IDAHO GEOLOGICAL SURVEY DIGITAL WEB MAP 122 MOSCOW-BOISE-POCATELLO WWW.IDAHOGEOLOGY.ORG EMBREE, PHILLIPS, AND WELHAN CORRELATION OF MAP UNITS EOLOGIC AP OF THE EWDALE UADRANGLE, REMONT AND ADISON OUNTIES, DAHO INTRODUCTION G M N Q F M C I Artificial Unit Alluvial Units Eolian Units Mass Wasting Volcanic The Newdale quadrangle lies on a dissected plateau locally known as the Units Units 1 2 3 Rexburg Bench. Geologic features of the Rexburg Bench are closely associ- Glenn F. Embree , William M. Phillips , and John A. Welhan Basalt Flows Rhyolitic Rock ated with water and agricultural resources. Near-surface geology of the m Qtfb Qtfg Qtfs Qtff Qls bench consists of the Huckleberry Ridge Tuff and three basalt units. The HOLOCENE 2011 Huckleberry Ridge is a highly permeable and jointed ash flow tuff. Most Qa Qc Qes high production irrigation water wells in the region draw from the Huckle- 10.0 ka berry Ridge aquifer. A dam constructed by the US Bureau of Reclamation in Qgh late the Teton River canyon failed catastrophically in 1976, largely due to Qel PLEISTOCENE Qbc Qbm Qgh uncontrolled seepage of reservoir water through joints and shear zones in Qbsp Qa the Huckleberry Ridge Tuff. Bedrock outcrops are rare in the Newdale ? Qa 1 Qgh quadrangle outside of the Teton River canyon because of widespread loess ~15 - 140 ka 126 ka Qa cover. The loess is the parent material for the rich soils of the region. QUATERNARY Qgh Qgh Qgh Qgh middle Qes Qbc PLEISTOCENE Qyh GEOLOGIC HISTORY Qel/Qbc Qyh 256 +/- 14 ka3 Qgh Qbsp Qel/Qbsp 780 ka The Huckleberry Ridge Tuff was erupted from the Henrys Fork caldera of the early Qyh Qyh Yellowstone Plateau at 2.059 Ma (Lanpere and others, 2002; Christiansen, Qbsp Qbm ? PLEISTOCENE Qel/Qyh 2001). Well logs show that the tuff flowed over irregular topography ? Qyh 909 +/- 13 ka3 Qbsp Qbc containing at least one large lake, west-flowing streams, basaltic lava flows, 2 Qyh ? 2.059 +/- 0.004 Ma 2.60 Ma and rhyolitic rocks. Large scale gravity sliding and deformation occurred Qyh Qyh Tb Qbsp Qbc within the lake and stream deposits shortly after the Huckleberry Ridge Ts PLIOCENE TERTIARY Qyh ? Qbc Qyh ignimbrite flowed over them (Geissman and others, 2010; Embree and ? Qyh Hoggan, 1999). A minimum of 1 km (0.6 mi) of horizontal movement Qbsp caused significant deformation in the still partially fluid tuff sheet. Unusu- 1 Range of OSL ages, Phillips and others, 2009, Pierce and others, 2003. Qyh ally numerous open joints, shear zones, large scale >150 m (>490 ft) ampli- 2 Sanidine 40Ar/39Ar age, Lanphere and others, 2002. Qbsp Qyh Qyh tude overturned asymmetric antiforms, and a tectonically denuded valley 3 40Ar/39Ar ages, D. Champion, oral communication, 2010. Qyh Qyh (Hog Hollow) were created prior to complete welding and devitrification of Geologic time scale from Walker, J.D. and Geissman, J.W. compilers, 2009, the ignimbrite. Geologic Time Scale: Geological Society of America, doi: Qyh Qyh Qyh Qyh 10.1130/2009. CTS004R2C. Qbsp The basalt of Moody Creek was erupted from a presently obscured vent in Qbsp Qbsp Qyh Qyh the adjacent Moody quadrangle to the south. Following ancestral Moody Qgh Qbc Qyh Creek and flowing around uplands of Huckleberry Ridge Tuff, at least 3 lava Qyh Qel/Qyh flows were emplaced. Pillow basalts at the base of the flows suggest that Qyh they encountered the Teton River, diverting or blocking it temporarily at the Qyh mouth of the Teton Canyon (Jordan and others, 2010b). The basalt of Moody Qa Creek has not been dated. It has reverse magnetic polarity, indicating that it Qtfs is probably older than 780 ka. Sand (Holocene)—Sand and pebbly sand; forms parts of pendant bars in SYMBOLS Qyh Qyh canyon and sheet-like deposits at canyon mouth and farther downstream; Qyh The basalt of Snake River Plain is tentatively correlated with flows erupted downstream deposits derived from scour of sandy road fills, irrigation Contact: dashed where approximately located. Qel/Qbc Qbsp Qyh Qyh from a low shield volcano in the Ashton quadrangle at 909 ka (D. Cham- canals, and terrace scarps; canyon deposits as thick as 3 m (10 m); deposits Qyh pion, oral communication, 2010). The basalt flowed south down the downstream <1 m (3.3 ft) thick. Normal fault: ball and bar on downthrown side; dashed where Qel/Qyh Qyh Henrys Fork drainage. It has reverse magnetic polarity. approximately located; dotted where concealed. Qtff Fine sand and silt (Holocene)—Fine sand, silt, and clay in deposits; <20 cm (8 Qel/Qbsp The basalt of Chester occupies an abandoned stream valley between the in) thick on flood plains, in depressions, and in fields surround by dikes, Lateral faults; arrows indicate direction of motion; dashed where Qyh highlands of the Huckleberry Ridge and the lowlands of the