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<br />(Farnsworth and others, 1982, map 3). Annual precip-
<br />itation along the San Juan River averages about 8 in.
<br />and average potential evaporation is about 65 in. Max-
<br />imum annual precipitation may exceed 30 in. in the
<br />adjacent mountains, most of which falls as snow
<br />between October and April. Average daytime summer
<br />temperatures in the study area reach 90 to 100 OF
<br />
<br />GeolClgic Setting
<br />
<br />Consolidated sedimentary rocks of Jurassic and
<br />Cretaceous age crop out in the study area (fig. 3). The
<br />largest percentage of outcropping rock consists of the
<br />different members of the Jurassic-age Morrison Forma-
<br />tion, particularly the Brushy Basin, Westwater Canyon,
<br />and Recapture Members (fig. 4). Strata are predomi-
<br />nantly sandstone, with intervening beds of siltstone,
<br />variegated shale and mudstone, and conglomerate.
<br />East of the community of Montezuma Creek, the San
<br />Juan River downcuts through the lower part of these
<br />shaly units. The basal Bluff Sandstone Member of the
<br />Morrison Formation crops out in the southwestern and
<br />western parts of the study area, panicularly along the
<br />San Juan River downstream from its confluence with
<br />Montezuma Creek. It is the primary water-bearing unit
<br />of the Morrison Formation in the subsurface where it is
<br />overlain and confined by other members of the Morri-
<br />son Formation. Maximum thickness of the Bluff Sand-
<br />stone is about 350 ft near the community of Bluff. The
<br />Cretaceous-age Burro Canyon Formation and Dakota
<br />Sandstone cap the Morrison Formation (Brushy Basin
<br />Member) on buttes and mesas throughout the study
<br />area (figs. 3 and 4).
<br />Quaternary-age alluvial deposits overlie the con-
<br />solidated rocks in valleys and washes, and eolian
<br />deposits locally cover the consolidated rocks on upland
<br />areas, particularly in the southwestern pans of the study
<br />area. Fine-grained alluvial deposits are present along
<br />the floodplain of the San Juan River, and cobbly Qua-
<br />ternary-age deposits cover adjacent terraces. The allu-
<br />vial deposits are less than 30 ft thick, except along the
<br />San Juan River, and generally yield freshwater to wells.
<br />Triassic-to Jurassic-age rocks underlie the study
<br />area to a depth of about 2,600 ft and include the princi-
<br />pal hydrogeologic units, the Wingate, Navajo, and
<br />Entrada Sandstones (fig. 3). The Navajo and Entrada
<br />Sandstones crop out only in the extreme southwestern
<br />part of the study area. Strata consist primarily of mas-
<br />sive beds of eolian sandstone, with interbeds of silt-
<br />stone and shale. The Kayenta and Carmel Formations
<br />
<br />are semiconfining units between the Wingate and
<br />Navajo, and the Navajo and Entrada Sandstones,
<br />respectively. Average combined thickness of all of
<br />these formations in the Aneth area is about 925 fl. The
<br />Triassic-age Chinle and Moenkopi Formations underlie
<br />the Wingate Sandstone (fig. 3). This massive sequence
<br />of shales and siltstones averages about 1,165 ft thick
<br />throughout the study area and is the principal confining
<br />unit between aquifers in Mesozoic and Paleozoic rocks.
<br />Permian-age rocks of the Cutler Formation,
<br />which includes the DeChelly Sandstone Member,
<br />underlie the Moenkopi Formation (fig. 3) but generally
<br />contain saline water. The Cutler Formation is about
<br />1,950 ft thick in the vicinity of the Greater Aneth Oil
<br />Field and thickness of the DeChelly Sandstone ranges
<br />from 60 to about 230 fl. To the north of the Greater
<br />Aneth Oil Field in T 38 S., R. 23 and 24 E., and T 39
<br />S., R. 25 E., water is withdrawn from aquifers in the
<br />Cutler Formation and lower part (Shinarump Member)
<br />of the Chinle Formation for use in injection operations.
<br />Rocks of the Pennsylvanian-age Hermosa Group
<br />underlie the Cutler Formation (fig. 3) and consist of
<br />interbedded shale, sandstone, and carbonates (lime-
<br />stone and dolomite), with lesser thicknesses of evapor-
<br />ites (gypsum, anhydrite, and salt). Hydrocarbons are
<br />present in carbonate rocks of the Ismay and Desert
<br />Creek zones of the Paradox Formation at depths of
<br />5,000 to 6,000 ft below land surface and are the princi-
<br />pal oil-bearing units in the Greater Aneth Oil Field and
<br />vicinity. The underlying Mississippian-age Leadville
<br />Limestone contains only saline water within the study
<br />area boundaries.
<br />The study area encompasses the southern part of
<br />the west-east trending Blanding Basin, a structural sub-
<br />basin of the Paradox Basin (Howells, 1990, fig. 6, p.
<br />12). During the Middle Pennsylvanian Period, the Par-
<br />adox Basin developed a restricted-circulation hypersa-
<br />line environment with periodic influxes of normal
<br />marine water, which resulted in cyclic deposition of a
<br />black shale-carbonate-evaporite sequence (Howells,
<br />1990, p. 24). The deepest part of the Blanding Basin is
<br />about 15 mi northeast of Bluff, Utah, and strata gener-
<br />ally dip toward the San Juan River from the north and
<br />south, at less than 2 degrees (fig. 3).
<br />The Comb Ridge Monocline is a prominent
<br />north-trending structural feature west of the study area.
<br />Pennsylvanian-age strata that are more than I mi deep
<br />in the Montezuma Creek/Aneth area are at the surface
<br />only 25 mi to the west because of the upwarp. The
<br />adjacent Abajo, Sleeping Ute, and Carrizo Mountains,
<br />to the north, east, and south of the study area, respec-
<br />
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