Late Cenozoic Basic Volcanism in Northwestern Colorado and Its Implications Concerning Tectonism and the Origin of the Colorado River System

Abstract

Upper Cenozoic terrestrial basin-fill sedimentary and basic volcanic rocks are common in the 20,700 km 2 Basalt area, which includes parts of the Gore, Sawatch, and southern Park Ranges, Elk Mountains, Grand Mesa, and White River Plateau. Principally on the basis of whole-rock K-Ar ages from basalt flows, the rocks can be placed in four groups. Group 1 rocks attain a thickness of 210 m and range in age from 24 to 20 m.y. (early Miocene). They consist primarily of flows of alkali-olivine basalt or of basalt flows interlayered with cross-bedded sandstone of the Browns Park Formation. Group 2 rocks, 14 to 9 m.y. old (late Miocene and perhaps early Pliocene), attain a maximum thickness of 180 m and are composed largely of basalt, basaltic andesite, and fine-grained tuffaceous fluvial, lacustrine, and eolian sedimentary rocks containing a rich vertebrate fauna indicative of a semiarid climate and a steppe vegetation. Toward the end stages of the accumulation 155 on July 6, 2015 memoirs.gsapubs.org Downloaded from 156 LARSON AND OTHERSof group 2 rocks, about 10 m.y. ago, the region was subjected to major tectonism, including uplift, reactivation of some Laramide (Late Cretaceous-early Tertiary) structural features, and creation of new warps, folds, and faults. The initiation of the Colorado River system apparently occurred at that time in response to increased precipitation stemming from an increase in elevation and relief. Downcutting began at this time and has continued intermittently to the present. By about 8 m.y. ago (late Miocene or early Pliocene, depending on boundary interpretation), the Roaring Fork River had downcut 600 m and had formed a broad flood plain, upon which thin alkali-olivine basalt flow units (group 3 rocks) were erupted. Downcutting appears to have been slow between about 8 and 1.5 m.y. ago; since then, an additional 300 m of valley deepening has occurred. Sporadic volcanism since 1.5 m.y. has accounted for several small cinder cones and flows of nephelinenormative alkali basalt (group 4 rocks); the last eruption occurred about 4,000 yr ago. on July 6, 2015 memoirs.gsapubs.org Downloaded from LATE CENOZOIC BASIC VOLCANISM, NORTHWESTERN COLORADO 167structural and physiographic development of this region during the latter half of Cenozoic time. The area under discussion covers approximately 20,700 km 2 and includes the White River Plateau, parts of the Park, Gore, and Sawatch Ranges, the Elk Mountains, and Grand Mesa (see Fig. 1 for pertinent geographic names). For simplification, this area is hereafter termed the Basalt area, in reference to the centrally located town of Basalt. The name is particularly appropriate because most of the conclusions in this paper have resulted from investigation of basic lava flows.A generalized geologic map of the Basalt area is shown in Figure 2. Because the scale is small, many geologic elements have been eliminated or minimized: faults and fold axes have been omitted, and pre volcanic rocks are simply designated as Precambrian, Paleozoic, Mesozoic, and Tertiary...

Remedial action plan and site conceptual design for stabilization of the inactive uranium mill tailings sites at Rifle, Colorado. Appendix D, Final report

“…The following discussion is excerpted from articles by Hunt (1956), Lohman (1981Lohman ( , 1965, Yeend (1969), and Sinnock (1981a,b;. For a broader understanding of the Cenozoic and Quaternary geomorphic history of Colorado, the reader is also referred to Epis et al (1980), Meierding and Birkeland (1980), Larson et al (1975), and Richmond (1965).…”

“…Since late Miocene time, erosion rates have been controlled by the Colorado River, which acts as the regional base level for streams. Potassium-argon dates of basalt flows' indicate that the Colorado River drainage above Grand Junction was established about 10 million years ago (Larson et al, 1975), whereas drainage integration through the Grand RFL001F2.AD3…”

“…Relatively rapid erosion has resulted from the combination of mechanically weak bedrock (Schumm and Chorley, 1966) and rapid tectonic uplift (Larson et al, 1975;Luchitta, 1972;Hunt, 1969). Broad spatial and temporal variations in erosion rates are due to differences in local rock type, changes in uplift rate, and climatic fluctuations.…”

“…basalt near Aspen (site 1) and 8 m.y. basalt near Glenwood Springs (site 3) (Larson et al, 1975) Two basalt flows above channel (Larson etal., 1975) Lava Creek B ash above channel (Izett and Wilcox, 1982) 215-<420 0-0.62 Ancestral Colorado River (?) gravel 5280 overlying basalt of Grand Mesa (Yeend, 1969); incision timing from Larson et al (1975) Lava Creek B ash above channel 300 (Whitney et al, 1983); age from Izett and Wilcox (1982) Late Pliocene -early Pleistocene 200 gravels above channel (Hunt, 1969) Alluvium with reversed magnetic po-850 larity above channel (WoodwardClyde Consultants, 1982); reversal age from Johnson (1982) Alluvium with reversed magnetic po-550 larity above channel (WoodwardClyde Consultants, 1982); reversal age from Johnson (1982) Bishop ash above Paradox Creek 280 (Cater, 1970) Channel downcutting estimated from 4000 series of erosion surfaces, oldest overlain by dated basalt (Stokes, 1973;Damon et al, 1974) 0-10 Tree-ring dating of exposed tree roots (Carrara and Carroll, 1979) Pearlett 0 ash above channel of 400 Little Snake River (Madole, 1976) Larson et al (1975) Bilateral separation of basalt remnants on Battlement Mesa and Mt.…”

“…gravel 5280 overlying basalt of Grand Mesa (Yeend, 1969); incision timing from Larson et al (1975) Lava Creek B ash above channel 300 (Whitney et al, 1983); age from Izett and Wilcox (1982) Late Pliocene -early Pleistocene 200 gravels above channel (Hunt, 1969) Alluvium with reversed magnetic po-850 larity above channel (WoodwardClyde Consultants, 1982); reversal age from Johnson (1982) Alluvium with reversed magnetic po-550 larity above channel (WoodwardClyde Consultants, 1982); reversal age from Johnson (1982) Bishop ash above Paradox Creek 280 (Cater, 1970) Channel downcutting estimated from 4000 series of erosion surfaces, oldest overlain by dated basalt (Stokes, 1973;Damon et al, 1974) 0-10 Tree-ring dating of exposed tree roots (Carrara and Carroll, 1979) Pearlett 0 ash above channel of 400 Little Snake River (Madole, 1976) Larson et al (1975) Bilateral separation of basalt remnants on Battlement Mesa and Mt. Callahan, if they were formerly continuous (Yeend, 1969); incision timing from Larson et al (1975) Unilateral valley widening since Colorado River established in late Pliocene or earliest Pleistocene (Hunt, 1969) Bilateral valley widening since Colorado River established in mid to late Tertiary (Hunt, 1969) Colorado River at Green Bilateral widening of inner canyon River, UT since late Pliocene (Hunt, 1969) Colorado River at Grand Canyon, AZ Bilateral widening since canyon cutting began in late Miocene (Hunt, 1969) > 51,000 ( Ages of fossil packrat middens in cliffs (Cole and Mayer, 1982); possibly a minimum rate (Haman, 1983) …”