Geologic Map of the Goodwin Meadows Quadrangle, Idaho
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IDAHO GEOLOGICAL SURVEY DIGITAL WEB MAP 97 MOSCOW-BOISE-POCATELLO WWW.IDAHOGEOLOGY.ORG KAUFFMAN AND OTHERS CORRELATION OF MAP UNITS Alluvial Mass Movement Volcanic Rocks Intrusive Island-Arc Metavolcanic Amphibolite-Facies Metamorphic GEOLOGIC MAP OF THE GOODWIN MEADOWS QUADRANGLE, IDAHO COUNTY, IDAHO Deposits Deposits Rocks and Metasedimentary Rocks Rocks of Unknown Affinity rounded ridges within the KPgs unit. Bonnichsen and Godchaux (1994) studied the ultramafic rocks northeast of Corduroy Meadows (NE¼, sec. 1, Qam Qas Qaf Holocene T. 28 N., R. 3 E.) and reported anthophyllite, chlorite, tremolite, and talc, Qls QUATERNARY Disclaimer: This Digital Web Map is an informal report and may be Pleistocene with minor amounts of opaque oxides and serpentine. They also report revised and formally published at a later time. Its content and format hornblende-garnet ("blackwall zone") rock with ilmenite and spinel. An John D. Kauffman, Reed S. Lewis, David E. Stewart, and Keegan L. Schmidt may not conform to agency standards. Ts Pliocene exposure about 1 mile to the east-northeast (southern part of sec. 31, T. 29 N., R. 4 E.) was studied by Hoover (1986) and Bonnichsen and Godchaux Columbia River Basalt Group (1994) who reported talc-olivine-anthophyllite rock and pyroxenite. Two 2008 Latah Formation additional pyroxenite-bearing (orthopyroxene) bodies were discovered Sediments Grande Ronde Basalt A during our mapping west of Twin Cabins Creek in the southwest part of Tgr2 map. 65 Miocene TERTIARY Tgn1 CENOZOIC TRPc Tgr1 50 75 Tgr1 KPgs Gneiss and schist (Permian to Cretaceous)—Fine- to medium-grained 34 Tim 36 Tli Tgr Qls Qls 52 1 plagioclase-hornblende gneiss that grades into hornblende ± biotite ± 79 chlorite schist and muscovite-plagioclase-quartz ± biotite ± zoisite schist. Tgn Imnaha Basalt KPgs 1 Qam Also includes calc-silicate rocks and zoisite-plagioclase quartzite. Light Tim gray or green to black, weathers to a fine gray brown soil. Calc-silicate Tgr Tim 1 Qls rocks contain zoisite, epidote, chlorite, plagioclase, quartz, pyroxene, and 75 sphene. Garnet is common. Although some plagioclase-amphibole gneiss Qaf may be metavolcanic in origin, most (if not all) of the unit is probably Tgr Kto CRETACEOUS 1 Tgr1 metasedimentary. Kto JURASSIC MESOZOIC 02JB042 Kto STRUCTURE Tgr1 KPgs KPum 06JK139 6 1 TRIASSIC MOUNT IDAHO STRUCTURE TRPsd TRPc Tgn1 PERMIAN PALEOZOIC Faults and folds of the southwest-northeast-trending Mount Idaho structure 81 (Schmidt and Lewis, 2007) cross the northwest corner of the map and ? 80 probably extend at depth under the northern part of the map (see cross 02JB041 section A-A'). The structure has a poorly understood, complex and lengthy 5 Qls history of deformation. Included in this feature are the Mount Idaho shear zone, a sheared, mylonitized zone as much as 1.2 miles wide within the 75 INTRODUCTION basement metamorphic rocks, and the Mount Idaho fault, which offsets 55 The geologic map of the Goodwin Meadows quadrangle depicts rock magnetic polarity, although field magnetometer readings commonly give both basement rocks and the overlying Columbia River basalts. The units exposed at the surface or underlying thin surficial cover of soil and weak normal or conflicting results, particularly near the top of the R2 Miocene and younger Mount Idaho fault, whose trace is interpreted to be colluvium. Thicker surficial deposits of alluvium and landslides are also 86 section. Consists of one or two flows in the central and southwest parts of just north of the quadrangle, represents south-side-up displacement on a depicted where they mask or modify the underlying rock units or form 70 the quadrangle. Unit thickness generally less than 100 feet. Local fault connected at depth to the Cretaceous-Paleogene Hammer Creek 80 62 significant mappable units. The map is the result of field work conducted thickening and thinning may result from structures developed on earlier thrust, which is the basal fault of the Mount Idaho shear zone. Monoclinal Tgr1 Kto in 2003, 2006, and 2007 by the authors. Previous work includes that of Grande Ronde Basalt or from erosion on the earlier basalt surface. to synclinal flexures that flank the south and east side of Mount Idaho in Tim? Bond (1963), reconnaissance mapping and sampling in the area from Ts the northwest part of the map are related to development of the Mount 1978 to 1980 (Camp, 1981; Swanson and others, 1981), mapping and Tgn1 Grande Ronde Basalt, N1 magnetostratigraphic unit (Miocene)—Dark Idaho fault. Tim geochemical analysis near the eastern edge of the map (Hoover, 1986), gray, fine-grained generally aphyric to plagioclase-microphyric basalt. unpublished mapping by Paul Myers (1977-1982), and reconnaissance Normal magnetic polarity. Number of flows not determined, but estimated Southeast of the Mount Idaho shear zone, higher-grade, hornblende- 64 73 sampling by John Bush in 2002. at three to five. Flows near