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Geologic Map of the Sandpoint 30' X 60' Quadrangle, Idaho And

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Geologic Map of the Sandpoint 30' X 60' Quadrangle, Idaho And Geologic Map of the Sandpoint 30´ x 60´ Quadrangle, Idaho and Montana, and the Idaho Part of the Chewelah 30´ x 60´ Quadrangle Mapped and Compiled by Reed S. Lewis, Russell F. Burmester, Roy M. Breckenridge, Mark D. McFaddan, and William M. Phillips Idaho Geological Survey Third Floor, Morrill Hall University of Idaho Digital Web Map 189 Moscow, Idaho 83844-3014 2020 Cover photo: View southwest over river delta (Qad) built by the Clark Fork into Lake Pend Oreille. Geologic Map of the Sandpoint 30´ x 60´ Quadrangle, Idaho and Montana, and the Idaho Part of the Chewelah 30´ x 60´ Quadrangle Mapped and Compiled by Reed S. Lewis, Russell F. Burmester, Roy M. Breckenridge, Mark D. McFaddan, and William M. Phillips 2020 INTRODUCTION o Geology depicted on this map is based partly on 48 30' 9 2 7 previous 15´ mapping by Harrison and Jobin (1963, 11 1965) and on unpublished 7 ½´ mapping by F. K. Miller 4 and others. Figures 1, 2, and 3 are index maps showing 10 the area covered by our STATEMAP-supported work 2 and mapping by previous workers. Remapping of 11 3 bedrock in 2003-2008 applied some different unit 7 definitions and contact placements for consistency with 1 M o n t a n a recent mapping to the south. We also made additional 2 subdivisions within the Prichard Formation based on a s h i n g t i o n W 9 6 mapping to the north by Cominco geologists (Michael 8 5 Zientek, written commun., 2003). Overall, the bedrock 9 areas of the eastern part of the map differ little from 48 o Harrison and Jobin (1963, 1965); visual differences are 117 o 116 o attributable to slight changes in placement of contacts 1. Boleneus and others, 2001. and major changes in unit assignment, which affect 2. Doughty, 1995. 3. Etienne, 1988. implied structures. Quaternary deposits were delineated 4. Harrison, 1969. by R.M. Breckenridge during several field seasons 5. Harrison and Jobin, 1963. from the 1980s through 2007. This map replaces a 6. Harrison and Jobin, 1965. preliminary version (Lewis and others, 2008) that 7. Harrison and Schmidt, 1971. 8. Miller, D.M. , unpublished mapping lacked cross sections and unit descriptions. 1980-1993. 9. Miller, F.K., unpublished mapping The most abundant rocks in the Sandpoint area are 1972-1995. low metamorphic grade metasedimentary rocks of 10. Miller, F.K., 2000. 11. Zientek, M.L. and Finch, J.M., the Mesoproterozoic Belt Supergroup (in Canada unpublished compilation, 2003. referred to as the Purcell Supergroup). Most of the high metamorphic grade gneiss and schist of the Priest Figure 1. Primary sources of geologic mapping. Idaho Geological Survey Digital Web Map 189 o 48 30' River complex in the western part of the area appear to 11 12 have protoliths of the same age, consistent with their M o n t a n a derivation from lower strata of the Belt Supergroup. Archean and Paleoproterozoic rocks are present in the 9 10 4 area but probably are tectonic slices of basement rocks. Some igneous rocks date from deposition of the Belt Supergroup, but most granitoids are Cretaceous in age and most hypabyssal rocks are Eocene in age. 7 8 3 2 1 a s h i n g t i o n W 13 5 6 DESCRIPTION OF ROCK UNITS 48 o 117 o 116 o 1. DWM-24, Burmester and others, 2004c. 2. DWM-25, Burmester and others, 2004a. Intrusive rock classification follows International Union 3. DWM-26, Burmester and others, 2004b. of Geological Sciences nomenclature using normalized 4. DWM-58, Lewis and others, 2006a. values of modal quartz (Q), alkali feldspar (A), and 5. DWM-59, Burmester and others, 2006a. plagioclase (P) on a ternary diagram (Streckeisen, 6. DWM-60, McFaddan and others, 2006. 1976). Mineral modifiers appear in order of increasing 7. DWM-74, Lewis and others, 2006c. 8. DWM-75, Burmester and others, 2006b. abundance for both igneous and metamorphic rocks. 9. DWM-76, Lewis and others, 2006b. Grain size classification of unconsolidated and 10. DWM-88, Lewis and others, 2007c. consolidated sediment use the Wentworth (1922) scale 11. DWM-89, Lewis and others, 2007a. (Lane, 1947). Bedding thicknesses and lamination 12. DWM-90, Lewis and others, 2007b. types are after McKee and Weir (1963) and Winston 13. DWM-91, Burmester and others, 2007. (1986). Grain sizes and bedding thicknesses are Figure 2. Idaho Geological Survey Statemap sources of given in abbreviation of metric units (for example, geologic mapping. dm=decimeter). Unit thicknesses, distances, and 48 o30' elevations are in both metric and English units. 