IDAHO GEOLOGICAL SURVEY TECHNICAL REPORT 11-2 MOSCOW-BOISE-POCATELLO WWW.IDAHOGEOLOGY.ORG KEELEY AND OTHERS Geologic Map of the Thatcher Hill Quadrangle and portions of the Treasureton and Cottonwood Creek Quadrangles, Franklin, Bannock and Caribou Counties, Idaho DESCRIPTION OF MAP UNITS ALLUVIUM (HOLOCENE) - Unconsolidated, poorly to moderately well sorted, CADDY CANYON QUARTZITE, UPPER MEMBER (CRYOGENIAN?) - White, light Qal Zccu matrix- to clast-supported sand and gravel with silt sized loess matrix and brown to blue-gray, well-sorted, fine- to medium-grained quartz arenite. Joshua A. Keeley , David W. Rodgers , Paul K. Link , and Skye Cooley This Technical Report is a reproduction of independent mapping by moderately well-developed soil. Clasts are rounded brown, purple, pink and Medium- to thick-bedded with trough and tabular cross stratification. Up J.A. Keeley, D.W. Rodgers, P.K. Link, and Skye Cooley. Its content white fine- to coarse-grained quartzite cobbles derived from the Neoprotero- section are increasingly more granule conglomerates at the base of thick and format may not conform to agency standards. 2011 zoic to Cambrian Brigham Group and overlying Cambrian carbonate units. beds of quartz arenite. Granules are white, vitreous and pink quartz with Includes stream channel and floodplain deposits. Maximum thickness is 10-15% angular coarse-grained feldspars. Unit is purple to maroon to the Correlation of Map Units 3669 III SE o 4 o 4 111°47’30’’ 37 111 45’ 111 52’30’’ 4 000m 4 4 4 (GRACE POWER PLANT) 34 about 20 m. east. Lower contact is placed at the top of underlying carbonate rocks of the 29 E 30 31 111°50’ 32 o Qls o 38 42 30’ lower member. Forms ledges and slopes. 325 m thick. Qtc 42 30’ 55 Cn Qal Holocene Csw LANDSLIDE DEPOSITS (HOLOCENE) - Unconsolidated, poorly sorted, clast- Qf Cbo Qmc Qls Qt Ql Cn 24 Ql Qbo and matrix-supported pebble to boulder conglomerate and diamict with CADDY CANYON QUARTZITE, LOWER MEMBER (CRYOGENIAN) - White Cwp Qc Qtc? silty loess matrix and well-developed soil. Where above 5440 ft elevation: Zccl orthoquartzite with orange to brown weathered surfaces and liesegang Qoa 47 000 Qc Qbo Cenozoic 05 m N CZcm 47 Qfg Qfg Qbo Ql 05 localized headwall scarps, slump and flow toe geomorphology with clasts banding. Mostly comprised of thick-bedded, medium- to coarse-grained, Pleistocene Qoa Qb derived from Qfg unit. Where on terrace slopes along the Bear River and well-sorted, feldspathic and quartz arenite. Lower portion has several 0.5-2 Qmc Qal Qb lower elevations: mass wasting products derived from Quaternary lake beds meter beds of fine-grained brown argillite with disrupted beds of siltite with 36 Qfg of Qmc and Qbo. Average thickness is about 5-10 m. Maximum thickness in sand injections and convoluted bedding. Top is marked by a 1.5 m thick Tsu Pliocene 25 Cottonwood Valley is 50 m. carbonate unit that contains thin-bedded pink dolostone, massive white to 41 Qoa Cbo yellow recrystallized limestone, and gray to pink dolostone with thin beds of Tsl Miocene 48 Cbl ALLUVIAL FAN DEPOSITS (HOLOCENE) - Low-relief, fan- or apron-like carbonate pebble conglomerate with rare ooids. Thin-bedded yellow argillite Unconformity 60 Qf 32 Qbo landforms that form the transition between valley alluvium and upland and siltite bound this unit above and below. Lower contact placed at the first Cbl sediments. Composed of reworked material from unit that lies immediately tan to white quartite bed above the siltite of the Papoose. Forms ledges and Csw Upper 36 upslope. slopes. 600 m thick. Cambrian 4704 Cn 41 Qfg 47 Qmc 04 CALCAREOUS TRAVERTINE (HOLOCENE-UPPER PLEISTOCENE) - Tufa, calcrete PAPOOSE CREEK FORMATION (CRYOGENIAN) - Brown to gray, fine- to Unconformity 55 Qal Qtc and calcareous travertine. Porous, gray to white to yellowish-gray calcium Zpc medium-grained, moderately well-sorted to poorly sorted, sublitharenite Clb Qc 34 carbonate hotspring and warmspring deposits that form mounds and interbedded with black, brown, and gray-blue, slightly phyllitic siltstone and Cbo 40 Qls Paleozoic 50 benches along valleys. Flowing springs commonly associated with the unit. shale. Sedimentary structures include sand injections, syneresis cracks, slump Middle 40 m thick in Mound Valley. folds and ball and pillow structures.Also contains minor beds of purple Cbl Cambrian 32 33 Ctk argillite and several beds of very thick-bedded, well-sorted white quartz ? Qmc Clb Qbo LOESS (HOLOCENE AND PLEISTOCENE) - Massive to stratified, unconsoli- arenite. Forms platy to blocky talus. Lower contact is not exposed in the field B Clb Clb 32 Ql Csp dated, tan wind blown silts overlying Gem Valley Basalt or older bedrock area but should be placed above gray to black quartz arenite or black to Ctk ? units. Ranges from several centimeters to 8 m thick (Bright, 1967). silver shale of the upper member of the Pocatello Formation. Ledge and 46 Cbl Clb 32 steep hill former. The unit thins from 480 m thick in Cottonwood Creek to 300 42 Clb Csp 47 COLLUVIUM (HOLOCENE AND PLEISTOCENE) - Angular to rounded, pebble- m in the northwest portion of the map area. 03 Clb Qc Lower Cambrian 46 4703 Cwp to cobble-sized quartzite clasts with well-developed loess and soil cover. Cwp ? 