Conglomerate and sandstone (Holocene)-Alluvium: shingly and detrital sediments, gravel, sand more abundant than silt and clay Fan alluvium and colluvium (Holocene and late Pleistocene)-Fan alluvium and colluvium: shingly and detrital sediments, gravel, sand, clay Conglomerate and sandstone (late Pleistocene)-Alluvium: shingly and detrital sediments, gravel, sand more abundant than silt and clay Loess (late Pleistocene)-Loess more abundant than sand, clay Conglomerate and sandstone (middle Pleistocene)-Alluvium: shingly and detrital sediments, gravel, sand more abundant than silt and clay Travertine (middle Pleistocene)-Travertine Conglomerate and sandstone (early Pleistocene)-Alluvium: shingly and detrital sediments, gravel, sand more abundant than silt and clay Conglomerate and sandstone (Pliocene)-Gray conglomerate, grit, sandstone more abundant than siltstone, clay, limestone, marl; gypsum, salt; acid to mafic volcanic rocks Conglomerate and sandstone (Miocene)-Red conglomerate, sandstone more abundant than siltstone, clay; acid and mafic volcanic rocks; limestone, marl; olivine basalt, trachybasalt, andesitic basalt (Taywara Series) Granite (Oligocene)-Granite (Phase III) Granodiorite and granosyenite (Oligocene)-Granodiorite, alaskite, granosyenite more abundant than granite (Phase II) Granodiorite (Oligocene)-Granodiorite (Phase I) Granite and granodiorite (Oligocene)-Granite, granite porphyry, granodiorite more abundant than quartz syenite, granosyenite Sandstone and siltstone (Oligocene)-Sandstone, siltstone more abundant than clay, conglomerate, limestone, marl; acid and mafic volcanic rocks Rhyolite lava (Oligocene and Eocene)-Basaltic andesite, basalt, trachyte, dacite, rhyolite, ignimbrite, tuff; conglomerate, sandstone, siltstone, limestone Andesite lava (Oligocene and Eocene)-Basaltic andesite, basalt, trachyte, dacite, rhyolite, ignimbrite, tuff; conglomerate, sandstone, siltstone, limestone Ultramafic intrusions (Eocene)-Dunite, peridotite, serpentinite Clay and shale (Eocene)-Clay, shale, siltstone more abundant than sandstone, limestone, marl, gypsum, conglomerate Conglomerate and sandstone (Paleocene)-Conglomerate, sandstone more abundant than siltstone, limestone, shale; mafic volcanic rocks Gabbro and monzonite (Paleocene and Late Cretaceous)-Gabbro, monzonite more abundant than diorite, granite, granosyenite, syenite porphyry, syenite Sandstone and siltstone (Late Cretaceous)-Limestone (Middle Afghanistan); redstone, siltstone, conglomerate (Khashrud tectonic zone) Gabbro and monzonite (Early Cretaceous)-Gabbro, monzonite more abundant than diorite, granodiorite Basalt lava (Late Triassic)-Shale more abundant than phyllite, andesite to basalt (greenstone altered), limestone (Kotagai Series) Siltstone and sandstone (Late Triassic (Norian and Rhaetian))-Siltstone, sandstone more abundant than shale, conglomerate Limestone and dolomite (Late Triassic (Carnian and Norian))-Limestone, dolomite Sandstone and siltstone (Late and Middle Triassic)-Limestone, dolomite, marl (Kabul Massif and Kunar tec...

