By P. Kyle House, Barbara E. John, Daniel V. Malmon, Debra Block, L. Sue Beard, Tracey J. Felger, Ryan S. 

Crow, Jonathan E. Schwing, and Colleen E. Cassidy

Pamphlet to accompany

Scientific Investigations Map 3411

2018

U.S. Department of the Interior

U.S. Geological Survey

Geologic Map of the Castle Rock 7.5’ Quadrangle, Arizona 

and California

Kayaker's view of Castle Rock (unit Tac) looking north-northeast from Castle Rock Bay, Arizona. The northern Mohave Mountains are 

visible in left background, and sediments of the Chemehuevi Formation (unit Qch) are seen along right edge of photo. Photograph taken 

August 27, 2014, by Kyle House, USGS. 
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

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Suggested citation:

House, P.K., John, B.E., Malmon, D.V., Block, Debra, Beard, L.S., Felger, T.J., Crow, R.S., Schwing, J.E., and Cassidy, C.E., 2018, 

Geologic map of the Castle Rock 7.5’ quadrangle: U.S. Geological Survey Scientific Investigations Map 3411, 15 p., 

1 sheet, scale 1:24,000, https://doi.org/10.3133/sim3411.

ISSN 2329-132X (online)
iii

Contents

Abstract...........................................................................................................................................................1

General Overview of Bedrock Geology......................................................................................................1

General Overview of Surficial Geology......................................................................................................1

Acknowledgments.........................................................................................................................................2

Description of Map Units..............................................................................................................................2

Miscellaneous Deposits ......................................................................................................................2

Piedmont Alluvium of the Mohave and Chemehuevi Mountains..................................................2

Alluvium of the Colorado River and Related Deposits....................................................................5

Basin-fill Sediments (Post-extensional)............................................................................................9

Alluvial and Megabreccia Deposits (Syn-extensional)................................................................10

Cenozoic Volcanic Rocks and Sediments.......................................................................................11

Mesozoic Plutonic Rocks ..................................................................................................................12

Whale Mountain Sequence .....................................................................................................12

Proterozoic Rocks...............................................................................................................................12

References Cited..........................................................................................................................................14

Table

1. Data for microfossil samples collected by P.K. House and analyzed by Kristin McDougall....13

1

Abstract

The Castle Rock quadrangle is in the northeast corner of 

Chemehuevi Valley, California and Arizona. It includes the 

Colorado River’s entrance to the valley at the mouth of Topock 

Gorge and the northern outskirts of Lake Havasu City, Arizona, 

and the Chemehuevi Indian Tribe community of Havasu Lake, 

California. The map includes large parts of the Chemehuevi 

Indian Reservation and the Havasu National Wildlife Refuge. 

Upon its exit through the mouth of Topock Gorge, the Colorado 

River enters Chemehuevi Valley where its floodplain (now 

submerged under Lake Havasu) is flanked by alluvial piedmonts 

of the Chemehuevi and Mohave Mountains to the west and east, 

respectively. This abrupt transition offers a useful perspective 

into the structural evolution of the Colorado River extensional 

corridor and of the Colorado River itself. It contains key 

structural and stratigraphic elements recording a complex history 

of Cretaceous plutonism and deformation, significant tectonic 

extension, volcanism, and sedimentation in the Miocene, and, 

ultimately, the evolution of the Colorado River from the latest 

Miocene to the present. Lake Havasu submerged the axis of 

Chemehuevi Valley following the completion of Parker Dam 

in 1938, and the Colorado River now feeds a verdant delta 

marsh that composes part of the map. Important bedrock units 

include the Cretaceous Chemehuevi Mountains Plutonic Suite, 

the 18.78 Ma Peach Spring Tuff, and thick overlying sequences 

of interlayered Miocene megabreccia and fanglomerate. The 

exposure of these units is closely linked to extension along the 

Chemehuevi-Whipple Mountains detachment fault system. The 

complex bedrock geologic framework serves as the structural 

and topographic foundation for the key strata chronicling the 

evolution of the lower Colorado River. Important stratigraphic 

units that bear on its evolution to the present day include the 

Bouse Formation, the Bullhead Alluvium, and the Chemehuevi 

Formation. The map area also contains the river’s modern delta at 

the head of Lake Havasu.

General Overview of Bedrock Geology

The Castle Rock quadrangle includes many spectacularly 

well exposed rocks that record the effects of tectonic extension 

and unroofing and tectonic denudation of a part of a major 

metamorphic core complex in the lower Colorado River 

extensional corridor. Tectonic extension was accompanied by, 

and interspersed with, regional volcanic activity and accelerated 

sedimentation related to increasing relief, crustal thinning, and 

seismicity (John, 1987b, 1988; Miller and John, 1999; Howard 

and others, 2013).

Rapid tectonic extension in the region began approximately 

18–19 Ma (Howard and others, 2013) around the time of the 

eruption and deposition of the Peach Spring Tuff, an important 

marker bed in the map area and the region. Progressive crustal 

extension along a system of low-angle detachment faults 

resulted in the exhumation of deep-seated Proterozoic and 

Mesozoic crystalline rocks (lower plate rocks) from beneath a 

cover of various sedimentary and volcanic rocks (upper plate 

rocks). Low-angle separation of the upper plate and lower plate 

assemblages was accompanied by high-angle normal faulting 

and related rotation of upper plate rocks. Erosion of upper- and 

lower-plate rocks was rapid during tectonic extension and related 

faulting and seismic activity. Syn-extensional sedimentary 

deposits include rocks that reflect the formation of relief, erosion 

of uplifting rocks, and deposition of sedimentary detritus in 

evolving fault-bounded basins. The syn-extensional sedimentary 

rocks include lacustrine and fluvial siltstones and sandstones; 

alluvial fan conglomerates (fanglomerates); and rock avalanche 

megabrecccias. The fanglomerates and megabreccias are 

interstratified and have lithologic compositions that record the 

progressive denudation of the Chemehuevi Mountains to deeper 

structural levels. As such, their compositions are inverted relative 

to the rocks in the mountain core. Increasingly younger sediments 

are composed of progressively structurally deeper rocks in the 

mountains (John, 1988; Miller and John, 1999; Howard and 

others, 2013). Significant tectonic extension had ceased by the 

late Miocene.

General Overview of Surficial Geology

Surficial geologic deposits in the Castle Rock quadrangle 

encompass all of the mostly undeformed, post-extensional 

sedimentary deposits and carbonate deposits derived from the 

Chemehuevi and Mohave Mountains and the nascent to modern 

Colorado River. The map area contains a suite of surficial strata 

that records the transition of northern Chemehuevi Valley from 

an actively extending intermontane basin into a tectonically 

quiescent valley, flanked by rugged mountains and the alluvial 

fans draining them. The largely post-tectonic influences of the 

early Pliocene arrival of the Colorado River, and its subsequent 

evolution, have dominated the evolution of the Chemehuevi 

Valley landscape.

The pre-Colorado River strata in the valley show decreasing 

tectonic influence over time, so progressively younger strata are 

progressively less deformed. The oldest unit in the map area that 

begins the post-tectonic regime is Tfo (older fanglomerate). Unit 

Tfo includes alluvial fan, wash, and bouldery paleochannel-fill 

deposits. The unit has sedimentary fabrics indicating north-south 

transport of fluvial gravel and sand, but none of them contains 

particles indicative of Colorado River origin. Wave-worked 

sediment, marl, and muds of the Bouse Formation overlie the 

Tfo package of units. The Bouse Formation (unit Tbo) is the 

primordial deposit of the nascent lower Colorado River, and it is 

likely that it was deposited as lacustrine and deltaic deposits into 

standing water, probably a lake but possibly an estuary.

The end of Bouse deposition in the valley was a likely 

response to a major drop in local base level. The newly integrated 

lower Colorado River was directly linked with the proto-Gulf 

of California and deposited the Bullhead Alluvium (unit Tcb). 

