Arizona Geological Survey Open File Report, OFR-15-10, 24 P

Fall 2015 Arizona Geological Society fieldtrip guide: Northern Plomosa Mountains & Bouse Formation in Blythe Basin

Authors Spencer, J.E.; Pearthree, P.A.

Citation Spencer, J.E. and Pearthree, P.A., 2015, Fall 2015 Arizona Geological Society field-trip guide: Northern Plomosa Mountains and Bouse Formation in Blythe Basin. Arizona Geological Survey Open File Report, OFR-15-10, 24 p.

Download date 06/10/2021 04:36:39

Link to Item http://hdl.handle.net/10150/629564 Fall 2015 Arizona Geological Society field- trip guide: Northern Plomosa Mountains & Bouse Formation in Blythe Basin

Jon E. Spencer and Phil A. Pearthree Arizona Geological Survey

Jon Spencer (left) and Phil Pearthree (right) at the Bouse Fisherman site, western Arizona

OPEN-FILE REPORT OFR-15-10

November 2015

Arizona Geological Survey

www.azgs.az.gov | repository.azgs.az.gov Arizona Geological Survey

M. Lee Allison, State Geologist and Director

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Recommended Citation: Spencer, J.E. and Pearthree, P.A., 2015, Fall 2015 Arizona Geological Society field-trip guide: Northern Plomosa Mountains and Bouse Formation in Blythe Basin. Arizona Geological Survey Open File Report, OFR-15-10, 24 p. Fall 2015 AGS field-trip guide

Arizona Geological Society

Northern Plomosa Mountains (day 1) Bouse Formation in Blythe Basin (day 2)

November 14-15 (Saturday-Sunday), 2015 Trip leaders: Jon Spencer and Phil Pearthree, Arizona Geological Survey

Schedule: Saturday, Nov. 14 – Northern Plomosa Mountains

Meet 10:00 AM at the BLM Bouse Fisherman geoglyph site on Plomosa Road (paved) (see Figure 1). This is a fenced Indian geoglyph (aka “intaglio”) several hundred yards by easy trail from a BLM parking lot on Plomosa Road. Good meeting spot with big parking lot. Spencer will give an overview of the area from the parking lot. Also, we will drive by this spot after the day’s field-trip stops so if you don’t have time to visit the fisherman you can do it on the way out. (To get to Bouse Fisherman, drive 5.6 miles north of Quartzsite on State Highway 95, turn right on paved Plomosa Road, drive about 8 miles on Plomosa Road. Parking lot will be on the left, just after entering the mountains and crossing a wash.)

Stop 1. Overview of Northern Plomosa geology from the Bouse Fisherman parking lot.

Stop 2. Barite veins and disseminated manganese mineralization in Miocene conglomerate (Fig. 2).

Stop 3. Stratigraphy of tiltblocks above the Plomosa detachment fault, and the upward appearance of mylonitic debris derived from the footwall block (Fig. 3).

Stop 4. Stratigraphy of the base of the Miocene section – basal conglomerate grading up into sand, silt, and limestone (Fig. 4).

Stop 5. Rock-avalanche breccia derived from metamorphic rocks that were subjected to hydrothermal alteration and later metamorphism, producing unusual mineral assemblages (including svanbergite) (Fig. 4).

Saturday evening drive to Blythe, California, for dinner and hotel, or camp south of Blythe near Cibola Wildlife Refuge (see Figures 5, 6).

Blythe has something like a dozen hotels. One that looks pretty good online (but not too expensive) and maybe good for AGS is: Days Inn Blythe, East Hobson Way (exit I-10 and go north on Intake, west on Hobson to 1673 East Hobson Way, 855-799-6859). Garcia’s Restaurant has been good – located at 231 W Hobson Way.

Sunday, Nov. 15 – Bouse Formation, Cibola and northwestern Dome Rock Mountains

Those staying in Blythe should drive to the meeting spot near the Cibola National Wildlife Refuge (shown on map Figure 6). Those camping out will already be there.

Stop 1. Assemble Sunday morning by 9:00 AM for a ~2.5 mile (round trip) hike down Hart Mine Wash to look at various facies of the Bouse Formation, from the underlying, locally reworked fanglomerate, the basal bioclastic limestone and tufa, marl, mudstone, and fine quartz-rich sand with Colorado River affinity. We will then cross the wash to visit an intensely deformed Bouse siliciclastic section and mildly deformed overlying strata (Fig. 6).

Stop 2. Exposures of upper bioclastic Bouse Formation, including reworked carbonate debris and locally- derived gravel (Fig. 6). We will consider relationships of this unit with the basal carbonate and siliciclastic units, and its significance for the evolution of the Bouse water body.

Stop 3. Return to I-10, drive east into California, turn south (right) at Tom Wells Road (see map Figure 7). Manganese oxides at the base of the Bouse Formation.

Stop 4 (optional). More manganese oxides at the base of the Bouse Formation (Fig. 7). DAY 1: Northern Plomosa Mountains Introduction

The northern Plomosa Mountains are a small metamorphic core complex in La Paz County, western Arizona. The dominant structure of the range is the Plomosa detachment fault, a low-angle normal fault that separates a single tilted footwall block from a highly extended hanging wall with diverse stratigraphy and complex structure. The northern Plomosa Mountains were first mapped by Scarborough and Meader (1983), with later detailed mapping of smaller areas by Stoneman (1985) and Steinke (1997). The Arizona Geological Survey (AZGS) mapped the northern Plomosa Mountains at 1:24,000 scale (Spencer et al., 2014) with funding from the STATEMAP program, a component of the National Geologic Mapping Act of 1992. This field trip is largely a showcase for geology mapped during the AZGS mapping project.

