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Geologic Map of the Fenn Quadrangle, Idaho County, Idaho

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Geologic Map of the Fenn Quadrangle, Idaho County, Idaho IDAHO GEOLOGICAL SURVEY DIGITAL WEB MAP 87 MOSCOW-BOISE-POCATELLO KAUFFMAN AND OTHERS GEOLOGIC MAP OF THE FENN QUADRANGLE, IDAHO COUNTY, IDAHO CORRELATION OF MAP UNITS Disclaimer: This Digital Web Map is an informal report and may be revised and formally published at a later time. Its content and format Artificial Alluvial Mass Movement Intrusive Volcanic Rocks Island-Arc Metavolcanic Deposits Deposits Deposits and Metasedimentary Rocks John D. Kauffman, Keegan L. Schmidt, Dean L. Garwood, and Kurt L. Othberg may not conform to agency standards. Rocks m Qam Qaf 2007 Qas Holocene Qamo Qls Qt Qafo QUATERNARY Qag Pleistocene 02DG023 Columbia River Basalt Group 80 Tgn1 Tgv 02DG024 Saddle Mountains Basalt 02DG022 VC79-410 Tgv Tgn1 Tgn Qaf 1 Tgv 02DG021 Qas Grande Ronde Basalt 5 CENOZOIC A Tgr2 Tli Miocene TERTIARY FAULT Tgn1 Tgbd Tgn 02DG043 Tgr1 1 Tgv Tgr 2? Tgv 1 Imnaha Basalt Tgv 02DG020 Tim Tgr2 Tgv CREEK KJap CRETACEOUS Seven Devils Group Tgr2 JURASSIC MESOZOIC Qafo TRIASSIC TRPsd 02DG044 Tgv PERMIAN PALEOZOIC Qas Qas 02DG019 Qas GRAVE Tgr2 INTRODUCTION magnetostratigraphic units (Swanson and others, 1979). Of these units, flows SYMBOLS m of the R , N and R are exposed in the quadrangle. No Wanapum Basalt 02DG018 Tgn1 1 1 2 units occur in the quadrangle. A single flow of Saddle Mountains Basalt, the The geologic map of the Fenn quadrangle depicts rock units exposed at the 80 Contact: Line showing the approximate boundary between one Tgr ? Qaf 2 surface or underlying thin surficial cover of soil and colluvium. Thicker basalt of Grangeville, forms a thin cap on Grande Ronde Basalt units on the map unit and another. surficial deposits of alluvium and landslide deposits are also depicted where plateau surface. Basalt units were identified using hand sample characteristics, Qaf Tgr2 A they mask or modify the underlying rock units or form significant mappable paleomagnetic signatures, geochemical signatures, and compilation of Fault, approximately located; dotted where concealed; bar and 02DG042 Tgv units. The map is the result of field work conducted in 2002 and 2006 by previous data. Representative samples of most basalt units were collected ball on downthrown side; arrow indicates dip of fault. the authors and compilation from previous work, including that of Bond for chemical analysis. These samples supplement previous ones collected (1963) and reconnaissance mapping and sampling in the area from 1978 by V. E. Camp (written commun., 2002). Our sample locations and those to 1980 (Camp, 1981; Swanson and others, 1981). of Camp are identified on the map. Analytical results are listed in Table 1. Qas Fold axis, approximately located; dotted where concealed; arrow Samples were analyzed at Washington State Universitys GeoAnalytical indicates direction of plunge. Tgv The quadrangle is mostly underlain by Miocene basalt flows of the Columbia Laboratory. River Basalt Group. Exposures of prebasalt rocks are restricted to the Salmon Qls 02DG017 River canyon in the south part of the quadrangle and consist of Permian to Saddle Mountains Basalt Anticline. Triassic island-arc rocks of the Seven Devils Group, including metamorphosed Basalt of Grangeville (Miocene)Medium to dark gray, fine- to medium- volcanic flows, sediments, and intrusive units. The island-arc rocks were Tgv Monocline, synclinal flexure; shorter arrow on steeper limb. grained basalt with common plagioclase phenocrysts 1-4 mm in length and accreted to the North American continent during the Jurassic to Cretaceous 06JK004 scarce to common olivine grains generally <1 mm in diameter that tend to Syncline. Tgv and were later intruded by aplitic dikes, probably in the late Jurassic or Tgbd m weather pinkish or orangish. Reverse magnetic polarity as determined in the 02DG016 Cretaceous. Prior to eruption of the Columbia River Basalt Group, the land field and the laboratory. Consists of one flow ranging from about 30-80 feet Tgn1 mass was eroded to form steep, rugged topography. During the Miocene, Estimated strike and dip of bedding or volcanic flow. thick. Forms the capping unit along the rim of Rock Creek canyon. Pinches the basalts were erupted from linear vents south and west of the quadrangle. Tgbd out northward just beyond the northern edge of the quadrangle. Probably m The flows invaded the area, filling the irregular topography and covering the Horizontal basalt flow. Qaf filled a broad flat synclinal trough and was restricted to the east side of Grave Tgr 2? older rocks. Near the contact of the basalt and the older rocks, discontinuous 02DG015 Creek by the Grave Creek fault and monocline and to the north side of the Strike and dip of bedding or volcanic flow. sediments were deposited and later were covered by subsequent basalt Salmon River by anticlinal and monoclinal flexures. Equivalent to the flows, forming interbeds within the basalt sequence. Structural warping of Tgr2 Grangeville Member of Camp (1981) and the Amphitheater flow of Bard the basalt occurred both during and after emplacement, in part controlling (1978). the distribution of younger basalt units, steam development, and the cutting Strike of vertical cleavage. 