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USGS Geologic Investigations Series I-2569, Sheet 1 of 2

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USGS Geologic Investigations Series I-2569, Sheet 1 of 2 U.S. DEPARTMENT OF THE INTERIOR GEOLOGIC INVESTIGATIONS SERIES U.S. GEOLOGICAL SURVEY MAP I-2569 SHEET 1 of 2 Pamphlet accompanies map 123° 122° 121° 46° 1 46° CORRELATION OF MAP UNITS SEDIMENTARY DEPOSITS AND VOLCANIC INTRUSIVE SEDIMENTARY ROCKS ROCKS ROCKS Basaltic and basaltic Andesitic Dacitic Rhyolitic andesitic rocks rocks rocks rocks Millions of Percent SiO2 years ago 57 62 70 0 Qt Qb1 Qa1 Qd1 Qr1 Holocene 0.012 Qb2 Qa2 Qd2 Qr2 0.025 Qb3 Qa3 Qd3 Qr3 0.120 QUATERNARY Qal Ql Qg Qs Qi Pleistocene Qb4 Qa4 Qd4 Qr4 0.780 Qb5 Qa5 Qd5 Qr5 2 Pliocene Ts1 Tb1 Ta1 Td1 Tr1 7 Tcu Ts2 Tb2 Ta2 Td2 Tr2 Miocene Tcl Basalt Group Columbia River 17 Ti TERTIARY Ts3 Tb3 Ta3 Td3 Tr3 25 Ts4 Tb4 Ta4 Td4 Tr4 Oligocene 35 ? Ts5 Tb5 Ta5 Eocene 45 1Time scale of Harland and others (1982) and Cande and Kent (1995) DESCRIPTION OF MAP UNITS Ta Tertiary andesitic rocks—Dominantly lava flows and domes but includes some SEDIMENTARY DEPOSITS AND ROCKS 1 Ta near-vent breccia and pyroclastic rocks. Rocks range in age from 2 to Qt Young tephra (Holocene)—Fallout lapilli and ash. Mapped only where large areas 2 approximately 45 m.y. , 2 to 7 m.y.; , 7 to 17 m.y.; , 17 to 25 m.y.; of bedrock are completely blanketed. Unit consists of Mazama ash bed (Powers Ta1 Ta2 Ta3 Ta3 , 25 to 35 m.y.; , 35 to 45 m.y. and Wilcox, 1964), which is a rhyolitic air-fall tephra (SiO = 70-71 percent) Ta4 Ta5 2 Ta 45° 14 4 45° erupted about 6,850 C years ago (Bacon, 1983) from Crater Lake caldera. Mazama ash thickly veneers areas as distant as 50 km north, northeast, and east Ta5 of caldera. Mazama ash has been identified as far away as Alberta, British Dacitic rocks Columbia and Utah; in British Columbia it is thicker than 1 cm (Powers and Qd1 Quaternary dacitic rocks—Dominantly lava flows and domes but includes some Limit of geologic mapping Wilcox, 1964). Thickest known deposits (20 m) lie just beyond caldera rim near-vent breccia and pyroclastic rocks. Rocks range in age from 0 to 2 m.y. (Bacon, 1983) and east of Mount Thielsen (Sherrod, 1991). Isopachs shown in Qd2 Qd , 0 to 12 k.y.; Qd , 12 to 25 k.y.; Qd , 25 to 120 k.y.; Qd , 120 to 780 k.y.; blue on map. Unit also includes undifferentiated Holocene pumice and ash on 1 2 3 4 Qd3 Qd , 780 k.y. to 2 m.y. upper flanks of Newberry volcano that accumulated during several eruptions 5 Qd from intracaldera vents; youngest eruption occurred about 1,300 14C years ago 4 (MacLeod and others, 1995) Qd5 Qal Alluvium (Holocene and Pleistocene)—Poorly to moderately sorted sand, gravel, Tertiary dacitic rocks—Dominantly lava flows and domes but includes some Td1 and silt deposited in valleys of active major streams. Many small areas omitted near-vent breccia and pyroclastic rocks. Rocks range in age from 2 to for clarity. Locally includes glacial and alluvial fan deposits Td2 approximately 35 m.y. Td , 2 to 7 m.y.; Td , 7 to 17 m.y.; Td , 17 to 25 m.y.; Landslide deposits (Holocene and Pleistocene)—Debris from mass-wasting 1 2 3 Ql Td3 Td , 25 to 35 m.y. processes, including slumps, earth flows, block glides, and rock falls. Landslides 4 Td4 are most common where competent lava flows overlie weaker sediment or ash Rhyolitic rocks flows. Landslides that cover less than a few square kilometers are generally not Quaternary rhyolitic rocks—Dominantly lava flows and domes but includes shown Qr1 some near-vent breccia and pyroclastic rocks. Rocks range in age from 0 to 2 Glacial deposits (Holocene and Pleistocene)—Unconsolidated to moderately Qg Qr2 m.y. , 0 to 12 k.y.; , 12 to 25 k.y.; , 25 to 120 k.y.; , 120 to 780 indurated, slightly to deeply weathered till that forms moraines and drift sheets Qr1 Qr2 Qr3 Qr4 Qr k.y.; , 780 k.y. to 2 m.y. mainly limited to middle elevations of High Cascades subprovince. Locally 3 Qr5 includes glacial outwash and related glaciofluvial and glaciolacustrine deposits. Qr4 Many small areas omitted for clarity, especially moraines and till in mountain Qr5 valleys and cirques. Chiefly younger than about 140,000 years (140 k.y.). Older Tr Tertiary rhyolitic rocks—Dominantly lava flows and domes but includes some tills, which in large part are unmapped and unstudied, crop out in lower 1 near-vent breccia and pyroclastic rocks. Rocks range in age from 2 to elevations of major canyons in Western Cascades subprovince but are included in Tr2 approximately 35 m.y. , 2 to 7 m.y.; , 7 to 17 m.y.; , 17 to 25 m.y.; unit Qs Tr1 Tr2 Tr3 Tr , 25 to 35 m.y. Quaternary sediment and sedimentary rocks, undivided—Mostly unconsolidated 3 Tr4 Qs sand, gravel, and lacustrine deposits. Commonly includes deposits mapped by Tr4 original authors as older alluvium, terrace deposits, upland gravel, basin-filling sediment, or undifferentiated Quaternary sediment. Along Columbia River, Patterns—In