U.S. DEPARTMENT OF THE INTERIOR U. S. GEOLOGICAL SURVEY Preliminary Geologic Map of the SILT Quadrangle, Garfield County, Colorado by RALPH R. SHROBA, GEORGE M. FAIRER, AND MORRIS W. GREEN(1) Open-File Report 94-696 This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although these data have been used by the U.S. Geological Survey and have been successfully imported into data base programs, no warranty, expressed or implied, is made by the USGS as to how successfully or accurately the data can be imported into any specific application software running on any specific hardware platform. The fact of distribution shall not constitute any such warranty, and no responsibility is assumed by the USGS in connection therewith. 1994 (2) This database of the Silt 7.5 minute quadrangle, originally released as U.S Geological Survey Open-File Report 94-696, has been updated to include a digitized database. The digitized version is also identifed as OFR 94-696. The Digital database is in ARC/INFO Format. Although this database has been subjected to rigorous review and is substantially complete, the USGS reserves the right to revise the data pursuant to further analysis and review. Furthermore, it is released on condition that neither the USGS nor the United States Government may be held liable for any damages resulting from its authorized or unauthorized use. This digital geologic map does not include a digital (DRG) topographic data file as it is currently unavailable. When the DRG database becomes available it can be combined with the geologic database. The geologic database can be downloaded via 'anonymous ftp' from a USGS system named greenwood.cr.usgs.gov (136.177.48.5). The files are located in a directory named /pub/open-file-reports/ofr-94-696. The database manager is: Ralph R. Shroba (303) 236-1293 U.S. Geological Survey Box 25046, Mail Stop 913 Denver, CO 80225 This geologic map was prepared as part of a study of digital methods and techniques as applied to complex geologic maps. The geologic map was scanned from the original greenline sheets used to prepare the published Open-File map of the Silt quadrangle (Shroba and others, 1994). Consequently, the digital version is at 1:24,000 scale using the Polyconic map projection parameters of the topographic base map. Stable base contact prints of the greenline sheets were scanned on an Ana Tech Eagle 408ET scanner to produce a .tif file. These files were then transferred to a Sun SPARCstation 20 computer where they were converted into ARC/INFO format. Each vector and polygon was given attributes derived from the original 1993 geologic map using ALACARTE (Wentworth and Fitzgibbon, 1991) software. This database was developed on a SPARCstation 20 computer system using ARC/INFO 7.0.2 software. The lineset and shadeset files are coded for a HP DesignJet 650C plotter using HPGL2 software. List of files: alcgeol.e00(a) symbol set alcwrg.e00(a) line set ctr269.e00(b) shade set fnt003.e00(c) font set loadsk.aml load program loess.e00 shade set mapbar.aml(b) mapbar program st.box index box program st.text index text st3.e00 line and polygon data set stex.eml map assembly program stl.e00 loess polygon data set stlb.box loess index box program ststr.e00 structure data set wyo.e00(c) text set st.doc text (this file) README.SK readme text seals.e00(c) logo polygon set (a) T.T. Fitzgibbon (b) W.R. Beaman (c) G.R. Green The authors acknowledge the assistance of G.N. Green in preparation of the digitized version of this geologic map. Line Types and Attributes Line Types ALCWRG Contact, certain 25 Contact, concealed 29 Normal fault, certain 1 Normal fault, inferred 3 Normal fault, concealed 5 Anticline 31 Syncline 31 Monocline 31 Water boundary 25 Polygon Types and Attributes P-types ALCWRG af 24 Qty 38 Qto 34 Qtt 40 Qg 60 Qfy 1 Qac 3 Qp 218 Qc 2 Qsw 5 Qls 4 Qta 36 Qdo 65 Qlo 37 Tw 62 Kmv 78 Km 80 (2) Denver, Colorado Preliminary Geologic Map of the Silt Quadrangle, Garfield County, Colorado By Ralph R. Shroba, George M. Fairer, and Morris W. Green CORRELATION OF MAP UNITS Refer to non-digitized version of this open-file report for a diagram showing correlation of map units DESCRIPTION OF MAP UNITS [Surficial deposits shown on the map are estimated to be at least 3 ft thick. Thinner deposits are not shown. Fractional map symbols (for example, Qlo/Qty) are used where loess mantles older surficial deposits and these underlying deposits have been identified. Thin, discontinuous colluvial deposits and residual material on bedrock were not mapped. Also not mapped are four elongate areas above mined coal beds on the Grand Hogback, near Harvey Gap and the Tipple Mine near the northwestern corner of the map area, where the ground surface is highly oxidized, fractured, and prone to subsidence (Stover and Soule, 1985). Divisions of Pleistocene time correspond to those of Richmond and Fullerton (1986). Age assignments for surficial deposits are based chiefly on the degree of modification of original surface morphology, height above stream level, and degree of soil development. Age assignments for units Qtt and Qdo are inferred chiefly on the basis of regional rates of stream incision of about 45 ft/100 k.y. (k.y., thousand years) and 530 ft/m.y. (m.y., million years) and on a regional rate of tectonic uplift of about 600 ft/m.y. The first incision rate is based on an average of three values for stream incision since the deposition of the 620-ka (ka, thousand years) Lava Creek B volcanic ash: (1) about 300 ft along the Colorado River near the east end of Glenwood Canyon (Izett and Wilcox, 1982), (2) about 290 ft along the Roaring Fork River near Carbondale, Colo. (Piety, 1981), and (3) about 260-280 ft along the White River near