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Fluvial deposits of Yellowstone tephras: Implications for late Cenozoic history of the Bighorn basin area, Wyoming and Montana
Quaternary International, Vol. 13/14, pp. 19-22, 1992. Printed in Great Britain.
1040-6182/92 $15.00 1992 INQUA/Pergamon Press Ltd
FLUVIAL DEPOSITS OF YELLOWSTONE TEPHRAS: IMPLICATIONS FOR LATE CENOZOIC HISTORY OF THE BIGHORN BASIN AREA, WYOMING AND MONTANA
Marith C. Reheis U.S. Geological Survey, MS 913, Box 25046, Denver Federal Center, Denver, CO 80225, U.S.A.
Several deposits of tephra derived from eruptions in Yellowstone National Park occur in the northern Bighorn basin area of Wyoming and Montana. These tephra deposits are mixed and interbedded with fluvial gravel and sand deposited by several different rivers. The fluvial tephra deposits are used to calculate stream incision rates, to provide insight into drainage histories and Quaternary tectonics, to infer the timing of alluvial erosion-deposition cycles, and to calibrate rates of soil development.
INTRODUCTION
110°
The Bighorn basin of Wyoming and Montana contains numerous sequences of well-preserved fluvial terraces along several major rivers and has for decades been a classic area for the study of fluvial geomorphology and Quaternary stratigraphy (for example, Alden, 1932; Mackin, 1937; Andrews et al., 1947; Ritter, 1967; Moss, 1974). Several deposits of tephra derived from caldera eruptions in Yellowstone National Park, northwest of the Bighorn basin (Fig. 1), have been identified in the past 15 years. These deposits provide age control for the terrace deposits in which they occur and permit ages to be estimated for older and younger terraces using calculations of incision rates derived from the heights and ages of dated terraces (Palmquist, 1983). The incision-rate method must be restricted to terraces along a reach where the climate and local bedrock do not vary significantly. The use of average stream incision rates to estimate the ages of terraces and to correlate terrace remnants in different areas depends upon the assumption that the region either is tectonically stable or is experiencing uplift at a constant rate. Although such conditions are difficult to prove, streamincision rates do appear reliable in Wyoming-Montana basins (Reheis et al., 1991). This paper summarizes recent publications that (1) infer the ages of the terrace deposits and terrace surfaces in the northern Bighorn basin area (Palmquist, 1983; Reheis, 1987a,b; Agard, 1989; Reheis et al., 1991) and (2) use the ages of the terrace surfaces to provide insight into drainage histories (Reheis et al., 1991) and Quaternary tectonics (Reheis, 1985), to infer the timing of alluvial erosion-deposition cycles (Reheis et al., 1991), and to calibrate rates and infer processes of soil development (Reheis, 1987a,b, 1990). Readers are referred to these recent publications for detailed discussions and information on site descriptions and methods. The tephra layers were identified mainly by glass chemistry, shard shape, and phenocryst assemblage. Those at sites 1, 2, 6, and 7 were identified
109 °
108 °
107 °
46 °
45 °
44 °
43 °
FIG. 1. Sketch map of principal rivers and mountain ranges (shaded) in the Bighorn basin area. Numbers are tephra sites described in text.