06JK050 of the Salmon River canyon and Rock Creek canyon (also called Rocky Swanson and others (1981) note a feeder dike for the Grangeville unit in 02DG014 Canyon). The Grave Creek fault in the northwest part of the quadrangle Von Berge Gulch and cite P. Hooper, C. Knowles, and J. Bond, oral Tgr2 developed after extrusion of Grande Ronde R2 basalt and prior to extrusion communication, as the reference. Recent communications with Peter Hooper of the basalt of Grangeville. Vertical displacement along this segment of the and Charles Knowles indicate the dike in question probably is not a Grangeville fault is about 200 feet. Landslides primarily occur where a thick weathered dike. A breccia dike (Tgbd) sampled at the location noted on Swanson and Headwall scarp of landslide. zone in basalt and thin Miocene sediments are exposed in the upper slopes others map has Grande Ronde chemistry (sample 06JK058; Table 1). We 2 of Rocky Canyon. The plateau soils include loess parent material (Barker, suspect a cartographic error, wherein a Tgr (for Grande Ronde) label may 1982), but loess deposits are thin and not included on this map. In addition Qaf have been misinterpreted as a Tgv (Grangeville) label when their map was 02DG013 to its present channel, Salmon River alluvial deposits form terrace remnants compiled. No source dike for the basalt of Grangeville was found during Qaf Tgv of at least two older regimes of the river. this project. Tgr1 Closed or nearly closed topographic depression in surface of Qaf Qag Tgb Qaf d basalt. 06JK058 DESCRIPTION OF MAP UNITS Grande Ronde Basalt 02DG012 Qafo Qas Qas Tgr2 Tgr2 Grande Ronde R2 magnetostratigraphic unit (Miocene)Medium to dark Tim Tgr1 Tgn ARTIFICIAL DEPOSITS Qaf 1 Qls gray, fine-grained basalt, commonly with a sugary texture. Uncommon to common plagioclase phenocrysts 1-2 mm long. Reverse magnetic polarity, Qls Tli m Made ground (Holocene)Artificial fills composed of excavated, transported, Sample location and number. Qaf although field magnetometer readings commonly give weak normal or Qls Tgn1 and emplaced construction materials of highly varying composition, but Tgr1 conflicting results, particularly near the top of the R section. Unit consists Tim Qaf 06JK062 typically derived from local sources. 2 06JK023 of one to three flows and thins from west to east and pinches out in the Tgr2 SEDIMENTARY AND northeast corner of the quadrangle. Thickness on the east rim of Grave Creek Tgbd canyon is about 200-250 feet. Thins eastward to about 40 feet before pinching Qas Tgr2 MASS MOVEMENT DEPOSITS out near Meyer Gulch in upper Rock Greek. Tgbd Alluvial Deposits Qls Qls Tgn1 Grande Ronde N1 magnetostratigraphic unit (Miocene)Dark gray, fine- Qls grained generally aphyric to plagioclase microphyric basalt. Normal magnetic Alluvium of the Salmon River (late Holocene)Channel and flood-plain deposits Qam polarity. Consists of four to six flows (possibly some with multiple flow Qaf that are part of the present river system. Two grain-size suites are typically units) with a total thickness of 800-900 feet. Well exposed in the canyons present: well-sorted and rounded pebble to boulder gravel in river bars and ACKNOWLEDGMENTS of Rock Creek and Salmon River. Individual flows range from 50 feet to more islands, and coarse sand in thin shoreline deposits. Gravel includes clasts Tgr1 than 150 feet thick. Flows near the top of the sequence are commonly 50- Tgn of primarily basaltic and metamorphic rocks. We thank the many landowners in the area for access to their property. V.E. 1 70 feet thick and typically sugary textured with scarce small plagioclase Camp provided copies of his field notes and map of the area and gave Qls phenocrysts 1-3 mm in length. Flows lower in the sequence, such as the Tgr Qamo Older alluvium of the Salmon River (early Holocene)Primarily stratified sand permission to publish his sample analytical results. We would like to thank 2 Johns Creek flow of Bond (1963), are typically thicker, generally 100-200 Qaf and well-rounded pebble to boulder gravel of point-bar and terrace remnants Peter Hooper and Charles Knowles for their input regarding the Grande that are above modern levels of the Salmon River. Gravel clast lithology feet. Tgn1 Qas Ronde dike in Von Berge Gulch that was previously mapped as basalt of Tgv similar to Qam. May be capped by thin loess and eolian sand. Height above Grangeville. Tgr Grande Ronde R1 magnetostratigraphic unit (Miocene)Mostly dark gray, present mean water level is approximately 40 feet. Interfingers with colluvium 1 B fine-grained aphyric to microphyric basalt. Very rare plagioclase phenocrysts and alluvial-fan deposits at toe of canyon slope. Thickness 5-20 feet.
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