The 18.8 Ma Peach Spring Tuff’s presence over nearly 20,000 km2 of the southern Basin and Range province makes it the principal strain and time marker in one of the world’s most intensively studied extensional belts (Glazner et al.,1986). After decades of intensive study however, the search for its source caldera yielded nothing. Then, during a routine, 2006 highway survey suggested and funded through the vision of ADOT geological engineer Nicolas M. Priznar. the elusive caldera was identified by Arizona Geological Survey geologists along the eastern edge of the Colorado River Extensional Corridor (CREX) near Oatman, Arizona.

Mineral Resources of the Warm Springs Wilderness Study Area (AZ-020-028/029), Mohave County, Arizona MLA 48-88 (1988)

Mineral Investigation of a Part of the Mount Nutt Wilderness Study Area (AZo020-024), Mohave County, Arizona MLA 35-88 (1988)

The STATEMAP Program has facilitated a tremendous amount of geologic mapping in the past 25 years, but it imposes a rather relentless schedule on state geological surveys. STATEMAP is part of the National Cooperative Geologic Mapping Program administered by the US Geological Survey (USGS). Federal funding is allocated annually, beginning in September for the AZGS, and is matched dollar-for-dollar by the state; we typically agree to deliver 4 or 5, 1:24,000-scale geologic maps at the end of the funding cycle. These products must be delivered to the USGS in fairly presentable draft form at the end of the contract period, or funding is not released for the following year.

The Mount Nutt 7 ½' Quadrangle is located in the Black Mountains just west of Oatman, AZ in Mohave County, western Arizona. The Quadrangle encompasses portions of the Mount Nutt and Warm Springs Wilderness areas and includes the east flank of the northern Black Mountains and the piedmont that extends eastward from the foot of the range into Golden Valley. Geologic mapping was done under the joint State-Federal STATEMAP program, as specified in the National Geologic Mapping Act of 1992, and was jointly funded by the Arizona Geological Survey and the U.S. Geological Survey under STATEMAP assistance award #G14AC00424. Mapping was compiled digitally using ESRI ArcGIS software.

These geologic map sheets and report encompass the proposed State Route 95 Relocation Corridor in Mohave Valley, western Arizona, between the Black Mountains on the east and the Colorado River on the west (Figure 1). The maps were developed through compilation of existing geologic mapping and extensive new geologic mapping. The map area includes a portion of the historical floodplain of the Colorado River, a broad swath of the piedmont east of the river, parts of western flank of the Black Mountains, the channels and terraces of Sacramento Wash, and the northernmost piedmont of the Mohave Mountains (the Powell Mountain area). The corridor is predominantly covered with late Miocene and younger alluvium deposited by local tributary washes and the Colorado River. Deposits range in age from modern to at least 8 Ma (millions of years ago), and deposit characteristics and composition vary substantially depending on whether they were emplaced by the modern Colorado River, older versions of the river, or tributary washes derived from the Black Mountains or the northern end of the Mohave Mountains. The northern, northeastern, and southernmost parts of the map include a variety of bedrock types ranging in age from early Proterozoic to middle Miocene. This map and report describe the physical characteristics of geologic units of different ages and sources, the mapped extent of various geologic units, and potential geologic resource and geologic hazard implications of this mapping. These maps include modified versions of portions of several published maps that have been completed in this area since 1997 (Howard et al., 1997; Howard et al., 1999; Faulds et al., 2001; Murphy et al., 2004; Pearthree and House, 2005; Pearthree, 2007; Spencer et al., 2007; Malmon, in review). Extensive new surficial geologic mapping and bedrock mapping was done where needed.

The Oatman and Katherine districts have produced more than thirty million dollars worth of gold and silver yet no complete report on the geology and ore occurrence of this interesting region is available though numerous short papers on various phases of its geology or the mining operations conducted there have been published from time to time. A thorough study of the Oatman District was made by Dr. F. L. Ransome and a preliminary report based on that study was issued by the United States Geological Survey in 1923. The detailed descriptions of the rocks and the excellent map of the district accompanying this report have been of great value to the operators in this district. As Dr. Ransome has left the Geological Survey, it is hardly likely that a complete report will be written by him and published. Among the earlier accounts of the district, that by Mr. F. C. Schrader is, by far, the most complete. Although Schrader's report is based upon a reconnaissance examination of the district, as stated in the introduction, it served a useful purpose until superseded by Ransome's more detailed studies. Schrader's work, which preceded Ransome's by fifteen years, was done at a time when a large number of the properties were operating. The bulletin, therefore, contains much information on the character of the vein fillings and the grade: of· the' ore mined. Both Ransome's and Schrader's bulletins have been drawn on freely for information on,the underground workings of properties now inaccessible. Frequent short notices in the technical press deal chiefly with mining or metallurgical problems, and rarely make more than a brief mention of the geology, and then usually treat of the geology of some particular mine rather than of the district,as a whole.

Gold placers, or deposits such as gravel and sand which contain notable concentrations of gold, all result from the slow milling and concentration processes incident to natural erosion of pre-existing gold-bearing rocks. The origin of many gold placers is traceable directly to auriferous veins, lodes, or replacement deposits which, in most instances, were not of high grade. According to Emmons, placers are not apt to form from goldbearing outcrops that contain abundant manganese, iron sulphides, and chlorides, unless precipitating agents such as calcite, siderite, rhodochrosite, pyrrhotite, chalcocite, nepheline, olivine, or leucite are abundant, or unless erosion is very rapid. In other words, the gold may be dissolved and carried below by means of natural chlorination processes that are established when solutions containing chlorides, together with sulphuric acid from the oxidation of iron sulphides, act upon manganese dioxide; but this process is neutralized if precipitating agents are present, and may be ineffective if erosion is very rapid

The fame of Arizona as a copper producing state is so great and widespread that sometimes her deposits of other minerals do not receive their due share of attention. It is not very generally known that within the boundaries of Arizona are some of the largest and richest gold placers in the country. The early prospector discovered these placers and worked the richest spots with his pan and rocker. Considerable sums of money have been spent in attempts to develop certain of the deposits but either through mismanagement or unwise expenditure of the money because of insufficient investigation, few of these ventures have paid. In the exploitation of placer ground the question of an easily available and abundant water supply is important. It is preeminently the great problem in the development of Arizona placers where there is so little rainfall and so few permanent streams. To the man who is able to solve this difficulty with a reasonable expenditure of capital, Arizona gold placers will give up millions of dollars worth of gold.