IDAHO GEOLOGICAL SURVEY GEOLOGIC MAP 43 MOSCOW-BOISE-POCATELLO LEWIS AND OTHERS GEOLOGIC MAP OF THE SIXMILE CREEK QUADRANGLE, CORRELATION OF MAP UNITS Surficial Columbia River Intrusive CLEARWATER, IDAHO AND LEWIS, COUNTIES, IDAHO Deposits Basalt Group Rocks Qal HOLOCENE QUATERNARY Qls Qtg Reed S. Lewis, John D. Kauffman, Keegan L. Schmidt, and Russell F. Burmester PLEISTOCENE unconformity 2006 Saddle Mountains Basalt Tcsc 50 Qls Tcsc Twe VC78-289 75 30 IDSM-5 Taw CENOZOIC 70 Tpr 80 Wanapum d 45 Twe Tprd 65 Basalt 55 Taw A’ Tprd MIOCENE TERTIARY KJqd Grand Ronde 55 KJqd Basalt KJqd VC78-291 35 75 Qtg 65 Tgn1 Tprd 50 50 Tgr1 45 55 Imnaha 55 Qls Basalt 70 80 Tim 20 Twe 80 85 unconformity 65 Kbt Kpeg Ktr Kbt KJpeg CRETACEOUS VC78-320 60 Kpeg KJqd MESOZOIC JURASSIC 65 25 70 Kbt 35-55 IDSM-4 Kbt Twe 76 Kpeg 60 20 similar in both major oxide and trace elements to Pomona Member near 55 INTRODUCTION SYMBOLS Lewiston (Swanson and others, 1979a) and included in the Pomona Member Tgn 1 NI-8 80 by Camp (1981). Kauffman (2004) suggests the two units may not be coeval Qal The geologic map of the Sixmile Creek quadrangle is based largely on field Qal on the basis of paleomagnetic directions, although a K/Ar age determination Contact: dashed where approximately located. work completed in the summer of 2002, but includes reconnaissance basalt Tgn1 for the Weippe of 12.9 ± 0.8 Ma (Kauffman, 2004) is not significantly different 60 45 85 mapping and sampling in the area from 1978 to 1980 (Swanson and others, Fault: dashed where approximately located; dotted where concealed. Kpeg from the 12 Ma K/Ar age reported for the Pomona Member by McKee and KJqd Tgn 1979a; Camp, 1981). Mapping of prebasalt basement rocks was supplemented 1 others (1977). Consists of one flow generally less than 100 feet thick. with previous work by Hietanen (1962) in the northern part of the quadrangle Fold axis. 55 Commonly weathers to saprolite. Overlies Taw unit north of Lolo Creek, but 75 Twe 30 and reconnaissance mapping by Paul Myers (written commun., 1999) along rests on Tgn1 west of the Clearwater River. Pinches out against Grande Ronde Syncline. Twe 40 the Clearwater River. Lee (2001, 2004) conducted a geochemical study of Basalt along a northwest-trending structural uplift in the west-central and basement rocks in Clearwater County. Alluvial and landslide deposit Anticline. southeast parts of the quadrangle. 60 descriptions were taken in part from Othberg and others (2003). 60 Monocline: shorter arrow indicates steeper dips. Asotin Member and Wilbur Creek Member, undivided (Miocene)Fine- Basalt units were identified using hand sample characteristics, paleomagnetic Taw Qls grained basalt with scattered plagioclase phenocrysts 1-5 mm in length, and 52 75 and geochemical signatures, and data from previous work. Representative Strike and dip of foliation. a few olivine phenocrysts about 1 mm in diameter. Upper parts of flows Twe 50 62 samples of most basalt units were collected for analysis. These samples commonly very vesicular, with abundant small spherical vesicles, commonly Strike and dip of primary flow foliation. supplemented previous ones collected by V.E. Camp (written commun., 85 10 with a pale bluish coating. Both members have normal magnetic polarity. 2002). Our sample locations and those of Camp are identified on the map Occurs only in the northeast corner of the map. One or two flows with a 30 Strike and dip of foliation in narrow discrete zones. 83 and analyses are listed in Table 1; these were performed at Washington State 45 total thickness of about 200 feet. Universitys GeoAnalytical Laboratory. Analyses of Lees (2001) basement Strike of vertical foliation. rock samples are reproduced with permission in Table 2. Additional Kpeg Wanapum Basalt IDSM-3 geochemical data are available in Lee (2004). Mineral modifiers are listed Strike and dip of mylonitic foliation. 74 Ktr 75 80 in increasing order of abundance for both intrusive and metamorphic rocks. Priest Rapids Member dikes (Miocene)Several basalt dikes occur in an 50 Tprd Approximate strike and dip of basalt flows visually estimated from Kbt Grain sizes of unconsolidated and consolidated sediments are classified on unnamed drainage north of Fivemile Creek along the narrow county road 20 slope of upland surfaces or from flow boundaries on canyon walls. Twe the Wentworth scale (Wentworth, 1922). from Greer to the upper reaches of Fivemile Creek and along a tributary to 82 30 VC78-364 56 Sixmile Creek. These dikes, mapped and sampled by Camp in 1978 and Strike and dip of fracture cleavage in narrow discrete zones. 02JK646 Ktr DESCRIPTION OF MAP UNITS 1979 and found to have Priest Rapids chemistry (see Table 1), were not 80 VC79-695 Kpeg examined in detail during this project. The Priest Rapids Member has reverse Strike of vertical cleavage in narrow discrete zones. magnetic polarity. QUATERNARY DEPOSITS 65 Bearing and plunge of lineation, type unknown. 