Meeker, Colo. (J.W. Whitney, oral commun., 1992; Whitney and others, 1983). The second incision rate is based on about 4,000 ft of down cutting by the Colorado River since the deposition of the 7.5-Ma (Ma, million years) basalt on Battlement Mesa (Stover, 1984, 1988), about 8 mi southwest of the map area. The rate of tectonic uplift of about 6,000 ft/10 m.y. was determined for the Derby Peak fauna in the Flat Tops area (Colman, 1985), about 37 mi northeast of the map area. Soil-horizon designations are those of the Soil Survey Staff (1975) and Guthrie and Whitty (1982). Most of the surficial deposits are calcareous and contain different amounts of primary and secondary calcium carbonate; stages of secondary calcium carbonate morphology are those of Gile and others (1966). Grain-size terminology for the surficial deposits is based on visual estimates and follow the modified Wentworth grade scale (American Geological Institute, 1982). In descriptions of surficial deposits, the term clasts refers to the fraction greater than 0.08 in. (2 mm) in diameter, whereas the term matrix refers to particles less than 2 mm in size. Dry matrix colors of the surficial deposits in the map area were determined by comparison with Munsell Soil Color Charts (Munsell Color, 1973). These deposits are commonly light brownish gray (2.5Y 6/2), pale yellow (2.5Y 7/4), light gray (10YR 7/2), very pale brown (10YR 7/3, 8/3, 7/4, and 8/4), pale brown (10YR 6/3), light yellowish brown (10YR 6/4), light brown (7.5YR 6/4), and pink (7.5YR 7/4). manmade deposits-Sand and finer material and rock fragments in fills in the valley of the Colorado River and on the Grand Hogback af Artificial fill (latest Holocene)--Compacted and uncompacted fill material composed mostly of varying amounts of silt, sand, and rock fragments. The unit includes major fills beneath segments of Interstate 70; minor fills beneath segments of U.S. Highway 6 and the adjacent tracks of the Denver and Rio Grande Western Railroad; the dam at the south end of Grass Valley Reservoir; and dump fills at coal mines on the Grand Hogback near the northwestern corner of the map area. Thickness generally less than 40 ft Alluvial deposits-Silt, sand, and gravel beneath flood plains, in stream channels, and beneath terraces along the Colorado River and Mamm Creek; in alluvial fans on flood plains and terraces; and in pediment deposits on gently sloping surfaces cut on bedrock Qfp Flood-plain and stream-channel deposits (Holocene and late Pleistocene)--Along the Colorado River, the upper 5-20 ft of the unit is commonly slightly silty sand to slightly clayey, silty, very fine to medium sand that locally contains a minor amount of pebbles and cobbles. The lower 10-15 ft of the unit is chiefly clast-supported, slightly bouldery, cobbly pebble gravel with a sand matrix. Deposits along Mamm Creek commonly contain more sand and silt than those along the Colorado River. Poorly to moderately well sorted and poorly to well stratified. Clasts are commonly subangular to rounded; those along the Colorado River are derived from a variety of sedimentary, igneous, and metamorphic rocks exposed upstream, whereas clasts along Mamm Creek are mostly sandstone and basalt. Low-lying areas of unit Qfp are prone to periodic flooding. Flood-plain deposits along Mamm Creek that are composed of sandy silt are prone to extensive piping and gullying. Locally includes unmapped alluvial-fan deposits (Qfy), low terrace deposits that are commonly less than 15 ft above stream level, and sheetwash deposits (Qsw) that are too small to map separately. The upper part of the unit may be a complex of alluvial cut-and-fill deposits of Holocene and late Pleistocene age. The lower part of the unit is probably equivalent, at least in part, to the younger terrace alluvium (Qty). The unit is tentatively correlated with deposits of terrace T8 along the Roaring Fork River between Glenwood Springs and Carbondale, Colo. (Piety, 1981). Thickness of channel deposits along the Colorado River is about 10-15 ft and thickness of adjacent flood- plain deposits is about 15-30 ft (Colorado Highway Department, unpublished data); maximum thickness along the Colorado River probably be about 50 ft Qty Younger terrace alluvium (late Pleistocene)--Stream alluvium that underlies terrace remnants that are about 30- 40 ft above the Colorado River and about 50 ft above Mamm Creek. On the south side of the Colorado River, the unit consists mostly of a poorly sorted, clast-supported, slightly bouldery, cobbly pebble gravel with a sand matrix. The clasts are commonly subrounded to rounded fragments of sedimentary, igneous, and metamorphic rock that were deposited by the Colorado River. On the north side of the Colorado River, the upper 30 ft of the unit commonly consists of sand, silty sand, and sandy silt that contain thin beds and lenses of cobbly pebble gravel to pebbly sand. The lower part of the unit is mostly a poorly sorted, clast- supported, slightly bouldery, cobbly pebble gravel with a sand matrix. The clasts are almost entirely sandstone and were deposited by south-flowing tributary streams. Along Mamm Creek, the unit is mostly a poorly sorted, clast- supported, slightly bouldery, cobbly pebble gravel with a sand matrix. The clasts along Mamm Creek are mostly sandstone and basalt and moderate to minor amounts of siltstone and marlstone. The unit is mapped in deep gullies and on a gently sloping stream cut near Rifle. Much of the unit is overlain by one loess sheet (Qlo) about 6-10 ft thick and locally by younger fan alluvium (Qfy). Unit Qty is probably equivalent in part to outwash of the Pinedale glaciation, which is about 12-35 ka (Richmond, 1986, chart 1A). The unit is tentatively correlated with deposits of