Large flood events that deposited boulder and large-cobble 

conglomerates, interspersed with finer grained fluvial deposits of 

cross-bedded sand and pebble gravels, may have accompanied 

the initiation of river integration. The early Pliocene aggradation 

of the Bullhead Alluvium culminated in an alluvial fill as 

thick as 200 m between about 4.5 and 3.5 Ma. High-standing 

remnants of this deposit were not identified in the Castle Rock 

quadrangles, but detrital clasts of Colorado River gravel have 

been identified in unit QTa at relatively high positions in the 
2

landscape. Subsequently, a protracted episode of net incision into 

the fill extended into the late Pleistocene. In this long interval, 

only one early Pleistocene Colorado River stratigraphic unit, 

the alluvium of Palo Verde Mesa (unit Qcp), was deposited and 

tentatively identified in this map area. Otherwise, the long-term 

trend of deep, net incision into the Bullhead alluvial fill was not 

significantly reversed until the deposition of the Chemehuevi 

Formation (unit Qch) in the late Pleistocene (70 ka). PostChemehuevi erosion in the late Pleistocene left behind a series 

of sandy gravel terraces, both cut and fill, which line piedmont 

slopes leading to the shores of Lake Havasu. Late Pleistocene 

erosion possibly continued into the early Holocene and was 

followed by an aggradational episode that culminated in the 

modern floodplain, which has been submerged by the lake for 80 

years. Deposits of the Colorado River delta are its youngest in the 

map area.

Acknowledgments

Mapping was supported by the U.S. Geological Survey 

(USGS) National Cooperative Geologic Mapping Program, the 

USGS Mendenhall Fellowship Program (to D.V. Malmon) and the 

National Science Foundation (grants EAR-9405175 and 1145183 

to B.E. John). Elizabeth Mennow provided support with samples 

and field data organization. Frederick Rivera of the Chemehuevi 

Indian Tribe Dept. of Conservation generously provided boat 

support on Lake Havasu. Keith Howard and Phil Pearthree 

contributed to field observations in the map area by providing field 

notes and participation in some reconnaissance work with House. 

Jonathan Schwing provided field assistance. Howard and Pearthree 

also provided detailed reviews of a preliminary version of the map. 

Stephanie Dudash reviewed the GIS data. Jan Zigler and Claire 

Landowski provided editorial and cartographic assistance.

DESCRIPTION OF MAP UNITS

[Note: Not shown on this map are swarms of Tertiary and Cretaceous(?) dikes that intrude rocks below the 

Chemehuevi detachment fault (LaForge and others, 2017). The dikes are dark greenish gray to black, gray, 

and tan. The dikes are highly variable in composition and texture; range in composition from coarse-grained 

hornblende diorite and olivine gabbro to biotite-bearing dacite porphyry; and are centered in the Chemehuevi 

Mountains Plutonic Suite. Dark-weathering diabase dikes form a major intrusive phase. Diabase dikes are 

typically fine to medium grained and subophitic and contain plagioclase and pyroxene. In the western part of 

the Chemehuevi Mountains, massive, subvertical northeast-trending lamprophyre dikes are common. They 

are characterized by two generations of amphibole, a very heterogeneous texture, and extreme local alteration. 

Intermediate to silicic andesite to dacite dikes are gray to tan weathering, with variable phenocryst mineralogy 

including plagioclase, hornblende, biotite, and potassium feldspar]

MISCELLANEOUS DEPOSITS

d Disturbed ground (recent)—Areas extensively altered by anthropogenic activities and development, including parts of Lake Havasu City, Arizona, and Havasu Lake, California

Qe Eolian sand deposits (Holocene to upper(?) Pleistocene)—Fine- to medium-grained eolian 

sand deposits, mostly active. Very well sorted and unconsolidated. Eolian sand is closely 

associated with the sand-dominated facies of the Chemehuevi Formation (Qchs) and 

typically forms drapes and climbing ramps on remnants of that deposit. Portions of the 

deposits may have also been derived from active washes and the pre-dam floodplain of 

the Colorado River. Relatively large eolian deposits are limited to the Arizona side of 

the map, the predominantly downwind side of the valley, and where the most extensive 

Qchs deposits occur. Includes local and patchy swaths of coppice dunes surrounding 

swaths of creosote bushes and irregular and sparsely vegetated sheets on slopes and in 

small valleys. Dune orientations and coppice-dune morphology indicates predominantly 

southwest to northeast transport. Maximum thickness generally less than 3 m

PIEDMONT ALLUVIUM OF THE MOHAVE AND CHEMEHUEVI MOUNTAINS

Piedmont alluvium includes sediments deposited by ephemeral washes and alluvial fans emanating from 

the Chemehuevi and Mohave Mountains and from erosion of relict fan and terrace landforms on the piedmont. 

This group of units and associated landforms postdate significant tectonic activity in the valley and spans the 

late Miocene to the present, with the bulk of the suite dating to the Holocene and late Pleistocene. Deposits are 

subdivided chronologically based on stratigraphic and morphologic relations with each other and with Colorado 

River deposits. Morphology-based mapping criteria include type of active surface drainage pattern (for example, 

distributary-constructional or dendritic-erosional); depth of dissection by surface drainage; extent and degree of 

smoothing of primary depositional topography; presence and nature of surface pavement development; degree of 

clast varnish development; and soil development, particularly soil carbonate horizon development. Aspects of these 

criteria are time dependent and follow relatively predictable trajectories on piedmont landforms that are not actively 

forming through primary depositional processes. Lithologic composition and sedimentary texture of the deposits 
3

and their substrates affect their morphological development; for example, piedmont deposits in the map area that are 

underlain by sediment derived from volcanic rocks typically support more stable surfaces over time than do areas 

underlain by sediment derived from coarsely crystalline rocks. In general, piedmont deposits underlain by relatively 

thick exposures of moderately soft- and fine-grained sediments of the Chemehuevi and Bouse Formations are less 

stable over time than those overlying more indurated strata.

Qy Young piedmont alluvium, undivided (recent to upper Pleistocene)—Sand, gravel, silt, and rare 

boulder deposits of young washes and alluvial fans on the piedmonts of the Chemehuevi and 

Mohave Mountains. This undivided unit includes deposits of the youngest active alluvial 

systems, relatively young abandoned or infrequently active alluvial surfaces, and some areas of 

young, but abandoned alluvial surfaces. These surfaces and deposits are characterized by active 

or relict distributary drainage patterns

Qy2 Active piedmont alluvium (upper to middle Holocene)—Sand, gravel, silt, and rare boulder 

deposits of active washes and alluvial fans on the Chemehuevi and Mohave Mountains 

piedmonts. Sediments are poorly to moderately sorted and bedded, subangular to 

subrounded, and locally derived. Surfaces have pristine bar-and-channel to weakly subdued 

bar-and-swale topography. Bar-and-channel topography can have up to 1.5 m of relief. 

Surfaces are moderately vegetated overall with both dispersed and aligned concentrations 

of desert riparian trees and shrubs. Desert varnish is mostly lacking and deposit surfaces 

are generally light toned in the field and in aerial imagery. Soil development is also mostly 

lacking in these deposits, but cryptogamic crust may be present on older bars and swales 

that impart a slightly dark tone. Surfaces of active alluvial fans have distinctive distributary 

patterns in aerial imagery. Surfaces of active washes are typically braided. Broadly 

correlative to unit Q4 of Bull (1991) and Dickey and others (1980). Thickness estimated to 

be 1 to 3 m

Qy1 Inactive piedmont alluvium (middle Holocene to upper Pleistocene)—Sand, gravel, silt, and 

rare boulder deposits of relatively young, inactive washes and alluvial fans on piedmonts 

of the Chemehuevi and Mohave Mountains. Sediments are poorly to moderately sorted 

and bedded, subangular to subrounded, and locally derived. Relict depositional fabric is 

discernible on most surfaces of this unit. Minimal to no evidence of recent fluvial activity 

is present. Surfaces have obvious, but somewhat muted, bar-and-channel to bar-and-swale 

topography with up to 1 m relief. Surface clasts commonly are weakly to moderately 

varnished with a light- to moderate-brown tint. Deposit surfaces are sparsely vegetated 

relative to Qy2. Soil development is weak. Stage 1 carbonate horizons (for example, 

Machette, 1985) are common, and incipient Av horizons are present in loosely paved 

swales. Unit is dispersed over large extent of the Chemehuevi Mountains piedmont in a 

pattern suggesting an original distribution similar to Qy2. Remnants are located adjacent 

to active washes, are dispersed within broad active flow swaths, and span the extent of 

abandoned flow swaths. Age uncertain but surface character suggests latest Pleistocene 

through middle Holocene. May correlate to units Q2c and Q3a of Bull (1991), whose age 

is estimated to be between 11 and 2 ka, primarily based on soil and surface characteristics. 

