Geologic Setting

This study site is located at the eastern edge of the western Snake River Plain in southern Idaho. The basal portion of the plain is composed of siliceous Idavada volcanics. A series of sediments named the Idaho and Snake River Groups have been deposited non-conformably on the Idavada volcanics (Figure 2). The Idaho Group is composed of seven formations identified by Malde and Powers (1962), and covers several thousand square miles in a wide area of the western Snake River Plain. The Glenns Ferry and Tuana Formations crop out within the study site along the west banks of the Snake River. These Cenozoic sediments are a combination of lake, stream and flood plain deposits inter-bedded with an occasional basalt flow, silicic volcanic ash and basaltic pyroclastic deposits.

Figure 2. Sequence of upper Cenozoic rocks in the western Snake River Plain, Owyhee County, Idaho (modified from Malde, 1991)

The Snake River Plain is a major late Cenozoic tectonic/volcanic feature in the northern portion of the Basin and Range geologic region (Bonnichsen and Breckenridge, 1982). The plain extends across southern Idaho for roughly 300 miles in a crescent shape. It is divided into two main sections identified as the western and eastern Snake River Plain. The western portion is about 40 miles wide, bounded by normal faults and has a northwest-southeast trend. Malde and Powers (1958) recorded at least 9,000 feet of displacement between the highlands to the north and the elevation of the plain today and concluded about 5,000 feet of displacement occurred in the early and middle Pliocene. The displacement started about 17 million years ago by rifting and down warping of the plain. The subsequent stretching of the crust produced a basin that began filling with sedimentary and volcanic rocks of considerable thickness during the Miocene, Pliocene and Pleistocene.

Explosive rhyolitic volcanism associated with the Yellowstone-Snake River Plain hotspot deposited the Idavada Volcanics from 14 to 9 MA (Figure 2) in southwestern and south-central Idaho during middle Miocene time (Malde and Powers, 1962). The Snake River Plain near Hagerman subsided in the wake of thermal uplift associated with the hotspot and also due to northeast/southwest extension that formed the southeast propagating western Snake River Plain graben or rift (Malde, 1991). These eruptions continued through much of the Miocene epoch and filled the basal portion of the western Snake River Plain with silicic lavas, welded and vitric tuffs. The Idavada Volcanics are commonly exposed to the north and south of the plain as local highlands. The Mount Bennett Hills north of the city of Hagerman are primarily composed of Idavada Volcanics (Maley, 1987).

Eleven million years ago deposition of the Idaho Group began on the Idavada Volcanics. Cope (1883) identified and named these sediments "The Idaho Group" and the body of water where these sediments collected "Lake Idaho". He correctly dated a portion of the Idaho Group as Pliocene based on fish fossils. The Idaho Group was deposited in lakes, flood plains and streams. The base level and sediment load in this environment was affected by local basaltic volcanism and subsidence of the western Snake River Plain as well as silicic volcanism in the eastern Snake River Plain (Malde, 1991).

Malde and Powers (1962) divided the Idaho Group into seven formations ranging in age from 11 million to 700,000 years old. In ascending order they are Poison Creek/Banbury Basalt, Chalk Hills, Glenns Ferry, Tuana Gravel, Bruneau and Black Mesa Gravel (Figure 2). These formations are composed of clastic sedimentary lithologies and inter-bedded olivine basalt flows, silicic volcanic ashes and basaltic pyroclastic material with an aggregate thickness up to 1500m (Malde and Powers, 1962). Most of the sediments are poorly consolidated and range in texture from clays to gravels. Only the Glenns Ferry and Tuana Formations of the Idaho Group, along with a thick caliche cap and overlying soils are exposed at the study site. Figure 3 illustrates a generalized geologic map in the vicinity of the study area based on a map by Malde (1972).

Figure 3 Geologic Map (omitted from on-line edition)

The age of the Glenns Ferry Formation is broadly constrained from Pliocene to early Pleistocene or 5 to 1.5 MA. (Malde, 1991). It exhibits four major environments including sandy fluviatile, muddy flood plain, lacustrine and valley border facies (Malde, 1972). Primarily fluviatile and flood plain environments are represented in the study area. The flood plain deposits of the Hagerman area are marginal to and east of the lacustrine facies that crop out near the town of Glenns Ferry and continue westward as illustrated in Figure 4 (Malde, 1972; Malde, 1991). Lacustrine deposits consist of massive tan silt and fine-grained sand forming monotonous outcrops and were deposited in ancient "Lake Idaho". The outcrops of fluvial facies are predominately found east of the town of Glenns Ferry. Coarse-grained arkosic sands and cobble gravels of the valley border facies are present at both the northern and southern margins of the western Snake River Plain (Figure 4) (Malde, 1972).

