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Geologic Map of the Ritz Quadrangle, Boundary County, Idaho

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Geologic Map of the Ritz Quadrangle, Boundary County, Idaho IDAHO GEOLOGICAL SURVEY DIGITAL WEB MAP 119 MOSCOW-BOISE-POCATELLO WWW.IDAHOGEOLOGY.ORG BRECKENRIDGE, BURMESTER, AND LEWIS CORRELATION OF MAP UNITS GEOLOGIC MAP OF THE RITZ QUADRANGLE, BOUNDARY COUNTY, IDAHO Made Alluvial and Colluvium and Deposits Lacustrine Deposits Mass Wasting Deposits Glacial and Related Deposits Disclaimer: This Digital Web Map is an informal report and may be m revised and formally published at a later time. Its content and format Qal Qaf Qlm Qc Qglc Holocene may not conform to agency standards. Quaternary Roy M. Breckenridge, Russell F. Burmester, and Reed S. Lewis Qgl Qgo Qgt Qgk Qgls Pleistocene 2010 Intrusive Rocks Kgbf Kgc Kgc Cretaceous 75 Qgl Qgt Qglc Metasedimentary Kgc Rocks Kgc Kgc Ymi Qglc Qgl Qgt Ypm Ypab Mesoproterozoic Qglc Kgc 75 Qgk INTRODUCTION Kgc Granodiorite of Copeland (Cretaceous)—Porphyritic, medium- to coarse- Qgt grained hornblende-biotite and biotite granodiorite that underlies eastern Qlm half of quadrangle. Microcline phenocrysts are as long as 4 cm. Plagioclase Quaternary deposits on the Ritz 1:24,000-scale quadrangle were mapped has strong oscillatory zonation and average composition of an28. Quartz Qlm in 1997-8 and 2009 by R.M. Breckenridge. Bedrock was mapped in 2009 typically in aggregates 4-8 mm across. Myrmekitic intergrowth of quartz by R.F. Burmester and R.S. Lewis to augment mapping by R.F. Burmester and feldspar common. Color index 13-17; biotite is more than twice as Qlm (1982, 1985-86) and F.K. Miller (1994). Plutonic rock descriptions are abundant as hornblende in most of pluton; hornblende sparse or lacking at modified from Miller and Burmester (2004). Cretaceous granodiorite northern edge of map. Accessories include abundant epidote and sphene Qgl 60 probably underlies most of the Quaternary deposits in the quadrangle. and subordinate zircon, apatite, opaque minerals, and allanite. Hornblende Qgt Kgc Qlm Biotite grade Precambrian metasedimentary rocks of the Prichard Forma- and biotite give potassium-argon ages of 95 Ma and 90 Ma respectively Kgc m tion, Belt-Purcell Supergroup, and more resistant mafic sills within the (Miller and Engels, 1975), recalculated using current IUGS constants Prichard crop out in the southeastern corner of the map. (Steiger and Jaeger, 1977). These are considered cooling ages, not emplace- 70 ment ages. Qal The Ritz quadrangle is located in the Purcell Trench, a major structural and Qlm physiographic feature that trends north-south more than 80 miles within the Ymi Mafic sills (Mesoproterozoic)—Coarse gabbro, fine diabase, and granophyric Qal Qlm Idaho Panhandle and northward into Canada (Fig. 1). This part of the trench quartz diorite. All are probably Moyie sills of tholeiitic composition intrud- Qal 55 Qglc Kgc 85 75 ing Prichard Formation. Composed of hornblende, biotite, plagioclase, Qaf is drained by the Kootenai River, which flows west from Montana to Kgc quartz, and opaque minerals (Bishop, 1973, 1976). Discontinuously and Qlm Bonners Ferry and then northward into British Columbia. During Pleisto- Qgl cene glaciations a lobe of the Cordilleran Ice Sheet repeatedly advanced incompletely exposed in map area, but elsewhere thicker sills are texturally 45 southward along the Purcell Trench from Canada. Tributary valley glaciers and compositionally zoned; fine-grained diabasic borders, irregularly Qlm Qgk Qlm from the Selkirk Range to the west and Cabinet Range to the east contrib- grading into coarse gabbro and quartz dioritic granophyre, which is uted to the ice stream. The ice dammed the Kootenai River, formed Glacial concentrated in upper parts. Elsewhere, contacts are normally sharp, Kgc Lake Kootenai, and diverted the outlet southward down the trench into the although some sills low in the Prichard disrupt sedimentary rocks in a Pend Oreille and Spokane River drainages. Thick sections of glacial till, manner consistent with intrusion into non-lithified sediment that probably Qlm outwash, and lacustrine deposits filled the depression of the Purcell Trench. contained interstitial water (Cressman, 1989). Zircons from sill east of Qal After retreat of the continental ice, the northward drainage of the Kootenai Bonners Ferry (Crossport-C sill of Bishop, 1973) gave a discordant Kgc River was restored. The presence of Glacier Peak tephra (11,200 B.P.) gives uranium-lead age of 1,433 Ma (Zartman and others, 1982), which may be low because of lead loss (Anderson and Davis, 1995). More likely the age m Qlm a minimum date for retreat of the ice lobe from the trench (Richmond, 1986). Alpine valley glaciers persisted until nearly 10,000 years ago in the of synsedimentary sills is 1468 Ma as determined for sills at the same Qal Qglc higher cirques of the Selkirk and Cabinet Ranges. Holocene alluvium, stratigraphic level to the north (Anderson and Davis, 1995) and to the colluvium, and lacustrine sediments are mostly the product of reworked southeast (Sears and others, 1998). Qlm 80 glacial deposits. Qglc Qlm BELT-PURCELL SUPERGROUP Qgl Ymi DESCRIPTION OF MAP UNITS Kgc Ypab Prichard Formation, members a and b (Mesoproterozoic)—Rusty weathering, dark gray siltite and argillite couplets, siltite, and rare lighter quartzite. 