38 High-elevation surfaces incised deeply by Pleistocene to Holocene streams. POCATELLO FORMATION (CRYOGENIAN) (Only present in cross section) - Zp 50 Qbo Includes cultivated land and hillsides with thick vegetation and soil cover. Upper member is exposed immediately southeast of the map area where it is CZcm 42 Qmc 48 Thickness is unknown. a thin-bedded, black to brown phyllitic shale interbedded with medium- to Unconformity thick-bedded, black to brown, medium- to coarse-grained sandstone. TERRACE GRAVELS (PLEISTOCENE?) - Unconsolidated, moderately to poorly Regionally present below the upper member is interbedded conglomerate, Zm Csp Qal Qt CZcm Qbo sorted cobble conglomerate that forms terraces up to 8 m above present day diamictite and quartz arenite of the Scout Mountain Member and mafic Zmr Cottonwood Creek. Matrix- to clast-supported, rounded and commonly volcanic rocks of the Bannock Volcanic Member. Total thickness to the north Ediacaran 36 imbricated quartzite cobbles with a fine- to coarse-grained sand matrix. Clast and west, outside the study area is 1,920 m (Link and Stanford, 1999). Zm 33 Neoproterozoic Cwp lithologies are brown, purple, pink, green and white Brigham Group quartz- Brigham Group Unconformity Cwp 4702 ites. A meter of peat locally comprises the top of the terrace. In Mound and GEOLOGIC HISTORY Ctk 47 33 38 Qt 02 Gentile Valleys the unit forms the first terrace topographically up from active Zi floodplain. Up to 8 m thick. Introduction - The map area is located within the southern Portneuf Range Z Undifferentiated Z Undifferentiated Unconformity ? 25 of southeastern Idaho about 70 km southeast of the city of Pocatello. The ALLUVIAL FAN GRAVELS AND SANDS (PLEISTOCENE) - Rounded, poorly map area is bounded on the west and south by Cottonwood Creek, on the 35 Qfg Zccu Qmc sorted cobble to boulder conglomerate with loess and soil matrix. Clasts are east by State Highway 34 and the Bear River, and on the north by 42° 26’ 45’’ CZcm Qmc Qal derived from the Brigham Group and overlying Cambrian carbonate units. latitude. Geologically, the map area is located in the northeastern corner of Zccl Shale chips are also present. Distinguished from the Tertiary Salt Lake the Great Basin physiographic province, in the late Cenozoic Basin and Range Cryogenian Qls Qfg Formation by the absence of red ilmenitic matrix and structural tilting, by the Province, and in the late Mesozoic Sevier thrust belt. Zpc presence of loess and soil, and by the fan morphology. Crops out on the east Zp flank of the range above 5440’ elevation and on the west flank above 6120’ Neoproterozoic to late Cambrian Sedimentation - The Neoproterozoic- 4701 29 41 elevation. Interfingers with fine-grained lake sediments of the Main Canyon Cambrian succession in southeastern Idaho includes the Pocatello Formation CZcm 47 41 Qls 01 Formation. Thickness unknown. (Ludlum, 1942), the Brigham Group (Link et al, 1985), and overlying carbon- ate rocks. The Pocatello Formation includes the basal Bannock Volcanic Qbo Member consisting of greenstones and mafic volcanic rocks, the Scout 42°27’30” 54 Qmc 42°27’30’’ OLDER ALLUVIUM (PLEISTOCENE) - Poorly sorted boulder to fine-grained Qal Qoa deposits on gently sloping valley-floor surfaces, slightly dissected by modern Mountain Member composed of sandstone, argillite, conglomerate, carbon- streams. Only present in Densmore Creek where the current drainage likely ate and diamictite, and the upper member comprising thin-bedded black did not create the observed valley width nor boulder gravels. Deposited in an shale. This succession records the initial rifting of the supercontinent Rodinia Qf Map Symbols incised valley dissecting the Main Canyon Formation. Unit grades into and (Link, 1987; Fanning and Link, 2004) to form a NNW-SSE trending rifted 27 may incise Bonneville Lake sediments. Thickness unknown. margin. The Bannock Volcanic Member represents a rift-related magmatic Qal 39 50 Qal episode prior to or coeval with the deposition of shallow marine glaciogenic Strike and dip of bedding Qmc BONNEVILLE LAKE SEDIMENTS (PLEISTOCENE) - Poorly consolidated stratified and reworked lodgment till and debris flow diamictite (Link; 1983; Crittenden 4700 Qbo gravel and sand deposited along the periphery of Lake Bonneville (30 to et al., 1983; Harper and Link, 1986; Link, 1987). None of the Pocatello CZcm 4700 Contact 10ka; Oviatt and Miller, 1997). Thin-bedded, generally flat lying, light red to Formation is exposed in the map area, but it is inferred to exist at shallow depth and shown in cross sections. Contact, approximately located 35 orange to pink, silts and fine sands that form gently valley-ward sloping 34 37 benches and dissected benches inset into Main Canyon Fm, Gem Valley The cyclic Blackrock Canyon Limestone (Corsetti et al., 2007) overlies the Basalt, and older bedrock units.