terraces T7 and T6 along the Roaring Fork River between Glenwood Springs and Carbondale (Piety, 1981), and with deposits of terraces A and B farther upstream between Woody Creek and Aspen (Bryant, 1979). Exposed thickness 12-35 ft along the Colorado River and 7 ft along Mamm Creek. Maximum thickness along the Colorado River probably about 55 ft Qto Older terrace alluvium (middle Pleistocene)--Stream alluvium that underlies terrace remnants about 120-160 ft above the Colorado River and Mamm Creek. Along the Colorado River, the upper part of the unit was deposited by tributary streams and the lower part was deposited by the Colorado River. The upper 30-40 ft of the unit commonly consists of slightly silty to silty sand that is locally slightly pebbly and locally contains thin, discontinuous beds and lenses of poorly to moderately well sorted, clast-supported, cobbly pebble gravel and pebble gravel with a sand matrix. The lower 10-30 ft of the unit is mostly poorly to moderately well sorted, clast-supported, cobbly pebble gravel with a sand matrix. Locally thin layers near the base of the unit are cemented by fine-grained calcium carbonate. The clasts in the upper part of the unit are mostly sandstone on the north side of the Colorado River and basalt and sandstone on the south side. The clasts in the lower part of the unit are chiefly subrounded to rounded sandstone, gneiss, quartzite, basalt, granodiorite(?), limestone, and dolomite. Along Mamm Creek, the unit commonly consists of poorly sorted, clast-support, cobbly pebble gravel with a sand matrix, and locally thin beds and lenses of slightly bouldery, cobbly pebble gravel, pebble gravel, and pebbly sand. The clasts in the unit along Mamm Creek are angular to subrounded basalt, sandstone, siltstone, and marlstone. The unit is mantled by about 5-19 ft of loess (Qlo) that commonly consists of two sheets and locally by sheetwash deposits (Qsw) and colluvium (Qc). The north-trending, loess-mantled deposit of older terrace alluvium in the eastern parts of secs. 17, 20, and 29, T. 6 S., R. 92 W. marks a former course of Mamm Creek. A buried soil is locally formed in the upper part of the unit. This soil has an argillic B horizon, 24 in. thick, that is formed in silty alluvium or in the overlying loess. It overlies a stage III K horizon, 48 in. thick, that is formed in the upper part of the underlying gravel. The morphologic development of the soil and the presence of the two overlying loess sheets suggest that the unit is of Bull Lake age (Shroba, 1989; Pierce and others, 1982) and may be about 140-150 ka (Pierce and others, 1976; Pierce, 1979) or about 130-300 ka (late middle Pleistocene; Richmond, 1986, chart 1A). The unit is tentatively correlated with deposits of terraces T5 and T4 along the Roaring Fork River between Glenwood Springs and Carbondale (Piety, 1981), and with deposits of terrace C farther upstream between Woody Creek and Aspen (Bryant, 1979). Exposed thickness 20-70 ft along the Colorado River and 5- 30 ft along Mamm Creek. Maximum thickness along the Colorado River probably about 80 ft Qtt Oldest terrace alluvium (middle Pleistocene)--Stream alluvium that underlies terrace remnants that are about 240, 300, 380, and 500 ft above the Colorado River, about 300 and 380 ft above Rifle Creek in the adjacent Rifle quadrangle, and about 160 ft above Dry Hollow Creek near its confluence with the Colorado River in the southeastern part of the map area. Along the Colorado River, the unit is mostly a poorly to moderately well sorted, clast-supported, non-bouldery to slightly bouldery, cobbly pebble gravel with a sand matrix. The unit locally consists of thin beds and lenses of pebble gravel and silty sand. It locally grades upward into moderately well sorted, clast-supported, pebble gravel with a sand matrix or poorly sorted, matrix-supported, cobbly and pebbly sand that is overlain by non-pebbly to slightly pebbly, slightly silty sand. Clasts are mostly subrounded to rounded and were derived from a variety of sedimentary, igneous, and metamorphic rocks exposed upstream. Biotite- bearing granitic clasts near the top of the unit are extensively weathered and are easily disintegrated. Deposits east of Rifle Creek in the northwestern part of the map area and the deposit along Dry Hollow Creek are mostly poorly sorted, clast-support, slightly bouldery to bouldery, cobbly pebble gravel with a sand matrix. The clasts in the deposits east of Rifle Creek are mostly of subangular to subrounded sandstone along with a minor amount of limestone and a trace amount of quartzite. The clasts in the deposit along Dry Creek are mostly subangular to subrounded sandstone and basalt. On the south side of the Colorado River, the upper part of the unit locally includes interbedded sand and gravel that was deposited by tributary streams. The clasts in these deposits are mostly subangular to subrounded sandstone and basalt. A stage III K soil horizon, 24-36 in. thick, is locally present in the top of the unit and the lower 3-6 ft of the unit is locally cemented by fine-grained calcium carbonate. The unit is locally mantled by 3-16 ft of loess (Qlo) that commonly consists of two or more sheets and locally by colluvium (Qc). Small deposits of oldest terrace alluvium near the western boundary of the map area, north of the Colorado River, may mark a former course or former courses of Rifle Creek. The unit may be correlative in part with other terrace deposits within 30 mi of the map area that contain or are overlain by the 620-ka Lava Creek B volcanic ash. The unit is tentatively correlated with deposits of terraces T3 and T2 along the Roaring Fork River between Glenwood Springs and Carbondale (Piety, 1981) and with deposits of terrace D