Unit may be equivalent to parts of the young alluvium (Q3) of Dickey and others (1980) 

and (or) unit 3 (Qs3) of Howard and others (1999). Typically, 1–4 m thick above active 

washes; greater thicknesses are probably concealed

Qi Intermediate-age piedmont alluvium, undivided (lower Holocene(?) to middle(?) 

Pleistocene)—Unconsolidated to weakly consolidated sand, gravel, silt, and rare 

boulder deposits of terraces and alluvial fans on the piedmonts of the Chemehuevi and 

Mohave Mountains. Sediments are poorly to moderately sorted and bedded, subangular 

to subrounded, and locally derived. Intermediate-age surfaces include relict alluvial fan 

and fan terraces that are isolated from active, constructional fluvial processes but do host 

local, typically dendritic drainage networks. Three divisions of this unit are present in this 

map: Qi2, Qi3, and Qi4. Only Qi2 definitively predates the Chemehuevi Formation

Qi4 Unit 4 (lower Holocene(?) to upper Pleistocene)—Sand, gravel, silt, and rare boulder 

deposits of inactive washes and alluvial fans on the Chemehuevi and Mohave Mountains 

piedmonts. Sediments are poorly to moderately sorted and bedded, subangular to 

subrounded, and locally derived. Weak soil development with moderate Bw horizons and 

Stage I to II soil carbonate horizon development. This unit postdates unit Qcr3 and cuts 

and is inset against Qi3 and older units in most locations. Moderate varnish on clasts. 

Moderate gravel lags to weak pavements particularly in swales. Depositional topography 

weakly to moderately muted; coarse bars and weakly to moderately paved swales are 
4

evident in field. Equivalent to Qi3c on Arizona Geological Survey maps. Unit Qi4 is not 

included as subunit of Qi3 though its morphologic characteristics are similar to Qi3a

and Qi3b and the three units can be difficult to distinguish in upper piedmont areas 

where their topographic separation can be minor. Qi4 is generally similar to Q2b and Q3 

of Dickey and others (1980), unit Qs3 of Wilshire and Reneau (1992) and Howard and 

others (1999), and unit Qi3c of Gootee and others (2016)

 Qi4b Unit 4b (lower Holocene(?) to upper Pleistocene)—Weakly consolidated to 

unconsolidated, poorly sorted and bedded, subangular to subrounded locally derived 

sand, pebbles, cobbles, and rare boulders. Deposit surfaces have subdued bar-and-swale 

topography and distinct pinnate texture in aerial imagery. Surface clasts have moderately 

dark varnish and swales between bar crests have moderate to strongly developed 

pavements. Deposits have stage I to II pedogenic carbonate morphology. Correlative 

to unit Q3b of Dickey and others (1980), unit Q3b of Wilshire and Reneau (1992) and 

Howard and others (1999), and units Q2b and Q2c of Bull (1991)

Qi4a Unit 4a (upper Pleistocene)—Weakly consolidated to unconsolidated, poorly sorted 

and bedded, subangular to subrounded locally derived sand, pebbles, cobbles, and rare 

boulders. Surfaces show minimal primary depositional topography with well-developed, 

strongly varnished pavements. Unit Qi4a deposit surfaces resemble those of Qi3a and 

Qi3b but cut them and deposits of unit Qch in middle to lower piedmont area. Deposits 

have stage II to III pedogenic carbonate morphology. Correlative to unit Q1b of Dickey 

and others (1980), unit Qi3a of Wilshire and Reneau (1992), and units Q2b and (or) Q2c 

of Bull (1991)

Qi3 Unit 3 (upper Pleistocene)—Poorly sorted, poorly bedded, subangular to subrounded 

locally derived sand, pebbles, cobbles, and rare boulders. Deposits are as thick as 20 m, 

but typically less. Unit may contain rare pebbles from older Colorado River deposits. 

Division of unit Qi3 into two subunits is made on the basis of observable relations to the 

Chemehuevi Formation but is an inference in some places. Subunit Qi3a appears to be 

roughly coeval with peak aggradation of Chemehuevi sediments. Near the north edge 

of the Arizona portion of the quadrangle, Qi3a surfaces may be coeval with the highest 

strata of Qch on the piedmont (approximately 205–210 m in that area). In contrast, 

Qi3b alluvium truncates Qch deposits such that pristine surface remnants atop unit Qch

extend well below the 205–210 m level. Undivided variant (Qi3) of this unit shown in 

areas where difficult or impractical to differentiate subunits. Morphological variability is 

minor and soil development is similar between the subunits. On the Mohave Mountains 

piedmont, Qi3 deposits are smooth with well-developed and darkly varnished pavements. 

On the central to southern Chemehuevi Mountains piedmont, unit Qi3 surfaces are 

slightly to moderately degraded. Tight desert pavements are common on well-preserved 

surfaces. Qi3 deposits host moderately developed soils with stage II to III pedogenic 

carbonate morphology. Unit may be equivalent to parts of the intermediate alluvium 

(Q2a and Q2b) of Dickey and others (1980) and (or) unit 2 (Qs2) of Wilshire and Reneau 

(1992) and Howard and others (1999)

Qi3b Unit 3b (upper Pleistocene)—Weakly consolidated to unconsolidated, poorly sorted 

and bedded, subangular to subrounded locally derived sand, pebbles, cobbles, and rare 

boulders. Surfaces have well-developed, strongly varnished pavements, are mostly to 

completely lacking original depositional bar-and-swale topography, and exhibit stage 

II to III pedogenic carbonate morphology. Locally includes a series of similar, inset fan 

lobes in some areas, for example, south of Trampas Wash where a slightly higher and 

detached lobe is shown as Qi3. This unit is distinctly inset to unit Qi3a in middle and 

lower piedmont areas. Pristine surfaces of this unit extend well below the top of the 

Chemehuevi Formation and may be graded to unit Qcr3, though the youngest variants 

may postdate it somewhat. Thickness varies considerably from about 1 to at least 8 m. 

This unit is equivalent to parts of the intermediate alluvium (Q2a and Q2b) of Dickey and 

others (1980); unit Qs2b of Wilshire and Reneau (1992) and Howard and others (1999)

Qi3a Unit 3a (upper Pleistocene)—Weakly consolidated to unconsolidated, poorly sorted 

and bedded, subangular to subrounded locally derived sand, pebbles, cobbles, and rare 

boulders. Surfaces have well-developed, strongly varnished pavements lacking original 

depositional bar-and-swale topography and exhibiting stage II to III pedogenic carbonate 

morphology. Unit is interbedded with the Chemehuevi Formation (unit Qch) at various 

levels and may also postdate that unit’s climactic aggradation since it extends slightly below 
5

the elevation of the highest Qch remnants in the map area. This unit is equivalent to parts 

of the intermediate alluvium (Q2a and Q2b) of Dickey and others (1980), and unit 2 (Qs2a) 

of Wilshire and Reneau (1992) and Howard and others (1999). Thickness as much as 20 m. 

Younger parts of unit may be graded to Colorado River unit

Qi2 Unit 2 (upper middle to middle(?) Pleistocene)—Relatively high standing fan and fan 

terrace remnants composed of weakly consolidated to unconsolidated, poorly sorted, 

poorly bedded, subangular to subrounded sand, gravel, silt, and rare boulders of local 

origin. Surfaces are smooth overall with rough pavements and darkly varnished clasts. 

