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QUATERNARY GEOLOGY OF THE HOTCHKISS-PAONIA RESERVOIR AREA <br />• DELTA AND GUNNISON COUNTIES, COLORADO <br />GENERAL <br />The Hotchkiss-Paonia Reservoir area includes the North Fork <br />Gunnison River and its tributaries in Northwestern Gunnison and <br />eastern Delta Counties. Six contiguous 7~S-minute quadrangles are <br />in the area studied; they are Gray Reservoir, Bowie, Somerset, <br />Paonia Reservoir, Hotchkiss, and Paonia. The quaternary geology <br />of each quadrangle is mapped at 1:24,000 and represents the <br />general geologic evolution of the area during the latest period <br />of geologic time. Maps of quaternary geology were prepared to <br />define young geologic units with common characteristics and to <br />identify potentially hazardous geologic conditions. The geologic <br />relationships of the Quaternary depicted on these maps are used a <br />technical basis for the compilation and interpretation of the <br />companion geologic-hazard maps. <br />MAP UNITS <br />The general geologic setting of this area is relatively simple: <br />flat-lying Cretaceous and Tertiary strata locally intruded by <br />igneous rocks. The sequence was sculpted during the late <br />Tertiary and Quaternary by erosional processes that formed the <br />• present mesa and mountain landscape. Various thin Quaternary <br />deposits, probably related to glacial stages, suggest that <br />fluvial and mass-wasting activities were the dominate geomorphic <br />processes in the formation of the present landscape. <br />Glacial, colluvial, and alluvial Quaternary deposits are shown on <br />the maps and described in the Explanation of Mao Units. Glacial <br />deposits include those surficial units which were transported and <br />deposited by ice movement or by glacial meltwater. Most of these <br />deposits probably are late Wisconsin, although older deposits may <br />be present locally. Alluvial deposits are clay to boulder-size <br />material transported and deposited by running water (fluvial <br />processes). Although age of the alluvial deposits is unknown, <br />the map units are placed in a general time framework based upon <br />available geomorphic evidence. Age and correlation of specific <br />deposits within the alluvial gravels (Qag) are based on soil <br />development, in-place weathering of igneous-rock fragments, and <br />position above modern streams. Diverse lithology of the <br />deposits, caused by extreme variations in local source areas, <br />makes definite correlations difficult and thus those indicated <br />are only approximate. Additionally more than one depositional <br />cycle, as indicated by several terrace levels, may be included in <br />a specific map unit. colluvial deposits generally are <br />unconsolidated soil and/or rock detritus deposited chiefly by <br />mass-wasting processes. Relative age of these deposits is noted <br />. by subscripts after the map symbol. Subscripts range from 1 to <br />scnxxx wecxcArxon EXH-12 - 7 - <br />