by Izett and Wilcox (1982); references for the other sites are given in the site descriptions below. DESCRIPTION OF TEPHRA LOCALITIES The 2.02 Ma Huckleberry Ridge tephra site (site 1, Fig. 1) was first located and identified by Izett and Wilcox (1982). This tephra deposit consists of about 5 m of ashy fluvial sand that rests on well-rounded andesitic stream gravel deposited by the ancestral Shoshone River when it flowed northward through the 19
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Pryor Mountains near the present course of Pryor Creek. Five sites contain deposits of the 0.62 Ma Lava Creek A and B ashes in the northern Bighorn basin (Fig. 1). The stratigraphic positions of these fluvial tephra deposits relative to the glaciofluvial stream gravels with which they are associated generally depend on the distance of the deposits from glacial source areas. Tephra deposits tend to overlie stream gravel close to glacial sources but are within or at the base of stream gravels far from glacial sources. In addition, two deposits (sites 2 and 6, Fig. l) are buried by sidestream alluvial-fan deposits. Site 2 is on lower Shell Creek about 25 km downstream of the glaciated headwaters of Shell Creek in the Bighorn Mountains. Fluvial deposits of the Lava Creek A and B ashes are interbedded with sand, silt, and gravel and overlie 5-7 m of cemented stream gravel of Shell Creek. Deposits of the Lava Creek A tephra, as much as 3 cm thick, are abruptly overlain by those of the Lava Creek B tephra, as much as 2 m thick (Izett and Wilcox, 1982). The tephra layers are in turn overlain by at least 10 m of locally derived colluvium consisting of red, gravelly silt (Palmquist, 1983). Sites 3 and 4 (Fig. 1) are on the lower Roberts terrace of the Clarks Fork of the Yellowstone River, respectively about 60 and 40 km downstream of the nearest glaciated sources in the Beartooth Mountains and the Absaroka Range. Site 3 contains fluvially reworked deposits of the 0.62 Ma Lava Creek A and B tephra (Reheis, 1987b), identified by R. E. Wilcox (pers. c o m m u n . , 1983). These tephra deposits, as much as 30 cm thick, include about 15 cm of silty Lava Creek A tephra abruptly overlain by about 15 cm of sandy Lava Creek B tephra. The tephra beds are overlain by a thin (0.5 m) layer of fluvial sand and gravel and are underlain by 2 m of well-sorted cross-bedded fluvial sand, which in turn overlies several meters of fluvial gravel. The lower Roberts terrace at this site is 93 m above the modern river. Site 4 contains a 10 m thick fluvial deposit of probable Lava Creek B ash (G. A. lzett, pers. c o m m u n . , 1990) that overlies about 7.5 m of stream gravel (S. S. Agard, pers. c o m m u n . , 1990). Site 5 is on the lower Greybull River near the confluence with the Bighorn River (Fig. 1), about 135 km downstream of the glaciated source of the Greybull River in the Absaroka Range. Here, lenticular beds of tephra as much as 2 m thick are interbedded with stream gravel near the base of a terrace deposit (Palmquist, 1983). The color, size, thickness, and stratigraphic position of this tephra indicate that it is probably the Lava Creek B, although apparently no laboratory work has been done. The exposure is adjacent to bedrock that forms the paleo-valley wall and locally the tephra abuts this bedrock. Site 6 is at the Kane fans on the Bighorn River at the confluence of the Shoshone River, about 120 km downstream of the nearest glaciated areas in the Bighorn Mountains and the Absaroka Range. The Lava Creek B tephra (Izett and Wilcox, 1982) occurs as
lenses in a stream gravel of the Bighorn River (Palmquist, 1983) and as a blanket as much as 30 m thick overlying the stream gravel. The tephra is in turn overlain conformably by 4-15 m of alluvial-fan gravel derived from the adjacent Bighorn Mountains (Reheis, 1987a). Site 7, on the lower Bighorn River (Fig. 1), is at least 180 km downstream from the nearest glaciated area in the headwaters of Shell Creek in the Bighorn Mountains. Cemented pumiceous sand and gravel of the Lava Creek B tephra (Izett and Wilcox, 1982), 4-6 m thick, overlies bedrock and underlies about 5 m of stream gravel (Hamilton and Paulson, 1968; Agard, 1989). DRAINAGE HISTORY OF THE NORTHERN BIGHORN BASIN
The Huckleberry Ridge and the Lava Creek ash beds have been used to reconstruct the timing of several major stream captures in the northern Bighorn basin (Reheis et al., 1991). The two most significant captures will be discussed here. The lithologic compositions of gravel carried by the modern Bighorn River, Shoshone River, Clarks Fork, Yellowstone River, and Rock Creek (Fig. 1) are distinctive and permit the discrimination of gravel deposited by different rivers and different combinations of rivers. Mackin (1937) first demonstrated this when he showed that the Shoshone River once flowed northward to the Yellowstone River through Pryor Gap in the Pryor Mountains before it was captured to its present position as a tributary to the Bighorn River. He also suggested that a combined Clarks Fork-Shoshone River flowed through the Pryor Mountains at an earlier time, but found no deposits to support this idea. The Shoshone River terrace gravel containing the Huckleberry Ridge tephra (site 1, Fig. 1) is discontinuously traceable northward to an ancestral terrace gravel of the Yellowstone River that stands 295 m above the modern Yellowstone River (Reheis et al., 1991). The height and age of this terrace yields a local average incision rate of 0.15 m/ka for the Yellowstone River and for the adjacent reach of the ancestral Shoshone River. A terrace gravel with clast lithologies indicating deposition by a combined Clarks ForkShoshone River stands 30 m higher than the 2.0 Ma Shoshone River terrace near the Yellowstone River. Using the average incision rate, the age of the Clarks Fork-Shoshone terrace is about 2.2 Ma. Hence, the ancestral Clarks Fork was captured from a course through the Pryor Mountains to its present position sometime between 2.2 and 2.0 Ma. The Shoshone River continued to flow through the Pryor Mountains for a time after the capture of the Clarks Fork. The time of capture of the Shoshone River from the Pryor Mountains to its modern course is constrained by the ages of terraces along the ancestral course of the Shoshone River and along the Bighorn River.
