Washington Division of Geology and Earth Resources Open …

 

 

STATE OF WASHINGTON
DEPARTMENT OF NATURAL RESOURCES
BRIAN J. BOYLE, Commissioner of Public Lands
ART STEARNS, Department Supervisor

DIVISION OF GEOLOGY AND EARTH RESOURCES
Raymond Lasmanis, State Geologist

THE CHUMSTICK AND WENATCHEE FORMATIONS:
FLUVIAL
AND LACUSTRINE ROCKS OF EOCENE AND OLIGOCENE AGE
IN THE CHIWAUKUM GRABEN, WASHINGTON

by

Randall L. Gresens,1 Charles W. Naeser,2
and John T. Whetten3

1Department of Geological Sciences
University of Washington
Seattle, Washington 98195

2u.s. Geological Survey
Denver, Colorado 80225

3Department of Geological Sciences
University of Washington
Seattle, Washington 98195
and
U.S. Geological Survey
Seattle, Washington 98105

Washington Department of Natural Resources
Division of Geology and Earth Resources
Olympia, Washington 98504

Open File Report 78-0

1978

Abstract: The Chiwaukum graben in central Washington contains two

unconformity-bounded fluvial and lacrustrine units, here named the

Chumstick and Wenatchee Formations, dated at 45 m.y. (middle to late

Eocene) and 34 m.y. (early Oligocene) by the fission-track method on

zircons from tuffs. Previously, both formations were thought to be

part of the Swauk Formation, which is older.

The Chumstick Formation rests on weathered crystalline basement and

is thousands of meters thick. Fanglomerate occurs at the base and along

the margins. Most of the formation consists of feldspathic sandstone

and pebbly sandstone of fluvial origin; but within the upper part is a

lacustrine unit, here designated the Nahahum Canyon Member. Tuff is

common in the lower part of the formation. Both of the bounding faults,

the Leavenworth and the Entiat, were active during deposition of the

Chumstick Formation, but relief was greatest on the northeast side.

The Wenatchee Formation is restricted to the vicinity of Wenatchee,

Washington, and is >300 m thick. It unconformably overlies the Chumstick

Formation within the graben but overlaps the northeast side of the graben,

where it lies directly on weathered metamorphic basement. The Wenatchee

Formation, like the Chumstick, was deposited primarily in fluvial and

lacustrine environments; unlike the Chumstick, much of the sediment is

mature quartz sandstone. Relief in the source area probably was very

subdued during its deposition.

INTRODUCTION AND HISTORICAL BACKGROUND

The Chiwaukum graben in the central Cascade Range of Washington

(Fig. 1) contains two Tertiary fluvial and lacustrine formations formerly

considered to be part of the Swauk Formation, a unit of nonmarine sand

stone, conglomerate, and shale. The Swauk was named by Russell (1900)

for exposures along Swauk Creek.

Its type locality is west of the graben.

It is overlain unconformably by Teanaway Basalt, which, in turn, is over

lain conformably by the Roslyn Formation. Rocks within the Chiwaukum

graben (Willis, 1953) were called Swauk largely on the basis of litho

logic similarity and evidence from fossil leaves (e.g., Chappell, 1936;

Page, 1939; Willis, 1953), even though the Teanaway Basalt and Roslyn

Formation were not known to occur there.

In this paper, we present

evidence that the rocks in the graben constitute two formations that are

younger than the Swauk Formation.

Russell (1900), who mapped in the area of the western border of the

Chiwaukum graben, suggested that there were two sedimentary units within

the Swauk, which he called Wenache sandstone phase and Camas sandstone

phase.

In deference to Smith (1904), who considered differences between

the sandstones to reflect facies differences in a·large lake, Russell

did not formally propose these names. Alexander (1956), in an unpublished

M.S. thesis, suggested that the Swauk be retained and that the younger

formation should be called Camas.

In essence, we agree with Alexander,

but since the name Camas is preempted, we use the name Chumstick Formation

for the Camas sandstone of Russell (1900) and._Alexander (1956). We use

the name Wenatchee Formation for a third unit that is younger than the

Swauk and Chumstick Formations (Fig. 2).

2

R.W. Tabor and V.A. Frizzell, Jr., currently working on the Swauk

Formation in the type area and elsewhere, report a fission-track age of

49±5 m.y. on zircon from a tuff which they believe to be in the upper

part of the Swauk (Tabor and Frizzell, 1977, p. 89). Additional details

concerning previous work on the Swauk Formation are given in Gresens et

al. (1977) and Tabor and Frizzell (1977).

CHUMSTICK FORMATION (JTW)

Name and General Character

The Chum.stick Formation is named for Chum.stick Creek, north of

Leavenworth, Washington (Fig. 1). The formation is a thick deposit of

interbedded sandstone, conglomerate, shale, and very minor tuff; sedi

mentary structures and plant remains indicate deposition in environments

ranging from alluvial fan and fluvial to lacustrine. Fanglomerate occurs

locally at the base of the formation and along its margins. The age of

the formation is late middle to late Eocene on the basis of fission-track

dating of zircons from tuffs.

Sandstone is the most common lithology. Thick whitish to buff-gray,

crossbedded to massive beds typically grade from pebbly sandstone at the

base to medium or fine-grained sandstone at the top. Conglomerate and

shale are present locally. Most sandstones are feldspathic; clast com

positions and paleocurrent determinations suggest that sediment was derived

from nearby sources, mainly from the area of the Entiat Mountains.