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Volcanic rocks near Yampa, Colorado (USA), represent one of several small late Miocene to Quaternary alkaline volcanic fi elds along the northeast margin of the Colorado Plateau. Basanite, trachybasalt, and basalt collected from six sites within the Yampa volcanic fi eld were investigated to assess correlations with late Cenozoic extension and Rio Grande rifting. In this paper we report major and trace element rock and mineral compositions and Ar, Sr, Nd, and Pb isotope data for these volcanic rocks. High-precision 40 Ar/ 39 Ar geochronology indicates westward migration of volcanism within the Yampa volcanic fi eld between 6 and 4.5 Ma, and the Sr, Nd, and Pb isotope values are consistent with a primary source in the Proterozoic subcontinental lithospheric mantle. Relict olivine phenocrysts have Mg-and Ni-rich cores, whereas unmelted clinopyroxene cores are Na and Si enriched with fi nely banded Ca-, Mg-, Al-, and Ti-enriched rims, thus tracing their crystallization history from a lithospheric mantle source region to one in contact with melt prior to eruption. A regional synthesis of Neogene and younger volcanism within the Rio Grande rift corridor, from northern New Mexico to southern Wyoming, supports a systematic overall southwest migration of alkaline volcanism. We interpret this Neogene to Quaternary migration of volcanism toward the northeast margin of the Colorado Plateau to record passage of melt through subvertical zones within the lithosphere weakened by late Cenozoic extension. If the locus of Quaternary alkaline magmatism defi nes the current location of the Rio Grande rift, it includes the Leucite Hills, Wyoming. We suggest that alkaline volcanism in the incipient northern Rio Grande rift, north of Leadville, Colorado, represents melting of the subcontinental lithospheric mantle in response to transient infi ltration of asthenospheric mantle into deep, subvertical zones of dilational crustal weakness developed during late Cenozoic extension that have been migrating toward, and subparallel to, the northeast margin of the Colorado Plateau since the middle Miocene. Quaternary volcanism within this northern Rio Grande rift corridor is evidence that the rift is continuing to evolve.

Mf of a thin coal bed approximately 250 ft above the base of the middle Bloyd sandstone. In 5 3 thin beds of sandstone. Shale and siltstone are dark gray and fissile to thin, The study area preserves an approximately 1,600-ft-thick record of early and late 1545 5 1530 Mf This work was conducted in a cooperative project between the U.S. Geological Survey 4 the Boxley quadrangle, the Bloyd-Atoka contact is placed at a similar level within a shale Qty Mbv ripple bedded. Sandstone is tan, very fine to fine grained, thin bedded with Paleozoic deposition on what is now the southern margin of the North American continent. and the U.S. National Park Service. We thank D.L. Zachry for discussions about the 1940 5 Oe Qal Qc Ql QUATERNARY hhg interval beneath an approximately 100-ft-thick interval of sandstone that underlies a ripple marks. Limestone includes medium to thick beds of red-brown Provincial series for Pennsylvanian and Mississippian units are from McFarland (1988). Pennsylvanian stratigraphy. Helpful reviews were provided by Angela Chandler and Paul Carrara. Mbv Qto 5 1945 prominent topographic ledge. This sandstone sequence is typical of Atoka facies in being fine to very fine grained, thin to medium bedded with ripple laminations (fig. 3A) and being Mb conglomerate, with clasts of fossil fragments and subrounded sandstone and The Middle Ordovician Everton Formation is a heterogeneous sandstone and carbonate Mbv Unconformity siltstone. The Brentwood Limestone Member at the base of formation (not unit that Suhm (1974) interpreted to have been deposited in barrier island and tidal flat hhc 2 2 laterally extensive across the quadrangle. An alternative contact that was considered, about Mbs 1550 5 2 REFERENCES CITED 5 mapped) is a 5-to 20-ft-thick limestone interval varying from massive gray depositional environments. The Everton Formation is unconformably overlain by the Upper hhg hbu Of ha Atokan 50 ft lower, would shift the sandstone interval including fucoid trace fossils (fig. 3B) from the 5 micrite to reddish-gray, coarse bioclastic limestone. Unit is conformable with Ordovician Fernvale Limestone. 1500 Angelier, Jacque, 1990, Inversion of field data in fault tectonics to obtain the regional 5 1500 upper Bloyd to the Atoka Formation (ha). This option was not favored because this Mbs Of Mp hhg 2 underlying Hale Formation. Forms moderate to steep slopes and is poorly The Mississippian Boone Formation is widespread within the northern part of the 5 Unconformity stress-III, A new rapid direct inversion method by analytical means: Geophysical

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