Fluvial Deposits of Yellowstone Tephras The 2.0 Ma terrace of the Shoshone River stands about 80 m above the Polecat Bench terrace of the Shoshone River along Pryor Creek (Fig. 1; Reheis et al., 1991). The Polecat Bench terrace is the youngest deposit of the Shoshone River along its ancestral course through Pryor Gap, and its age is estimated to be about 1.45 Ma using the incision rate of the Yellowstone River to which the Shoshone was tributary. North of the Bighorn Mountains, the ages of several Bighorn River terraces are estimated from incision rates calculated from the height of the terrace containing the Lava Creek B tephra (site 7, Fig. 1) and from the height of a terrace correlated with the 2.0 Ma terraces of the Shoshone and Yellowstone Rivers (Agard, 1989). The lithologies of most terrace gravels of the Bighorn River are similar to those of the modern river, except for the gravels older than 1.2 Ma which are considerably less andesitic than the younger gravels. Because the modern Shoshone River carries gravel almost exclusively composed of andesite, the 1.2 Ma and younger terraces of the Bighorn River record the influx of Shoshone River andesite gravel. Hence, the capture of the Shoshone River occurred after 1.45 Ma, the age of the youngest Shoshone River terrace north of Pryor Gap, but before 1.2 Ma, the age of the oldest Bighorn River terrace containing Shoshone River gravel (Reheis et al., 1991).
INCISION RATES AND TECTONIC IMPLICATIONS The average incision rates of streams in the northern Bighorn basin were faster from 0.6 Ma to the present than from 2.0 to 0.6 Ma. Along Rock Creek, the pre0.6 Ma incision rate was about 0.05 m/ka, whereas the post-0.6 Ma incision rate was about 0.07 m/ka. The incision rate of the Clarks Fork along its modern course increased from 0.12 to 0.15 m/ka and that of the lower Bighorn River from 0.15 to 0.19 m/ka. The post-0.6 Ma incision rate of the upper Bighorn River was about 0.15 m/ka (Reheis et al., 1991). Moreover, terrace surfaces along Rock Creek (Fig. 1) diverge downstream, and the degree of divergence is progressively larger with increasing age (Reheis, 1985). The rapid incision rates and the divergence of terraces downstream along Rock Creek and perhaps along the Shoshone and Greybull Rivers (Pierce and Morgan, 1990) suggest ongoing uplift of the northern Bighorn basin. On a more local level, the Polecat Bench terrace of the ancestral Shoshone River appears to be warped over an anticline south of the Pryor Mountains and may have been offset by faulting along the range-front fault bounding the north side of the Pryor Mountains (Reheis, 1985). Because the age of the Polecat Bench terrace is about 1.45 Ma, such warping and faulting must be Quaternary in age. Faulting in the area of the Pryor Mountains may provide a mechanism to explain the capture of the Clarks Fork and Shoshone Rivers away from their ancestral course through the Pryor Mountains.