The footwall of the Plomosa detachment fault consists of granitic and metamorphic rocks in a southward tilted fault block, with progressively deeper crustal levels represented by more northern exposures. The northern part of the fault block includes mylonitic fabrics with a northeasterly trending lineation. The middle part consists of a complex, probably Cretaceous thrust zone containing folded Paleozoic to Jurassic metasedimentary and metavolcanic rocks. The southern part consists of a three part stratigraphic sequence of upper Oligocene(?) to lower to middle Miocene strata: (1) basal conglomerate, arkosic sandstone, siltstone, and limestone, (2) mafic lava flows and rock avalanche breccias derived from diverse lithologies, including Paleozoic metasedimentary rocks, Jurassic volcanic rocks, and granitoids, and (3) intermediate volcanic rocks consisting largely of hornblende dacite.

Rocks above the Plomosa detachment fault consist of numerous fault blocks with similar basement and stratigraphic sequence except for some notable differences in the highest stratigraphic assemblage. Specifically, hornblende dacite and other intermediate to felsic volcanic rocks are less significant while conglomerate and sandstone are common. Some of the conglomerate contains mylonitic debris interpreted as derived from the detachment-fault footwall. This is significant because it indicates that many kilometers of displacement on the Plomosa detachment fault must have occurred to uncover the footwall mylonites. Furthermore, the basin that earlier received assemblages 1 and 2 was broken apart by extensional faulting so that assemblage 3 can’t be readily correlated from fault block to fault block. Note also that fission track cooling ages from apatite and zircon collected from footwall rocks vary in age from ~15 Ma at the northern end of the range to ~23 Ma just north of the Bouse fisherman geoglyph (Foster and Spencer, 1992). This is interpreted as the time of cooling of footwall rocks due to tectonic exhumation.

References Cited

Foster, D.A., and Spencer, J.E., 1992, Apatite and zircon fission-track dates from the northern Plomosa Mountains, La Paz County, west-central Arizona: Arizona Geological Survey Open-File Report 92-9, 11 p.

Scarborough, R.B., and Meader, N., 1983, Reconnaissance geology of the northern Plomosa Mountains, La Paz County, Arizona: Tucson, Arizona Geological Survey Open-File Report 83-24, 35 p. Spencer, J.E., Youberg, A., Love, D., Pearthree, P.A., Steinke, T.R., and Reynolds, S.J., 2014, Geologic map of the Bouse and Ibex Peak 7 ½' Quadrangles, La Paz County, Arizona: Arizona Geological Survey Digital Geologic Map DGM-107, scale 1:24,000. Steinke, T.R., 1997, Geologic map of the eastern Plomosa Pass area, northern Plomosa Mountains, La Paz County, Arizona: Arizona Geological Survey Contributed Map CM-97-A, scale 1:3570. Stoneman, D.A., 1985, Geologic map of the Plomosa Pass area, northern Plomosa Mountains, La Paz County, Arizona: Arizona Geological Survey Miscellaneous Map MM-85-C, scale 1:12,000.

Figure 1. Location map for field trip Day 1.

Figure 2 (Stop 2). Geologic map of the area of barite and manganese oxide veins and disseminated manganese oxides. Mineralization and alteration are associated with the Plomosa detachment fault and are generally thought to be related to Miocene basin brines circulating as a result of igneous and extensional tectonic activity (see section on IOCG deposits below).

Iron-oxide copper gold (IOCG) deposits in western Arizona

(modified from Spencer and Duncan, 2015)

The Colorado River Extensional Corridor is one of the most severely extended regions in western North America (Howard and John, 1986; Spencer and Reynolds, 1991). Felsic to mafic igneous activity occurred intermittently during the 15-20 million year period of extension (Shafiqullah et al., 1980; Spencer et al., 1989b, 1995). Widespread iron-oxide and copper mineralization during extension produced deposits that have yielded significant copper and gold and minor silver, lead, and zinc. These deposits are a type of iron-oxide copper gold (IOCG) deposit (e.g., Barton, 2014) with the distinction that they are associated with extensional detachment faults. Numerous bedded and fault-zone-related manganese- oxide deposits of the same age form a separate group of deposits in the same general area (Fig. 2a). Sedimentary basins that formed during tectonic extension accumulated sedimentary and volcanic rocks that, in many areas, were moderately to severely affected by potassium metasomatism, with K/Na ratios of 10:1 to even 100:1 (e.g., Chapin and Lindley, 1986; Brooks, 1986). It is suspected that all three alteration and mineralization processes are related (Fig. 2a), but it has not been possible to demonstrate genetic relationships with existing data.

Figure 2a. Suite of mineralization and alteration types that affect Miocene rocks and structures in core complexes of western Arizona. All of these except chloritic alteration of detachment-fault footwall breccias are thought to be related to basin brines. Brines circulated along and above extensional detachment faults in response to high thermal gradients associated with magmatism and with tectonic exhumation of hot footwall rocks. These brines were probably associated with potassium metasomatism but this has not been demonstrated.

Mineral deposits related to detachment fault. The association of mineral deposits with extensional detachment faults in the lower Colorado River extensional corridor has been known since the 1980s (e.g., Wilkins and Heidrick, 1982; Ridenour et al., 1982; Wilkins et al., 1986; Spencer and Welty, 1986, 1989; Spencer et al., 1988; Duncan, 1990; Salem, 1993; Evenson et al., 2014). These deposits generally consist of massive specular hematite with associated quartz, calcite, barite, and rare fluorite. Pyrite and chalcopyrite are present locally and formed early during mineralization, whereas common fracture- filling chrysocolla and malachite formed late. Of the ~53 million pounds of copper produced historically from the deposits, most came from chalcopyrite at the Swansea, Planet, and Mineral Hill Mines in the Buckskin Mountains (Spencer and Welty, 1989). Production at Swansea came largely from replacement deposits in metamorphosed Paleozoic carbonates directly above the detachment fault, but the structural and lithologic associations of the deposits are diverse. Gold mineralization was by far most significant at Copperstone mine, which yielded ~516 thousand ounces of gold (mostly during 1988-1993) primarily from amethystine quartz veins within specular hematite. Gold production in other deposits is minor and mostly came from four mines, at least two of which contain more quartz than typical (Spencer and Welty, 1989).

