Big Sandy Formation 

Near Wikieup, 

Mohave County, 

Arizona

y RICHARD A. SHEPPARD and ARTHUR J. GUDE, 3D

:ONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1354-C

description of a newly named lacustrine 

ormation of Pliocene age in the valley of 

he Big Sandy River of western Arizona
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary

GEOLOGICAL SURVEY 

W. A. Radlinski, Acting Director

Library of Congress catalog-card No. 70-179693

UNITED STATES GOVERNMENT PRINTING OFFICE 

WASHINGTON : 1972

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.G. 20402 - Price 15 cents (paper cover)

Stock Number 2401-2041
CONTENTS

Page

Abstract ______________..____..________________ Cl

Introduction ____________________________________ 1

Lithology, thickness, and boundaries of the Big Sandy Formation __ 2

Age of the Big Sandy Formation ______________________ 9

References cited ________________________________ 10

ILLUSTRATIONS

Page 

FIGURE 1. Index showing the locations of the type and reference

sections of the Big Sandy Formation ________ C2

2. Photograph of a typical outcrop of the Big Sandy

Formation near the type section ___________ 5

3. Generalized columnar section of the Big Sandy

Formation ___________________________ 8

III

CONTRIBUTIONS TO STRATIGRAPHY

BIG SANDY FORMATION NEAR WIKIEUP, 

MOHAVE COUNTY, ARIZONA

By RICHARD A. SHEPPARD and ARTHUR J. GUDE, 3D

ABSTRACT

The Big Sandy Formation, a name here applied to a chiefly lacustrine deposit of Pliocene age, crops out in the valley of the Big Sandy River near 

Wikieup, Mohave County, Ariz. The formation has a maximum exposed thickness of about 245 feet and consists mainly of green and brown mudstone and 

thin interbedded tuffs, most of which are zeolitic. Coarser clastic rocks, including conglomerate, are in marginal parts of the formation and may locally 

be fluviatile. The formation is unconformably overlain by Quaternary sedimentary deposits, and it unconformably overlies unnamed Tertiary sedimentary and volcanic rocks and Precambrian granitic rocks.

INTRODUCTION

Lacustrine deposits of late Tertiary age crop out along the Big 

Sandy River in Mohave County, Ariz., from about 5 miles north 

of Wikieup to about 7 miles south of that town. These lacustrine 

sediments were deposited in a basin elongated north to south 

between Hualapai Mountain on the west and Aquarius Cliffs on 

the east. Lee (1908) briefly mentioned these deposits in his geologic reconnaissance report of part of western Arizona. More 

recently, Morrison (1940) mapped sedimentary deposits in the 

valley of the Big Sandy River and included the lacustrine deposits 

in a widespread unit he called older fill.

We became interested in these lacustrine deposits in 1966 when 

an investigation was initiated to study the diagenetic alteration of 

interbedded silicic vitric tuffs. Ross (1928, 1941) had earlier 

described the formation of analcime, a zeolite, in tuffs of the 

lacustrine deposit near Wikieup, and he (1941, p. 627) also described a partial stratigraphic section, about 80 feet thick, of the 

lacustrine deposits.

Cl
C2 CONTRIBUTIONS TO STRATIGRAPHY

LITHOLOGY, THICKNESS, AND BOUNDARIES 

OF THE BIG SANDY FORMATION

The Big Sandy Formation is herein named for the Big Sandy 

River, a southward-flowing tributary of the Bill Williams River. 

The formation extends over an area of about 30 square miles, 

and its distribution is shown in figure 1. The formation is well

113-35' R. 12 W

T. 16'/z N.

34-40' 

T. 15 N

3 MILES 

J

FIGURE 1. Index map showing the distribution of the Big 

Sandy Formation (stippled) and locations of the type section (T) and reference sections (R-l, R-2, and R-3). X, 

fossil locality.
BIG SANDY FORMATION, WIKIEUP, MOHAVE COUNTY, ARIZ. C3

exposed in the steep bluffs east of the Big Sandy River, southward 

from Wikieup, and in the walls of many washes that head eastward 

from the river. Exposures of the formation lie at elevations of 

about 1,800-2,500 feet.

The greatest thickness of the Big Sandy Formation that was 

observed is about 245 feet, but the full thickness must be much 

greater. An unknown thickness of the formation was removed by 

erosion prior to deposition of the overlying Quaternary gravel. 