farther upstream between Woody Creek and Aspen (Bryant, 1979). Exposed thickness 10-45 ft along the Colorado River, 5-17 ft east of Rifle Creek, and 9-12 ft along Dry Hollow Creek. Maximum thickness along the Colorado River possibly about 60 ft Qg Gravelly alluvium (middle Pleistocene)--Small scattered deposits of alluvium on ridges and hilltops about 40, 80, 200, 240, and 320 ft above unnamed intermittent streams and about 20-160 ft above adjacent pediment deposits (Qp) in the area between the Grand Hogback and the Colorado River. Much of the unit is poorly exposed, but it appears to consist mostly of poorly sorted, clast-supported, slightly bouldery to bouldery, cobbly pebble gravel with a silty sand matrix. The unit locally contains thin beds and lenses of cobbly pebble gravel and non-pebbly to pebbly, silty sand. Clasts are mostly subangular to subrounded sandstone along with a minor amount of limestone and trace amounts of chert and quartzite. Some of the sandstone boulders are as long as 8 ft. Locally there is greater than 6 ft of relief on the underlying Wasatch Formation (Tw). The unit is probably stream-channel deposits that accumulated during periods of stream incision that occurred prior to periods of extensive lateral stream cutting and deposition of pediment deposits (Qp). It is locally mantled by about 3-6 ft of loess (Qlo). Exposed thickness 3-63 ft; maximum thickness possibly about 80 ft Alluvial and colluvial deposits--Clay, silt, sand, and gravel in valley bottoms and pebbly, silty sand and sandy silt in sheets that locally mantle valley bottoms and the adjacent valley sides Qfy Younger fan alluvium (Holocene and latest Pleistocene)- -Mostly poorly sorted, clast- and matrix-supported, slightly bouldery, cobbly pebble gravel with a silty sand matrix, and locally pebbly and cobbly, silty sand that contains thin lenses of cobbly pebble gravel, pebble gravel, and sand. Near the mouth of Mamm Creek the unit consists mostly of sand to clayey silt (Colorado Highway Department, unpublished data). The unit locally contains boulders as long as 8 ft. Some of the larger boulders were probably deposited by debris flows. Poorly bedded; beds are commonly less than 3 ft thick. Clasts are commonly angular to subangular sandstone north of the Colorado River and angular to subrounded sandstone and basalt south of the Colorado River. Low-lying areas of unit Qfy are prone to periodic flooding. The unit is undissected and was deposited chiefly by small intermittent streams graded to the flood plains of modern streams (Qfp) and locally to the tops of terraces that are underlain by younger terrace alluvium (Qty). Locally includes unmapped deposits of intermittent streams and colluvium (Qac), younger debris-flow deposits (Qdy), and sheetwash deposits (Qsw) that are too small to map separately. The unit may include hyperconcentrated-flow deposits, which have characteristics that are intermediate between those of stream-flow and debris-flow deposits. Although older fan alluvium was not identified in the map area, the unit is named younger fan alluvium because it is correlative with younger fan alluvium (Qfy) mapped in the adjacent New Castle quadrangle where older fan alluvium (Qfo) is also mapped (Green and others, 1993a). Exposed thickness 10-50 ft; maximum thickness probably about 80 ft Qac Undivided alluvium and colluvium (Holocene and late Pleistocene)--Chiefly undifferentiated alluvial flood-plain and stream-channel deposits (Qfp) and younger fan alluvium (Qfy), and colluvial younger debris-flow deposits (Qdy), sheetwash (Qsw) deposits, and possibly hyperconcentrated- flow deposits. Some of these deposits probably grade laterally and vertically into each other. The alluvial deposits commonly consist of interstratified silty sand, sandy clayey silt, sandy silty clay, and thin beds and lenses of cobbly pebble gravel, pebble gravel, and pebbly sand. Sheetwash deposits are typically pebbly silty sand and sandy silt. Alluvial and colluvial deposits derived from the Mancos Shale (Km) commonly contain more silt and clay than those derived from the other bedrock units. Some of the alluvial deposits derived from the Mancos contains thin, buried, soil A horizons in the upper part of the unit and may contain expansive clays and have high shrink-swell potential. Unit Qac is prone to extensive gullying and piping and low-lying areas of the unit are prone to periodic flooding. Alluvial deposits form flood plains, low terraces, and small alluvial fans along intermittent streams. Sheetwash deposits locally mantle the valley bottoms and the adjacent valley sides. Exposed thickness of the alluvium 3-10 ft; maximum thickness possibly about 50 ft. Exposed thickness of the colluvium 3-5 ft; maximum thickness possibly about 20 ft Qp Pediment deposits (middle Pleistocene)--Gravelly alluvium and debris-flow deposits at two or more levels that overlie gently sloping surfaces cut on Mancos Shale (Km) and Wasatch Formation (Tw). Locally there is as much as 20 ft of relief on the underlying bedrock. Mostly poorly sorted, clast-supported, slightly bouldery, cobbly pebble gravel with a silty sand matrix, and lenses of poorly sorted, clast- and matrix-supported, cobbly pebble gravel to pebbly, silty sand. Clasts are chiefly angular to subrounded sandstone. Deposits north of the Colorado River also commonly contain a minor amount of limestone and locally minor to trace amounts of siltstone and quartzite. Deposits south of the Colorado River near Mamm Creek also contain moderate to minor amounts of basalt, siltstone, and marlstone. The unit probably includes minor, unmapped sheetwash deposits (Qsw) and colluvium (Qc), and possibly hyperconcentrated-flow deposits. Bouldery, debris-flow deposits are