Pavements have moderately to strongly weathered protruding cobble clasts and related 

fragments, overturned pebble and cobbles, and some carbonate detritus. Deposits have 

soils with up to stage III pedogenic carbonate. Correlative to unit Q1b of Dickey and 

others (1980) and tentatively to older parts of unit Qs2a of Wilshire and Reneau (1992) 

and Howard and others (1999)

QTa Older piedmont alluvium (middle Pleistocene to upper Pliocene(?))—Poorly sorted, poorly 

bedded silt, sand, gravel, and rare to locally common boulders of local origin. Clasts are 

angular to subrounded. Contains rare clasts of reworked Bouse limestone and at least one 

exposure contains rare, rounded quartzite pebbles probably reworked from Tcb. Weakly 

indurated in some areas, poorly indurated in most. Preserved mainly as a relatively 

thin, sometimes ridge-like mantle (up to 5 m) atop remnants of units Tfo and Tgf near 

the Chemehuevi Mountains front. Unit QTa also cuts across small remnants composed 

of several facies of the Bouse Formation that onlap underlying Tfo sediments. Basal 

contact with them is erosional and has basal cobble conglomerate at some localities. 

Heavily eroded fan surfaces lack pavement or clear pedogenic carbonate morphology, 

but varnished clasts and patches of retrograde pavement are present on some sloping 

surfaces. Deeply weathered clasts (mainly granitic) are relatively common in loose 

deposits atop ridge crests. May be equivalent to parts of unit C of Metzger and Loeltz 

(1973); unit QT1 of Dickey and others (1980); unit QTa of Miller and John (1999); and 

unit QTs1 of Wilshire and Reneau (1992) and Howard and others (1999). Thickness at 

least 10 m

ALLUVIUM OF THE COLORADO RIVER AND RELATED DEPOSITS

Qcbd Alluvium of Blythe, Lake Havasu deltaic facies (1938 to present)—Fine sand, silt, and clay 

deposited by the modern Colorado River at the head of Lake Havasu. Supports a delta 

marsh containing low-lying fluvial islands with transient boundaries, isolated backwaters, 

and extensive grassy marshes. Thickness of unit probably as much as 12 m based on 

outcrop locations and pre-dam valley-bottom elevations. Unit also includes fine-grained 

deposits at tributary mouths and small embayments inundated by the lake; these areas may 

not contain significant amounts of Colorado River sediment

Qcr Riverside terrace sediments, undivided (upper Pleistocene)—Unconsolidated Colorado River 

sand and gravel deposits that constitute all or parts of a discontinuous suite of fluvial 

terraces. Composed of minor fill terraces (up to 6 m), thin gravelly and sandy strath veneers, 

and vanishingly thin deposits or continuous lags of rounded gravel. Terrace sediments have 

Stage II to III carbonate soil horizons and degraded rhizolith-bearing horizons are present 

on some older terraces. Large terrace remnants have flat surfaces that slope gently downvalley. Surfaces support moderate to very well developed pavements of moderately to 

very darkly varnished, well-rounded pebbles and cobbles. Some Qcr deposits may include 

older, exhumed river-lain strata and thin layers of locally reworked gravels. Surfaces of 

Riverside terraces (Qcr3, Qcr2, Qcr1) postdate the climactic aggradation of the Chemehuevi 

Formation, and pre-reservoir topographic maps indicate that additional terraces are present 

below the surface of Lake Havasu in the map area. The unit has three numbered subunits in 

order of deposition and decreasing elevation above the lake. Equivalent to young deposits 

grouped in unit QTr in Dickey and others (1980); unit Qrd of Howard and others (1999); 

and Qcts of Gootee and others (2016), House and others (2008), House and Faulds (2009), 

and Spencer and others (2015).

Qcr3 Terrace 3—Unconsolidated to moderately consolidated deposits of sandy, fine- to mediumgrained gravel and sand of the Colorado River. Terrace surfaces have tight, ornate 

pavements of moderately to very darkly varnished Colorado River gravel clasts. Iridescent 

varnish is common on these clasts. Calcified wood fragments and rhizoliths are present, 

though rare. Unit ranges in elevation from 151 to 157 m along its extent in the map area. 
6

The large flat surfaces on the California shore of the lake in the southeast corner of the map 

are the most extensive examples. There, unit Qcr3 ranges from vanishingly thin to as thick 

as 8–10 m of Colorado River sand and gravel overlying boulder conglomerate (unit QTcb), 

Bullhead Alluvium (Tcb), Bouse Formation (Tbo), and late Miocene fanglomerate (Tfo). 

The thickness of the unit appears to increase toward the south end of the terrace where it 

may approach 10 m

Qcr2 Terrace 2—Thin, moderately indurated deposits of sandy pebble-cobble gravel that overlie 

thick sequences of Qchs or Qch. Locally associated indurated horizons are rich with 

calcified wood and rhizoliths. Forms flat discontinuous surfaces atop unit Qchs at the 

183–189 m level in the Arizona part of the map. A prominent erosional scarp in unit Tfo

occurs at this level on opposite side of the river, but the gravels inset to that scarp appear 

older and exhumed (see unit Qchg)

Qcr1 Terrace 1—Thin, moderately indurated deposits of sandy pebble-cobble gravel that overlie 

thick sequences of Qchs or Qch. Locally associated indurated horizons are rich with 

calcified wood and rhizoliths. Forms flat discontinuous surfaces atop unit Qchs at the 

192–198 m level in the Arizona part of the map. These deposits lie at elevations at least 

10 m below the highest extant Qch and Qchm deposits in the map area. They occur as 

semi-consolidated beds of sandy, rounded Colorado River pebble to small cobble gravels. 

Wind-worn clasts, ventifacts, are relatively common on the surface, as are calcified 

wood fragments and rhizoliths. These surfaces may be exhumed by deflation, but their 

continuity suggests a terrace landform. Unit generally coincides with the downslope 

terminus of Qi3a piedmont deposits. Extent is limited to isolated patches atop thick 

Qchs deposit remnants. Surfaces commonly deflated to carbonate-enriched soil horizons 

or strata rich with rhizoliths and calcified wood. These terrace remnants are possibly 

coeval with piedmont unit Qi3a

Qch Chemehuevi Formation, undivided (upper Pleistocene)—Thick and complex sequence of 

mud, sand, and gravel deposited by the Colorado River in the late Pleistocene. Includes 

coeval interbedded tributary gravel deposits on outer margins of its outcrop extent and 

in some deep exposures near the lake. Also includes thin beds and lenses of far-traveled 

fluvial gravel. Thick and extensive deposits of the Chemehuevi Formation are distributed 

along a 4- to 5-km-wide, north-south-oriented band on the Mohave Mountain piedmont 

in Arizona and as a less conspicuous band that crops out in association with a series of 

scarps on the Chemehuevi Mountains piedmont in California. The stratigraphic base of 

the unit is not exposed in the map area, but well logs near Castle Rock Bay suggest that 

Qch may extend up to 20 m below the level of Lake Havasu and as much as 15 m below 

the historical floodplain as shown on historical topographic maps. Maximum thickness 

in the map area is approximately 105 m. Exposed basal contacts vary considerably in 

elevation, extending below the lake surface in places and spanning up to 10 m of vertical 

relief on paleotopography at several sites. Exposures typically include a distinctive 

stack of two primary facies: a lower, orangish-pink bluff-forming mud-dominated facies 

(Qchm) capped with a lighter-toned, slope-forming sand-dominated facies (Qchs). There 

is substantial (up to 10 m) relief along the basal contact of the sequence in the map area. 