Heating and freezing experiments with fluid inclusions from quartz and fluorite in mineral deposits associated with detachment faults indicate a distinctive high salinity and moderate temperature mineralizing environment (Wilkins et al., 1986; Roddy et al., 1988; Duncan, 1990; Salem, 1993; Spencer and Duncan, 2015). Mineralizing fluids circulated along detachment faults, and were probably heated by footwall rocks which were uplifted from middle-crustal depths during tectonic exhumation and isostatic uplift. Tectonic ascent of mylonitic footwall rocks now exposed in metamorphic core complexes was analogous to ascent of large, moderate- to low-temperature intrusions. Faulting repeatedly crushed rocks in fault zones and by this process probably maintained open spaces for fluid circulation. The origin of the consistently saline fluids is less well understood.

Manganese mineralization. Bedded and shear-zone-hosted manganese-oxide deposits are widespread in central western Arizona and adjacent southeastern California, largely in the same area as detachment-fault-related deposits (Fig. 2a). These deposits, present in about 24 mining districts, have yielded over 200 million pounds of Mn, mostly during a US government buying program in 1953-1955 (Spencer, 1991). Genesis of manganese-oxide deposits is generally thought to begin with iron and manganese mobilization by warm to hot, mildly acidic solutions, followed by selective precipitation of iron and retention of manganese until manganese-rich solutions deposit manganese at locations removed from the iron deposits (e.g., Krauskopf, 1957; Hewett, 1966). Analysis of fluid inclusions in calcite, barite, and quartz from four samples associated with manganese deposits in the Artillery Mountains indicated fluid salinities of <3% NaCl equivalent (Spencer et al., 1989a). This means that the saline fluids that yielded iron oxide in detachment-fault-related deposits were not the same fluids that yielded manganese oxides in near-surface or surficial environments, at least for the two manganese deposits studied. In addition, manganese-oxide deposits in the Metate district in Arizona (east of Blythe) are hosted by Pliocene strata (AZGS DGM map of the Dome Rock Mountains SW 7 ½ʹ Quadrangle, in preparation). These observations indicate that manganese mineralization is not associated with saline, detachment-related mineralizing fluids in a simple manner, and in at least one district is not related at all.

Potassium metasomatism. K-metasomatism has drastically altered the chemistry of volcanic and sedimentary rocks in Miocene basins and volcanic fields in western Arizona, including in areas of detachment-fault mineralization and manganese mineralization (Roddy et al., 1988; Brooks, 1988; Spencer et al., 1989b; Hollocher et al., 1994; Caudill, 2015). Drastic chemical modification of large volumes of volcanic and sedimentary rocks occurred at about the same time as detachment and manganese mineralization, leading to the inference that K-metasomatism mobilized metals that were later precipitated in detachment deposits and manganese deposits (Spencer and Welty, 1989; Hollocher et al., 1994). It has not been possible to establish geochemical links between these different processes of mineralization and alteration (Caudill, 2015), however, although more studies could conceivably establish such links.

Conclusion. Detachment-fault-related mineralization, manganese mineralization, and potassium metasomatism affected Miocene rocks over a wide area in central western Arizona. Although it has been proposed that the three phenomena are related, it has not been possible to conclusively demonstrate this with available data. New fluid-inclusion data (Spencer and Duncan, 2015) indicate that saline fluids were widespread, but mixing trends between different salinity or different temperature fluids are not apparent from data derived from deposits associated with detachment faults. Such mixing trends are more common, but still rare, in other epithermal deposits of similar age in western Arizona. High salinities in some of these other deposits indicate that basin brines were widespread and present in basins not obviously related to detachment faults.

References Cited

Barton, M.D., 2014, Iron oxide(-Cu-Au-REE-P-Ag-U-Co) systems: Elsevier, Treatise on Geochemistry, 2nd edition, v. 11, chap. 23, p. 515-541. Brooks, W.E., 1986, Distribution of anomalously high K2O volcanic rocks in Arizona: Metasomatism at the Picacho Peak detachment fault: Geology, v. 14, n. 4, p. 339-342. Brooks, W.E., 1988, Recognition and geologic implications of potassium metasomatism in upper-plate volcanic rocks at the detachment fault at the Harcuvar Mountains, Yavapai County, Arizona: U.S. Geological Survey Open-File Report 88-17, 9 p. Caudill, C.M., 2015, K-metasomatism as related to manganese and copper deposits in upper-plate Miocene sedimentary units in central-western Arizona: Tucson, University of Arizona M.S. thesis, 183 p. Chapin, C.E., and Lindley, J.I., 1986, Potassium metasomatism of igneous and sedimentary rocks in detachment terranes and other sedimentary basins: economic implications, in Beatty, Barbara, and Wilkinson, P.A.K., eds., Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest 16, p. 118-126. Duncan, J.T., 1990, The geology and mineral deposits of the Northern Plomosa mineral district, La Paz County, Arizona: Arizona Geological Survey Open-File Report 90-10, 55 p., 1 sheet, scale 1:9,318. Evenson, N.S., Reiners, P.W., Spencer, J.E., and Shuster, D.L., 2014, Hematite and Mn oxide (U-Th)/He dates from the Buckskin-Rawhide detachment system, western Arizona: Gaining insights into hematite (U-Th)/He systematics: American Journal of Science, v. 314, p. 1373-1435; DOI 10.2475/10.2014.01. Hewett, D.F., 1966, Stratified deposits of the oxides and carbonates of manganese: Economic Geology, v. 61, n. 3, p. 431-461. Hollocher, K., Spencer, J.E., and Ruiz, J., 1994, Composition changes in an ash-flow cooling unit during K- metasomatism, west-central Arizona: Economic Geology, v. 89, p. 877-888. Howard, K.A., and John, B.E., 1987, Crustal extension along a rooted system of imbricate low-angle faults: Colorado River extensional corridor, California and Arizona, in Coward, M.P., Dewey, J.F., and Hancock, P.L., eds., Continental Extensional Tectonics: Geological Society Special Publication No. 28, p. 299-311.