Snake River roads or irrigation canals. Found where slack water was present during approximately located; dotted where concealed. Qyh Plain. Petrologically similar to the basalt of Moody Creek, the basalt of flooding. Mapped where thicker than 5 cm (2 in). Sources were compacted Chester has an age of about 256 ka (D. Champion, 2010) and normal loess core of the Teton Dam, eolian sand, fine-grained artificial fills, and the Qbc Qyh magnetic polarity. fine fraction of gravelly embankments. Fold axis; arrow indicates direction of plunge. Qel/Qbsp Qbsp Qyh Downdropping of the upper Snake River Plain and the Teton Basin relative Qls Landslide deposits (Holocene)—Landslides resulting from the rapid drawdown Overturned antiform; dashed where approximately located; Qyh Qyh Qel/Ts to the adjacent Big Hole Mountains caused the Rexburg Bench to be of the Teton Reservoir after the failure of Teton Dam (see Figure 1). Majority dotted where concealed. Qa incised by the Teton River. Most of the incision was accomplished between of landslides are translational earth slides with failure surfaces near contact Qbsp Ts Antiform; dashed where approximately located; dotted where Qbsp eruption of the Huckleberry Ridge Tuff and the eruption of basalt of Moody between overburden (loess, colluvium, and/or slope wash) and underlying concealed. Creek. Huckleberry Ridge Tuff. Thickness from 0.3-0.6 m (1-2 ft) to ~3 m (10 ft.) Ts 10P01 Qyh Where overburden was >3m (10 ft), landslides began as shallow rotational 60 Strike and dip of bedding. During the glacial periods of the middle and late Pleistocene, a large ice 25 slumps 6-7.5 m (20-25 ft) thick, then evolved into earth flows and debris Qbc Qyh sheet and numerous alpine valley glaciers occupied much of the headwa- flows. Some flows reached out into the river and caused temporary Strike and dip of vertical bedding. Qa Qyh Qyh ters of Henrys Fork and Teton River in the Yellowstone and Grand Teton damming or changed stream configurations. Rock falls and slides involving Qbsp Qyh areas. Outwash streams from these glaciers deposited thick fills of gravel tuff bedrock also occurred. Landslides of all types are more numerous on 22 Strike and dip of eutaxitic foliation. Qel/Qbc Qyh along the Henrys Fork. At the same time, strong northeast-directed winds south side of Teton River because of greater thickness of unconsolidated Strike and dip of vertical eutaxitic foliation. deposited loess over the Rexburg Bench. Part of the surface of the basalt of material from preferential deposition of loess. Most slides occurred at or Qbsp Snake River Plain was probably scoured by outwash streams, as shown by below the maximum reservoir elevation of 5,301.7 ft. Slides were mapped Strike and dip of overturned eutaxitic foliation. the anomalously thin loess cover on the flow. 78 Qyh using air photos taken in June 1976; many of the slides remain visible on Qyh 83 images taken in 2004. Boundary of Teton Reservoir on June 5, 1976, elevation 5301.7 ft. Qel/Qbsp UNST-8 TETON DAM DISASTER Boundary of area inundated by floodwater from Teton Dam Flood Qyh Qyh of June 1976. Because of errors in the original quadrangle On June 5, 1976, the Teton Dam failed catastrophically, killing 14 people ALLUVIAL, COLLUVIAL AND LACUSTRINE DEPOSITS Qa base map, the extent of flooding may not agree with topogra- Qbc Qyh and causing $400 million to $1 billion dollars (1976 dollars) in flooding Alluvium of the Teton River, Falls River, and Henrys Fork (Holocene)—Unconsolidated phy in all areas. Qyh damage. The causes of the disaster have been extensively studied (e.g. Seed clayey silt, silty sand, and gravel. Generally less than 3 m (10 ft) thick. and Duncan, 1987) and show that poor understanding of the geologic Asymmetric ripples in deposits of the Teton Dam failure; ripples 34 conditions at the dam site contributed directly to the disaster. Qc Colluvium and slope wash (Holocene)—unconsolidated angular blocks of tuff are 0.3 to 3 m thick and 2 to 50 m long. The larger ripples in silty tuffaeous matrix at base of steep slopes in Teton River canyon. occur on bars in the Teton River canyon and at the canyon Qbc The dam site in the Teton River canyon (NE/4, Section 30, T. 7 N., R. 42 E.) Generally <5 m thick (16 ft). mouth; smaller ripples occur in downstream areas. consists of Huckleberry Ridge Tuff and a maximum of about 30 m (100 ft) 22 of alluvium in the river channel. Uplands adjacent to the canyon are Qgh Gravel alluvium of the Henrys Fork and Falls River (late Pleistocene)—Poorly Thermal wells showing maximum temperature (°C). Qbc Qel/Qbc covered with up to 9 m (30 ft) of loess. The loess was used to construct most to moderately sorted gravel and sand; thickness from <10 m to about 50 m Qtff Qyh 87 Test wells.