The name Humboldt Group was applied to upper Tertiary rocks in western Utah and Eastern Nevada by members of the Fortieth Parallel Survey (King, 1878). Sharp (1939b) suggested a type section for the Humboldt Formation along Huntington Creek about 2 miles east of the Dixie Flats Quadrangle in Elko County.

The Humboldt Formation is Upper Miocene in age. Smith and Ketner (1976) restricted the Humboldt to the middle and part of the upper member of the Humboldt defined by Sharp (1939b). The upper part of the Raine Ranch and Carlin Formations of Regnier (1960) were included in the Humboldt by Smith and Ketner (1976). A zircon fission-track age of 9.5 +/- 1.9 Ma was determined for the Humboldt Formation (Coats, 1985). Humboldt sediments near Thurston Spring in the Wine Cup Ranch Southwest Quadrangle contain a Clarendonian flora according to D. I. Axelrod (in Coats, 1985).

Sharp (1939b) divided the Humboldt Formation into three regional members. The lower member is shale, oil shale, fresh-water limestone and conglomerate, about 800 to 1,000 feet thick. This unit was later broken into various units and formations including the Elko Formation and is not included with the Humboldt Formation as restricted by Smith and Ketner (1976). The middle member of Sharp (1939b) is characterized by rhyolitic tuff and ash, which has a maximum thickness of about 1,300 feet. The upper member is composed of up to 3,600 feet of conglomerate, sandstone, siltstone, mudstone and shale.

There are no persistent marker beds within the Humboldt Formation, and rapid lateral facies changes frustrate attempts to subdivide or correlate individual Humboldt sections. Sections vary greatly in thickness but have rarely been measured. The upper contact of the formation is rarely exposed, and the basal contact is an irregular unconformity surface developed on both Paleozoic and older Tertiary topography. Many exposures are small, irregular and isolated patches. The Humboldt commonly dips 10 to 20 degrees over the entire evaluation area, but may locally attain steeper dips along high-angle faults that have tilted and rotated beds.

The majority of sections are made up of interbedded, gray, fine to medium grained, evenly and thinly bedded vitric ash and tuff a few inches to 100 feet in thickness, and soft, light gray, green, or red tuffaceous siltstone. Conglomerate, sandstone, siltstone, mudstone and limestone are common in the upper portion of the Humboldt Formation. Dense dark gray to black, aphanitic basaltic andesite and rhyolite flows and plugs are interstratified throughout the formation. These volcanics include about 800 feet of the Palisade Canyon Rhyolite, which forms a wedge in the Humboldt at the northern end of Pine Valley (Smith and Ketner, 1976).

Lenticular channel-form conglomerates in the Humboldt Formation contain pebbles and cobbles of angular to well rounded Paleozoic limestone, red, green, and black chert and metamorphic fragments, commonly 2 to 4 inches in diameter. The conglomerates are in beds that are seldom more than 20 feet in thickness. Clast type varies in response to the local source terrain and can provide a means of reconstructing erosional histories (Gilluly and Masursky, 1965; Smith and Ketner, 1976).

Gray, white, brown, and pink sandstones are fine to medium-grained, tuffaceous and calcareous, crossbedded on a small scale, and are often lensoidal and locally pebbly. Mudstone and claystones are commonly white, gray or brown and thin bedded or massive in texture. These units occasionally contain both dinosaur bones and plant fragments confirming the terrestrial origin of these sediments.

A small percentage of dense cream to gray, locally cherty or sandy limestone is in lenses from a few feet to perhaps 30 feet long, and 2 inches to 3 feet thick. Minor amounts of diatomite are also locally present in the Humboldt Formation, such as along the western flank of the Carlin-Pinon Range and in the Suzie Creek drainage (Regnier, 1960). The sandstone, conglomerate, siltstone and tuff all grade laterally into one another in various places and are locally lenses within basaltic andesite and rhyolite tuff sequences.

Along the northwestern flank of the Medicine Range lacustrine calcareous siltstone, sandstone, yellow-gray limestone and vitric rhyolite nonwelded tuff are correlated with the Humboldt Formation (Collinson, 1968). The basal unit is composed of cross-stratified sandstone that contains large silicified logs. To the north along the flank of the Elk Mountains about 300 feet of Humboldt are exposed, and are composed of light tan tuffaceous stratified sandstone with silicified wood fragments, white friable stratified vitric tuff and reddish-brown fissile silty mudstone (Fifer, 1960). These lithologies are typical of Humboldt exposures within northeastern Elko County.

Immediately to the west and east of the Snake Mountains, the Humboldt Formation is relatively well exposed with poorly consolidated, massive to very thin bedded and cross-bedded, gray, yellow, green, and orange vitric tuff. The tuff is mostly composed of fresh to altered glass shards. Phenocrysts of quartz, sanidine, oligoclase, and bioite form about 5 percent of the rock. Pumice fragments compose about 10 percent, and detrital quartz and chert grains up to 25 percent of these porous and friable tuffs (Gardner, 1968). Dark gray shale is locally interbedded with the tuff, and a chert pebble conglomerate occurs in several localites. This conglomerate contains angular to sub-rounded, sand to boulder-size fragments of Valmy quartzite, chert, and lesser limestone and siltstone. These lithologies are typical of the Humboldt along the western flank of the HD Range as well where thin fresh-water limestones are also interbedded with the vitric tuff (Riva, 1962). Near the Leach Mountains about 190 feet of nearly horizontal tuffaceous sediments assigned to the Humboldt are exposed (LeCompte, 1978).