Krauskopf, K.B., 1957, Separation of manganese from iron in sedimentary processes: Geochimica et Cosmochimica Acta, v. 12, p. 61-84. Ridenour, J., Moyle, P.R., and Willett, S.L., 1982, Mineral occurrences in the Whipple Mountains Wilderness Study Area, San Bernardino County, California, in Frost, E.G., and Martin, D.L., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado River region, California, Arizona, and Nevada: San Diego, Cordilleran Publishers, p. 69-75. Roddy, M.S., Reynolds, S.J., Smith, B.M., and Ruiz, J., 1988, K-metasomatism and detachment-related mineralization, Harcuvar Mountains, Arizona: Geological Society of America Bulletin, v. 100, p. 1627-1639. Salem, H.M., 1993, Geochemistry, mineralogy, and genesis of the Copperstone gold deposit, La Paz County, Arizona: Tucson, Arizona, University of Arizona Ph.D. dissertation, 206 p. Shafiqullah, M., Damon, P.E., Lynch, D.J., Reynolds, S.J., Rehrig, W.A., and Raymond, R.H., 1980, K-Ar geochronology and geologic history of southwestern Arizona and adjacent areas, in Jenny, J.P., and Stone, C., eds., Studies in Western Arizona: Arizona Geological Society Digest, v. 12, p. 201-260. Spencer, J.E., 1991, The Artillery manganese district in west-central Arizona, in Arizona Geology: Arizona Geological Survey, v. 21, no. 3, p. 9-12. Spencer, J.E., and Duncan, J.T., 2015, Fluid-inclusion characteristics of Oligocene to Miocene iron-oxide Cu-Au (IOCG) deposits in central western Arizona: Arizona Geological Survey Open-File Report 15-05, v. 1.0, 18 p., with appendices. Spencer, J.E., and Reynolds, S.J., 1991, Tectonics of mid-Tertiary extension along a transect through west-central Arizona: Tectonics, v. 10, p. 1204-1221. Spencer, J.E., and Welty, J.W., 1986, Possible controls of base- and precious-metal mineralization associated with Tertiary detachment faults in the lower Colorado River trough, Arizona and California: Geology, v. 14, p. 195-198. Spencer, J.E., and Welty, J.W., 1989, Geology of mineral deposits in the Buckskin and Rawhide Mountains, in Spencer, J.E., and Reynolds, S.J., Geology and mineral resources of the Buckskin and Rawhide Mountains, west-central Arizona: Arizona Geological Survey Bulletin 198, p. 223-254. Spencer, J.E., Duncan, J.T., and Burton, W.D., 1988, The Copperstone Mine: Arizona's new gold producer, in Arizona Geology: Arizona Geological Survey, v. 18, no. 2, p. 1-3. Spencer, J.E., Grubensky, M.J., Duncan, J.T., Shenk, J.D., Yarnold, J.C., and Lombard, J.P., 1989a, Geology and mineral deposits of the Central Artillery Mountains, in Spencer, J.E., and Reynolds, S.J., Geology and mineral resources of the Buckskin and Rawhide Mountains, west-central Arizona: Arizona Geological Survey Bulletin 198, p. 168-183. Spencer, J.E., Richard, S.M., Reynolds, S.J., Miller, R.J., Shafiqullah, M., Grubensky, M.J., and Gilbert, W.G., 1995, Spatial and temporal relationships between mid-Tertiary magmatism and extension in southwestern Arizona: Journal of Geophysical Research, v. 100, p. 10,321-10,351. Spencer, J.E., Shafiqullah, M., Miller, R.J., and Pickthorn, L.G., 1989b, K-Ar geochronology of Miocene extensional tectonism, volcanism, and potassium metasomatism in the Buckskin and Rawhide Mountains, in Spencer, J.E., and Reynolds, S.J., eds., Geology and mineral resources of the Buckskin and Rawhide Mountains, west-central Arizona: Arizona Geological Survey Bulletin 198, p. 184-189. Wilkins, J., Jr., and Heidrick, T.L., 1982, Base and precious metal mineralization related to low-angle tectonic features in the Whipple Mountains, California and Buckskin Mountains, Arizona, in Frost, E.G., and Martin, D.L., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado River region, California, Arizona, and Nevada: San Diego, Cordilleran Publishers, p. 182-203. Wilkins, J., Jr., Beane, R.E., and Heidrick, T.L., 1986, Mineralization related to detachment faults: A model, in Beatty, Barbara, and Wilkinson, P.A.K., eds., Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona Geological Society Digest, v. 16, p. 108-117.

Figure 3 (Stop 3). Geologic map showing road access to Stop 3 where we will look at conglomerate in the upper part of the tilted Miocene stratigraphic section. Mylonitic debris appears up section and indicates the degree of tectonic exhumation that occurred during sedimentation of this unit.

Figure 4 (Stops 4 and 5). At stop 4 we will walk up section to the southwest to look at the stratigraphy of Miocene basal arkosic sandstone and conglomerate and overlying sandstone, siltstone, and limestone. At stop 5 we will look at a rock avalanche breccia that is within mafic lava flows. The breccia contains probably Jurassic volcanic rocks that were altered and metamorphosed (Table 1) before incorporation into the Miocene rock avalanche deposit.