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TIMING OF ALLUVIAL DEPOSITION Deposits of Lava Creek A and B ashes in the Bighorn basin are interbedded with, or overlie stream gravel that is probably glaciofluvial in origin. Few moraines of this age are preserved in the mountains around the Bighorn basin. However, there appears to have been widespread glaciation in the western United States just prior to 0.6 Ma (Richmond and Fullerton, 1986). At one site, north of Rock Creek, a till of preBull Lake age (Reheis, 1987a), buried under Bull Lake till (Ritter, 1967; most Bull Lake deposits are probably Illinoian in age, or 0.18--0.13 Ma; Richmond and Fullerton, 1986), appears to grade to a 0.62 Ma lower Roberts terrace. Thus, it is probable that stream gravels containing and underlying Lava Creek ash in the Bighorn basin are glaciofluvial. The stratigraphic position of deposits of Lava Creek A and B ashes described above is related to distance downstream from the nearest glacial source area: in general, the tephra layers overlie glaciofluvial gravel close to glacial sources but lie within or at the base of stream gravel far from glacial sources. In addition, two tephra deposits (sites 2 and 6) are buried by thick deposits of sidestream alluvial fans. These relations suggest that (1) gravel deposition occurs earlier near glaciated source areas than farther downstream, and (2) sidestream deposition continues for some time after mainstream deposition ceases. These relations are supported by similar observations on younger terraces in the Bighorn basin that can be physically traced into moraines (Reheis et al., 1991). RATES AND PROCESSES OF SOIL DEVELOPMENT Along Rock Creek and at the Kane fans, terrace surfaces dated by tephra-derived incision rates provide time calibrations for studies of soil development rates and processes in different climatic regimes (Reheis, 1987a,b, 1990). These studies are being used to estimate ages of undated deposits in areas of similar climate and parent material and to make inferences about paleoclimate from preserved properties of relict soils. Surficial A horizons of soils in all three climatic regimes studied (moist, semiarid, and arid) reflect the progressive influences of accumulation of eolian dust and surface erosion. Properties of B and C horizons, in contrast, reflect the climatic conditions under which the soils formed. Field and chemical properties of B and C horizons of soils in a moist climatic regime change at logarithmic rates that reflect relatively rapid chemical weathering, whereas properties of B and C horizons of soils in a dry climatic regime change at linear rates that reflect progressive dust additions and slower chemical weathering. Zirconium-poor dust is an important constituent of the soils in all three climates. The clay mineralogy of the soils reflects the local climate, but the most
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crystalline clays form in the clay-poor basal B and C horizons rather than in the clay-rich upper B horizons. REFERENCES Agard, S.S. (1989). Map showing Quaternary and late Tertiary terraces of the lower Bighorn River, Montana. United States Geological Survey Miscellaneous Field Studies Map MF-2094, scale 1:100,000. Alden, W.C. (1932). Physiography and glacial geology of eastern Montana and adjacent areas. United States Geological Survey Professional Paper 174, 133 pp. Andrews, D.A., Pierce, W.G. and Eargle, D.H. (1947). Geologic map of the Bighorn basin, Wyoming and Montana, showing terrace deposits and physiographic features. United States Geological Survey Oil and Gas Investigations Preliminary Map 71, scale 1:126,720. Hamilton, L.J. and Paulson, Q.F. (1968). Geology and ground-water resources of the lower Bighorn valley, Montana. United States Geological Survey Water-Supply Paper 1876, 39 pp. 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. United States Geological Survey Miscellaneous Investigations Map 1-1325, scale 1:4,000,000. Mackin, J.H. (1937). Erosional history of the Big Horn Basin, Wyoming. Geological Society of America Bulletin, 48, 813-894. Moss, J.H. (1974). The relation of river terrace formation to glaciation in the Shoshone River basin, western Wyoming. In: Coates, D. (ed.), Glacial Geomorphology, pp.293-314. Publications in Geomorphology, New York.
Palmquist, R.C. (1983). Terrace chronologies in the Bighorn Basin, Wyoming. Guidebook, Wyoming Geological Association 34th Annual Field Conference, pp. 217-231. Pierce, K.L. and Morgan, L.A. (1990). The track of the Yellowstone hotspot: volcanism, faulting, and uplift. United States Geological Survey Open-File Report 0F-90-415, 68 pp. Reheis, M.C. (1985). Evidence for Quaternary tectonism in the northern Bighorn basin, Wyoming and Montana. Geology, 13, 364-367. Reheis, M.C. (1987a). Gypsic soils on the Kane alluvial fans, Big Horn County, Wyoming. United States Geological Survey Bulletin 1590-C, 39 pp. Reheis, M.C. (1987b). Soils in granitic alluvium in humid and semiarid climates along Rock Creek, Carbon County, Montana. United States Geological Survey Bulletin 1590-D, 71 pp. Reheis, M.C. (1990). Influence of climate and eolian dust on the major-element chemistry and clay mineralogy of soils in the northern Bighorn basin, U.S.A. Catena, 17, 219--248. Reheis, M.C., Palmquist, R.C., Agard, S.S., Jaworowski, C., Mears, B.M. Jr, Madole, R.L., Nelson, A.R. and Osborne, G. (1991). Non-glacial Quaternary history of some Rocky Mountain basins. In: Morrison, R.B. (ed.), Quaternary Non-Glacial History of the Conterminous United States, Decade of North American Geology Volume, K-2, pp. 407-440. Geological Society of America, Boulder, Colorado. 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, pp. 3--10. Pergamon Press, New York. Ritter, D.F. (1967). Terrace development along the front of the Beartooth Mountains, southern Montana. Geological Society of America Bulletin, 78, 467-484.