locally common in the upper part of the unit. Some of the sandstone boulders in the upper part of the unit near the Grand Hogback are as long as 14 ft. A stage III K soil horizon is formed in the upper 20-30 in. of the unit and some of the sandstone pebbles and cobbles in the upper 6 ft have weathered to sand-size particles. The unit is well exposed in steep-sided gullies and roadcuts, but only one deposit is mapped near the western boundary of the map area. Mantled locally by about 6-12 ft of loess (Qlo) that locally consists of two or more sheets. A reddish-yellow (5YR 6/6), argillic B horizon, 25 in. thick, is locally present in the top of the basal loess sheet. The lower limits of the unit are about 120-360 ft above the Colorado River and about 160 ft above Mamm Creek. The lower pediment deposits along the Colorado River and Mamm Creek appear to be graded to terrace remnants composed of older terrace alluvium (Qto) that are about 120-160 ft above the Colorado River and Mamm Creek. Exposed thickness commonly 6-24 ft; maximum observed thickness 37 ft; maximum thickness possibly about 50 ft Colluvial deposits-Clay, silt, sand, gravel, and angular rock fragments on valley sides and hill slopes that were mobilized, transported, and deposited by gravity and sheet erosion Qc Colluvium, undivided (Holocene and late Pleistocene)-- Mostly clast-supported, pebbly, cobbly, and bouldery gravel with a silty sand matrix, and gravelly, silty sand to clayey silt. Deposits derived from the Mancos Shale (Km) commonly contain more silt and clay than those derived from the other bedrock units. Some of the deposits derived from the Mancos Shale may contain expansive clays and have high shrink-swell potential. Typically unsorted to poorly sorted and unstratified to poorly stratified. Clasts are typically angular to subrounded; their lithologic composition reflects that of the bedrock units and the surficial deposits from which the colluvium was derived. The unit locally includes creep, sheetwash (Qsw), younger debris-flow (Qdy), landslide (Qls), and possibly hyperconcentrated-flow deposits, talus (Qta), thin loess (Qlo) on older, gently sloping colluvial deposits; deposits of alluvium and colluvium (Qac) in and along minor drainageways; and pediment deposits (Qp) on the north side of the Grand Hogback. These deposits are too small to map separately or lack distinctive surface morphology and could not be distinguished in the field or on aerial photographs. Exposed thickness 3-5 ft; maximum thickness probably about 20 ft Qdy Younger debris-flow deposits (Holocene and latest Pleistocene)--Small, lobate and digitate masses of debris and bouldery levees with well preserved surface morphology that were deposited by sediment-charged flows at and near the mouths of small valleys along the southern margin of the Grand Hogback near the northwestern corner of the map area. Much of the unit is poorly exposed, but it appears to consist mostly of very poorly sorted and very poorly stratified boulders to granules supported in a matrix of silty sand, and locally includes poorly sorted, clast- supported, bouldery, cobbly pebble gravel with a silty sand matrix in levees and lenticular beds. Clasts are mostly subangular sandstone that are as long as 6 ft. The top of the unit is less than 60 ft above stream level and is irregular due primarily to the (1) numerous boulders that mantle and protrude through the ground surface and (2) lack of a loess (Qlo) mantle. The unit may locally include minor stream-flow and hyperconcentrated-flow deposits. Exposed thickness 2-7 ft; maximum thickness possibly 50 ft Qsw Sheetwash deposits (Holocene and late Pleistocene)-- Mostly pebbly, silty sand and sandy silt that are derived chiefly from weathered bedrock and loess (Qlo) by sheet erosion. Common on gentle to moderate slopes in areas underlain by Mancos Shale (Km), Wasatch Formation (Tw), and loess. Low-lying areas of unit Qsw are prone to periodic sheet flooding. The unit locally includes deposits of loess, undivided alluvium and colluvium (Qac) in and along minor drainageways, and outcrops of Mancos and Wasatch that are too small to map separately. Exposed thickness 6-15 ft; maximum thickness probably about 30 ft Qls Landslide deposits (Holocene and late Pleistocene)-- Chiefly unsorted and unstratified rock debris that is commonly characterized by its lobate form and hummocky topography. Many of the landslides are complex (Varnes, 1978) and commonly formed on steep, unstable slopes that are underlain by Mesaverde Group rocks (Kmv) and the Wasatch Formation (Tw) on the south side of the Grand Hogback near the northwestern corner of the map area. Younger landslide deposits are commonly bounded upslope by crescentic headwall scarps and downslope by lobate toes. The unit includes debris-slide, rock-slide, debris-slump, earth-slump, slump- earth-flow, earth-flow, debris-flow deposits (Varnes, 1978), and possibly hyperconcentrated-flow deposits. The sizes and lithologies of the clasts and the grain-size distributions of the matrices of these deposits reflect those of the bedrock units and surficial deposits that were displaced by sliding, slumping, and flowing. The unit locally includes minor sheetwash (Qsw) and creep (colluvium, Qc) deposits that are too small to map separately. Exposed thickness 3-10 ft; maximum thickness possibly about 200 ft Qta Talus deposits (Holocene and late Pleistocene)--Mostly crudely sorted and stratified, angular, bouldery to pebbly rubble on steep slopes produced chiefly by rock fall from outcrops of Mesaverde Group rocks (Kmv). The matrix is mostly sand and silt; some of it may be of eolian origin. The upper part of the unit locally lacks matrix. Only two deposits were mapped near the