Stop 5. Aluminous metasomatic rocks. At Stop 5 we will see a small example of aluminous metasomatic mineralization that has been identified in perhaps a dozen mountain ranges in southwestern Arizona and southeastern California. Typical assemblages are quartz-kyanite, quartz-pyrophyllite, and quartz- muscovite. Accessory minerals include K-feldspar, magnetite, andalusite, sillimanite, tourmaline, rutile, 2+ ilmenite, biotite, lazulite ((Mg,Fe )Al2(PO4)2(OH)2), apatite, dumortierite (Al7BO3(SiO4)3O3), staurolite 2+ (Fe 2Al9O6(SiO4)4(O,OH)2), svanbergite (SrAl3(PO4)(SO4)(OH)6), topaz (Al2SiO4(F,OH)2), and pyrite (Reynolds et al., 1988). Host rocks commonly consist of Jurassic volcanic rocks. These unusual mineral assemblages are thought to result from Jurassic alteration by acidic hydrothermal solutions that leached mobile elements and left behind clays and immobile elements such as aluminum, titanium and phosphorous. Later metamorphism of these altered rocks yield the unusual mineral assemblages.

Reynolds, S.J., Richard, S.M., Haxel, G.B., Tosdal, R.M., and Laubach, S.E., 1988, Geologic setting of Mesozoic and Cenozoic metamorphism in Arizona, in Ernst, W.G., ed., Metamorphism and crustal evolution of the western United States (Rubey volume 7): Englewood Cliffs, New Jersey, Prentice Hall, p. 466-501. Table 1 (Stop 5): Electron microprobe data from Ken Domanic (UA Lunar and Planetary Laboratory) (N Plomosa altered rocks collected by Jon Spencer, 2013, at stop 5)

Thin section 2 27 13 – 1 Thin section 2 27 13 – 5A

Main Groundmass Minerals Main Groundmass Minerals

Kaolinite (Al2Si2O5(OH)4) – 57% Kaolinite (Al2Si2O5(OH)4) – 40% Quartz – 32% Quartz – 37% Mixed Clay Minerals – 5% Major Accessory Minerals Hematite – 4% (relatively large crystals) Major Accessory Minerals

Rutile (TiO2) – 3% (small scattered clusters) Hematite – 5% (relatively large crystals)

Topaz (Al2SiO4(F,OH)2) – 2% (in veins) Rutile – 3% (small scattered clusters) Muscovite 5% (altering to phyrophyllite) Minor Accessory Minerals Topaz – 3% (in veins and groundmass)

Svanbergite (SrAl3(PO4)(SO4)(OH)6) – 1% (Interstitial to Kaolinite) Minor Accessory Minerals Zircon – <1% (Small crystals scattered in groundmass, high Svanbergite – <1% (Interstitial to Kaolinite) Hf and probably Pb) Zircon - <1% (small crystals scattered in groundmass, rich in Hf and probably Pb) Barite - <1% (rare, large crystals) Thin section 2 27 13 – 2 Thin section 2 27 13 – 5B Main Groundmass Minerals

Alunite (KAl3(SO4)2(OH)6) – 41% Main Groundmass Minerals

Quartz – 42% Pyrophyllite (Al2Si4O10(OH)2) – 48% Quartz – 33% Major Accessory Minerals Hematite – 4% (relatively large crystals) Major Accessory Minerals Rutile – 3% (small scattered clusters) Hematite – 4% (relatively large crystals)

Topaz (Al2SiO4(F,OH)2) – 4% (in veins and groundmass) Rutile – 3% (small scattered clusters) Diaspore (AlO(OH)) – 3% (in veins) Muscovite 4% (altering to phyrophyllite) Topaz – 2% (in veins and groundmass) Minor Accessory Minerals Svanbergite 4% (relatively large crystals with variable Sr)

Svanbergite (SrAl3(PO4)(SO4)(OH)6) - <1% (diffuse patches in Alunite) Minor Accessory Minerals Zircon - 1% (Small crystals scattered in groundmass, rich in Zircon - <1% (small to medium sized crystals scattered in Hf and probably Pb) groundmass rich in Hf and probably Pb) Kaolinite <1% Barite - <1% (rare, large crystals)

DAY 2: Bouse Formation in the southern Blythe basin

Figure 5. General location map for Day 2. See Fig. 7 for details of Stops 1 and 2 in the Hart Mine Wash area.

Introduction

On Day 2 we will be visiting several outcrops of the controversial Bouse Formation in the southern part of the Blythe basin. Bouse deposits were first systematically investigated and formally defined as a geologic formation as part of USGS geohydrology investigations along the lower Colorado River (Metzger, 1968; Metzger et al., 1973; Metzger and Loeltz, 1973). These investigations described the basic characteristics of the Bouse Formation, including: (1) basal carbonate deposits including limestone, tufa (travertine) and marl; and (2) much thicker fine-grained siliciclastic deposits consisting of interbedded clay, silt, and quartz-rich sand that are always stratigraphically above the basal carbonate. Based primarily on interpretations that some of the microfauna found in southern Bouse deposits were marine organisms (Smith, 1970), and to a lesser degree on interpretations of sedimentary environments (e.g., Buising, 1990), the Bouse Formation was interpreted to be entirely marine/estuarine. Based on this interpretation, the Bouse was regarded as evidence for a substantial Neogene marine incursion up the Colorado River Valley generally associated with rifting in the Gulf of California/Salton Trough. Since Bouse deposits are found as high as 560m above sea level in northern Mohave Valley, this was used as evidence for Pliocene-Quaternary uplift of the entire region, increasing to the north (Lucchitta, 1979). These views were later challenged by analyses of Sr ratios from the Bouse carbonate, which are quite close to modern Colorado River water and are not close to marine values (Spencer and Patchett, 1997). They proposed an alternative hypothesis wherein the Bouse Formation was deposited in a series of progressively lower lakes filled by the developing Colorado River as it made its way south. They suggested that apparently marine fauna in the southern Bouse (Blythe) basin were introduced by avian transport from the nearby Gulf of California into a large, saline lake. In this model, no Pliocene- Quaternary uplift is required to explain the modern altitudes of the Bouse Formation. Much recent work has supported and fleshed out the lacustrine model (Spencer et al., 2008; House et al., 2008; Roskowski et al., 2010; Spencer et al., 2013; Pearthree and House, 2014); but some recent paleontological studies have again concluded that the southern Bouse Formation was deposited all or in part in an estuary (Miller et al., 2013; McDougall and Miranda-Martinez, 2014). Current Ph.D. research by Jordon Bright at the University of Arizona is attempting to reconcile the apparently contradictory implications of paleontology and isotope values.

