u.s. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY PRELIMINARY GEOLOGIC MAP OF THE MOUNT HOOD 30- BY 60-MINUTE QUADRANGLE, NORTHERN CASCADE RANGE, OREGON By David R. Sherrod and William E. Scott1 OPEN-FILE REPORT 95-219 1995 This map is preliminary and has not been reviewed for confonnity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use oftrade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 1Both at Cascades Volcano Observatory, 5400 MacArthur Blvd., Vancouver, WA 98661 PRELIMINARY GEOLOGIC MAP OF THE MOUNT HOOD 30- BY 60-MINUTE QUADRANGLE, NORTHERN CASCADE RANGE, OREGON By David R. Sherrod and William E. Scott ABOUT THIS MAP of the Columbia River Basalt Group in the This map shows the geology ofthe central and Salmon River valley were mapped in detail by eastern parts of the Cascade Range in northern Burck (1986), the overlying middle and upper(?) Oregon. The Quaternary andcsitic stratovolcano Mioccnc lava flows. volcaniclastic strata. and of Mount Hood dominates the northwest quarter intrusions havc never been studied. The other ofthe quadrangle, but nearly the entire arca is un­ poorly known area, the Mutton Mountains in the derlain by arc-related volcanic and volcaniclastic southeastern part of the map area, consists of rocks of the Cascade Range. Most stratigraphic Oligocenc and lowcr Mioccnc volcanic and units were emplaced since middle Miocene time, volcaniclastic rocks. Overlying lava flows ofthe and all arc Oligocene or younger. Despite tbe Columbia River Basalt Group were mapped in proximity of the map area to the Portland some detail by Anderson (in Swanson and others, metropolitan area. large parts remained virtually 1981). unstudied or known only from limited reconnais­ sance until the late 1970s. A nntable exception is the area surrounding Mount Hood, where map­ AGE DETERMINATIONS Ages for many units were detennincd by iso­ ping and ebemical analyses by Wise (1969) pro­ vided a framework for geologic interpretation. topic methods, ebiefly potassium-argon (K-Ar). For many samples. fusion and gas extraction was Mapping since 1975 was conducted first to un­ conducted on sample splits, tbereby improving derstand tbe stratigraphy and structure of the the prccision of thc age detcnnination. In these Columbia River Basalt Group (Anderson, 1978; cases, a weigbted mean age is reported (table I), Vogt, 1981; J.L. Anderson, in Swanson and oth­ weighted by the inverse of tbe variance of indi­ ers, 1981; Vandiver-Powell, 1978; Burck, 1986) vidual runs (a standard statistical method; for ex­ and later to examine the gcotbcnnal potential of ample, Taylor, 1982). The age of young Mount Mount Hood (Priest and otbers, 1982). Addi­ Hood deposits is known from earbon-14 (14C) tional mapping was completed in 1985 for a geo­ dating ofcharcoal and wood. logic map of the Cascade Range in Oregon (Sberrod and Smith, 1989). From 198710 1990, Magnetic polarity was determined for many detailed mapping was conducted in three 15­ samples to further cbaracterize stratigraphic units. minute quadrangles on a limited basis (D.R. Sher­ In some areas. a cbronostratigrapbic framework rod, unpublisbed mapping) (sec fig. 1 for index was developed by combining isotopic ages and to mapping). An ongoing volcanic hazards study sequential polarity cbanges; this framework al­ of Mount Hood by the U.S. Geological Survey lowed us to assign minimum ages for some un­ (Scott and otbers, 1994) bas provided the catalyst dated stratigraphic units. We use 0.78 million for completing the geologic map of the Mount years as thc age of tbe most recent major ebron Hood 3Q-minute by 6Q-minute quadrangle. boundary (base of Brunbes Normal-Polarity Cbron) (Sbaekleton and others, 1990; Baksi and As ofJune 1994, only two broad arcas still re­ others, 1992). The correlation chart shows ages main largely unmapped. One of these areas, la­ for other ebron and subcbron boundaries in beled "unmapped" on tbe geologic map, lies in tbe Brunhes, Matuyama. and Gauss time (from Salmon River valley soutb of Zigzag along the Cande and Kent, 1992). west margin of tbe quadrangle. Altbough strata STRATIGRAPHIC SYNOPSIS ism in the map area have shifted sporadically. The Cascade Range volcanic arc has been ac­ Andesitic eruptions were predominant in the tive since about 40 million years (m.y.) ago western part from about 14 to 10 m.y. ago (Duncan and Kulm, 1989), altbougb the oldest (Salmon and Sandy Rivers area), preducing the arc-related rocks. if present in northern Oregon. Rhododendron Formation and overlying lava are deeply buried. Only one canyon in tbe map flows. From about 8 to 6.5 m.y. ago, lithic py­ area (the Claekamas River canyon in the south­ roclastic debris oftbe Dalles Formation was shed west comer) has cut deep enough to expose lower by chiefly andesitic volcanoes in the north-central Miocene volcaniclastic strata in the Cascade part ofthe map area (Hood River valley escarp­ Range (unit