Chapter 4 Geologic Setting Alan Ver Ploeg, Karl Taboga and Paul Taucher 4-27 he Platte River Basin drainage basin configuration of these elements and relationships comprises approximately 24,106 square among them influence the availability of miles (15.43 million acres) in Wyoming, groundwater. The geologic history relevant to Teffectively covering the southeast corner of groundwater resources of the Platte River Basin the state. The geologic setting for the basin is, begins with the nonconformable deposition of therefore, both extensive and complex in that it transgressive marine sediments onto Precambrian encompasses 4 major Laramide structural basins, basement rocks during Middle Cambrian time overlies small parts of five other Laramide basins (Libra and others, 1981; Richter, 1981; Snoke, and includes five mountain ranges of the southern 1993). From that time forward, a general geologic Rocky Mountains as well as several smaller history that describes the development of the uplifts. So, a complete description of the geologic stratigraphic and structural elements the Platte framework of the Platte River Basin must include River Basin is as follows: summary accounts of the assemblages of geologic and hydrogeologic units and structural elements 1. Paleozoic strata in the Platte River Basin that define each of these major structures. To were deposited in marine and nonmarine accomplish this, an extensive set of figures and transgressive /regressive environments. maps, presented as plates, have been prepared for Marine limestones and dolomites are the inclusion in this report: dominant lithologies of the Paleozoic sequence, with less extensive sandstones • Plate 1 illustrates the bedrock geology and shales that represent beach and near- of the Platte River Basin in Wyoming, shore environments. Deposition in the Colorado and Nebraska overlain on a base Paleozoic Era was broken by long periods map that shows highway, township, state of erosion, indicated in the geologic record and county data. Inset maps present the by several regional unconformities. elevations of the Precambrian basement 2. The early Mesozoic Era was a time when and lineaments (linear geologic features). shallow seas deposited interbedded layers Appendix A contains detailed descriptions (in decreasing abundance) of sandstone, of the geologic units shown in Plate 1. siltstone, shale, carbonates, and evaporites. • Plate 2 displays an outcrop map of An emergent transition to a terrestrial hydrogeologic units in the Platte River environment during the Late Triassic Basin developed by correlating the and Early Jurassic Epochs resulted in the geospatial data of hydrogeologic units with deposition of marginal marine, eolian, hydrostratigraphic nomenclature charts fluvial, and paludal sandstones and shales. (Plates J, K, M, S, T, and U; Figure 7-2). 3. During the Early Cretaceous Epoch The general hydrogeology of the Platte a thick section of interbedded shale, River Basin is discussed in Chapter 7. sandstone, siltstone, and claystone was Individual Platte River Basin aquifers are deposited under terrestrial, shallow marine discussed in detail in Chapter 7, also. and deltaic conditions. Late Cretaceous • Nineteen cross sections, contained in this transgressions and regressions resulted in a chapter (Figures 4-2 through 4-20), show thick sequence of interbedded sandstone, typical subsurface structure in the Platte siltstone, claystone, and shale deposited River Basin. Isopach maps with substantial in marine, marginal marine, coastal coverage of the major aquifers in the Platte plane, and deltaic environments. Crustal River Basin are not available. deformation associated with the Laramide Orogeny began in the Late Cretaceous; 4.1 General/historical geology the Lance Formation recorded the final, eastward retreat of the Cretaceous seas The Platte River Basin contains simple to followed by the deposition in terrestrial complex stratigraphic and structural elements. The environments that would prevail 4-28 throughout the Tertiary Period. feet (Richter, 1981) of Cenozoic, Mesozoic, and 4. Laramide compressional deformation Paleozoic sediments deposited on Precambrian continued through the early Eocene crystalline basement rocks (Libra et al., 1981; with large-scale reverse and thrust faults Richter, 1981). The structural basins are bordered forming the basement-cored mountain by compressional uplifts cored by Precambrian ranges and uplifts that surrounded and granite and mantled by moderately to steeply separated the concurrently subsiding dipping sedimentary formations (Libra et al., 1981). structural basins that make up the Platte Paleozoic and Mesozoic formations exposed along River Basin. The uplifted areas were the flanks of the mountain ranges surrounding