We have recently proposed a more elaborate model of downstream-spilling lakes that accommodates the relative volumes of water and sediment supplied by the Colorado River (Pearthree and House, 2014; Fig. 6). The volume of water supplied annually by the Colorado River to this region is orders of magnitude greater than the sediment transported by the river, so when the river spilled through the Lake Mead area and into the formerly closed basins of the lower Colorado River Valley, each basin would have filled relatively rapidly with water (depending on the size of the basin, of course). Once the basin filled to a spillover point, it would have begun to fill the next lower basin and erode the spillover. This would have resulted in a progressive lowering of the water surface of the upper lake as the next lower lake increased in size. As long as the upstream lake existed, it trapped nearly all of the siliciclastic sediment supplied by the river; thus, the downstream lake or lakes were dominated by carbonate deposition. A delta was deposited at the northern end of the upper lake and gradually built southward as the spillover elevation lowered, filling the lake with sediment as it diminished in size and volume. Eventually, the lowering spillover met the level of the siliciclastic sediment filling the lake, resulting in the demise of the upper lake. After that time, continued lowering of the spillover would have resulted in

15 erosion and recycling of siliciclastic sediment into the next basin downstream. At any particular locality in the Bouse basins, the basal limestone, tufa, and marl reflect deposition in clear water conditions, prior to the arrival of siliciclastic sediment. Carbonate deposition would have occurred throughout the Blythe basin as long as the upstream lakes existed, and would have persisted for much longer in areas far from the input of river sediment such as the Cibola area. Carbonate deposition was shut off in any particular area by the arrival of substantial siliciclastic deposits, initially very fine-grained suspended sediment (pro-delta deposits), and eventually sand-dominated proximal delta deposits.

The other major active controversy regarding the Bouse Formation is its age and the timing of arrival of Colorado River sand in the Salton Trough. Identification of a 5.5-5.6 Ma tephra in deposits beneath the Bouse Formation constrains the age of the northern Bouse deposits to less than 5.5 Ma (House et al., 2008). In the Blythe basin, the 4.83 Ma Lawlor Tuff has been found in two widely spaced localities just below the highest Bouse outcrops, apparently providing a fairly tight age constraint for the time of maximum high water (Spencer et al., 2013). Dorsey et al. (2011) have developed an alternative chronology for a thick sequence of primarily Colorado River-derived sediment in the Salton Trough, based primarily on 2 dated tephra deposits high in the section and paleomagnetic reversal stratigraphy. Based on their analyses, they estimate an age of 5.35 Ma (Miocene-Pliocene boundary) for the first arrival of Colorado River sand in the Salton Trough. In order to explain this apparent age discrepancy, Dorsey (written comm.) has proposed that the Colorado River was dammed or diverted to some other area for several hundred thousand years. Discovery of a new subunit of the Bouse Formation (named the “upper bioclastic limestone” by Homan, 2014) has been used to develop an argument that there were 2 periods of inundation associated with the southern Bouse basin separated by hundreds of thousands of years. Based on further mapping and investigations that we have conducted in 2015, we suggest that this upper bioclastic unit records the erosion and redeposition of carbonate material derived from Bouse deposits left draped on the landscape as the southern Bouse lake was draw down and eventually ceased to exist.

Today we will visit a very small sample of outcrops of the Bouse Formation in the Hart Mine Wash area that nonetheless gives a reasonable flavor for the character and variety of Bouse deposits in the Blythe basin.

Figure 6. Schematic longitudinal sections oriented N-S along the lower Colorado River Valley, illustrating the new and shinier lacustrine model of Bouse deposition. The upper illustration shows the maximum and minimum altitudes of Bouse deposition in each basin, showing how the general concept of downstream-spilling lakes could work. Numbered diagrams show a proposed temporal sequence from (1) the time when the northernmost lake was spilling, the spillover was lowering, the lake was filling with sediment, and the southern basin was filling with water but receiving no siliciclastic sediment from upstream; to (4) when the spillover of the southern basin met the rising level of siliciclastic sedimentation and the Bouse lakes were no more. Panel 5 shows the deep erosion that immediately followed Bouse deposition, prior to massive river aggradation recorded as the Bullhead Alluvium (panel 6). Modified from Pearthree and House (2014).

References Cited

Bright, J., Cohen, A.S., Dettman, D.L., Pearthree, P.A., Dorsey, R.J., and Homan, M.B., in review, Catastrophic lake spillover integrates the late Miocene-early Pliocene Colorado River and the Gulf of California: microfaunal and stable isotope evidence from Blythe basin, California-Arizona, USA: Palaios.

Buising, A.V., 1990, The Bouse Formation and bracketing units, southeastern California and western Arizona: Implications for the evolution of the proto-Gulf of California and the LCR, Journal of Geophysical Research, v. 95, pp. 20,111-20,132.