1040-6182/92 $15.00 1992 INQUA/Pergamon Press Ltd
FLUVIAL DEPOSITS OF YELLOWSTONE TEPHRAS: IMPLICATIONS FOR LATE CENOZOIC HISTORY OF THE BIGHORN BASIN AREA, WYOMING AND MONTANA
Marith C. Reheis U.S. Geological Survey, MS 913, Box 25046, Denver Federal Center, Denver, CO 80225, U.S.A.
Several deposits of tephra derived from eruptions in Yellowstone National Park occur in the northern Bighorn basin area of Wyoming and Montana. These tephra deposits are mixed and interbedded with fluvial gravel and sand deposited by several different rivers. The fluvial tephra deposits are used to calculate stream incision rates, to provide insight into drainage histories and Quaternary tectonics, to infer the timing of alluvial erosion-deposition cycles, and to calibrate rates of soil development.
INTRODUCTION
110°
The Bighorn basin of Wyoming and Montana contains numerous sequences of well-preserved fluvial terraces along several major rivers and has for decades been a classic area for the study of fluvial geomorphology and Quaternary stratigraphy (for example, Alden, 1932; Mackin, 1937; Andrews et al., 1947; Ritter, 1967; Moss, 1974). Several deposits of tephra derived from caldera eruptions in Yellowstone National Park, northwest of the Bighorn basin (Fig. 1), have been identified in the past 15 years. These deposits provide age control for the terrace deposits in which they occur and permit ages to be estimated for older and younger terraces using calculations of incision rates derived from the heights and ages of dated terraces (Palmquist, 1983). The incision-rate method must be restricted to terraces along a reach where the climate and local bedrock do not vary significantly. The use of average stream incision rates to estimate the ages of terraces and to correlate terrace remnants in different areas depends upon the assumption that the region either is tectonically stable or is experiencing uplift at a constant rate. Although such conditions are difficult to prove, streamincision rates do appear reliable in Wyoming-Montana basins (Reheis et al., 1991). This paper summarizes recent publications that (1) infer the ages of the terrace deposits and terrace surfaces in the northern Bighorn basin area (Palmquist, 1983; Reheis, 1987a,b; Agard, 1989; Reheis et al., 1991) and (2) use the ages of the terrace surfaces to provide insight into drainage histories (Reheis et al., 1991) and Quaternary tectonics (Reheis, 1985), to infer the timing of alluvial erosion-deposition cycles (Reheis et al., 1991), and to calibrate rates and infer processes of soil development (Reheis, 1987a,b, 1990). Readers are referred to these recent publications for detailed discussions and information on site descriptions and methods. The tephra layers were identified mainly by glass chemistry, shard shape, and phenocryst assemblage. Those at sites 1, 2, 6, and 7 were identified
109 °
108 °
107 °
46 °
45 °
44 °
43 °
FIG. 1. Sketch map of principal rivers and mountain ranges (shaded) in the Bighorn basin area. Numbers are tephra sites described in text.
by Izett and Wilcox (1982); references for the other sites are given in the site descriptions below. DESCRIPTION OF TEPHRA LOCALITIES The 2.02 Ma Huckleberry Ridge tephra site (site 1, Fig. 1) was first located and identified by Izett and Wilcox (1982). This tephra deposit consists of about 5 m of ashy fluvial sand that rests on well-rounded andesitic stream gravel deposited by the ancestral Shoshone River when it flowed northward through the 19
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Pryor Mountains near the present course of Pryor Creek. Five sites contain deposits of the 0.62 Ma Lava Creek A and B ashes in the northern Bighorn basin (Fig. 1). The stratigraphic positions of these fluvial tephra deposits relative to the glaciofluvial stream gravels with which they are associated generally depend on the distance of the deposits from glacial source areas. Tephra deposits tend to overlie stream gravel close to glacial sources but are within or at the base of stream gravels far from glacial sources. In addition, two deposits (sites 2 and 6, Fig. l) are buried by sidestream alluvial-fan deposits. Site 2 is on lower Shell Creek about 25 km downstream of the glaciated headwaters of Shell Creek in the Bighorn Mountains. Fluvial deposits of the Lava Creek A and B ashes are interbedded with sand, silt, and gravel and overlie 5-7 m of cemented stream gravel of Shell Creek. Deposits of the Lava Creek A tephra, as much as 3 cm thick, are abruptly overlain by those of the Lava Creek B tephra, as much as 2 m thick (Izett and Wilcox, 1982). The tephra layers are in turn overlain by at least 10 m of locally derived colluvium