Nowhere are the basal beds exposed in the axial part of the basin.

The following section is the thickest continuous section measured, is typical of the formation, and is designated the type 

section:

Type section of the Big Sandy Formation

[Measured in amphitheater, about 2 miles east of Wikieup, along the section line between 

sees. 24 and 25, T. 16 N., R. 13 W., Wikieup quad., from about 1,200 ft east of section 

corner to about 2,400 ft east of section corner]

Quaternary gravel. Thickness 

Unconformity. (feet) 

Big Sandy Formation:

Mudstone, light-green; breaks with conchoidal fracture; contains 

lenses of green siltstone and fine- to medium-grained sandstone; 

abundant clusters of calcite crystals occur 6 ft above base of 

unit. Top of unit eroded ________________ ________ 38.0

Mudstone, brownish-gray, poorly exposed; contains thin lenses of 

siltstone and fine-grained sandstone. Thin analcimic tuffs occur 

5 and 9 ft above base of unit; lower tuff (0.3 ft thick) is greenish yellow; upper tuff (0.2 ft thick) is green in lower part and 

white in upper part __________________________ 19.0

Tuff, yellowish-green, analcimic, laminated; lower part has small

irregular cavities lined with calcite _______________ 1.1

Mudstone, grayish-brown and reddish-brown; small amount of 

greenish-brown mudstone in lower part; breaks with conchoidal 

fracture; contains thin lenses of siltstone and fine-grained 

sandstone; local calcareous concretions. The base of a discontinuous bed (0.0-0.5 ft thick) rich in altered pumice lapilli is 

19 ft above base of unit. A gray to greenish-gray analcimic 

tuff (0.3-0.5 ft thick) occurs 28 ft above base of unit ____ 64.0

Limestone, white; weathers pale gray; thin to medium bedded; 

contorted and fragmental internal structures; irregular cavities 

contain large yellowish-brown calcite crystals __________ 3.0

Mudstone; mostly brownish gray to greenish gray but grades upward to green; contains thin lenses of siltstone and sandstone; 

local gray calcareous concretions; bedding contorted in upper 8 

ft of unit. A yellowish-green analcimic tuff (0.0-0.3 ft thick) 

occurs 21 ft above base of unit ___________________ 26.0

Mudstone, brown; breaks with conchoidal fracture; contains thin 

lenses of siltstone and fine-grained sandstone. A gray to green 

analcimic tuff (0.7 ft thick) occurs at top of unit; middle part 

of tuff rich in detrital grains; tuff probably equivalent to upper 

marker tuff ______________________________ 11.7
C4 CONTRIBUTIONS TO STRATIGRAPHY

n. n i -n ~ Thickness Big Sandy Formation Continued (ieet)

Mudstone, brown to reddish-brown; breaks with conchoidal fracture; local contorted bedding; common light-gray calcareous 

concretions. A light-green and white analcimic tuff (0.2 ft 

thick) occurs at top of unit ______________________ 12.2

Sandstone, brown, fine-grained, thin-bedded, wavy-bedded, 

calcareous ________________________________ 1.0

Mudstone, brown to reddish-brown; breaks with conchoidal fracture; local contorted bedding; common light-gray calcareous 

concretions. A yellowish-green analcimic tuff (0.1-0.2 ft thick) 

occurs at top of unit __________________________ 12.1

Lower marker tuff; green in lower part but white to light gray in

upper part; multiple bedded; analcimic ______________ 1.5

Mudstone, brown; breaks with conchoidal fracture; contains numerous thin calcareous lenses. A conspicuous fine-grained sandstone (0.3 ft thick) occurs 3 ft from top of unit. Base of unit 

not exposed _______________________________ 7.0

Total Big Sandy Formation measured __________.____ 196.6

The following short section of the Big Sandy Formation, designated reference section R-l (fig. 1), was measured about 5 miles 

south of the type section, and it includes the lowest exposed part 

of the formation.

Reference section R-l of the Big Sandy Formation

[Measured along bluffs east of the Big Sandy River in SW%SW% 

sec. 18, T. 15 N., R. 12 W., Wikieup quad.]