1 Multiple lithologies within a rock unit appear in order of decreasing abundance, and descriptions of stratigraphic 8 4 units are from bottom to top where possible. Soil series are from Weisel and others (1982). Interpretations 5 of kinematic indicators follow Simpson and Schmid (1983) for ductile fabrics, and Petit (1987) and Doblas M o n t a n a (1998) for brittle ones. Magnetic susceptibilities of hand samples or rock faces measured with a 1995 KT-9 Kappameter from Exploranium G.S. Ltd., appear in 3 a s h i n g t i o n 2 5 Table 1. W 7 6 48 o 117 o 116 o MAN-MADE DEPOSITS 1. Bennett and others, 1975. 2. Clark, 1967. 3. Green, 1976. m—Historical man-made deposits (Holocene)—Fills 4. Harms, 1982. 5. Harrison and others, 1972. along highways, bridges, railroad right of way, and tres- 6. Hoffer, 2005. tles. Not shown are numerous small fills in and around 7. Miller, 1974. Sandpoint and recent construction along U.S. Highway 8. Miller, 1982. 95. Figure 3. Secondary sources of geologic mapping. 2 Idaho Geological Survey Digital Web Map 189 Table 1. Magnetic susceptibility of intrusive and gneissic rocks in the Sandpoint quadrangle. Values determined from hand specimens or rock faces as measured with a 1995 KT-9 Kappameter from Exploranium G.S. Ltd. and reported in SI units (x 10-3). Magnetite concentration is presumed to be proportional to magnetic susceptibility. Map units ordered the same as in rock descriptions. Unit Min. Max. Mean n Classification Comments Tl (mt-poor) 0.75 1.8 1.3 2 moderate Tl (mt-rich) 15 19 17 3 high Tum 0.57 4.9 2.7 5 moderate Td 3.3 13 7.6 19 high Tqm (mt-poor) 0.52 1.9 1.3 3 moderate Tqm (mt-rich) 14 25 18 3 high Tggd--Bodie Canyon 6.3 12 9.1 2 high stock Tggd--Wrencoe 2.8 21 12 9 high excluding one diorite sample pluton (0.39) and one fine-grained granite sample (0.07) Kgf 0.02 0.43 0.09 12 low Kmg 0.03 0.89 0.25 16 low excluding 1 mylonitic sample with value of 4.4 Kbg (mt-poor) 0.01 0.48 0.12 32 low Kbg (mt-rich) 1.0 4.7 2.7 6 moderate Kbog 0.02 0.30 0.13 16 low excluding 1 sample with value of 2.9 Khog (mt-poor) 0.03 0.50 0.22 3 low Khog (mt-rich) 14 20 17 3 high Kbgd--Kelso Lake 7.6 27 17 14 high pluton Kbgd--Rapid Lighting 1.7 17 6.0 14 high excluding 1 fine-grained sample Creek pluton with value of 0.09 Kgdf 28 28 28 1 very high Khgd--other plutons 4.6 20 11 7 high Khgd--Salee Creek 0.25 12 4.3 3 moderate pluton Kt 0.35 0.35 0.35 1 low Kgdp--Packsaddle 8.0 13 11 2 high Mtn stock Kgdp--Whisky Rock 2.3 19 12 3 high stock Kqm--Benning Mtn 35 35 35 1 extreme stock KYdi 0.41 0.41 0.41 1 low Ymi 0.06 0.96 0.45 33 low Yam 0.37 0.59 0.45 3 low Yag (mt-poor) 0.09 0.13 0.11 2 low Yag (mt-rich) 13 13 13 1 high Wgn 6.4 7.9 7.2 2 high 3 Idaho Geological Survey Digital Web Map 189 ALLUVIAL, LACUSTRINE, AND Qtl—Talus deposits (Holocene)—Blocky and tabular, poorly sorted angular clasts of Belt Supergroup rocks as MASS-MOVEMENT DEPOSITS talus deposits below cliffs oversteepened by glaciation and flood scour. Locally mined for decorative stone. Qal—Alluvium (Holocene)—Silt, sand, and gravel Mining of these deposits has revealed scour features deposits in modern stream drainages. Moderately- to under the talus aprons. Generally, no soil development. well-sorted silt, sand, and pebble and cobble gravels Shown only in vicinity of State Highway 200 northwest with rare boulders. Mostly reworked glacial deposits in of Cabinet, Idaho. Varied thickness typically 3 to 9 m the lowlands and postglacial colluvium in the mountains. (10 to 30 ft). Typical soils are silt loam to sandy and gravelly loam. Qlm—Lacustrine and fluvial mud deposits (Holo- Soil series of Hoodoo and Wrencoe. Deposits several cene)—Consist of soft clayey silt; locally underlain meters thick. by late glacial outwash, till, or Missoula flood depos- Qaf—Alluvial-fan deposits (Holocene)—Mixed peb- its. Upper limit of the unit along Lake Pend Oreille is ble to cobble gravel deposited as fans at the mouth of controlled by Albeni Falls Dam. The maximum water local drainages. Mostly subangular to angular clasts level is 631.5 m (2,071.7 ft) and the normal minimum derived locally from colluvium and glacial deposits on is 625.1 m (2,051.0 ft). Unit thickens into Lake Pend steep slopes.
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