consisting of red, gravelly silt (Palmquist, 1983). Sites 3 and 4 (Fig. 1) are on the lower Roberts terrace of the Clarks Fork of the Yellowstone River, respectively about 60 and 40 km downstream of the nearest glaciated sources in the Beartooth Mountains and the Absaroka Range. Site 3 contains fluvially reworked deposits of the 0.62 Ma Lava Creek A and B tephra (Reheis, 1987b), identified by R. E. Wilcox (pers. c o m m u n . , 1983). These tephra deposits, as much as 30 cm thick, include about 15 cm of silty Lava Creek A tephra abruptly overlain by about 15 cm of sandy Lava Creek B tephra. The tephra beds are overlain by a thin (0.5 m) layer of fluvial sand and gravel and are underlain by 2 m of well-sorted cross-bedded fluvial sand, which in turn overlies several meters of fluvial gravel. The lower Roberts terrace at this site is 93 m above the modern river. Site 4 contains a 10 m thick fluvial deposit of probable Lava Creek B ash (G. A. lzett, pers. c o m m u n . , 1990) that overlies about 7.5 m of stream gravel (S. S. Agard, pers. c o m m u n . , 1990). Site 5 is on the lower Greybull River near the confluence with the Bighorn River (Fig. 1), about 135 km downstream of the glaciated source of the Greybull River in the Absaroka Range. Here, lenticular beds of tephra as much as 2 m thick are interbedded with stream gravel near the base of a terrace deposit (Palmquist, 1983). The color, size, thickness, and stratigraphic position of this tephra indicate that it is probably the Lava Creek B, although apparently no laboratory work has been done. The exposure is adjacent to bedrock that forms the paleo-valley wall and locally the tephra abuts this bedrock. Site 6 is at the Kane fans on the Bighorn River at the confluence of the Shoshone River, about 120 km downstream of the nearest glaciated areas in the Bighorn Mountains and the Absaroka Range. The Lava Creek B tephra (Izett and Wilcox, 1982) occurs as
lenses in a stream gravel of the Bighorn River (Palmquist, 1983) and as a blanket as much as 30 m thick overlying the stream gravel. The tephra is in turn overlain conformably by 4-15 m of alluvial-fan gravel derived from the adjacent Bighorn Mountains (Reheis, 1987a). Site 7, on the lower Bighorn River (Fig. 1), is at least 180 km downstream from the nearest glaciated area in the headwaters of Shell Creek in the Bighorn Mountains. Cemented pumiceous sand and gravel of the Lava Creek B tephra (Izett and Wilcox, 1982), 4-6 m thick, overlies bedrock and underlies about 5 m of stream gravel (Hamilton and Paulson, 1968; Agard, 1989). DRAINAGE HISTORY OF THE NORTHERN BIGHORN BASIN
The Huckleberry Ridge and the Lava Creek ash beds have been used to reconstruct the timing of several major stream captures in the northern Bighorn basin (Reheis et al., 1991). The two most significant captures will be discussed here. The lithologic compositions of gravel carried by the modern Bighorn River, Shoshone River, Clarks Fork, Yellowstone River, and Rock Creek (Fig. 1) are distinctive and permit the discrimination of gravel deposited by different rivers and different combinations of rivers. Mackin (1937) first demonstrated this when he showed that the Shoshone River once flowed northward to the Yellowstone River through Pryor Gap in the Pryor Mountains before it was captured to its present position as a tributary to the Bighorn River. He also suggested that a combined Clarks Fork-Shoshone River flowed through the Pryor Mountains at an earlier time, but found no deposits to support this idea. The Shoshone River terrace gravel containing the Huckleberry Ridge tephra (site 1, Fig. 1) is discontinuously traceable northward to an ancestral terrace gravel of the Yellowstone River that stands 295 m above the modern Yellowstone River (Reheis et al., 1991). The height and age of this terrace yields a local average incision rate of 0.15 m/ka for the Yellowstone River and for the adjacent reach of the ancestral Shoshone River. A terrace gravel with clast lithologies indicating deposition by a combined Clarks ForkShoshone River stands 30 m higher than the 2.0 Ma Shoshone River terrace near the Yellowstone River. Using the average incision rate, the age of the Clarks Fork-Shoshone terrace is about 2.2 Ma. Hence, the ancestral Clarks Fork was captured from a course through the Pryor Mountains to its present position sometime between 2.2 and 2.0 Ma. The Shoshone River continued to flow through the Pryor Mountains for a time after the capture of the Clarks Fork. The time of capture of the Shoshone River from the Pryor Mountains to its modern course is constrained by the ages of terraces along the ancestral course of the Shoshone River and along the Bighorn River.