Tephra layers in Chemehuevi Formation sediments upstream of this map area correlate 

reasonably well with tephra in Owens Lake, California, and Walker Lake, Nevada, 

whose ages are constrained to 70–74 ka (Malmon and others, 2012). The Chemehuevi 

Formation includes predominant lithofacies separated by a widespread (regional) 

unconformity. At several sites in this map area, the contact between the mud-dominated 

and sand-dominated facies is associated with indurated and oxidized beds of cross-bedded 

Colorado River sand and gravel. The level of this distinct unconformity within unit Qch

also corresponds to erosional surfaces cut on older, indurated deposits (for example, unit 

Tac near Castle Rock Bay and unit Tfo on the Chemehuevi Mountains piedmont near the 

central part of the map

Qchs Sand-dominated facies—Well-sorted, medium sand, scattered gravel, and discontinuous minor 

mud beds, having high proportion of rounded quartz-sand grains deposited by the ancestral 

Colorado River. This facies is typically weakly consolidated to unconsolidated with 

poorly exposed stratification, but some exposures show meter-scale tabular cross-bedding; 

discontinuous beds of imbricated, rounded pebble and cobble gravel; interbedded lenses of 

locally derived, angular gravel; reddish and pebbly carbonate-rich layers; and discontinuous 

layers of horizontally bedded and laminated silt and clay <0.5 m thick. Found in steep, loose 
7

slopes lying close to angle of repose of well-sorted, dry sand (about 32o). Voluminous unit 

Qchs remnants are only present on the Arizona side of the quadrangle where they probably 

track at least one deep paleochannel course on the east side of the steep bluffs southeast 

along the river from Blankenship Bend. Gravel ventifacts are common on unit Qchs slopes 

and, particularly, on upper surfaces where aeolian activity is focused. This facies was likely 

deposited within active channels of a complexly aggrading and degrading Colorado River. 

Thickness at least 20 m

Qchm Mud-dominated facies—Well-bedded sand, silt, and clay deposited by ancestral Colorado 

River. Commonly consists of discrete beds 5–50 cm thick of alternating layers of mud 

and fine to very fine sand. Some interbeds of fine sand exhibit various styles of crossbedding. Large exposures along the Arizona shore of Lake Havasu form distinct lightbrown to pinkish-brown badlands and bluffs. Mud facies exposed beneath thick deposits 

of sand facies (Qchs) at the apex of Blankenship Bend and near the southeastern corner 

of quadrangle. Deposited within floodplains bordering an aggrading Colorado River. The 

highest remnants of Qchm are on the Arizona side of the river on the flanks of steeply 

dipping flatirons of the Peach Spring Tuff (Tps) near the mouth of Topock Gorge. These 

remnants are as much as 10 m higher than the highest Qchs deposits. Locally as thick as 

15 m. Some exposures of unit resemble mud deposits of the Bouse Formation. A small 

fault in muds mapped as Qchm is present in bluff exposures along Lake Havasu along 

the Arizona shoreline in the southeastern part of the map area

Qchg Gravel facies—Cross-bedded sand and pebble to cobble gravel of the Colorado River, 

moderately to well sorted, commonly oxidized, and loose to moderately indurated. On the 

Arizona side of the river, unit is exposed in bluffs along the Arizona shore of Blankenship 

Bend in the north-central part of the map. There, it separates Qchm from Qchs along 

an erosional unconformity. Exhumed remnants of this oxidized gravelly stratum are also 

present along Qchm ridges extending from the bluffs. Unit also appears as isolated lenses 

higher in the Qchs section, but exposures are poor. Similar exhumed gravels are present 

atop the Castle Rock bluffs along the river’s edge extending south from Blankenship 

Bend. In those areas, the gravel is concordant with fluvial erosion surfaces atop unit Tac. 

Good exposures of this relation are on the north and south sides of Castle Rock Bay. On 

the California side of the map, a thin (<1 m) veneer of darkly varnished and moderately 

to strongly oxidized gravel sits on a flat surface beveled into unit Tfo. Beveled surfaces 

are bounded by a prominent fluvial scarp at approximately 189 m elevation, which is 

coincident with unit Qcr2. Farther south on the Chemehuevi piedmont, several lowerlying fluvial scarps are associated with thin veneers of fluvial gravel interlayered with 

linear bands of finer-grained Qch outcrops. It is possible that these gravels are exhumed 

strata or reworked lags from gravelly intervals of Qchs. However, it is possible (or 

likely) that the gravels include parts of a Qcr-type unit. It remains indeterminate at this 

time. Thickness less than 5 m

Qco Older sand and mud deposits (upper Pleistocene)—Weakly to moderately indurated 

sequence of thin interbedded layers of mud and silt to silty fine sand. Some intervals of 

rhythmically laminated mud and silt are locally present. In the Blankenship Bend area, 

this unit underlies a thick bed of piedmont alluvium that underlies Qch in Blankenship 

Valley. Lithological similarities to unit Qchm (and Tboss) makes identification and 

division uncertain, and the unit may be more extensive than indicated on the map. Unit 

may correlate to Needles beds of Malmon and others (2009); Cottonwood beds of House 

and Faulds (2009); and Riverside beds of Faulds and others (2001) and House and others 

(2005). It is possible that deposits on the east shore of Lake Havasu, ~1 km south of 

Blankenship Point (lat 34.5813° N., long 114.4185° W.), correlate to Bouse Formation 

(unit Tboss), but this is presently indeterminate. Thickness less than 4 m

Qcp Alluvium of Palo Verde Mesa (lower(?) Pleistocene)—Local mantles of darkly varnished and 

well-rounded gravel and sand deposited by the ancestral Colorado River. Present only 

at the extreme southern edge of the California part of this quadrangle as lag drapes 

on irregular, hummocky remnants of fine-grained siliciclastic deposits of the Bouse 

Formation (Tboss). Unit is intact and more extensive in the adjacent quadrangle 

(Havasu Lake 7.5ʹ). Includes rich assortment of pebbles and cobbles originally derived 

from distant sources. Quartzite cobbles very common. Gravel clasts have moderate to 

dark varnish. Slight to moderate reddening of varnished clasts is common. Unit bears 

strong similarities to Bullhead Alluvium gravels in gross appearance, but are associated 
8

with an array of younger fluvial terrace remnants immediately south of the map area in 

the Havasu Lake quadrangle. Treads of those terraces stand approximately 107–110 m 

above Lake Havasu, contain moderately rare clasts of re-worked and rounded petrified 

wood fragments, and are overlain by old piedmont alluvial deposits that postdate the 

Bullhead Alluvium (House, 2016). In the Havasu Lake quadrangle, immediately to the 

south, terraces of Qcp truncate thin tributary fan deposits containing reworked pea gravel 

and small cobbles of the Colorado River. These tributary deposits truncate the Bouse 

Formation (Tboss). Thickness in this map area less than 1 m

QTcb Boulder conglomerate (lower(?) Pleistocene to Pliocene)—Boulder and coarse-cobble-enriched 

deposits of the ancestral Colorado River. Poorly to moderately sorted deposits of locally 

derived, predominantly granitic boulders in a matrix of Colorado River gravel and 

sand from distant sources. Unit appears to contain no far-travelled clasts larger than 

medium cobbles, but includes boulders up to 1.75 m in diameter from local sources. 

Some exposures in the Blankenship Bend area include discontinuous trains of large, 

imbricated and subrounded boulders. Imbrication indicates south, southeast, and east 

directed transport paths that conform to local topographic constraints. Boulder fraction of 

this unit chiefly derived from erosion of underlying boulder fanglomerate units Tfo and 

Tgf. Maximum exposed thickness is about 30 m near Blankenship Bend. Conglomerate 

is mostly flat lying with an irregular and locally deep erosional contact in underlying 

strata. Locally it grades upwards into finer gravel. On the California side of the river 

a similar boulder conglomerate is exposed as patchy remnants atop unit Tfo upslope 

from the prominent scarp mentioned in the description of unit Qchg. These boulders are 

moderately weathered and occupy a landscape position at least 20 m above the highest 

boulders at Blankenship Bend. Clast imbricated at this level indicate a south-southeast 

transport direction. Boulder conglomerate may be correlative to the Conglomerate of 

Laughlin (Faulds and others, 2001) and the Boulder conglomerate of Bat Cave Wash 

(Howard and others, 2015), but these possible correlations are unresolved at this time. 