northwestern corner of the map area. Maximum thickness possibly about 50 ft Qdo Older debris-flow deposits (early? Pleistocene)-- Mostly debris-flow deposits and a minor amount of stream alluvium that underlie a gently sloping geomorphic surface near the southwestern corner of the map area. The lower limit of the surface is about 840 ft above the Colorado River. The debris-flow deposits are chiefly very poorly sorted and very poorly stratified boulders to granules supported in a matrix of slightly clayey, silty sand to sandy, clayey silt, and locally includes lenticular beds of poorly sorted, clast-supported, bouldery, cobbly pebble gravel with a silty sand matrix. The deposits are commonly about 3-8 ft thick and are locally overlain by layers of slightly silty sand about 2-15 in. thick. Clasts are commonly randomly oriented and are angular to rounded siltstone, marlstone, sandstone, and basalt. Basalt clasts are as long as 5 ft. Stream alluvium is locally present near the top of the unit and commonly consists of poorly sorted, poorly stratified, clast-supported, slightly bouldery to bouldery, cobbly pebble gravel with a sand matrix, and lenses of cobbly pebble gravel and pebble gravel. The alluvium is mostly stream-channel deposits that are about 6-12 ft thick. The unit may locally include hyperconcentrated-flow deposits. The soil formed in the top of the unit consists of a stage IV K horizon 35 in. thick that overlies a stage III K horizon 4 in. thick and a stage II Bk horizon 20 in. thick. No buried soils were noted in the unit. The unit overlies the Wasatch Formation (Tw), but near its lower limit in the adjacent Rifle quadrangle it overlies unmapped terrace alluvium that is about 600 ft above the Colorado River. The unit is mantled by greater than 3 ft of loess (Qlo) and may be similar in age to the high-level basaltic alluvium (QTba) in the adjacent New Castle quadrangle (Green and others, 1993a). Exposed thickness 100 ft; locally about 110-220 ft thick in the adjacent Rifle quadrangle and about 70 ft thick in the nearby North Mamm Peak quadrangle Eolian deposits--Wind-deposited clay, silt, and sand that mantle level to gently sloping surfaces Qlo Loess (late and middle Pleistocene)--Wind-deposited, nonstratified, friable when dry, slightly plastic to plastic when wet, calcareous (6-18 percent calcium carbonate), slightly clayey, sandy silt. The grain-size distribution of the carbonate-free fraction of unweathered loess in and near the map area commonly consists of 22-46 percent sand (2-0.05 mm), 43-62 percent silt (0.05-0.002 mm), and 15-18 percent clay (<0.002 mm). About 55-75 percent of the unweathered loess is composed of very fine sand (0.01-0.05 mm) plus coarse silt (0.05-0.02 mm). Median grain size ranges from 0.03 to 0.05 mm. The unit is prone to sheet erosion, gullying, and compaction when wet, due in part to its low dry bulk density of about 90 lbs/ft3. Locally includes loess-derived sheetwash (Qsw) and creep (colluvium, Qc) deposits that are too small to map separately. Deposited during five episodes of eolian activity (Shroba, 1994). Deposition may have continued into Holocene time. Possible sources of the loess include flood-plain deposits of the Colorado River and its major tributaries; sparsely vegetated outcrops of Tertiary siltstone and mudstone in the Piceance Basin west of the map area (Tweto, 1979); and large areas of exposed sandstone in the Canyonlands region in southeastern Utah (Whitney and Andrews, 1983). However, the relatively high content of very fine sand plus coarse silt and the relatively high coarse silt/total silt ratios (about 0.7) of the unweathered loess suggest: (1) a relatively short distance of eolian transport and (2) the flood plain of the Colorado River, which aggraded primarily during glacial times in response to glacial and periglacial activity farther upstream, is the likely source of much of the loess (Shroba, 1994). The mapped distribution of loess is approximate, because it lacks distinct topographic expression. The unit commonly mantles level to gently sloping surficial deposits that are as old or older than the younger terrace alluvium (Qty). Younger terrace alluvium is locally mantled by one loess sheet; older terrace alluvium (Qto) is locally mantled by two loess sheets; and pediment deposits (Qp) and the oldest terrace alluvium (Qtt) are locally mantled by two or more loess sheets. The soil that is formed in the upper loess sheet on the older terrace alluvium commonly consists of the following horizons: an organic-enriched A horizon about 8 in. thick; a cambic B horizon about 4-8 in. thick; a weak to moderate prismatic, argillic B horizon about 8-16 in. thick; and a stage I Bk horizon greater than 30 in. thick. The buried soil that is formed in the lower loess sheet on the older terrace alluvium contains more clay and calcium carbonate than the soil in the upper loess sheet and commonly consists of the following horizons: a cambic B horizon about 8 in. thick; a moderate to strong prismatic, argillic B horizon about 22-28 in. thick that contains weak stage I-II calcium carbonate; a weak stage III K horizon about 16 in. thick; and a stage I-II Bk horizon about 12 to greater than 24 in. thick. Where the upper loess sheet is composed of very sandy silt, the soil formed in it has a weakly developed, non-prismatic argillic B horizons that is about 14 in. thick. If the upper and lower loess sheets on the older terrace alluvium correlate with loess units A and B, respectively, that are on and adjacent to the eastern Snake River Plain in eastern Idaho, then (1) the upper loess sheet in the map area accumulated between about 10-70 ka and is of late Pleistocene age and (2) the lower loess sheet accumulated during