Fluvial Deposits of Yellowstone Tephras The 2.0 Ma terrace of the Shoshone River stands about 80 m above the Polecat Bench terrace of the Shoshone River along Pryor Creek (Fig. 1; Reheis et al., 1991). The Polecat Bench terrace is the youngest deposit of the Shoshone River along its ancestral course through Pryor Gap, and its age is estimated to be about 1.45 Ma using the incision rate of the Yellowstone River to which the Shoshone was tributary. North of the Bighorn Mountains, the ages of several Bighorn River terraces are estimated from incision rates calculated from the height of the terrace containing the Lava Creek B tephra (site 7, Fig. 1) and from the height of a terrace correlated with the 2.0 Ma terraces of the Shoshone and Yellowstone Rivers (Agard, 1989). The lithologies of most terrace gravels of the Bighorn River are similar to those of the modern river, except for the gravels older than 1.2 Ma which are considerably less andesitic than the younger gravels. Because the modern Shoshone River carries gravel almost exclusively composed of andesite, the 1.2 Ma and younger terraces of the Bighorn River record the influx of Shoshone River andesite gravel. Hence, the capture of the Shoshone River occurred after 1.45 Ma, the age of the youngest Shoshone River terrace north of Pryor Gap, but before 1.2 Ma, the age of the oldest Bighorn River terrace containing Shoshone River gravel (Reheis et al., 1991).
INCISION RATES AND TECTONIC IMPLICATIONS The average incision rates of streams in the northern Bighorn basin were faster from 0.6 Ma to the present than from 2.0 to 0.6 Ma. Along Rock Creek, the pre0.6 Ma incision rate was about 0.05 m/ka, whereas the post-0.6 Ma incision rate was about 0.07 m/ka. The incision rate of the Clarks Fork along its modern course increased from 0.12 to 0.15 m/ka and that of the lower Bighorn River from 0.15 to 0.19 m/ka. The post-0.6 Ma incision rate of the upper Bighorn River was about 0.15 m/ka (Reheis et al., 1991). Moreover, terrace surfaces along Rock Creek (Fig. 1) diverge downstream, and the degree of divergence is progressively larger with increasing age (Reheis, 1985). The rapid incision rates and the divergence of terraces downstream along Rock Creek and perhaps along the Shoshone and Greybull Rivers (Pierce and Morgan, 1990) suggest ongoing uplift of the northern Bighorn basin. On a more local level, the Polecat Bench terrace of the ancestral Shoshone River appears to be warped over an anticline south of the Pryor Mountains and may have been offset by faulting along the range-front fault bounding the north side of the Pryor Mountains (Reheis, 1985). Because the age of the Polecat Bench terrace is about 1.45 Ma, such warping and faulting must be Quaternary in age. Faulting in the area of the Pryor Mountains may provide a mechanism to explain the capture of the Clarks Fork and Shoshone Rivers away from their ancestral course through the Pryor Mountains.
