fall 2015 arizona geological society field- trip guide...

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OPEN-FILE REPORT OFR-15-10

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

Jon E. Spencer and Phil A. PearthreeArizona Geological Survey

November 2015

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

Arizona Geological Survey

M. Lee Allison, State Geologist and Director

Manuscript approved for publication in November 2015 Printed by the Arizona Geological Survey

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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.

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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

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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.

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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.

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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.

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Figure 1. Location map for field trip Day 1.

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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).

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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.

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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.

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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.

10

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.

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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.

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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.

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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,

ilmenite, biotite, lazulite ((Mg,Fe2+)Al2(PO4)2(OH)2), apatite, dumortierite (Al7BO3(SiO4)3O3), staurolite

(Fe2+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

Main Groundmass Minerals

Kaolinite (Al2Si2O5(OH)4) – 57%

Quartz – 32%

Major Accessory Minerals

Hematite – 4% (relatively large crystals)

Rutile (TiO2) – 3% (small scattered clusters)

Topaz (Al2SiO4(F,OH)2) – 2% (in veins)

Minor Accessory Minerals

Svanbergite (SrAl3(PO4)(SO4)(OH)6) – 1% (Interstitial to

Kaolinite)

Zircon – <1% (Small crystals scattered in groundmass, high

Hf and probably Pb)

Thin section 2 27 13 – 2


Alunite (KAl3(SO4)2(OH)6) – 41%

Quartz – 42%



Rutile – 3% (small scattered clusters)

Topaz (Al2SiO4(F,OH)2) – 4% (in veins and groundmass)

Diaspore (AlO(OH)) – 3% (in veins)


Svanbergite (SrAl3(PO4)(SO4)(OH)6) - <1% (diffuse patches

in Alunite)

Zircon - 1% (Small crystals scattered in groundmass, rich in

Hf and probably Pb)

Kaolinite <1%

Thin section 2 27 13 – 5A


Kaolinite (Al2Si2O5(OH)4) – 40%

Quartz – 37%

Mixed Clay Minerals – 5%




Muscovite 5% (altering to phyrophyllite)

Topaz – 3% (in veins and groundmass)


Svanbergite – <1% (Interstitial to Kaolinite)

Zircon - <1% (small crystals scattered in groundmass, rich in

Hf and probably Pb)

Barite - <1% (rare, large crystals)

Thin section 2 27 13 – 5B


Pyrophyllite (Al2Si4O10(OH)2) – 48%

Quartz – 33%




Muscovite 4% (altering to phyrophyllite)

Topaz – 2% (in veins and groundmass)

Svanbergite 4% (relatively large crystals with variable Sr)


Zircon - <1% (small to medium sized crystals scattered in

groundmass rich in Hf and probably Pb)

Barite - <1% (rare, large crystals)

14

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.

15

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

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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.

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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.

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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.

19

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-

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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.

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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.

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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

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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.

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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.

fall 2015 arizona geological society field- trip guide...

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