Relations observed in this quadrangle are consistent with the unit including strata that 

record individual flood events in the lower part of the Bullhead Alluvium

Tcb Bullhead Alluvium (lower Pliocene)—Ancestral Colorado River deposit composed of weakly 

to moderately cemented Colorado River gravel and sand, containing abundant rounded 

quartzite pebbles and cobbles mixed with locally derived sediments. Unit has a distinctive 

cherty pea-gravel-rich facies, a distinctive quartzite-cobble-rich facies, and a polylithic sand 

and gravel facies. The latter is the most common. It is typically light gray to light brown 

with local zones of orangish-yellow oxidation, which may contain petrified wood. The 

gravel fraction contains diverse assemblage of pebbles and cobbles from locations well 

out of the map area and large cobbles to medium boulders derived from local alluvial-fan 

deposits. Sand fraction typically medium to coarse sand (sugary texture common) with 

mixture of well-rounded quartz-rich sand with clear to pink translucent quartz grains mixed 

with well-rounded to subangular far-traveled and locally derived arkosic sands. Deposits 

are moderately to well-sorted. Strong imbrication fabric is common in gravelly intervals, 

while elaborate cross-bedding is common in sandy intervals. Some outcrops show excellent 

sorting and clast structure; others, particularly boulder-rich intervals, are moderately sorted 

and have sandy matrix. Outcrops of the unit are located near the southern edge of the map 

where at least 5 m of tabular bedded cross-stratified pebble-cobble gravels are exposed. 

A local exposure showing a tilted (dipping 10° SW), possibly faulted part of the unit, was 

found at lat 34.5749° N, long 114.4285° W. Pristine and presumably in situ petrified wood 

fragments have been identified in three locations in the quadrangle. One near Jops Harbor, 

Ariz., where the wood is present in a sandy gravel facies containing large rip-ups of Bouse 

Formation marl (Tbocm); the other two in the Blankenship Bend area in stratigraphic 

association with the boulder conglomerate unit QTcb. At Jops Harbor, the unit overlies 

Tbocm and includes large rip-up clasts of that unit within a sandy and pea-gravel-rich 

variant. This unit unconformably overlies Tfo and slightly tilted Tgf at Blankenship Bend, 

where it fills a paleochannel near the apex of the bend. It overlies the Bouse Formation 

(Tbocm and Tbogs) at Clear Bay, California; and it overlies Tac along parts of the top 

of the Castle Rock bluffs between the southeast edge of the map and Castle Rock Bay. 

The latter deposits contain rare rip-up blocks of Tbocm and some small, locally derived, 

boulders. South of Clear Bay, it overlies flat-lying Bouse Formation marl
9

Tbo Bouse Formation, undivided (lower Pliocene)—Well-sorted, well-bedded fine-grained sand, 

mud, and bedded limestone and marl. Underlain at many sites by distinctly oxidized zone 

in older units and, commonly, thin beds of oxidized sand and gravel reworked from those 

units, likely by wave action (Tbogs and Tbogg). Here, the Bouse is divided into two 

facies: a basal carbonate (Tbocm) composed of limestone and marl and a siliciclastic 

facies (Tboss) with interlayered beds of claystone and quartz-rich sand. In the highest 

outcrops in this quadrangle, unit Tbocm overlies relatively thin beds of indurated, 

calcareous, and cross-bedded sands. In these outcrops, the basal sands and the carbonate 

sediments exhibit intricate drapes that conform to irregular topography in underlying 

strata

Tbos Clay, silt, and sand—Moderately to very well sorted and interbedded fine sand, silt, and clay, 

possibly deposited in a delta of the nascent Colorado River. Includes sequences of quasirhythmic, laminated mudstones interbedded with planar-laminated to thinly bedded, 

fine- to medium-grained, rounded, quartz-rich sand. Muds are gypsiferous in places and 

alternate in color from light greenish gray to grayish orange pink. Sand beds are pink to 

pinkish gray and have thin oxidized laminae that are reddish yellow. Sand has abundant 

rounded quartz grains that range from translucent to pinkish. Sands are almost certainly 

derived from the Colorado River. Unit commonly contains some amount of subangular 

to subrounded locally derived sands as well. Paleontological analysis of two samples 

from this unit (see map and table 1) proved barren of microfossils. Maximum exposed 

thickness is about 15 m

Tbocm Marl or limestone—White to gray, thinly bedded to massive limestone, marl, and rare travertine. 

Occurs as onlapping beds on bedrock and fanglomerate. Locally shows obvious draping over 

irregular topography formed on bedrock and older basin deposits. In this quadrangle, the lowest 

Bouse Formation carbonate outcrop (146 m) is at the level of the lake but can be seen to extend 

a few meters below the lake surface and the highest outcrop (299 m) is on the upper piedmont 

of the Chemehuevi Mountains. These outcrops lie 6.7 km apart on a nearly east-west line just 

below the middle of the map. In some of the highest exposures on the Chemehuevi piedmont, 

a thick bed of well-sorted oxidized sand (Tbogs) is found between 1.5 m of marl and several 

meters of siliciclastic sediments (Tbos). At the lowest exposure on the shore of Lake Havasu 

at Clear Bay, flat-lying and cross-bedded, weakly to moderately oxidized yellowish sand 

underlies 2 m of well-bedded limestone. The sands contain rounded quartz grains mixed with 

subangular lithic-rich locally derived sands (Tbogs). Here, the Bouse may interfinger with 

fan gravel containing 10–20 cm rounded mud balls. Marl bed(s) with desiccation cracks and 

gastropod fossils are present at this location (K.A. Howard, U.S. Geological Survey, written 

commun., 2017)

Tbogs Golden sand facies—Moderately well sorted subangular to subrounded and strongly to 

moderately oxidized orange-yellow fine to coarse sand below the basal carbonate of the 

Bouse Formation. Cross-stratification common in good exposures. Rarely observed as 

interbeds within the marl. Commonly associated with Tbogg. Oxidation is common and 

locally pervasive, extending as a zone into underlying strata or as a stain on underlying 

bedrock. Most commonly associated with clastic substrates (fanglomerate or colluvium) 

from which particles were winnowed, milled, and sorted, likely by wave or current action 

(for example, Buising, 1990)

Tbogg Golden gravel facies—Moderately well sorted, subangular to subrounded, and strongly to 

moderately oxidized orange-yellow pebble to cobble gravel below the basal carbonate of 

the Bouse Formation. Cross-stratification common in good exposures. Rarely observed 

as interbeds within the marl. Commonly associated with Tbogs. Oxidation is common 

and locally pervasive, extending as a zone into underlying strata or as a distinct stain on 

underlying bedrock. Most commonly associated with clastic substrates (fanglomerate or 

colluvium) from which particles were winnowed, milled, and sorted by wave or current 

action

BASIN-FILL SEDIMENTS (POST-EXTENSIONAL)

Tfc Paleochannel-fill conglomerate (lower Pliocene to upper Miocene)—Boulder and cobble-rich 

alluvial fill in pre-Colorado River, north- to south-directed paleochannels. This includes 

an approximately 8-m-thick boulder conglomerate filling a distinct paleochannel that is 
10

cut into steeply dipping fanglomerate (Tgf) that forms steep bluffs along the south side 

of Trampas Wash at lat 34.600 N, long 114.454 W. The paleochannel contains locally 

derived, subrounded granitic boulders up to 1.25 m in diameter. Most, if not all, boulders 

are lithologies from the Chemehuevi Mountains Plutonic Suite. No far-travelled lithologies 

typical of Colorado River gravels observed in this unit. Approximate orientation of channel 

suggests south-southeast transport direction (140°). Unit also contains a set of several 

similar but smaller channels that also cut steeply dipping fanglomerate on the south side 

of the valley of Trampas Wash nearer its mouth. These channels lie lower in the landscape 

and are probably inset below the larger boulder-filled channel to the west. They contain 

matrix-supported, subangular, locally derived cobbles and are oriented in a south-southeast 

direction

Tac Alluvium of Castle Rock (upper Miocene)—Poorly sorted, moderately bedded sand, gravel, 

and silt of local origin, rare angular to subrounded boulders. Gravel clasts reflect locally 

derived lithologies. Deposits are moderately to strongly indurated and gently south 

dipping. Includes some thin intervals of dark staining and manganese cementation. 