Thicknesses are difficult to determine in the Humboldt since the upper and lower contacts are commonly eroded or covered, and outcrops are very soft and poorly exposed. Near the type section along Huntington Creek the Humboldt is about 1,800 to 2,000 feet thick (Smith and Ketner, 1976), 1,500 to 2,000 feet in the Maggie Creek and Suzie Creek drainages (Regnier, 1960), 750 feet along western Pine Valley (Gilluly and Masursky, 1965), about 500 feet along the northwestern flank of the Medicine Range (Collinson, 1968), up to 500 feet near the Snake Mountains (Gardner, 1968), and about 300 feet along the Elk Mountains in northern Elko County (Fifer, 1960).

The Humboldt Formation is present in numerous valleys centered near Elko and is exposed over an area of approximately 110,000 square kilometers in northeastern Nevada and westernmost Utah. It is present in northern Crescent, Pine, Huntington, Newark and Ruby Valleys, Maggie Creek-Suzie Creek drainage west of the Adobe Range, North Fork of the Humboldt River drainage on the east side of the Adobe Range, Marys River drainage area west of the Snake Mountains, between the Snake Mountains and HD Range and near Contact, Windermere Hills, southern Clover Valley near Spruce Mountain, northwestern flank of the Medicine Range, Antelope Valley, eastern Independence Valley along the western flank of the Pequop Mountains, west of the Leach and Delano Mountains, the Elk Mountains area, and near Sugar Loaf Mountain in the northeastern corner of the evaluation area.

The Humboldt Formation represents a series of interfingered fluviatile and lacustrine sediments, and ash-flow and water-lain tuffs deposited in a series of intermontane basins that formed during Miocene Basin and Range extensional faulting. These basins were periodically filled by lakes fed in part by meandering stream systems that deposited and reworked sediments.

Several other Miocene sedimentary units are exposed within northeastern Nevada including the Slide Creek Gravel in the northern portion of Elko County. The Slide Creek Gravel was designated for poorly sorted and unconsolidated gravels with rounded to subangular boulders up to 4 feet across in the Jarbidge Quadrangle (Coats, 1964). The unit unconformably overlies the Jarbidge Rhyolite, and is overlain unconformably by the Jenny Creek Tuff (Coats, 1985).

In the Rowland Quadrangle, Bushnell (1967) mapped a gravel, probably a fanglomerate deposited along a steep fault scarp, which he called the Young America Gravel. The Young America Gravel contains angular to subrounded reddish quartzite pebbles less than 1 inch, to boulders 3 feet across (Bushnell, 1967). It interfingers with the Humboldt Formation and tuffs and sediments of the Idavada Formation, and is locally overlain by the Cougar Point Tuff (Bushnell, 1967; Coats, 1985).

In the western Mountain City and eastern Owyhee Quadrangles, undated sediments are informally referred to as the sedimentary rocks of McCall Creek and are included here with Miocene sediments (Coats, 1985). This unit is composed of poorly sorted tuffaceous boulder gravel that locally has a tuffaceous matrix. It appears to be a volcanic mudflow that has been weathered to a coarse boulder conglomerate or gravel (Coats, 1985). Also included with the Miocene sediments in the Mountain City Quadrangle are poorly exposed green to white rhyolitic air-fall tuffs which contain black and white petrified logs, some are as big as 3 feet in diameter (Coats, 1985).

In White Pine County, rocks mapped as younger Tertiary sediments are mainly Miocene and possibly partly Pliocene in age. They occur in the Pancake, Maverick Springs, Egan, Schell Creek, Snake, Antelope, and southern Goshute Mountains (Hose and Blake, 1976). They are mainly composed of 50 to 300 feet of varying proportions of intercalated air-fall tuff, calcareous siltstone and fine grained sandstone, light-colored fresh-water limestone, conglomerate containing well-rounded Paleozoic sedimentary and Tertiary volcanic pebbles up to several inches in length, and monolithologic breccias of Paleozoic rocks. Also interbedded with these sediments are minor andesite and basalt flows dated at about 20 to 21 m.y. (Hose and Blake, 1976). In the Sacramento Pass area of the Snake Range these sediments attain a thickness of at lest 5,000 feet (Hose and Blake, 1976).

The Miocene Horse Camp Formation is present in a deformed basin about 5 by 10 miles across near Currant in the Grant Range (Moores and others, 1968). These fluvio-deltaic sediments are as much as 10,000 feet thick in the central portion of the basin. The upper part of the Horse Camp Formation is monlithologic breccia sheets related to low-angle normal faults within the Grant Range. The Horse Camp rocks were divided into four members (Moores and others, 1968). The lowest member is rhyolite-rich fluvial sandstone and conglomerate and fanglomerate, which interfingers with lacustrine siltstone and limestone. The overlying member 2 is about 6,000 feet of monolithologic breccia, sandstone and conglomerate that interfinger with lacustrine sediments, vitric ash, and volcanic-rich conglomerates and sandstones. Member 3 is about 3,000 feet of fanglomerate and landslide masses as much as one mile long, which are overlain by member 4, fluvial sandstone and conglomerate rich in Paleozoic clasts. Unconformities are present between members 1 and 2, and between members 2 and 3 (Moores and others, 1968).

Just south of the study area in the northern Pahroc Range and in several ranges in central and southern Lincoln County, Miocene lacustrine limestones are exposed in small discontinuous outcrops. These massive or laminated limestone lenses are gray, pink, yellow, or white, with algal and ostracodal debris. Local well-rounded cobble conglomerates and thick tuffaceous interbeds are present. The limestones range in thickness from 50 to 550 feet and are commonly about 100 to 200 feet thick (Tschanz and Pampeyan, 1970).