Mowry, Thermopolis); the aquifers are generally accessible in and near outcrop along the basin margins.

Where present with sufficient thickness, unconsolidated Quaternary alluvial aquifers and terrace deposits constitute important [Major] aquifers within the basin. The Upper Tertiary bedrock hydrostratigraphy of the Bighorn Basin includes the Oligocene White River [Marginal] aquifer and the upper Eocene Wagon Bed confining unit [Marginal aquifer]. The Tertiary Absaroka Volcanics represent a largely undeveloped groundwater resource; development is impeded by complex to chaotic stratigraphy within the deeply eroded, high-relief, and mostly inaccessible Absaroka Range.

Below the Wagon Bed, three hydrogeologic systems (each composed of several hydrogeologic units) separated by two regional confining units compose the hydrogeology of the Bighorn Basin. Hydrologic interconnection between units within the aquifer systems varies due to the presence or absence of local confining units and fracture zones. Likewise, hydrologic properties and water quality vary between the distinct units. In addition to the hydrogeologic systems and intervening regional confining units, two distinct hydrogeologic units above the Precambrian basal confining unit are also discussed below.

The lower Tertiary/Upper Cretaceous aquifer system comprises all Upper Cretaceous and lower Tertiary hydrogeologic units stratigraphically above the thick and widespread Cody regional confining unit. Aquifers within the system are lenticular, discontinuous sandstone bodies that are hydraulically isolated to various degrees by interbedded fine-grained confining units. The hydrogeologic units that compose the lower Tertiary/ Upper Cretaceous aquifer system above the Cody [Major] confining unit are, youngest to oldest:

  • the Willwood [Minor] aquifer
  • the Fort Union [Major Sandstone] aquifer
  • the Lance [Major] aquifer
  • the Meeteetse aquifer and [Major] confining unit
  • the Mesaverde [Major Sandstone] aquifer

Immediately below the Cody [Major] confining unit, the lower and middle Mesozoic aquifers and confining units system comprises Cretaceous through Jurassic hydrogeologic units. The basal unit of the system, the Nugget Sandstone, might span Triassic and Jurassic time. This hydrogeologic system is composed of [Major], [Minor], and [Marginal] aquifers and confining units dominated by sandstone, siltstone, and shale lithologies. While some of the aquifers are lenticular and discontinuous, the major aquifers are generally more continuous and laterally extensive than those in the lower Tertiary/Upper Cretaceous aquifer system above the Cody confining unit. The lower and middle Mesozoic aquifers and confining units include substantial confining units that hydraulically isolate some of the member aquifers. The hydrogeologic units of the system include, from youngest to oldest:

  • the Frontier [Minor] aquifer
  • the Mowry–Thermopolis [Major] confining unit, including:
    • The Mowry [Major] confining unit
    • The Muddy Sandstone aquifer
    • The Thermopolis [Major] confining unit
  • the Cloverly [Major Sandstone] aquifer
  • the Morrison confining unit and [Minor] aquifer
  • the Sundance confining unit and [Marginal] aquifer
  • the Gypsum Spring confining unit and [Marginal] aquifer
  • the Nugget [Major Sandstone] aquifer

The Triassic/Permian Chugwater-Dinwoody [Marginal] aquifer and confining unit, where it contains only the Chugwater aquifer, separates the overlying lower and middle Mesozoic aquifers and confining units from the underlying Paleozoic aquifer system. Where the Chugwater–Dinwoody contains both the Chugwater and Dinwoody hydrogeologic units, the Chugwater [Marginal] aquifer alone separates the two aquifer systems and the Dinwoody confining unit is included in the Paleozoic aquifer system.

The Paleozoic aquifer system comprises Permian through Ordovician carbonate and sandstone hydrogeologic units. The hydrogeologic units that compose the Paleozoic aquifer system are well known for producing high volumes of groundwater at and near the flanks of the Laramide uplifts surrounding the basins where permeability has been structurally enhanced by solution-enlarged fractures. Shale strata within the Phosphoria Formation, the Goose Egg Formation, and the Horseshoe Shale Member of the Amsden Formation constitute the only significant confining units within the Paleozoic aquifer system. The hydrogeologic units that compose the aquifer system are, from youngest to oldest:

  • the Goose Egg–Phosphoria [Marginal] aquifer and confining unit or Goose Egg [Marginal] aquifer and confining unit, including:
    • the Dinwoody confining unit [Marginal aquifer]
    • the Phosphoria [Minor] aquifer and confining unit
  • the Tensleep aquifer, including:
    • the Tensleep [Major Limestone] aquifer
    • the Ranchester Limestone Member of the Amsden Formation [Marginal aquifer]
  • the Amsden confining unit (Horseshoe Shale Member of the Amsden Formation) [Marginal aquifer]
  • the Madison–Bighorn [Major Limestone] aquifer, including: a basal confining unit
    • the Madison [Major Limestone] aquifer, including:
    • the Darwin Sandstone Member of the Amsden Formation [Marginal aquifer]
    • the Madison Limestone [Major Limestone aquifer]
  • the Darby [Major Limestone] aquifer
  • the Bighorn [Major Limestone] aquifer

The Paleozoic aquifer system is underlain by the Cambrian Gallatin–Gros Ventre confining unit, the Cambrian Flathead aquifer, and the Precambrian basal confining unit:

  • the Gallatin–Gros Ventre confining unit, including:
    • the Gallatin confining unit [Minor aquifer]
    • the Gros Ventre confining unit [Minor aquifer]
  • the Flathead [Major Limestone] aquifer
  • the Precambrian basal [Major] confining unit

The Precambrian basement complex is a marginal aquifer locally where it is exposed and fractured in the cores of the uplifts, where thin and disconnected soil, deeply weathered bedrock, and shallow fractures provide transmissive zones. Where these conditions do not exist at land surface, and virtually everywhere in the subsurface, the Precambrian basement is the basal confining unit for the structural groundwater basins in the WBRB and throughout Wyoming.


Reference View complete Wind/Bighorn Basin Water Plan