Within the fine-grained upper part of the Chum.stick Formation is a

lacustrine unit designated the Nahahum Canyon-.Member (Fig. 2) after Nahahum

.

Canyon, northeast of Cashmere (Fig. 3). The Nahahum Canyon Member grades

into the Chumstick Formation near the margins of the graben and near what

3

is assumed to have been a bedrock ridge within the graben. On the north

east side of the graben, where the member is best preserved and exposed,

normal faulting along the Entiat fault controlled the location of alluvial

and ·lacustrine depositional environments.

Type Section

We designate the section along Eagle Creek (Fig. 1) as the type

section of the Chumstick Formation, including the Nahahum Canyon Member,

beginning at the confluence of Eagle and Chumstick Creeks (Sec. 31, T25N,

Rl8E), approximately on the axis of the Peshastin syncline (Willis, 1953;

Fig. 3), and extending northeast through both limbs and the core of the

Eagle Creek anticline to the Entiat fault (Sec. 20, T25N, Rl9E). Meta

morphic rocks in the core of the anticline (Sec. 23, T25N, RlSE) are

excluded from the type section.

The type section affords the most accessible overview of the entire

unit, but not all lithologies are represented, and the section is incom

plete because of the fault that separates the Nahahum Canyon Member from

the rest of the Chumstick Formation (Whetten and Laravie, 1976).

In the following section, the major lithologies are described and

reference sections given for lithologies that are not easily seen at the

type section.

Lithologies and Depositional Environments

Redbed fanglomerate–The Chumstick Formation was deposited on Swakane

Biotite Gneiss. The basal deposit (map unit “Tf”, Whetten and Laravie,

1976; Figs. 3, 4) is a diamictite composed of.angular. to subrounded clasts of

gneissic detritus to 50 cm in diameter and smaller clasts of vein quartz in a

4

reddish sandy matrix. Although thin lenses of sandstone occur locally,

the fanglomerate is generally without bedding or other structures.

The rocks weather readily and are poorly exposed. A telltale

reddish soil commonly develops where the unit is present, however, and

slightly rounded clasts distinguish colluvium derived from it from that

of Swakane Biotite Gneiss.

The fanglomerate, exposed only near the core of the Eagle Creek

anticline, ranges in thickness from Oto 200 m. Rocks of this lithology

are not exposed at the type section, but a suitable reference section is

located at the head of Williams Canyon on the Blag Mountain road (SW~

Sec. 1, T24N, R18E).

The redbed fanglomerate is interpreted to be the remains of alluvial

fans that mantled knobs and ridges of gneiss on the floor of the graben.

The red color appears to be produced by alteration of ferromagnesian

minerals (mostly biotite) in the gneiss. Although red is the typical

color, some rocks are of other colors, including brown and gray.

Fanglomerate on the margins of the graben–Discontinuous lenses of

conglomerate and diamictite (hereinafter referred to as fanglomerate)

occur in the Chumstick Formation on the west margin of the graben along

the Leavenworth fault (Cashman, 1974; Cashman and Whetten, 1976; Tabor

and Frizzell, 1977; Fig. 4). These rocks intertongue with fluvial

deposits of the Chumstick Formation. Similar rocks occur along the Entiat

fault (map unit “Tc”; Whetten and Laravie, 1976; Whetten and Waitt, 1978)

and interfinger with the Nahahum Canyon Member.

·-

On the west side of the graben, the unit is predominantly serpentinite

fanglomerate derived from the Ingalls Peridotite of Ellis (1959), diorite

fanglomerate from the Mount Stuart Granodiorite, and schist fanglomerate

5

from the Chiwaukum Schist of Page (1939, 1940). Most fanglomerate is

essentially monolithologic; locally it is a mixture of two or more rock

types. The depositional environment was one of coalescing alluvial fans

along the graben margin fed-by-streams and debris flows from one or more

rapidly rising and eroding fault scarps. Maximum thickness of the

fanglomerate is about 50 m, and clasts range to 3 min diameter.

Some of the best exposures of fanglomerate are seen near Ingalls

Creek Lodge on U.S. Highway 97, 12 km south of the junction with U.S.

Highway 2 (SE~ Sec. 13, T23N, Rl7E). On the west side of Peshastin

Creek, a serpentinite fanglomerate occurs as a conspicuous reddish-brown

bed 25 to 30 m thick; a diorite fanglomerate exposed behind the lodge

forms a vertical cliff 50 m high.

Fanglomerate near the Entiat fault was mapped by Whetten and Laravie

(1976), and Whetten and Waitt (1978), and described by Laravie (1976).

The coarsest rocks, having clasts more than 2 min diameter, crop out

adjacent to the fault (Figs. 3, 4) and were probably deposited as alluvial

fans. Basinward, these rocks interfinger with fluvial sandstone, which,

in turn, interfingers with lacustrine rocks of the Nahahum Canyon Member.

Virtually all clast lithologies are known to occur in the Entiat Mountains.

The fanglomerate is poorly exposed, but resistant clasts that weather out

of the unit occur in soils and on the surface.

The core rocks of the Eagle Creek anticline formed a topographic

high during deposition of the Nahahum Canyon Member; coarse conglomerates

and sandstones occur adjacent to it. This suggests that the Chiwaukum

graben may have developed one or more subsidiary graben during deposition

of the Chumstick Formation.