the source of several thousand feet of the Platte River Basin were folded, faulted, and Tertiary sediments composed of Mesozoic, eroded from the highest areas of the uplifts during Paleozoic and Precambrian rocks that the Laramide Orogeny; they now dip basinward were eroded from the uplifts and filled at angles ranging from approximately 5 degrees the basins to the extent that all but the to vertical, and some are overturned. Strata of highest areas of the surrounding uplifts Paleocene through early Eocene age are also were buried. These strata are composed of deformed around the perimeters of these structural conglomerates, sandstones, and claystones basins but are mostly flat lying in the interior basin deposited primarily in fluvial, alluvial fan, areas. Numerous anticlinal structures with associated and lacustrine environments. faults that formed during the Laramide Orogeny 5. Late Tertiary normal faulting concurrent crop out along the margins of the structural basins. with modest extension occurred Section 5.4 discusses the substantial influence that throughout Wyoming. Uplift during the structures, primarily those located around the basin past 5 million years over a broad area perimeters, exert on groundwater recharge, flow and that encompasses the Platte River Basin storage. resulted in the erosion and removal of The topography of the Platte River Basin an enormous volume of Tertiary strata, generally reflects the structure and topography exhuming the Laramide framework and of the Precambrian basement surface formed by sculpting the present physiography of the uplift, folding, faulting, and erosion of the earth’s Platte River Basin crust under compressional stress during the Sevier 6. The youngest geologic units in the basin are and Laramide orogenies. Downwarping of the unconsolidated Pliocene and Quaternary structural basins and upwarping and faulting of the terrace deposits and Quaternary alluvial uplifts were concurrent; and the upper strata within deposits of various thicknesses. These the basin interiors are composed of Tertiary-age deposits, some as much as several hundred sediment that was eroded from the adjacent uplifts. feet thick, are composed of conglomerate, The structure contour map of the Precambrian gravel, sand, and finer-grained clastic basement surface in the Platte River Basin shown material. The age and occurrence of these on the lower inset map on Plate 1 shows a general deposits have been correlated with recent northwest-southeast structural trend. The geologic glacial and interglacial periods by Mackin cross sections on Figures 4-2 through 4-20 (1937). show Precambrian basement rocks overlain by varying thicknesses of Paleozoic through Cenozoic 4.2 Structural geology formations, all deformed by large-scale folding and faulting. The Laramide age basins, contained either The major Laramide structural elements of the wholly or partly within the Platte River drainage Platte River Basin (Figure 4-1) comprise: basins, are small (Carbon basin) to large (Denver • The folded and faulted Precambrian basin) asymmetric intermontane structural basins basement formed during the Laramide Orogeny (Late • The deeply buried downwarped areas of Cretaceous-Eocene) that contain up to 35,000 the Denver, Laramie, Hanna-Carbon, 4-29 Shirley, Wind River and Powder River in the basins is an ongoing process. Accordingly, basins the stratigraphic sections preserved in interior • The mountain ranges and uplifts that basin areas are most complete, and stratigraphic surround and separate the basins: sections are less complete to non-existent at o The Laramie, Medicine Bow and higher elevations in the surrounding mountain Sierra Madre/Park mountain ranges. In some places Tertiary and Quaternary ranges deposits directly overly Precambrian basement o Green Mountain, Ferris rocks. Mountains, Seminole, and Shirley Mountains 4.4 Granite Mountains, Green o The Granite Mountains Mountain, Ferris Mountains, Seminoe o The Casper Arch and Hartville Mountains, Shirley Mountains and Uplift Freezeout Mountains (Lillegraven and o The Rawlins Uplift Snoke, 1996; WSGS, 2013) o The Wind River Range o The structural basins Together, these Rocky Mountain foreland . Denver Basin features form the southern flank of the Sweetwater . Laramie Basin Arch which extends over 100 miles from the . Shirley Basin western edge of the Granite Mountains to the . Hanna-Carbon Basin eastern Freezeout Mountains. This area separates . Saratoga Valley the Wind River basin from the Great Divide and . Great Divide Basin Hanna basins, and in a general sense connects the . Powder River Basin southern end of the Wind River Range with the . Wind River Basin northern Laramie Mountains. There are many subsidiary structures within 4.4.1 Granite Mountains (Love,