Quaternary gravel. Thickness 

Unconformity. (feet) 

Big Sandy Formation:

Mudstone, greenish-gray; breaks with conchoidal fracture; poorly

exposed. Top of unit eroded ________ - 4.0

Lower marker tuff; white to light gray except for yellowishgreen middle; thin to medium bedded; multiple bedded; mostly 

resistant but locally friable and feldspathic; contains brownishgray mudstone interbed in lower part; upper part locally 

silicified _________ _________________-_____ 3.0

Mudstone, yellowish-gray; breaks with conchoidal fracture; 

abundant crystal molds filled with calcite. At base of unit is a 

light-gray tuff (0.2 ft thick) that contains abundant detrital 

grains and consists of authigenic analcime and potassium 

feldspar ___ ______________________________ 6.2

Mudstone, yellowish-brown and light-pink, massive; breaks with 

conchoidal fracture; local concentrations of crystal molds filled 

with calcite; calcareous in upper part ____ _ 20.0

Sandstone, light-brown, tuffaceous, thin-bedded, crossbedded; 

locally calcareous; local concentrations of biotite; contains thin 

brown mudstone interbeds ____________ - 2.0

Mudstone, pink and yellowish-brown, thin- to thick-bedded; breaks

with conchoidal fracture __________ - 6.5
BIG SANDY FORMATION, WIKIEUP, MOHAVE COUNTY, ARIZ. C5

Thickness 

Big Sandy Formation Continued (feet)

Mudstone, brown and yellowish-gray; small amount of light-red 

mudstone; breaks with conchoidal fracture. At top of unit is a 

yellowish-gray to pink tuff (0.5 ft thick) which consists chiefly 

of analcime and potassium feldspar and which grades upward 

into mudstone _____________________________ 7.0

Mudstone, yellowish-brown and olive-gray; breaks with conchoidal 

fracture; locally calcareous; local thin siltstone beds; base of 

unit not exposed. At top of unit is a pink to white tuff 

(0.2-0.3 ft thick) which has an uneven base and which grades 

upward into mudstone __________________________ 9.2

Total Big Sandy Formation measured ______________ 57.9

The Big Sandy Formation consists chiefly of green and brown 

lacustrine mudstone or a calcareous silty or sandy variant (fig. 2). 

These rocks grade laterally into coarser clastic rocks, including 

conglomerate. The coarse-grained rocks are probably a marginal 

facies of the mudstone; however, some sandstone and conglomerate included in the formation may be of fluviatile origin. The 

following reference sections, R-2 and R-3, were measured near 

the southern boundary of the formation (fig. 1) and are representative of the coarser clastic facies. Vertebrate fossils are 

common in the upper part of reference section R-3.

Tuffs are the most conspicuous and continuous strata in the 

formation. At least 13 tuffs were recognized, and they make up

FIGURE 2. Typical outcrop of the Big Sandy Formation about 0.5 mile west 

of the type section. Arrows point to the lower marker tuff.
C6 CONTRIBUTIONS TO STRATIGRAPHY

Reference section R-2 of the Big Sandy Formation

[Measured about 1 mile east of the Big Sandy River, along small washes from 

NE^i sec. 30 eastward to NW^NE^NE^i sec. 30, T. 16 N., E. 12 W., Greenwood 

Peak quad.]

Quaternary gravel. Thickness 

Unconformity. (feet) 

Big Sandy Formation:

Siltstone and sandstone, gray to greenish-gray, thin- to mediumbedded, zeolitic. Top of unit eroded _________________ 20.0

Upper marker tuff, yellow, multiple-bedded, zeolitic; breaks with 

a platy fracture; forms resistant ledge; locally silicified at 

top ______________________________________ 2.2

Siltstone and mudstone, grayish-brown; small amount lenticular 

brown sandstone. Green analcimic tuff (0.2 ft thick) occurs at 

base of unit ________________________________ 5.7

Sandstone, siltstone, and mudstone, brown to gray, thin- to 

medium-bedded; thin lenticular conglomerates. Sandstone is fine 

to medium grained, poorly sorted, calcareous, and analcimic _ 29.0

Lower marker tuff, green to light-gray, zeolitic; locally silicified 

in lowermost and uppermost parts; friable where green but 

breaks with a conchoidal fracture where light gray _______ .9

Sandstone, brownish-gray, thin- to medium-bedded, fine- to coarsegrained, locally calcareous, zeolitic; thin lenticular conglomerates _____________________________________ 11.0

