Geologic map of the Bruneau Formation in the Jackass Butte and Dorsey Butte quadrangles, southwestern Idaho
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Structural geology in the Crested Butte Quadrangle is more complex than previously reported. Outcrop relationships previously explained as unconformities are the result of large faults at Hunter's Hill and in Slate River valley. The following lithologic assemblages are recognized in the Absarokan rocks of Colorado: black shale and limestone, gypsum, fine buff clastic rocks, coarse buff reworked clastic rocks, coarse red reworked clastic rocks, coarse red arkosic clastic rocks, and red siltstone. All of these except the gypsum and red siltstone assemblages are present in the Crested Butte Quadrangle. The Belden shale is the black shale and limestone assemblage, and was deposited unconformably upon the Leadville limestone during the beginning of the invasion of the Absarokan seas. A new name, the Gothic formation, is introduced for the overlying coarse and fine buff clastic rock assemblages. The type section is at Copper Creek. This formation was deposited under predominantly marine conditions as the Ancestral ockies and the Central Colorado trough became sharply differentiated. The Maroon formation is restricted to the coarse red arkosic and reworked clastic rock assemblages above the Gothic. It was deposited as tectonic differentiation continued, but after the seas had withdrawn from the area. Red siltstone, not present in the Crested Butte district, was deposited during the final reduction of the Ancestral Rockies after tectonic differentiation ceased. To complete the cycle the Jurassic Entrada sandstone was deposited unconformably upon slightly deformed Absarokan rocks. The Gothic thins toward the northeast with concurrent thickening of the Maroon. Also the Maroon is absent in the southwest part of the area. These facts indicate a progressive northeastward shift of the site of deposition i this area during the Absarokan. Absarokan fossils from this area, separated on the basis of the type of enclosing rock, fall into more or less distinct groups. Although the fossils are mostly of Des Moinesian age, there are a few Morrowan species in the Belden and many post-Des Moinesian species in both the Belden and Gothic. It is suggested that the Morrowan species here may be restricted to black shale and limestone producing End_Page 543------------------------------ environments, and the post-Des Moinesian species here to environments characteristic of nearly filled or dried up seas. Their presence at Crested Butte may then be explained by the existence of such environments in this area during Des Moinesian time.
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Dome (geology)
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ABSTRACT Late Miocene volcaniclastics of the Ellensburg Formation (Washington, northwestern USA) are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a Cascade Range source area. Two facies associations are recognized. The first is composed of laterally restricted, well‐sorted, polymictic conglomerates representing a gravel‐bedload regime during inter‐eruption periods. The second comprises laterally extensive sheets, bounded by deeply‐developed paleosols, composed of monomictic pebbly, pumiceous dacitic sandstones with intercalated debris‐flow and hyperconcentrated flood‐flow deposits. These sheets aggraded in response to eruptive events that are sometimes recorded by air‐fall tephras at the bases of sequences. Debris‐flow and hyperconcentrated flood‐flow deposits occur as far as 120 km from source, but are uncommon beyond 50 km. Hyperconcentrated flood‐flow deposits initially increase in abundance away from source as debris‐flow deposits diminish, suggesting formation of hyperconcentrated flood flows by dilution of debris flows. Sandy facies form broad sheets dominated by scour‐fill bedding, in proximal and medial settings, and grade to narrower, trough cross‐bedded sheets in distal settings, suggesting moderation of flood discharges with distance. Base‐level changes associated with episodic sediment influx caused incision‐aggradation cycles in mainstream settings, and episodic impoundment of tributaries to form lakes or rapidly aggrading, poorly drained floodplains. Although volcanism was the primary control on depositional style, concurrent development of the Yakima fold belt produced a structure‐consequent drainage pattern that determined sediment dispersal, and basin subsidence permitted preservation of both syn‐eruption and inter‐eruption facies. Detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics, but dominates deposits of small tributary streams flowing toward, rather than from, the Cascades.
Hyperconcentrated flow
Aggradation
Debris flow
Mudflow
Denudation
Subaerial
Butte
Paleosol
Alluvial fan
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Butte
Colluvium
Berm
Alluvial fan
Pebble
Bedrock
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ABSTRACT Fluvial facies in the Sentinel Butte Formation include meandering-channel, channel-plug, natural-levee, floodbasin, and swamp facies. The channel facies comprises elongate, tabular, fining-upward sand beds that exhibit gently dipping point-bar surfaces (sigmoidal cross bedding). Trough cross bedding in the lower part of the sand beds grades upward to planar and ripple cross bedding. Paleocurrent data indicate current directions generally parallel to bed trends. Channel-plug facies consist of organic silty clay filling arcuate, asymmetrically trough-shaped abandoned meanders. Levee facies grade upward from channel facies and consist of ripple cross-bedded sandy silt and silty clay. Floodbasin facies comprise dark-gray, organic, bioturbated silty clay, and grade upward to woody lignite f the swamp facies. Petrified tree stumps are present in levee, floodbasin, and swamp facies. Deltaic facies include distributary-channel, channel-plug, natural-levee, marsh, delta-front, prodelta, interdistributary-bay, and crevasse-splay facies. The channel facies consists of elongate, narrow, trough-shaped, fining-upward sand beds. Planar cross bedding grades upward to ripple and climbing-ripple cross bedding. Paleocurrent indicators are parallel to sand bed trends. Most channel deposits grade upward to organic, disturbed clay and lignite of the marsh facies, but others grade upward to ripple cross-bedded sandy silt and silty clay of the channel-plug facies. Small levee facies of silty clay border the upper part of the channel facies and grade downward to planar cross-bedded silty sand and silty clay of the delta-front facies. The delta-front facies overlies laminated, spar ely fossiliferous silty clay of the prodelta facies and grades laterally to fissile silty clay of the interdistributary-bay facies. Thin, lens-shaped beds of silty sand within the interdistributary-bay facies represent the crevasse-splay facies.
Paleocurrent
Overbank
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Phreatomagmatic eruption
Maar
Butte
Pyroclastic fall
Lapilli
Lava field
Cinder cone
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