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Regional unconformities are evidenced in Lower Cretaceous sedimentary rocks in the following stratigraphic positions: 1) at the base of the Lakota sandstone; 2) at the base of the Fall River sandstone; 3) at the base of the Muddy sandstone and 4) possibly in the upper part of the Fall River Formation. The Lakota unconformity was eroded by north-flowing rivers into nonmarine Morrison mudstones; hence, it involves no marine sedimentary rocks. The other unconformities, including that at the base of the Wall Creek member of the Frontier Formation, involve mainly marine sedimentary rocks both above and below the unconformities. Lacking precise fossil evidence in the Lower Cretaceous unconformities, absolute proof of the presence of the regional unconformities in specific outcrops is generally interpretative and in part ambiguous. Above the Fall River and Muddy unconformities, fluvial sandstones are locally present in paleovalleys, but no fluvial sandstones have yet been identified in Central Wyoming above the Wall Creek unconformity. The locally preserved fluvial sandstones in paleovalleys evidence the streams and rivers that eroded the unconformities. Generally, the unconformities were eroded into brackish-marine shales and mudstones; however, in the Lander area, regressive marine sandstones are preserved below the lower Muddy unconformity and flood-plain mudstones are preserved above the fluvial Muddy sandstones. Apparently, there was considerable reworking of the thin nonmarine deposits on the unconformities by marine currents in the seas that transgressed into Central Wyoming because outside the paleovalleys, it is common to find transgressive marine sedimentary rocks immediately above the unconformities. Specific outcrops in the Grieve paleovalley are cited to illustrate an example of the variety of different depositional environments that accompanied the advance of the sea into the paleovalley and onto the paleohills of the lower Muddy unconformity of this area.
Acknowledgments and Associated Footnotes
1 William H. Curry, III: Consultant, Casper, Wyoming
© Wyoming Geological Association, 2015