75 Qgt Kgc Exposed in a small area near the southeast corner of the map where siltite Intrusive rocks are classified according to IUGS nomenclature using 75 (dm thick) and argillite (mm thick) couples alternate with mm thick siltite normalized values of modal quartz (Q), alkali feldspar (A) and plagioclase and argillite couplets. Paucity of quartzite, common association with mafic Kgc (P) on a ternary diagram (Streckeisen, 1976). Mineral modifiers are listed in Qlm sills and regional mapping by Cominco (Michael Zientek, written commu- order of increasing abundance for igneous rocks. Grain size classification Ymi nication, 2003) supports assignment to the lower Prichard. See Lewis and of unconsolidated and consolidated sediment is based on the Wentworth others (2006) for description of individual units in the Trout Peak quad- scale (Lane, 1947). Bedding thicknesses and lamination type are after Qal rangle to the southeast. Qgo McKee and Weir (1963), and Winston (1986). Thicknesses and distances Qgl are given in abbreviation of metric units (e.g., dm=decimeter). Unit Ypm Prichard Formation, massive unit (Mesoproterozoic)—Structureless, poorly m thickness and elevation are listed in both meters and feet. Multiple litholo- sorted quartzite and siltite to quartz-rich fine-grained biotite granodiorite. 40 Kgc Qglc Qgt gies within a rock unit description are listed in order of decreasing abun- Commonly has granofels texture with fine biotite and muscovite common. dance. Soil descriptions for Quaternary units are after Chugg and Fosberg Here apparently floored by a mafic sill with differentiated top, but also Kco? Qgl (1980) and Weisel (2005). possibly capped by another, and probably is laterally discontinuous. East of 70 Templeman Creek contains 5 percent clasts of laminated siltite and agillite Qgk Kgc a few cm thick, 3-10 cm long commonly with lighter colored rims. Qgl Ypab A Qgl MAN-MADE DEPOSITS m 70 m Made land (historical)—Highway fills, railroad rights of way, and levees of the SYMBOLS Qal Qglc Kootenai River alluvial flood plain. Ymi Contact: dashed where approximately located. Qal ALLUVIAL AND LACUSTRINE DEPOSITS Fault: dashed where approximately located; dotted where concealed. Qglc Kgc Qal Alluvium (Holocene)—Alluvial deposits of the Kootenai River and tributary Moraine crest or kame ridge. Qgt streams. Mostly finer grained in the Kootenai River alluvial plain and Qlm coarser grained in tributary drainages. Moderately sorted to well sorted silt, Area of subglacial tunnel erosion and scattered kame deposits. Kgbf sand, and local pebble and cobble gravels. Mostly reworked glacial depos- Qgl 65 its in the river valley and post glacial colluvium in the surrounding moun- Kettle. Qgl tains. Schnoorson-Ritz-Farnhampton soils association; typical soils are very deep silty clay loams, silt loams, and mucky silt loams in basins and swales Terrace escarpment . and on low terraces, flood plains, and natural levees. Thickness is several Qgo Kgbf 80 to more than 10 m (6 to >30 feet). 60 Strike and dip of jointing. 60 Ymi Qaf Kgc 65 Alluvial fan deposits (Holocene)—Mixed pebble to cobble gravel deposited as 20 Strike and dip of bedding. Qgls fans at the mouths of local drainages. Mostly subangular to angular clasts 85 Ypm A’ derived locally from colluvium and glacial deposits on steep slopes. Estimated strike and dip of bedding. Qglc Qgk Qgt Qgt 24 85 Qlm Lacustrine and mud deposits (Holocene)—Organic muck, mud, and peat bogs Strike and dip of foliation. in poorly drained paleoglacial outwash channels and kettle depressions. Strike of vertical foliation. Ypab Interbedded with thin layers of fine sand, silt, and clay. Soils of the Pywell Kgc 80 Ypab series. Thickness from 1 m to 5 m (3-16 feet). Estimated strike and dip of foliation. 20 Qgls 55 Ymi REFERENCES Kgbf Qal 80 COLLUVIAL AND MASS WASTING DEPOSITS Qc Colluvial deposits (Holocene)—Silt, sand, and gravel colluvium. Forms debris fans and colluvial aprons along steeper escarpments and gullies of terraces Anderson, H.E., and D.W. Davis, 1995, U-Pb geochronology of the Moyie sills, Qgl 80 Ymi Qglc and benches. Includes small unmappable mass movements. This unit southeastern British Columbia: implications for the Mesoproterozoic Qglc Qlm Qlm mostly in escarpments of Qglc where mappable. Varied thickness up to history of the Purcell (Belt) basin: Canadian Journal of Earth Science, v. 32, several meters (15 feet). p. 1180-1193. Bishop, D.T., 1973, Petrology and geochemistry of the Purcell Sills, Bound- Kgbf Kgc 65 Ymi Qglc Glaciolacustrine deposits (Pleistocene to Holocene)—Mixed deposits of silt, Qgl 85 ary County Idaho, and adjacent areas, in Belt Symposium v. 1: Depart- sand, and gravel colluvium, slope wash, and small landslides. Steep slopes ment of Geology, University of Idaho and Idaho Bureau of Mines and Qgls Qglc Ypab of reworked and locally transported Qgl.
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