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. 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.
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 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. 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.
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 (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 (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.