addition to primary age and composition of volcanic rocks shown by includes flood deposits resulting from numerous catastrophic emptyings of color, patterns may be used to indicate volcanic facies: Volcanic rocks deposited mostly near vents—Units shown without pattern are glacial Lake Missoula approximately 13,000 years ago (13 k.y.). In Willamette No pattern Valley, includes Willamette Silt (Allison, 1953; Hampton, 1972). Along west products of concurrent volcanism and deposition, and they generally margin of Cascade Range, includes high-standing terraces that probably correspond to vent facies of Smedes and Prostka (1972) or to core facies and developed during Pleistocene glaciations. Along east side of Cascade Range, proximal volcaniclastic facies of Vessell and Davies (1981). Also includes includes extensive distal outwash fans. In parts of La Pine valley and in Upper lava that flowed down valleys or spread over lowlands far removed from and Lower Klamath Lake basins, includes graben-filling sediment at least 400 m vents. Typical constructional volcanic features include composite volcanoes, thick derived from Cascade Range and Newberry volcano (Sammel and Peterson, shield volcanoes, domes, and smaller monogenetic volcanoes. Larger 1976; Veen, 1982; Couch and Foote, 1985) volcanic edifices commonly intertongue with each other and with rocks of Tertiary sedimentary rocks—Rocks ranging in age from 2 to approximately 45 other map units (see figure 2); most intertonguing relations are too detailed to show at map scale. Vent areas, particularly those of composite and shield Ts1 millon years (m.y.). Ts , 2 to 7 m.y.; Ts , 7 to 17 m.y.; Ts , 17 to 25 m.y.; Ts , 25 1 2 3 4 volcanoes, contain lava flows, flow breccia, and near-vent pyroclastic Ts to 35 m.y.; Ts , 35 to 45 m.y. 2 5 deposits, which generally cannot be separated at scale of this map. Ts3 Constructional volcanic edifices older than a few million years are generally Ts4 poorly preserved and difficult to recognize Volcanic diamicton—Unsorted to poorly sorted, poorly layered to well-layered Ts5 Patterns—In addition to primary age designation shown by color, patterns may be deposits of direct volcanic origin that contain a wide range of clast sizes. used to indicate different kinds of Tertiary sedimentary rocks: Corresponds to more distal part of vent facies of Smedes and Prostka (1972) and medial volcaniclastic facies of Vessell and Davies (1981). Deposited Dominantly volcaniclastic sandstone, siltstone, granule conglomerate, and from cold- or warm-fluid flows that moved rapidly downslope propelled by No pattern mudstone closely related to volcanic processes in time and gravity. Commonly described by other authors as catastrophic. Includes location—Sedimentary units shown without pattern correspond to fine alluvial mudflow deposits, some mass-flow deposits not further described in source facies and distal part of coarse alluvial facies of Smedes and Prostka (1972) and reports, debris flows, landslides triggered by volcanic action, some to distal volcaniclastic facies and outer part of medial volcaniclastic facies of block-and-ash flows, some deposits designated only as tuff breccia, and Vessell and Davies (1981). Sandstone, siltstone, granule conglomerate, and deposits mapped as volcanic diamicton but not more specifically identified. mudstone dominate. Most sequences include beds interpreted as debris-flow Also includes young mudflow deposits and debris-flow deposits downstream deposits, hyperconcentrated stream-flow deposits, and ash-flow tuff. Excluded from major volcanoes. Lava flows are rare are sedimentary sequences whose origin and mode of formation are not closely Pyroclastic-flow deposits—Most are poorly sorted, thin, rich in lithic clasts, related to active volcanism, even where individual beds contain substantial 44° 44° and nonwelded to partly welded. Pumice is common in ash-flow tuff but not volcanic detritus. Beds are generally a few centimeters to 1 m thick. Unit grades in block-and-ash deposits. Plagioclase is most common phenocryst and may laterally and vertically into other sedimentary or volcanic rock units as constitute as much as 20 percent of some deposits. Variable amounts of proportion of other rock types increases. clinopyroxene, orthopyroxene, and amphibole are also present but rarely total Sedimentary units shown without pattern formed predominantly by reworking more than 5 percent. Biotite or quartz present in only a few tuffs; sanidine of penecontemporaneous primary volcanic deposits or, less commonly, directly found only in upper Eocene and Oligocene tuff east of Cascade Range. from volcanic processes. Most beds formed as a result of erosion and Deposited from hot dry flows of particulate volcanic rock, intermixed air, and redeposition of primary volcanic or volcaniclastic deposits that were presumedly volcanic gas that move rapidly downslope propelled by gravity.
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