Forms steep cliffs and benches along a narrow band flanking the eastern shore of Lake 

Havasu. Castle Rock is an isolated remnant of this unit. Unit is probably an axial valley 

equivalent to the alluvial fan deposits of unit Tfo, but the boundary between them has 

not been confidently identified. The upper surface of the unit in the bluffs is a possible 

strath terrace, which corresponds closely to the contact between Qchm and Qchs in 

Castle Rock Bay. It forms a flat surface ranging from 25 to 30 m above Lake Havasu. It 

is covered with patchy remnants of the Riverside terrace alluvium (Qcr3) that locally 

overlies small deposits of Qch and Tcb that occupy erosional niches in the unit. Unit 

mostly postdates mid-Miocene extensional tectonic activity and predates the arrival of 

the Colorado River in Chemehuevi Valley. Some of what is mapped as Tac is probably 

equivalent to the upper part of Tfo, and it correlates to fanglomerate of Metzger and 

Loeltz (1973); the fanglomerate of Osborne Wash of Dickey and others (1980); and (or) 

the fanglomerate of Howard and others (1999). Exposed thickness as much as 30 m; total 

thickness of unit unknown

Tfo Older fanglomerate (upper Miocene)—Poorly sorted, poorly bedded, subangular to subrounded 

sand and gravel dominated by granitic clasts from the Chemehuevi Mountains. Includes 

scattered large boulders and bouldery lenses. Highly dissected surfaces with exposures 

up to 40 m thick are present south of Trampas Wash. Largely unburied remnants of this 

unit have a corrugated or ridge-and-ravine topographic form. Eroded surfaces lack clear 

soil structure and exhibit spotty retrograde pavement and varnish development on some 

ridge crests and in hollows on slopes. Overlain by Tcb at Blankenship Bend along a 

deep erosional unconformity, and overlain by the Bouse Formation in parts of the upper 

Chemehuevi Mountains piedmont. The former relation indicates the unit once extended 

across the modern valley axis; the latter relation indicates that the relict deposits were 

once part of a large alluvial fan complex at the foot of the Chemehuevi Mountains. It is 

likely that this unit was graded into unit Tac prior to the integration of the Colorado River. 

Colorado River sediments (Qch) are inset into valley-facing scarps 2–5 m high, cut into 

this unit near the 620-ft (189-m) contour on the California side of Lake Havasu. Isolated 

and irregular lags of rounded Colorado River gravels are scattered on slopes above (to west 

of) the scarps. Unit corresponds to unit To (Fanglomerate of Osborne Wash) of Dickey and 

others (1980) and may contain some amount of unit Tgf as described by John (1988) and 

shown on this map. May be stratigraphically equivalent to parts of unit Q1 of Bull (1991); 

units Q1b and (or) QT1a of Dickey and others (1980); unit QTa of Miller and John (1999); 

unit QTs1 of Howard and others (1999); unit Tf of Spencer and others (2015); and unit Tfg1 

of Gootee and others (2016). Thickness at least 30 m

ALLUVIAL AND MEGABRECCIA DEPOSITS (SYN-EXTENSIONAL)

Tgf Granite-clast fanglomerate (Miocene)—Light-tan- and orange-weathering, virtually monomictic 

alluvial-fan deposits, composed of white to tan granite and granodiorite clasts; clasts of 

dike rock types, equivalent of rocks below the Chemehuevi detachment fault; and clasts 

of altered cataclasite. Locally contains thin siliceous tuff beds and basalt flows. Miller and 

John (1999) report a 40Ar/39Ar date of 13.9±0.1 Ma for biotite in a tuff found in this unit

Tgm Granite megabreccia (Miocene)—Light-tan- and pale-orange-weathering, granite-clast landslidemegabreccia deposits; silicified. Granite blocks up to ~1 km × 20 m within the deposits. 
11

Megabreccia deposit is composed of granitic debris characteristic of rocks below the 

Chemehuevi detachment fault. Locally, includes altered granitic cataclasite debris, 

possibly derived from exhumed fault scarps

Tgnf Gneiss-clast fanglomerate (Miocene)—Dark-red- to red-brown-weathering, poorly sorted alluvial-fan 

deposits. Includes subangular to subrounded clasts of Proterozoic gneisses (Xgn), granite (Yg), 

and amphibolite, characteristic of rocks above the Chemehuevi detachment fault. Miller and 

John (1999) report a 40Ar/39Ar age of 15.24±0.02 Ma from biotite in a tuff in this unit

Tgnm Gneiss-clast megabreccia (Miocene)—Dark-reddish-brown-weathering, gneissic- and graniteclast, landslide-megabreccia deposits containing silicified landslide blocks up to 500 × 20 

m. Megabreccia deposits composed of debris of gneissic and granitic rocks characteristic 

of rocks above the Chemehuevi detachment fault

Tvf Volcanic-clast fanglomerate (Miocene)—Dark-reddish-brown, poorly sorted alluvial fan 

deposits, composed of mainly intermediate and mafic-volcanic clasts in a sandy 

matrix. The fanglomerate is in gradational contact with unit Tgnf and contains clasts 

characteristic of rocks above the Chemehuevi detachment fault, including Peach Spring 

Tuff (Tps). Boulders up to 1 m in diameter are present

Tvm Volcanic-clast megabreccia (Miocene)—Dark-reddish-brown-weathering, volcanic-clast, 

landslide-megabreccia deposits containing silicified landslide blocks up to 500 × 20 m. 

Megabreccia deposit is composed of volcanic debris characteristic of rocks above the 

Chemehuevi detachment fault

Tvpm Peach Spring Tuff megaclast (Miocene)—Single megaclast of Peach Spring Tuff measuring 

approximately 275 × 30 m

Tsb Sedimentary breccia (Miocene)—Angular to subrounded cobbles and pebbles in a poorly sorted, mudrich matrix; the rock is always matrix rich but may be matrix or clast supported. Crude parallel 

stratification on a 30-cm to 1-m scale is visible in some places (fig. 11B) but is usually poorly 

developed. Individual beds are generally massive; inverse to normal grading is rare. The higher 

proportion of matrix and less-well-developed stratification distinguish sedimentary breccia 

from conglomerate

CENOZOIC VOLCANIC ROCKS AND SEDIMENTS

Tb Basalt (Miocene)—Dark-purplish-brown, augite-olivine basalt flows, dikes, and plugs. Flows 

have conspicuous columnar joints. Locally intrudes and fuses cataclastites associated 

with the Chemehuevi detachment fault. K-Ar whole-rock age of 11.1±0.4 and 14.5±1.0 

Ma by M.A. Pernokas (written commun. in John, 1987); however, this sample is not from 

basalt exposed in the map area

Tss Sandstone (Miocene)—Pale-brown to very pale brown sandstone and siltstone. Sandstone is well 

bedded and moderately to well sorted. Composed of thin to medium beds of fine to medium 

sand with thin lenses to discontinuous medium beds of pebble gravel conglomerate and 

medium beds of siltstone. Planar bedding and cross-bedding are common, and rare oscillatory 

ripples and mud cracks are present. In at least one location, unit is overlain along an angular 

unconformity by incised gravelly paleochannels within unit Tvf

Tps Peach Spring Tuff (Miocene)—Non-welded to densely welded, white to pink, ash-flow and air-fall 

tuff. Commonly devitrified showing vapor-phase alteration. Phenocrysts 5 to 10 percent, 

predominately blue sanidine, plagioclase, and quartz, with rare biotite, hornblende, pyroxene, 

and sphene. Recent dating of this unit indicates an age of 18.78±0.02 and identifies a source 

vent near Bullhead City, Arizona (Ferguson and others, 2013)

Ts Sandstone and conglomerate (Miocene)—Epiclastic conglomerate and sandstone. Range in 

thickness from several meters where exposed in this quadrangle to several hundred 

meters in other areas. Includes interbedded volcanic rocks that commonly include 

andesitic lahar deposits low in the section, basalt and plagioclase-phyric (“jackstraw”) 

andesitic or latitic flows and plugs at intermediate levels, and fine-grained mafic flows 

high in the section. Sedimentary interbeds include arkose near the base of the volcanic 

section and volcanic-clast tuffaceous conglomerate high in the section

Tv Volcanic flows and intrusions (Miocene and Oligocene(?))—Mafic, intermediate and silicic volcanic 

flows and small hypabyssal intrusions. Locally includes intensely altered and fractured biotite 

dacite porphyry intrusions along and below the Chemehuevi detachment fault in the southern 

Chemehuevi Mountains. Some dacite flows and intrusions are syntectonic with detachment 

faulting
12

MESOZOIC PLUTONIC ROCKS

Chemehuevi Mountains Plutonic Suite of John and Wooden (1990)

Kg Two-mica granodiorite and monzogranite (Cretaceous)—Medium- to coarse-grained, 

leucocratic subequigranular to porphyritic two-mica granodiorite and monzogranite. 