an interval that ended shortly after 140- 150 ka and is partly or entirely of latest middle Pleistocene age (Pierce and others, 1982) Exposed thickness 2-20 ft; commonly 3-15 ft thick; maximum thickness possibly about 25 ft Tw Wasatch Formation (Eocene and Paleocene)--Interbedded, variegated reddish-brown, reddish-purple, yellowish-brown, tan, and white conglomerate, conglomeratic sandstone, sandstone, siltstone, mudstone, and claystone unconformably overlying Mesaverde Group rocks (Kmv). Sediment is first cycle, poorly sorted, and contains abundant metamorphic and granitic rock fragments; locally includes a basal cobble conglomerate as much as 50 ft thick, designated the Ohio Creek Conglomerate by Bass and Northrop (1963, p.J58). Coarse clastic beds are trough crossbedded and vary in thickness, whereas fine-grained clastic beds are commonly thin. All beds are laterally discontinuous and most are lenticular; generally, the sequence fines upward. About 5,000 ft thick Deposition of Wasatch clastic sediments occurred in high- to low-energy, stream-dominated, fluvial and lacustrine environments during the initial phase of contemporaneous development and infilling of the Piceance Basin during the Laramide orogeny. Sediments were derived from multiple source areas within the present Rocky Mountains while they were uplifted. Sediments were transported and deposited in braided stream, flood-plain lacustrine, and lacustrine environments. Differentiation of sediments into more discrete lithofacies within fluvial and lacustrine complexes was hindered by the relatively short distances from sediment sources to depocenters in the basin. Kmv Mesaverde Group rocks, undivided (Upper Cretaceous)-- Thin to thick beds of yellowish-brown and olive-gray, carbonaceous mudstone and fine- to medium-grained, silty sandstone, siltstone, and claystone; contains thin beds and lenses of silty and sandy pebble and cobble conglomerate; beds are generally massive. Locally contains thin beds and laminae of coal; clinker is common. Contains several beds of white, well-sorted, short forset-crossbedded, quartz sandstone in the lower part of the unit. Theses beds contrast strongly in appearance and physical character with surrounding silty, carbonaceous beds. These white sandstone beds are equivalent to similar beds in the Trout Creek Sandstone Member of the Isles Formation as mapped by O'Sullivan (1985) in the nearby Rio Blanco quadrangle. These beds thin and pinch out due to non-deposition and erosion in a southeasterly direction along the Grand Hogback east and southeast of the map area. Except for the white sandstone beds, clastic sediments are generally poorly sorted, subangular, and contain abundant coaly, carbonaceous particles derived from rip-up of well- and poorly drained swamp sediments in backshore areas. Some beds are silica cemented and form prominent ridges and cliffs. The upper part of this unit is equivalent to the Williams Fork Formation as mapped by O'Sullivan (1985). About 4,500 ft thick Mesaverde deposition occurred during the first major Cretaceous regressive marine cycle in the Rocky Mountain seaway following the Dakota transgressive cycle. Deposition occurred primarily in backshore areas and on deltas of the lower coastal plain where sediment-laden streams meandered among coal swamps, marshes, and mudflats behind a seaward-migrating shoreline. White sandstone beds in the lower part of the unit are the product of shoreline processes in which surf- and wave-zone activity winnowed, sorted, and otherwise concentrated sand along an upper shoreface. The extent and thickness of minable coal beds in the unit are unknown because most of them are covered. Correlation of coal beds in the map area with those in the nearby New Castle area is uncertain (Gale, 1910; p. 120) Km Mancos Shale (Upper Cretaceous)--Dominantly light- to dark-gray, carbonaceous shale locally containing thin lenticular beds of dark-gray and black, fossiliferous limestone and thin-bedded, very fine grained, silicious, silty sandstone; sediments are generally limy. Fossils in the limestone are highly fragmented. The unit contains white to yellowish-brown bentonite horizons (altered volcanic ash) a few inches thick. The upper and lower contacts of the unit are conformable. Only about the upper 1,000 ft of the unit are exposed in the map area. Deposition occurred primarily on the continental slope in transgressive (lower part of the unit) and regressive (upper part of the unit) submarine environments. Clastic deposition occurred by sediment settling and turbidity flow, whereas limestone formed by chemical precipitation. The dark-gray and black color of the rocks is attributed largely to the content of black, coaly, detrital organic matter apparently derived from the destruction of pre-Dakota, coaly, carbonaceous swamp deposits on the lower coastal plain by high-energy transgressive backshore and shoreline processes. REFERENCES CITED American Geological Institute, 1982, Grain-size scales used by American geologists, modified Wentworth scale, in Data sheets (2nd ed.): Falls Church, Va., American Geological Institute, sheet 17.1. Bass, N.W., and Northrop, S.A., 1963, Geology of Glenwood Springs quadrangle and vicinity, northwestern Colorado: U.S. Geological Survey Bulletin 1142-J, 74 p. Bryant, Bruce, 1979, Geology of the Aspen 15-minute quadrangle, Pitkin and Gunnison Counties, Colorado: U.S. Geological Survey Professional Paper 1073, 146 p. Colman, S.M., 1985, Map showing tectonic features of late Cenozoic origin in Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map I-1556, scale 1:1,000,000. Gale, H.S., 1910, Coal fields of northwestern Colorado and northeastern Utah: U.S. Geological Survey Bulletin 415, pl. 10, 