the top of the sequence are commonly 50-70 dominated gneiss and schist occur in the basement window in the Qls Qam feet thick and typically sugary textured with scarce small plagioclase northwest corner of the map. On the adjacent Grangeville East Much of the northern and central parts of the quadrangle are underlain by phenocrysts 1-3 mm in length. Flows lower in the sequence are typically quadrangle, these rocks show mylonite fabrics adjacent to the Mount Ts? Miocene basalt flows of the Columbia River Basalt Group. Basement rocks thicker, generally 100-200 feet. Colonnades of thin flows and entablatures Idaho shear zone that do not persist for more than 30-100 feet from the Qls are exposed in the eastern and southern parts and in two drainages in the VC79-454 of thick flows tend to form tiered cliffs on steep canyon slopes. Thickness contact and are replaced by northeast-striking, southeast-dipping gneissic northwest corner. Permian to Triassic island-arc rocks of the Seven Devils of the unit ranges from 400 to 600 feet. and schistose banding and foliation (Schmidt and others, 2007). Group are exposed in a window through the basalt in the northwest Ts corner of the map. Permian to Cretaceous amphibolite facies Tgr1 Grande Ronde Basalt, R1 magnetostratigraphic unit (Miocene)—Mostly ? metamorphic rocks crop out in the eastern and southern parts of the dark gray, fine-grained aphyric to microphyric basalt. Uncommon GOODWIN MEADOWS FAULT quadrangle and in thrust contact with the Seven Devils Group rocks in the plagioclase phenocrysts 2-4 mm in length in one or more flows. Reverse The Goodwin Meadows fault, which appears to be a minor vertical fault, 06JK141 northwest. Cretaceous intrusive rocks are found in the canyon of the South magnetic polarity, although flows near the R -N boundary commonly 1 1 offsets Grande Ronde Basalt units in the vicinity of Goodwin Meadows. Fork Clearwater River, in a window through the basalt on the western side have inconsistent and weak field magnetometer polarity readings; The fault trends northeast through Goodwin Meadows, then bends ? of the quadrangle, and in intrusive contact with the metamorphic rocks in therefore the mapped contact is poorly constrained. Outcrop northward along the South Fork Clearwater River. Maximum displacement the south and east. The basalt flows invaded ancestral drainages and characteristics of flows are similar to those in the Grande Ronde N unit. Kto 1 in the basalt is about 300 feet. The fault appears to die out southwest of flooded the paleotopography. Structural warping of the basalt occurred Exposed thickness is about 700 to 800 feet. Goodwin Meadows and also just north of the quadrangle boundary. both during and after emplacement, in part controlling the distribution of ? younger basalt units, stream development, and the cutting of the South Tim Imnaha Basalt (Miocene)—Medium- to coarse-grained, sparsely to abun- Fork Clearwater River canyon. Major landslides are associated with the dantly plagioclase-phyric basalt; olivine common; plagioclase phenocrysts generally 0.5-2 cm but some are as large as 3 cm. Flows examined in the KPgs contact between basalt and basement rocks. A saprolite surface produced by deep weathering is noted at many localities. Alluvial deposits are found field have normal polarity. Poorly exposed along Forest Service Road 2022 57 within the canyon of the South Fork Clearwater River 1,000 feet above below Cove Road in the northeast part of the quadrangle. Also found as SYMBOLS present river level and may represent a terrace remnant of an older regime float fragments at elevations as high as 5,200 feet on the north slope of Tgn1 65 Contact: dashed where approximately located. 60 of the river. A similar high gravel deposit at an elevation of 5,000 feet in Mount Idaho. Imnaha in the northeast corner of the quadrangle on the east side of the South Fork Clearwater River is from mapping by V.E. Camp the southwest corner of the map is of unknown origin, but may be related Fault: bar and ball on downthrown side; dashed where (Swanson and others, 1981). to early development of the White Bird Creek drainage system. approximately located; dotted where concealed. A/T on cross-section indicates strike-slip direction on fault: A = away from observer; T = toward observer. 73 Kto INTRUSIVE ROCKS 30 DESCRIPTION OF MAP UNITS Tgr1 73 Kto Tonalite and trondhjemite (Cretaceous)—Light gray biotite- and hornblende- Thrust fault: dashed were approximately located; dotted ? Throughout the following descriptions and later discussion of structure, 1 80 biotite tonalite cut in the northern part of map by dikes and pods of light where concealed; teeth on upper plate. 60 we use the metric system for sizes of mineral or clast constituents of units. gray, fine-grained to pegmatitic, muscovite-bearing trondhjemite. Tonalite However, we use English system for thickness and distance measurements characterized by a distinctive mottling due to the presence of large biotite Qls? because that system is used on the base map.