Figure 4. Study area location and distribution Glenns Ferry Formation sedimentary facies in the Glenns Ferry/Hagerman area (after Malde, 1972).

Malde (1972) describes the depositional setting as a highly sinuous meandering stream and flood plain which formed the delta plain marginal to a perennial lake to the west. The climate was predominately humid but also semi-arid at times.

"... the river flowed in a wide valley marked by temporary lakes and by broad stretches that were seasonally flooded. As the river shifted its course, the sedimentary environments changed correspondingly. Even so, the persistence of rather uniform environments in certain areas is shown by surprisingly thick sequences of fairly uniform deposits (Malde, 1972, page D13)."

Lee (1995) recognized two lithofacies associations, sandy and muddy. These are equated to represent a channel and flood plain environment within a meandering stream system. Sandy associations make up about 25% of the Glenns Ferry Formation at the northern end of the Hagerman Fossil Beds National Monument (Lee, 1995). The sandy fluviatile association contains lithofacies of gravely sand, trough cross-bedded sand, ripple-marked and scour-fill sand generally arranged in upward-fining successions. These are interpreted to represent point bar deposits in a mixed-load, highly sinuous meandering stream system. Bjork (1968) described these channel sands as grey, micaceous quartz sand that is uniformly fine-grained.

Muddy facies sequences with local organic rich and pedogenic facies make up about 75% of the Glenns Ferry Formation (Lee, 1995). The muddy facies association accumulated vertically in flood plains periodically inundated by water from floods in the fluvial system and consists mainly of pale olive colored silty clays and clayey silt beds arranged in upward fining cycles at a scale of decimeters to meters (Lee, 1995). These deposits are commonly characterized by monotonous fine-grained, graded, calcareous, pale-olive silt beds from one to three feet thick capped with a dark, carbonaceous clay from one to several inches thick (Malde, 1965).

The Tuana Gravel Formation rests on the Glenns Ferry Formation. Saddler (1997) describes the composition of the Tuana Gravels as coarser grained sediments in the silt, sand and gravel fractions. Thickness of the gravel varies up to 200 feet but is commonly about 50 feet thick within the study site (Bjork, 1968). Malde (1965) describes the Tuana as gradually rising in elevation and thickening southward and suggests that the ancestral Snake River deposited the gravels across the valley. The exposed base of the Tuana Gravel exhibits cut and fill stream channels in the underlying silts and clays of the Glenns Ferry Formation. These stream channels are commonly filled with fine sand.

A caliche layer has formed several feet below overlying soil horizons. It covers most of the study site but no formal mapping or characterization has been performed specifically to determine its areal extent, continuity, structure or thickness. The caliche does reflect a climatic change from the Tuana environment and is considered to have formed during an interglacial dry cycle of the Pleistocene (Bjork, 1968). Outcrop observations indicate the caliche is a very dense layer averaging several feet thick, but thins to less than one foot in thickness at some locations. It contains vertical fractures that are re-cemented in some places and not in others. It is resistant to weathering and forms a cap rock near the top of the hillsides in most of the study site and surrounding area.

Geologic Model

A six layer geologic model of the study site was developed and provides the foundation for the hydrostratigraphic model. The model is based on literature research and review, examination of geologic outcrops located at perched aquifer discharge zones and recharge areas and an investigation of geologic conditions for the Fossil Gulch Canal base. Geologic units within the plateau were inferred from geophysics and drill logs, discussed in Appendices A and C.

Literature research and review is based on the following sources.

1. General geology (Malde, 1972), (Bonnichsen and Breckenridge, 1982), (Malde, 1991), and (Malde and Powers, 1958).
2. Stratigraphy (Bjork, 1968), (Lee, 1995), (Malde, 1972) and (Malde and Powers, 1962).
3. Carbonaceous Paper Shales (Lorkowski and Hauser, 1996).
4. Tuana Gravel Formation (Saddler, 1997).
5. Drill logs and borehole geophysics (U.S.G.S., 1986 and 1994).
6. Seismic data (Michaels and Donaldson, 1994).

Outcrop elevations of the model layers were collected at the southeast end of the study site. There is a southerly dip to the formations based on the U.S.G.S geologic map (Malde, 1972) and stratigraphy studies (Bjork, 1968). A gradient of 0.006 (0.6%) was calculated for the Shoestring Basalt flow from the Fossil Gulch Pond (Elev. 3,280 feet) to outcrops located at the southeast end of the study site (Elev. 3,180 feet). It is assumed the other geologic units generally follow this trend within the plateau.