Tos). East ofthe range, mildly alka­ ment). Andesitic to dacitic volcanism was again line volcanic rocks of Oligocene and early predominant about 5 to 3 m.y. ago, with known Miocene age are found in the Mutton Mountains. eruptive centers located from Lookout Mountain By middle Miocene time, much oftbe map area westward to Lolo Pass, probably including the was flooded by tholeiitic basalt ofthe Columbia area now occupied by Mount Hood. A major River Basalt Group. The Columbia River Basalt episode of mafic volcanisrn--basalt and basaltic Group is Dot related to the volcanic arc but faIms andesite-began about 3-4 m.y. ago and lasted a useful stratigraphic marker. For infonnative until about 2 m.y. ago. Since about 2 m.y. ago summaries of the emplacement. distribution, and (Quaternary according to Harland and others, chemical evolution ofthe Columbia River Basalt 1982), volcanism has been concentrated along the Group. the interested reader is referred to studies axis of the High Cascades. North and south of by Beeson and others (1989), Nelson (1989), Mount Hood, Quaternary volcaoic rocks are pre­ Tolan and others (1989), Wright and others dominantly basaltic andesite lava flows; whereas (1989), or references cited therein. at Mount Hood itself, andesite is volumetrically Since the time of Columbia River Basalt, the prooOOlinant, fonning pyroclastic and debris flow locus and composition ofCascade Range volcan- deposits and lava flows. 2 7 8 8 ~ ... '-=__---L L---'-_-" --:---".J 1. Ancinon{l978) 1. O.R.Shtnod.~~.I985 2. ~ r., SwanIio"IIIn:l 0Ihers, 19811 a DR 5tlwIod. U1llJb. ~ ~enIy 3. 8Lwdl (19ll6) 1985-19lIl 4. Priesl n or-s 11982) 9. Vogt (1981) 5. W.E. Sc:ou. ~ ~ 1990-1993 10. wat.'s (19681) 6. C.R. Sherrod.~ ~ 1990-1993 11. Wise (1969) Figure 1. Index to mapping used on geologic map. 2 DESCRIPTION OF MAP UNITS [Volcanic rocks are named according to chemical classification, in terms ofweight percent SiOz, as fol­ lows: basalt, <52 percent; basaltic andesite, ~52 and <57 percent; andesite, ~57 and <63 percent; dacite, ~63 and <68 percent; rhyodacite, ~68 and <72 percent; rhyolite, ~72 percent. This scheme follows the recommendations ofthe ruGS Subcommission on the Systematics ofIgneous Rocks (Le Bas and Streck­ eisen, 1991) but is modified slightly to include a field for rhyodacite] SURFICIAL DEPOSITS FOUND IN grained detritus; typically forms ALL PARTS OF MAP AREA aprons below steep cliffs. Charac­ terized by open-work, unvegetated Qal Alluvium (Holocene and Pleisto- talus deposits, especially near tim­ cene)-Unconsolidated gravel, berline or on south- or west-facing sand, and silt. Includes sandy and slopes gravelly deposits along valley floors and sandy and muddy to Qls Landslide deposits (Holocene and peaty deposits of marshes and Pleistocene)---Poorly sorted de­ meadows. Near Mount Hood, posits of slumps and earthflows. this unit originates in several Generally poorly exposed; mapped ways: much in the upper forks of on basis of morphology. Com­ Hood River and the Muddy Fork monly found at the base of steep of the Sandy River is glacial out­ slopes where incompetent rocks wash and debris-flow deposits of underlie thick, more competent se­ neoglacial age (past 3,000 yr), but quences of lava flows. Map-unit in the White, upper Salmon, and symbol shown queried along West Zigzag Rivers much consists of Fork Hood River where a large debris-flow deposits and fluvial mass of material is of uncertain sediments related to erosion and origin reworking ofthe thick debris-flow and pyroclastic-flow deposits that were emplaced in these valleys Q 9 Glacial deposits (Holocene and during Holocene time Pleistocene)-Very poorly sorted pebbles, cobbles, and boul­ ders in fine-grained (silty sand) Qca Colluvial and alluvial slope de- matrix. Chiefly till; occurs as posits (Holocene and Pleis­ ground and lateral moraines in tocene)-Poorly sorted deposits map area. Includes minor allu­ that form sheets and fans on lower vium where reworked by streams. parts ofvalley walls; deposited by Grades upslope into talus and small streams and various slope colluvium. Circles indicate processes. Grades downslope morainal crests. Divided in Mount into alluvium ofvalley floors (Oal) Hood area into following units: and pinches out upslope against bedrock. May include till and Qgnt Till of neoglacial age outwash locally (Holocene)-Forms steep, sharp-crested, unvegetated to forested moraines that lie within Qt Talus (Holocene "'and Pleisto- 1.5 km of present glaciers. Lo- cene)-Blocky to platy, coarse- 3 cally includes small areas oftill of Tv Cinder cone or small volcano Evans Creek age (Qget) (Tertiary)-Cinders, scoria, and Qget Till of Evans Creek age agglutinate of vent deposits. (Pleistocene)--Forms moraines Composition chiefly basalt or that, in many areas, mark the basaltic andesite maximwn extent of glaciers about 20,000 yr ago during the Fraser Tiba Intrusive basalt and basaltic an­ glaciation.