Sandstone and conglomerate, gray, thin- to medium-bedded, 

calcareous, zeolitic. Sandstone is mostly coarse grained and 

poorly sorted and consists chiefly of volcanic detritus, much of 

which is pumiceous. Conglomerate is lenticular and consists 

chiefly of rounded to subangular volcanic pebbles and cobbles _ 4.0

Siltstone and sandstone, gray, thin-bedded; local small-scale crossbedding. Base not exposed _______________________ 5.0

Total Big Sandy Formation measured ______________ 77.8 

Reference section R 3 of the Big Sandy Formation

[Measured along a wash that parallels Signal Road about 2,600 ft northwest of the road, from 

NW^NW^iSW^i sec. 29 to NE^iSE^NW^i sec. 29, T. 16 N., E. 12 W., Greenwood 

Peak quad.]

Quaternary gravel. Thickness 

Unconformity. (feet) 

Big Sandy Formation:

Siltstone and sandstone, green, locally calcareous, thin- to 

medium-bedded; sandstone is fine grained. Top of unit eroded. 

A yellow clinoptilolitic tuff (0.2 ft thick) occurs at base of 

unit _____.._______________________________ 12.5

Sandstone and siltstone, green; abundant irregular calcareous 

concretions; poorly exposed ______________________ 19.5

Siltstone and sandstone, green, thin- to medium-bedded. Vertebrate fossils are common, especially in siltstone in lower and 

upper several feet of unit ________________________ 18.5

Sandstone and siltstone, green, thin- to medium-bedded; numerous 

gray silicified beds. Sandstone is fine to medium grained, poorly 

sorted, and zeolitic. Yellow to pale-gray tuffs occur 18 ft above 

base of unit and at top of unit; lower tuff (0.3 ft thick) consists
BIG SANDY FORMATION, WIKIEUP, MOHAVE COUNTY, ARIZ. C7

Thickness

Big Sandy Formation Continued (feet) 

chiefly of mixed zeolites; upper tuff (0.4 ft thick) consists 

chiefly of erionite _____________________________ 32.3

Sandstone, siltstone, and conglomerate, brown, thin- to mediumbedded. Sandstone is fine to medium grained, poorly sorted, 

locally crossbedded, and zeolitic. Pebbles in conglomerate are 

chiefly volcanic. A white zeolitic tuff (0.5 ft thick) occurs 10 ft 

above base of unit ________________________ _ 22.5

Upper marker tuff, white; breaks with platy fracture in upper 

and lower parts but with hackly fracture in middle; consists 

chiefly of clinoptilolite ______________________ _ 2.7

Sandstone, brown, thin- to medium-bedded, fine- to coarse-grained, 

zeolitic. Yellow zeolitic tuff (0.2 ft thick) occurs at base of 

unit ______________________________________ 5.2

Sandstone and conglomerate, brown, thin- to medium-bedded, 

zeolitic. Sandstone is fine to coarse grained, poorly sorted, and 

weakly indurated. Conglomerate is lenticular and consists of 

subangular volcanic pebbles. Base of unit not exposed _____ 21.0

Total Big Sandy Formation measured _____________ 134.2

about 2-3 percent of the exposed formation. All the tuffs are altered and now consist of zeolites, potassium feldspar, clay minerals, and silica minerals of authigenic origin. The distribution and 

genesis of these minerals in the altered tuffs are the subjects of a 

report in preparation and are not discussed here. The tuffs are pastel shades of gray, yellow, or green, and they generally form ledges. 

The tuffs are 1-40 inches thick, but most are less than 6 inches 

thick. Most tuffs do not persist throughout the extent of the formation; however, two consistently thick and persistent tuffs in the 

lower exposed part of the formation have been given informal 

field names lower marker tuff and upper marker tuff. The upper 

marker tuff occurs about 35 feet above the lower marker tuff 

(fig-3).

Erosion and dissection of the nearly flat lying formation have 

proceeded to such a degree that most of the formation is characterized by badlands or dissected gravel-capped pediments. The Big 

Sandy River and its nearly mile wide, alluvium-filled channel 

transect the formation north of Wikieup, but south of Wikieup 

the western boundary of the formation lies along the river or in the 

channel (fig. 1). Much of the formation is concealed by either 

gravel-capped pediments or relatively recent alluvium of the Big 

Sandy River and its many tributaries.