An investigation of the canal base geology was performed from the end of the concrete lined segment to an aqueduct approximately 2.5 miles downstream from the Fossil Gulch Pond (Figure 5). The canal base was sampled every 200 feet with several holes drilled from a power auger that has a 2.5-foot depth capability. The sampling occurred after Bell Rapids Irrigation Company cleaned the canal by scraping sediments out of the base.

Results indicate a layer of caliche in the base of the canal extending from the concrete lined section of canal, downstream to 1/3 mile before Fossil Gulch Pond. Then very dense silty clay was recorded to the pond. One 200-foot long area did reveal coarse sand with no caliche down to 2.5 feet. Caliche was observed in the sides of the canal at this location indicating that heavy equipment had broken through the layer of caliche during construction of the canal. Fossil Gulch Pond is in direct contact with the Shoestring Basalt flow based on observations made in February 1998. Basalt outcrops were exposed under approximately four feet of sediments during a dredging operation performed by the Bell Rapids Irrigation Co. to remove 28 years of sedimentation in Fossil Gulch Pond. Dense olive colored silty clays were recorded in the base of the canal from the Fossil Gulch Pond to 0.75 mile downstream. Basalt was observed in the base of the canal from 0.75 mile downstream of the pond, to 1.5 miles downstream from Fossil Gulch Pond.

Figure 5 map of canal base geology (omitted from on-line edition)

Layers of the geologic model have been identified at locations within the plateau from drill logs, borehole geophysics and surface geophysics. Appendix C provides illustrations of monitor well geologic logs and locations in Figure 10. The Shoestring Basalt has been inferred between the drill log locations within the plateau from a seismic geophysical study.

In 1994 Idaho Power Company mapped the surface of the Shoestring Basalt flow using seismics. The primary objectives were to determine the areal extent of the basalt flow in proximity to the Fossil Gulch Irrigation Canal to aid in assessment of ground water flow paths. White dashed lines with a line number mark the location of the seismic traverses illustrated in Figure 6. The surface of the Shoestring Basalt was identified from seismics along lines 1, 2, 4, 5 and 6. The basalt was not observed seismically near the canyon rim with line 3 but is known to exist from drill logs and outcrops in this area. Overall the surface of the Shoestring Basalt flow is continuous and smooth with a composite apparent dip to the south and southwest (Michaels and Donaldson, 1994).

The individual layers of the model are described in the following paragraphs and illustrated in Figure 7. Layer 1 of the model is the Tuana Gravel Formation that is composed of gravel, sand, and subordinate silt and clay. Paleo-stream channels have cut and filled into the underlying Glenns Ferry Formation at an elevation of about 3,325 feet (Saddler, 1997). Layer 1 is acting as an aquifer and transmitting water through paleo-stream channels, which show as localized point discharge locations on the hillsides.

Figure 6 Seismic line locations (omitted from on-line edition)

Layer 2 of the model is part of the Glenns Ferry Formation which consists of 75% silty clays (Lee, 1995) with the remaining 25% composed of mostly stream facies. This layer is about 125 feet thick and extends from an elevation of 3,325 feet down to the surface of the basalt flow at about 3,180 feet elevation (Lee, 1995). Layer 2 of the model is acting as an aquitard separating the Tuana Gravel Formation from Layer 3.

Layer 3 is the Shoestring Basalt flow, which averages 25 feet thick in outcrops and exhibits a one-half foot thick basal baked zone forming an aquitard. The fractured basalt flow is acting as an aquifer discharging up to 300 gallons per minute in one area. The basalt crops out at an elevation of 3,180 feet at the southeastern side of the study site and on the plateau surface (3,280 feet) for the northwestern side. Well logs and geophysics have identified the basalt at locations within the plateau. Layer 4 extends from the base of the basalt flow down to an elevation of about 3,110 feet and consists of more silty clays, typical of the Glenns Ferry Formation and forms an aquitard. Layer 5, from 3,110 feet to 3,100 feet elevation is dominated by a stream facie acting as an aquifer.

Lorkowski and Hauser (1996) mapped this layer and the underlying package of carbonaceous paper shales. The stream facie dominates this layer but is not laterally continuous. Paleo-stream channels have cut into the underlying carbonaceous paper shale and filled with fine sand.

Layer 6 is composed of carbonaceous paper shales that form an aquitard and failure plane for slope failures. The unit is about 20 feet thick, crops out from 3,100 feet to 3,080 feet in elevation, and is composed primarily of finely laminated clays with deposits of diatoms and ash (Lorkowski and Hauser, 1996). These deposits likely have low shear strengths and their elevations correspond to the lower rotational failure planes for the 1991 and ca. 1979 slope failures.

Figure 7. Geologic Model (omitted from on-line edition)

Table of Contents
Chapter 1 | 2 | 3 | 4 | 5
Appendix A | B | C | D

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