35 57 50 Grande Ronde Basalt 67 50 15 Alluvial deposits (Holocene)Stream alluvium. Mapped at several islands in Bearing and plunge of mylonitic lineation. Tgn 80 KJpeg 1 89 the Clearwater River and at one location on upper Fivemile Creek. Island Tgn Grande Ronde Basalt, N magnetostratigraphic unit (Miocene)Mostly Kbt 1 1 50 45 Qal sediments are primarily coarse channel gravels deposited during high-energy fine-grained, dark gray to black, aphyric to plagioclase-microphyric basalt. Bearing and plunge of small fold axis. 59 35 KJpeg Twe stream flow and consist of subrounded to rounded pebbles, cobbles, and However, uppermost flows are commonly medium gray and medium grained, IDSM-1 Quartz vein. 48 55 45 boulders in a sand matrix. Deposits on upper Fivemile Creek are similar but with plagioclase phenocrysts 2-5 mm long, or fine grained but with a sugary 45 finer grained. texture and diktytaxitic in places. Normal magnetic polarity. Flows range in 02RL802 Sample location and number. 60 thickness from about 50 feet to several hundred feet. Thin flows are 87 86 Qls Landslide and slump deposits (Pleistocene-Holocene)Poorly sorted and poorly characterized by large stocky columns grading upward into vesicular tops 0.7033 Initial Sr/ Sr ratio (from Criss and Fleck, 1987). 80 stratified angular basalt fragments mixed with silt and clay. Landslide deposits commonly with crude columnar structure. Thick flows have a lower colonnade IDSM-2 include debris slides as well as blocks of basalt and sedimentary interbeds with columns 2-5 feet in diameter and above that an abrupt change to a 818-11F that have been rotated and have moved downslope. Some may result from blocky, hackly thick entablature that becomes vesicular near the top. REFERENCES 0.7033 slumping of unmapped Latah Formation sediments. Entablature of one flow or series of flows forms cliffs along upper canyon walls. Probably two to four flows with a total thickness of 400-500 feet. Camp, V.E., 1981, Geologic studies of the Columbia Plateau: Part II: Upper 82 Terrace gravel deposits (Pleistocene-Holocene)Mostly stream alluvium but Qtg Miocene basalt distribution, reflecting source locations, tectonism, and 72 may include some slope-wash and fan deposits. Differs from Qal only in its 02JK649 Tgr Grande Ronde Basalt, R1 magnetostratigraphic unit (Miocene)Typically drainage history in the Clearwater embayment, Idaho: Geological Society location on terraces that are well above river level. Primarily coarse channel 1 Twe dense, dark gray to black, fine- to very fine-grained aphyric to microphyric of America Bulletin, Part I, v. 92, p. 669-678. gravels deposited during high-energy stream flow. Subrounded to rounded basalt. Less commonly medium grained with scattered small plagioclase Criss, R.E., and R.J. Fleck, 1987, Petrogenesis, geochronology, and hydrothermal Qls pebbles, cobbles, and boulders in a sand matrix. Includes intercalated KJqd phenocrysts. Reverse magnetic polarity, although field magnetometer readings alteration systems of the northern Idaho batholith and adjacent areas based colluvium and debris-flow deposits from steep side slopes. commonly inconsistent and weak. Flows with thick entablatures form steep on 18O/16O, D/H, 87Sr/86Sr, and 40Ar/39Ar studies, in T.L. Vallier and H.C. Kpeg slopes or cliffs 100-200 feet high. A thickness of about 600 feet occurs in 56 Brooks, eds., Geology of the Blue Mountains Region of Oregon, Idaho, and COLUMBIA RIVER BASALT GROUP Sixmile Creek, but the base of the sequence is not exposed. Forms irregular Tgr1 Qls 53 36 Washington: U.S. Geological Survey Professional Paper 1436, p. 95-138. 61 contact with underlying intrusive rocks. Many of the mapped landslides Hietanen, Anna, 1962, Metasomatic metamorphism in western Clearwater Tgn1 The stratigraphic nomenclature for the Columbia River Basalt Group follows (Qls) in this unit may originate on unmapped interbedded sediments of the County, Idaho: U.S. Geological Survey Professional Paper 344-A, 113 p., 59 that of Swanson and others (1979b) and used in Reidel and Hooper (1989). Latah Formation. scale 1:48,000. Qls In Idaho, the group is divided into four formations. From oldest to youngest, Kauffman, J.D., 2004, Geologic map of the Gifford quadrangle, Nez Perce Qls these are the Imnaha Basalt, Grande Ronde Basalt, Wanapum Basalt, and Imnaha Basalt (Miocene)Interpreted to be present at depth as illustrated in Tim County, Idaho: Idaho Geological Survey Geologic Map 36, scale 1:24,000. 61 Saddle Mountains Basalt. Imnaha Basalt does not crop out but is inferred cross section. Exposures of Imnaha Basalt to the west in the Clearwater River Lee, Robert, 2001, Petrologic and field analysis of a tonalite-trondhjemite Qls 29 to underlie R Grande Ronde Basalt in the southwest part of the quadrangle. canyon consist of medium- to coarse-grained, sparsely to abundantly 1 intrusion in accreted terrain in Clearwater County, Idaho: Washington State Kpeg 47 Grande Ronde Basalt, from oldest to youngest, has been subdivided into the plagioclase-phyric basalt.