Dorsey, R.J., Housen, B.A., Janecke, S.U., Fanning, C.M., and Spears, A.L.F., 2011, Stratigraphic record of basin development within the San Andreas fault system: Late Cenozoic Fish Creek–Vallecito basin, southern California: Geological Society of America Bulletin, v. 123, p. 771-793; doi:10.1130/B30168.1

House, P.K., Pearthree, P.A., and Perkins, M.E., 2008, Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona, in Reheis, M.C., Hershler, R., and Miller, D.M., eds., Late Cenozoic drainage history of the southwestern Great Basin and lower Colorado River region: geologic and biotic perspectives: Geological Society of America Special Paper 439, p. 335-353; doi: 10.1130/2008.2439(15).

Howard, K.A., House, P.K., Dorsey, R.J., and Pearthree, P.A., 2015, River-evolution and tectonic implications of a major Pliocene aggradation on the lower Colorado River: The Bullhead Alluvium: Geosphere; February 2015; v. 11; no. 1; p. 1–30; doi:10.1130/GES01059.1

Kimbrough, D.L., Grove, M., Gehrels, G.E., Dorsey, R.J., Howard, K.A., Lovera, O., Aslan, A., House, P.K., and Pearthree, P.A., 2015, Detrital zircon U-Pb provenance of the Colorado River: A 5 m.y. record of incision into cover strata overlying the Colorado Plateau and adjacent regions: Geosphere, v. 11, no. 6, p. 1–30, doi:10.1130/GES00982.1.

Lucchitta, I., 1979, Late Cenozoic uplift of the southwestern Colorado Plateau and adjacent LCR region: Tectonophysics, v. 61, p. 63-95.McDougall and Miranda-Martinez, 2014

McDougall, K., and Miranda Martinez,A.Y., 2014, Evidence for a marine incursion along the lower Colorado river: Geosphere; v. 10; no. 5; p. 842–869; doi:10.1130/GES00975.1.

Metzger, D.G., 1968, The Bouse Formation (Pliocene) of the Parker-Blythe-Cibola area, Arizona and California: U.S. Geological Survey Professional Paper 600-D, scale 1:603,000, p. D126-D136.

Metzger, D.G. and Loeltz, O.J., 1973, Geohydrology of the Needles area, Arizona, California, and Nevada: U.S. Geological Survey Professional Paper 486-J, 54 pp.

Metzger, D.G., Loeltz, O.J., and Irelan, B., 1973, Geohydrology of the Parker-Blythe-Cibola area, Arizona and California: U.S. Geological Survey Professional Paper 486-G, 130 p.

Miller, D.M., Reynolds, R.E., Bright, J.E., and Starratt, S.W., 2013, Bouse Formation in the Bristol basin near Amboy, California: Geosphere, v. 10, p. 462-475, doi: 10 .1130 /GES00934 .1 .

Pearthree, P.A., and House, P.K., 2014, Paleogeomorphology and evolution of the early Colorado River inferred from relationships in Mohave and Cottonwood valleys, Arizona, California, and Nevada: Geosphere, v. 10; p. 1139–1160; doi:10.1130/GES00988.1.

Roskowski, J.A., Patchett, P.J., Spencer, J.E., Pearthree, P.A., Dettman, D.L., Faulds, J.E., and Reynolds, A.C., 2010, A late Miocene-early Pliocene chain of lakes fed by the Colorado River: Evidence from Sr, C, and O isotopes of the Bouse Formation and related units between Grand Canyon and the Gulf of California: Geological Society of America, v. 122, n. 9/10, p. 1625-1636; doi: 10.1130/B30186.1.

Spencer, J.E., and Patchett, P.J., 1997, Sr isotope evidence for a lacustrine origin for the upper Miocene to Pliocene Bouse Formation, lower Colorado River trough, and implications for timing of Colorado Plateau uplift: Geological Society of America Bulletin, v. 109, p. 767-778.

Spencer, J.E., Patchett, P.J., Pearthree, P.A., House, P.K., Sarna-Wojcicki, A.M., Wan, E., Roskowski, J.A., and Faulds, J.E., 2013, Review and analysis of the age and origin of the Pliocene Bouse Formation, lower Colorado River Valley, southwestern USA: Geosphere, v. 9, p. 444–459, doi: 10 .1130 /GES00896 .1 .

Spencer, J.E., Pearthree, P.A., and House, P.K., 2008, An evaluation of the evolution of the latest Miocene to earliest Pliocene Bouse lake system in the lower Colorado River valley, southwestern USA, in Reheis, M.C., Hershler, R., and Miller, D.M., eds., Late Cenozoic drainage history of the southwestern Great Basin and lower Colorado River region: geologic and biotic perspectives: Geological Society of America Special Paper 439, p. 375–390; doi: 10.1130/2008.2439(17).

Figure 7. Geologic map of the Hart Mine Wash area, showing approximate path of our “Bouse hike” (Stop 1) and Stop 2 to take a look at the deposits that culminated the period of Bouse deposition.

Stop 1. Hart Mine Wash – Here we will walk across some middle to late Pleistocene alluvial fan deposits and drop into Hart Mine Wash (HMW) from the south. Once we get to the southern edge of HMW we will walk downstream to the west through a fairly complete section of the various facies of the Bouse Formation. Initially we will see the alluvial fan deposits that commonly underlie the basal Bouse carbonate deposits, with some examples of obvious reworking of the fan deposits in a near-shore environment (Figure 8). The first few meters of these deposits commonly are orange-stained or green-

19 tinged, presumably resulting from inundation associated with the Bouse – we have informally called this the “golden gravel”. We will then walk by a complete transgressive carbonate section, including nearshore bioclastic-rich limestone and tufa, and quieter/deeper water very fine marl facies. Marl beds transition upward into greenish mudstone, with several meters of interbedded marl and mudstone beds. The arrival of significant amounts of very fine siliciclastic material presumably signals the beginning of distant pro-delta deposition. We will look at a particular part of this transition where Jordon Bright has found evidence for fairly dramatic changes in stable isotope compositions in fauna and in the sediment, which he is suggesting is evidence for the initial spillover of the southern Bouse lake basin (Bright et al., in review). Fine siliciclastic (clay, then silt and clay) deposits are predominant above the isotopic transition, with fine to medium quartz-rich sand near the top of the section. Detrital zircon analysis of similar quartz-rich sand from Big Fault Wash, ~2.5 km south of this site, has an age spectrum essentially identical to the modern Colorado River and the earliest Colorado River deposits in this region (Kimbrough et al., 2015; Spencer, unpublished data).