The rocks weather light tan and contain microcline phenocrysts up to 1.5 cm across in a 

coarse-grained (5–8 mm) groundmass of quartz, plagioclase, orthoclase (2–4%), biotite 

(1–2%), muscovite (1–2%), and accessory magnetite, apatite, allanite, epidote, zircon, 

and rare sphene and monazite. Locally, surrounds irregular blocks of porphyritic biotite 

granodiorite and monzogranite (Kpg), suggesting that unit Kpg is older. Elsewhere the 

contact relations are ambiguous

Kpg Chemehuevi Peak Granodiorite (Cretaceous)—Porphyritic biotite granodiorite and 

monzogranite. Forms the most voluminous unit of the Chemehuevi Mountains Plutonic 

Suite: light-tan to gray, medium- to coarse-grained, subporphyritic to porphyritic, biotitesphene granodiorite and monzogranite. Zoned microcline megacrysts, measuring up to 6 

cm across, make up as much as 40% of the volume of the rock; they are set in a mediumgrained groundmass consisting of quartz, plagioclase, microcline, and biotite (5–12%), 

with coarse accessory sphene and allanite-cored epidote euhedra (up to 2 mm), magnetite, 

apatite, zircon, and rare primary muscovite

Kbg Biotite granodiorite (Cretaceous)—Light-gray to tan, subequigranular biotite granodiorite. 

Biotite is the most abundant mafic phase (up to 12%) and is typically euhedral (up to 

5 mm), associated with rare blue-green hornblende. Perthitic microcline and quartz 

are interstitial. Sphene (up to 2%) occurs as euhedral crystals (up to 2 mm) and as 

overgrowths on magnetite. Apatite and zircon occur in and near the magnetite. Metamict 

subhedral allanite grains up to 1–2 mm are rare throughout this phase and commonly 

have euhedral epidote overgrowths. Borders of this unit against the porphyritic 

hornblende-biotite granodiorite and Proterozoic gneisses are fine to medium grained 

and contain rare potassium feldspar megacrysts up to 4 cm long. Inward, away from the 

older rocks, these phenocrysts increase in abundance and primary igneous flow structures 

appear. Mineralogically, the biotite granodiorite is nearly identical to the porphyritic 

granodiorite, but has rare hornblende

Kgd Porphyritic hornblende-biotite granodiorite (Cretaceous)—Border unit of the Chemehuevi 

Mountains Plutonic Suite. Gray, medium-grained, quartz-poor, variably porphyritic, 

hornblende-biotite granodiorite. The granodiorite includes small equant microcline 

phenocrysts (up to 1 cm), stubby blue-green hornblende (4–20%), euhedral biotite 

(2–5%), coarse sphene (1–3%), and accessory magnetite, allanite, epidote, and zircon. 

The epidote is considered late magmatic based on textural relations. The granodiorite is 

deformed and bears a mylonitic foliation and subhorizontal lineation

Whale Mountain sequence

KJqd Hornblende-biotite quartz diorite and quartz monzodiorite (Cretaceous or Jurassic)—Darkgray to brown concordant bodies (up to 1 × 4 km in plan view) of hornblende-biotite quartz 

diorite and quartz monzodiorite. This phase is characterized by plagioclase phenocrysts up 

to 3–4 mm, in a matrix of 18–38% medium-grained, blue-green hornblende and blue-green 

biotite, with accessory sphene, magnetite, apatite, and zoned allanite with overgrowths of 

epidote. Along the north flank of the Chemehuevi Mountains, these rocks bear a steeply 

dipping mylonitic foliation and subhorizontal lineation. Rocks south of Chemehuevi Peak, 

in the lowlands along the south flank of the range, are undeformed and intrude nonmylonitic 

Proterozoic gneisses and amphibolites (Xgn)

qm Biotite monzogranite to syenogranite (Cenozoic or Mesozoic)—Tan, coarse-grained, weakly 

foliated, subequigranular monzogranite to syenogranite. Contains sparse euhedral biotite 

(up to 2%)

PROTEROZOIC ROCKS

Yg Porphyritic monzogranite (Mesoproterozoic)—Tan to pale-red, coarse-grained porphyritic 

biotite granodiorite to monzogranite. Dark-gray, tabular potassium-feldspar megacrysts 

(up to 4 cm), typically rimmed by plagioclase, sit in a matrix of plagioclase, quartz, 

potassium feldspar, biotite, rare hornblende, magnetite, and sphene. These rocks are 

considered equivalent to the 1.4- to 1.5-Ga anorogenic granites discussed by Anderson 
13

(1983), based on textural, compositional, and mineralogical similarities to granites at 

Parker Dam and Davis Dam

Xgn Gneiss and migmatite (Paleoproterozoic)—Heterogeneous crystalline rocks including migmatite 

granite and amphibolite-facies orthogneiss and paragneiss. Layered gneiss, the most 

common rock, consists of leucocratic, biotite ± garnet-bearing quartzo-feldspathic 

gneiss and is associated with subordinate coarse pegmatite, biotite schist, amphibolite to 

hornblendite, and rare augen gneiss. Lowlands along the south flank of the Chemehuevi 

Mountains expose a complex of garnet-bearing leucocratic granitic gneiss, amphibolite, 

pegmatite, and fine-grained, laminated, biotite quartzo-feldspathic gneiss. The age of the 

unit is uncertain; south of Red Rock Wash, near Devils Elbow in the Topock quadrangle, 

the gneisses are intruded by undeformed coarse-grained porphyritic monzogranite (Yg). 

This relation suggests that the unit of gneissic rocks is older than 1.4 Ga and, based on 

regional knowledge, is probably ~1.7 Ga (Howard and others, 2013)

Xmgn Mylonitized gneiss and migmatite (Paleoproterozoic)—Mylonitized, heterogeneous crystalline 

rocks including migmatite, granite, and amphibolite-facies orthogneiss and paragneiss. 

Probably mylonitic equivalent to Xgn. In the central and eastern Chemehuevi Mountains, 

layered gneiss has a superposed shallow-dipping, northwest-striking mylonitic foliation 

and, locally, a subhorizontal mylonitic lineation. Equivalent rocks in the northwestern 

part of the range near Whale Mountain have a steep, northeast-striking foliation and a 

subhorizontal mylonitic lineation

Table 1. Data for microfossil samples collected by P.K. House and analyzed by Kristin McDougall.

[USGS No., micropaleontology laboratory numbering system. Field No., sample identification designated by field geologist. Quadrangle, 7.5ʹ quadrangle and state 

where sample collected. Locality, description related to geographic or stratigraphic location of sample. Latitude and Longitude, latitude and longitude of sample 

location entered as decimal degrees. All locations are NAD83. Accuracy of location is approximate. Lithologic unit, formal stratigraphic unit name at time of work. 

Fossils, fossil groups observed in residues. Comments, any commentary related to sample including, but not limited to, lithology, elevation, age, and ecology. Table 

modified from McDougall and Martinez (2014)]

USGS 

No. Field No. Quadrangle Locality Longitude Latitude Lithologic unit Fossils Comments

Mf12892 2071304-05 Castle Rock, 

AZ

Sample from base of 

slope in greenishyellow, fissile clay 

(mud). Overlain, 

eroded, and loaded 

by 3–5 m thick sand 

body which makes up 

most of slope. Mud 

interval 2 m. Sand 

3–5 m, overlain by 

considerably more and 

younger sediment

-114.431 34.504 Bouse 

Formation

Barren of 

foraminifers 

and other 

organics

Sample elevation 

640 feet 

(195.72m)

Mf12895 2071302 Castle Rock, 

AZ

Metzger’s interbedded 

unit. Interbedded 

‘suicide mud’ and fine 

(likely) CR sand

-114.424 34.507 Bouse 

Formation

Barren of 

foraminifers 

and other 

organics

Sample elevation 

182.88 m
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