265 p. Gile, L.H., Peterson, F.F., and Grossman, R.B., 1966, Morphological and genetic sequences of carbonate accumulation in desert soils: Soil Science, v. 101, p. 347-360. Green, M.W., Fairer, G.M., and Shroba, R.R., 1993a, Preliminary geologic map of the New Castle quadrangle, Garfield County, Colorado: U.S. Geological Survey Open-File Report 93-310, 33 p., 1 pl., scale 1:24,000. ______1993b, Preliminary geologic map of the Rifle Falls quadrangle, Garfield County, Colorado: U.S. Geological Survey Open-File Report 93-700, 21 p., 1 pl., scale 1:24,000. Guthrie, R.L., and Witty, J.E., 1982, New designations for soil horizons and layers and the new Soil Survey Manual: Soil Science Society of America Journal, v. 46, p. 443-444. Izett, G.A., and Wilcox, R.E., 1982, Map showing localities and inferred distributions of the Huckleberry Ridge, Mesa Falls, and Lava Creek ash beds (Pearlette family ash beds) of Pliocene and Pleistocene age in the western United States and southern Canada: U.S. Geological Survey Miscellaneous Investigations Series Map I-1325, scale 1:4,000,000. Munsell Color, 1973, Munsell soil color charts: Baltimore, Md., Kollmorgen Corp., Macbeth Division. O'Sullivan, R. B., 1985, Preliminary geologic map of the Rio Blanco quadrangle, Rio Blanco and Garfield Counties, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF-1816, scale 1:24,000. Pierce, K.L., 1979, History and dynamics of glaciation in the northern Yellowstone National Park area: U.S. Geological Survey Professional Paper 729-F, 90 p. Pierce, K.L., Fosberg, M.A., Scott W.E., Lewis, G.C., and Coleman, S.M., 1982, Loess deposits of southeastern Idaho: age and correlation of the upper two loess units, in Bonnichsen, Bill and Breckenridge, R.M., eds., Cenozoic geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 717-725. Pierce, K.L., Obradovich, J.D., and Friedman, Irving, 1976, Obsidian hydration dating and correlation of Bull Lake and Pinedale glaciations near West Yellowstone, Montana: Geological Society of America Bulletin, v. 87, n. 5, p. 703-710. Piety, L.A., 1981, Relative dating of terrace deposits and tills in the Roaring Fork Valley, Colorado: Boulder, University of Colorado, M.S. thesis, 209 p. Richmond, G.M., 1986, Stratigraphy and correlation of glacial deposits of the Rocky Mountains, the Colorado Plateau, and the ranges of the Great Basin, in Sibrava, V., Bowen, D.Q., and Richmond, G.M., eds., Quaternary glaciations in the northern hemisphere: Quaternary Science Reviews, v. 5, p. 99-127. Richmond, G.M., and Fullerton, D.S., 1986, Introduction to Quaternary glaciations in the United States of America, in Sibrava, V., Bowen, D.Q., and Richmond, G.M., eds., Quaternary glaciations in the northern hemisphere: Quaternary Science Reviews, v. 5, p. 3-10. Shroba, R.R., 1989, Physical properties and laboratory data for soils formed in Pleistocene tills at Bull Lake, Dinwoody Lakes, and Fremont Lake, Fremont and Sublette Counties, Wyoming: U.S. Geological Survey Open-File Report 89-370, 14 p. Shroba, Ralph R., Fairer, George M., and Green, Morris W., 1994, Preliminary geologic map of the Silt quadrangle, Garfield County, Colorado: U.S. Geological Survey Open-File Report 94-696, 24 p., 1 pl., scale 1:24,000. ------ 1994, Quaternary loess stratigraphy along the Colorado River between Glenwood Springs and Rifle, Colorado: preliminary findings: [abs] American Quaternary Association, Biennial Meeting, 13th Minneapolis, Minn., Abstracts, p. 246 Soil Survey Staff, 1975, Soil taxonomy: U.S. Department of Agriculture Handbook 436, 754 p. Stover, B.K., 1984, Debris-flow origin of high-level sloping surfaces on the northern flanks of Battlement Mesa, and surfical geology of parts of the North Mamm Peak, Rifle, and Rulison quadrangles, Garfield County, Colorado: Boulder, University of Colorado M.S. thesis, 75 p. ----- 1988, A debris-flow model for the origin of high-level sloping surfaces on the northern flanks of Battlement Mesa, Garfield County, Colorado, in Holden, G.S. and Tafoya R.E., eds., Geological Society of America field trip guidebook 1988, Centennial Meeting, Denver, Colorado: Professional Contributions of Colorado School of Mines, no. 12, p 419- 425. Stover, B.K., and Soule, J.M., 1985, Surficial geologic map, in Surficial geology, geomorphology, and general engineering geology of parts of the Colorado River valley, Roaring Fork Valley, and adjacent areas, Garfield County, Colorado: Colorado Geological Survey Open File Report 85 1, scale 1:50,000. Tweto, Ogden, 1979, Geologic map of Colorado: U.S. Geological Survey, scale 1:500,000. Varnes, D.J., 1978, Slope movement types and process, in Schuster, R.L., and Krizek, R.J., eds., Landslides, analysis, and control: National Academy of Sciences, Transportation Research Board Special Report 176, p. 11 33. Wentworth, C.M. and Fitzgibbon, T.T., 1991, ALACARTE users manual: U.S. Geological Survey Open-File Report 91-587C, 268 p. Whitney, J.W., and Andrews, E.D., 1983, Past and present geomorphic activity in the Piceance Creek drainage basin, northwestern Colorado, in Gary, J.H., ed., Sixteenth Oil Shale Symposium Proceedings: Golden, Colorado School of Mines Press, p. 566-577. Whitney, J.W., Piety, L.A., and Cressman, S.L., 1983, Alluvial history in the White River basin, northwest Colorado [abs]: Geological Society of America Abstracts with Programs, v. 15, n. 5, p. 328. CONVERSION FACTORS ------------------------------------------------- Multiply By To obtain ------------------------------------------------- inches (in.) 2.540 centimeters (cm) feet (ft) 0.3048 meters (m) miles (mi) 1.609 kilometers (km) ------------------------------------------------- Refer to non-digitized version of this open-file report for a description of map symbols.