Figure 8. Locations of stratigraphic sections along the south side of HMW described by Jordon Bright. We will see all of these outcrops during our hike down HMW. The “distinctive clay layer” marks a dramatic change in stable isotope values in some microfaunal species that Jordon has interpreted as evidence for a transition from a stratified lake to more thoroughly mixed lake.

The second part of our hike will focus on a thicker section of deformed siliciclastic Bouse deposits and the units that immediately overlie it on the north side of HMW. In the western part of this large outcrop, several east- and west-dipping high-angle normal faults displace siliciclastic Bouse deposits, a local fan gravel that rests on an erosional unconformity over the Bouse deposits, overlain by a coarse gravel that includes a small percentage of exotic, well-rounded pebbles and cobbles and quartz-rich sand beds that are characteristic of the Bullhead Alluvium, the earliest definitive river deposits along the lower Colorado River Valley (Howard et al., 2015). Highly deformed fine-grained Bouse siliciclastic deposits are exposed upslope to the east. These contorted beds may be the expression of primary tectonic deformation during Bouse deposition. However, deformation of this sort is common in fine siliciclastic deposits in the Cibola area, and the exposures of intense deformation do not define obvious linear fault zones, so we suspect that these are large subaqueous slump features, possibly triggered by large earthquakes in the general vicinity. As we continue upslope toward our starting point, we will see extensive exposures of more basal Bouse carbonate, and eventually the “golden gravel” re-emerges. Locally, we find obviously rounded local gravel and tufa coatings on boulders indicative of the nearshore or beach environment.

Stop 2. Just north of Hart Mine Road. Our next field stop focuses on exposures of the youngest Bouse deposits, the upper bioclastic unit (UBU, labled Tbz on the geologic map). This unit was first recognized and described by Homan (2014) as the “upper bioclastic limestone”; it is characterized by abundant bioclastic material and possibly one or more species that have not been identified in the lower bioclastic Bouse deposits. Homan (2014) described it as having an erosional contact with underlying Bouse carbonate and siliciclastic deposits. From this relationship, Dorsey and Homan (written communication) inferred a substantial hiatus between deposition of the upper fine siliciclastic Bouse sediments and the upper unit.

Further investigations during 2015 reveal a more complex composition of this unit (>50% carbonate in some areas, but predominantly local gravel with a matrix of bioclastic material in others) and more complex spatial relationships with underlying Bouse units. In the outcrop we visit here, bioclastic-rich beds of UBU are interbedded with fine siliciclastic Bouse deposits. We have found similar interbedded deposits at a number of other localities at mid-range altitudes on the piedmont. We interpret this as indicating that at least some of the deposition of the UBU is contemporaneous with the highest level of fine siliciclastic Bouse deposition. Exposures farther upslope in this dissected landform reveal that the UBU has an erosional relationship with the lower Bouse carbonate higher on the piedmont [If time and interest permit we can visit at least one such exposure]. At lower altitudes closer to the valley axis, the UBU erosionally overlies Bouse fine siliciclastic deposits.

Figure 9. Stratigraphic column, a sequence-stratigraphic column, and a schematic cross-section of the eastern ½ of the Cibola area (developed by Brian Gootee, AZGS).

Thus, we have an apparent paradox of an erosional relationship between the UBU and older Bouse deposits both higher and lower in the paleolandscape, and an interfingering relationship in middle positions. We propose a scenario wherein the UBU is primarily reworked material from the lower Bouse carbonate deposits, which were exposed to erosion as the southern spillover was lowered by erosion and the lake level dropped. Areas high on the piedmont consist of bioclastic-rich UBU, including local siliciclastic material and lots of eroded fragments of Bouse carbonate deposits, in contact with the Bouse carbonate deposits that have been exposed to erosion by the lowering lake level. The areas of interbedded UBU and siliciclastic Bouse represent the last vestiges of the lake. Incision of the basin after the demise of the southern lake resulted in erosion of siliciclastic Bouse deposits in the basin axis, as tributary deposits rich in bioclastic material prograded across them

Figure 10. Stop 3. – Return to Blythe, take I-10 back into Arizona, turn off of I-10 at Tom Wells Road, go south to areas of historic manganese mining (Metate district) where manganese oxides fill fractures and open spaces in rubble beneath Pliocene Bouse carbonates, and also affect the carbonates.

Figure 11. Speculative model for mineralization. Bouse Formation carbonates and siltstone (~5 Ma) covered underlying Miocene alluvial-fan sediments, forming an aquitard. Deposition of Bullhead Alluvium caused sediment compaction below the Bouse Formation and expulsion of basinal fluids that traveled through fractured bedrock at the margins of the basin. Alternatively, changes in the hydrologic regime associated with Colorado River water influx and accumulation of Bullhead Alluvium resulted in expulsion of manganiferous basinal fluids. Basin fluids came into contact with Bouse carbonates at the margins of the basin where fluids could escape. Reaction of mildly acidic solutions with carbonates led to manganese-oxide precipitation. Expelled basin fluids then flowed into Bullhead Alluvium where atmospheric oxygen diffused into the top of the water table and triggered manganese-oxide precipitation within Bullhead Alluvium.