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TIMING OF ALLUVIAL DEPOSITION Deposits of Lava Creek A and B ashes in the Bighorn basin are interbedded with, or overlie stream gravel that is probably glaciofluvial in origin. Few moraines of this age are preserved in the mountains around the Bighorn basin. However, there appears to have been widespread glaciation in the western United States just prior to 0.6 Ma (Richmond and Fullerton, 1986). At one site, north of Rock Creek, a till of preBull Lake age (Reheis, 1987a), buried under Bull Lake till (Ritter, 1967; most Bull Lake deposits are probably Illinoian in age, or 0.18--0.13 Ma; Richmond and Fullerton, 1986), appears to grade to a 0.62 Ma lower Roberts terrace. Thus, it is probable that stream gravels containing and underlying Lava Creek ash in the Bighorn basin are glaciofluvial. The stratigraphic position of deposits of Lava Creek A and B ashes described above is related to distance downstream from the nearest glacial source area: in general, the tephra layers overlie glaciofluvial gravel close to glacial sources but lie within or at the base of stream gravel far from glacial sources. In addition, two tephra deposits (sites 2 and 6) are buried by thick deposits of sidestream alluvial fans. These relations suggest that (1) gravel deposition occurs earlier near glaciated source areas than farther downstream, and (2) sidestream deposition continues for some time after mainstream deposition ceases. These relations are supported by similar observations on younger terraces in the Bighorn basin that can be physically traced into moraines (Reheis et al., 1991). RATES AND PROCESSES OF SOIL DEVELOPMENT Along Rock Creek and at the Kane fans, terrace surfaces dated by tephra-derived incision rates provide time calibrations for studies of soil development rates and processes in different climatic regimes (Reheis, 1987a,b, 1990). These studies are being used to estimate ages of undated deposits in areas of similar climate and parent material and to make inferences about paleoclimate from preserved properties of relict soils. Surficial A horizons of soils in all three climatic regimes studied (moist, semiarid, and arid) reflect the progressive influences of accumulation of eolian dust and surface erosion. Properties of B and C horizons, in contrast, reflect the climatic conditions under which the soils formed. Field and chemical properties of B and C horizons of soils in a moist climatic regime change at logarithmic rates that reflect relatively rapid chemical weathering, whereas properties of B and C horizons of soils in a dry climatic regime change at linear rates that reflect progressive dust additions and slower chemical weathering. Zirconium-poor dust is an important constituent of the soils in all three climates. The clay mineralogy of the soils reflects the local climate, but the most
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crystalline clays form in the clay-poor basal B and C horizons rather than in the clay-rich upper B horizons. REFERENCES Agard, S.S. (1989). Map showing Quaternary and late Tertiary terraces of the lower Bighorn River, Montana. United States Geological Survey Miscellaneous Field Studies Map MF-2094, scale 1:100,000. Alden, W.C. (1932). Physiography and glacial geology of eastern Montana and adjacent areas. United States Geological Survey Professional Paper 174, 133 pp. Andrews, D.A., Pierce, W.G. and Eargle, D.H. (1947). Geologic map of the Bighorn basin, Wyoming and Montana, showing terrace deposits and physiographic features. United States Geological Survey Oil and Gas Investigations Preliminary Map 71, scale 1:126,720. Hamilton, L.J. and Paulson, Q.F. (1968). Geology and ground-water resources of the lower Bighorn valley, Montana. United States Geological Survey Water-Supply Paper 1876, 39 pp. 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. United States Geological Survey Miscellaneous Investigations Map 1-1325, scale 1:4,000,000. Mackin, J.H. (1937). Erosional history of the Big Horn Basin, Wyoming. Geological Society of America Bulletin, 48, 813-894. Moss, J.H. (1974). The relation of river terrace formation to glaciation in the Shoshone River basin, western Wyoming. In: Coates, D. (ed.), Glacial Geomorphology, pp.293-314. Publications in Geomorphology, New York.
Palmquist, R.C. (1983). Terrace chronologies in the Bighorn Basin, Wyoming. Guidebook, Wyoming Geological Association 34th Annual Field Conference, pp. 217-231. Pierce, K.L. and Morgan, L.A. (1990). The track of the Yellowstone hotspot: volcanism, faulting, and uplift. United States Geological Survey Open-File Report 0F-90-415, 68 pp. Reheis, M.C. (1985). Evidence for Quaternary tectonism in the northern Bighorn basin, Wyoming and Montana. Geology, 13, 364-367. Reheis, M.C. (1987a). Gypsic soils on the Kane alluvial fans, Big Horn County, Wyoming. United States Geological Survey Bulletin 1590-C, 39 pp. Reheis, M.C. (1987b). Soils in granitic alluvium in humid and semiarid climates along Rock Creek, Carbon County, Montana. United States Geological Survey Bulletin 1590-D, 71 pp. Reheis, M.C. (1990). Influence of climate and eolian dust on the major-element chemistry and clay mineralogy of soils in the northern Bighorn basin, U.S.A. Catena, 17, 219--248. Reheis, M.C., Palmquist, R.C., Agard, S.S., Jaworowski, C., Mears, B.M. Jr, Madole, R.L., Nelson, A.R. and Osborne, G. (1991). Non-glacial Quaternary history of some Rocky Mountain basins. In: Morrison, R.B. (ed.), Quaternary Non-Glacial History of the Conterminous United States, Decade of North American Geology Volume, K-2, pp. 407-440. Geological Society of America, Boulder, Colorado. 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, pp. 3--10. Pergamon Press, New York. Ritter, D.F. (1967). Terrace development along the front of the Beartooth Mountains, southern Montana. Geological Society of America Bulletin, 78, 467-484.