The Big Sandy Formation unconformably overlies unnamed 

Tertiary sedimentary and volcanic rocks to the east and Tertiary 

volcanic rocks and Precambrian granitic rocks to the south. Small 

remnants of basalt crop out beneath the Big Sandy- Formation in 

washes 2.5-3.0 miles east of Wikieup. These older rocks were 

probably islands in the lake during much of the deposition of the
C8 CONTRIBUTIONS TO STRATIGRAPHY

:30CXXXXXXX)<

Upper marker tuff

Lower marker tuff

EXPLANATION

Tuff

FEET

r 0

-20

- 30

-40

-50

FIGURE 3. Generalized columnar section of the Big 

Sandy Formation. Base of formation is not exposed; top is eroded.
BIG SANDY FORMATION, WIKIEUP, MOHAVE COUNTY, ARIZ. C9

Big Sandy Formation. The relationship of the formation to an 

unnamed thick, east-dipping fanglomerate deposit to the west is 

unknown, but the fanglomerate deposit is probably slightly older 

than the Big Sandy Formation. Compared with the Big Sandy 

Formation, the fanglomerate is more deeply dissected and it rests 

higher on the side of the valley of the Big Sandy River. The 

fanglomerate is intruded by a basaltic dike. Morrison (1940, p. 3) 

included the fanglomerate deposit in his older fill and concluded 

that the fanglomerate graded laterally into lacustrine rocks herein 

named the Big Sandy Formation. Our field evidence, however, 

does not confirm his conclusion.

Dips in the Big Sandy Formation are generally less than 2°, 

but steeper dips have been recorded. Local dips of as much as 

10° were measured in Natural Corrals Wash, about 0.5 mile west 

of U.S. Highway 93. Normal faults of slight displacement cut the 

formation, particularly north of Wikieup. The greatest displacement measured was only 14 feet.

AGE OF THE BIG SANDY FORMATION

The Big Sandy Formation contains a rich vertebrate fauna, but 

little has been published on the fossils. Lance (1960) showed an 

early or middle Pliocene fossil locality in sedimentary deposits 

at Wikieup. However, data on neither the fossils nor the exact 

locality were given. Several paleontologists from the Frick Laboratory of the American Museum of Natural History have studied 

the vertebrate fossils from the Big Sandy Formation, but reports 

of those studies have not been published. The fossils were collected 

from two pits in the NW^SW^NE^ sec. 29, T. 15 N., R. 12 W., 

Greenwood Peak quadrangle (fig. 1). The fossil-bearing strata 

are the same as those measured in the upper part of reference 

section R-3.

Ted Galusha of the Frick Laboratory (written commun., 1971) 

summarized his work and that of his colleagues as follows: "Based 

on studies of the Equidae, Camelidae, Antelocapridae, Canidae, 

Ursidae, Procyonidae, Mustelidae, and Felidae material from the 

Wikieup quarries, we are agreed that the fauna indicates a later 

Hemphillian age." Numerous teeth and bone fragments were 

collected in 1967 as float at the Frick Laboratory site. The fossils 

were identified as tPliohippus cf. P. leardi Drescher by G. 

Edward Lewis of the U.S. Geological Survey, who stated (written 

commun., 1968) that this species of horse occurs in the upper 

part of the early Pliocene and in the late Pliocene. Thus, the Big 

Sandy Formation is definitely Pliocene and probably late Pliocene 

in age.
CIO CONTRIBUTIONS TO STRATIGRAPHY

REFERENCES CITED

Lance, J. F., 1960, Stratigraphic and structural position of Cenozoic fossil

localities in Arizona: Arizona Geol. Soc. Digest, v. 3, p. 155-159. 

Lee, W. T., 1908, Geologic reconnaissance of a part of western Arizona:

U.S. Geol. Survey Bull. 352, 96 p. 

Morrison, R. B., 1940, Ground-water resources of the Big Sandy Valley,

Mohave County, Arizona: Arizona State Water Commissioner, 6 p. 

Ross, C. S., 1928, Sedimentary analcite: Am. Mineralogist, v. 13, no. 5,

p. 195-197. 

 1941, Sedimentary analcite: Am. Mineralogist, v. 26, no. 10, p. 627-629.

U. S. GOVERNMENT PRINTING OFFICE : 1971 O - 445-15^