The Lawson Creek 

Formation of

Middle Proterozoic Age in 

East-Central Idaho

GEOLOGICAL SURVEY BULLETIN 1482-E

The Lawson Creek 

Formation of

Middle Proterozoic Age in 

East-Central Idaho

By S. W. HOBBS

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1482-E

A description of a new post-Swauger Middle 

Proterozoic formation in east-central Idaho

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1980
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

H. William Menard, Director

Library of Congress Cataloging in Publication Data

Hobbs, Samuel Warren, 1911-

The Lawson Creek Formation of Middle Proterozoic age in east-central Idaho.

(Contributions to stratigraphy) (Geological Survey Bulletin 1482-E)

Bibliography: p. 12

I. Geology, Stratigraphic-Pre-Cambrian. 2. Geology-Idaho Custer Co. 

3. Geology-Idaho-Lemhi Co. 4. Geology-Idaho-Butte Co. I. Title.

II. Series III. Series: United States Geological Survey Bulletin 1482-E

QE75.B9 no. 1482-E [QE653] 557.3s [551.7'l 5*09796] 80-607156

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

Washington, D.C. 20402 

Stock No. 024-001-03304-6
CONTENTS

Page

Location and regional relations -

Definition and general character - - -- -- - - - -- 3

Lithology and contacts - - -- - - - - 5

The Lawson Creek Formation at other localities -- - -- - 8

Geologic history of the area and problems of correlation -- - - -- - 10

ILLUSTRATIONS

Page

FIGURE 1. Index map of east-central Idaho and southwestern Montana,

showing general distribution of Proterozoic rocks -- - -- 2 

2. Map showing distribution of Lawson Creek and Swauger Forma3. Incomplete columnar section and description of the Lawson Creek

TABLE

TABLE 1. Tentative regional correlation of Proterozoic rocks in Idaho and

.«»««»«--__--- __-.««««. -_-««»««» ««««..»« « 1 j_ 1j_

III

CONTRIBUTIONS TO STRATIGRAPHY

THE LAWSON CREEK FORMATION OF

MIDDLE PROTEROZOIC AGE IN

EAST-CENTRAL IDAHO

By S. W. HOBBS

ABSTRACT

The Swauger Formation, the youngest middle Proterozoic or Proterozoic Y unit 

heretofor identified in east-central Idaho, is overlain at four localities in the northernmost part of the Lost River Range and adjacent Salmon River Mountains by a 

heterogeneous sequence of strata that constitutes a new stratigraphic unit of postSwauger age. This unit is herein named the Lawson Creek Formation from an exposure of more than 1,200 m (meters) of strata that overlie the Swauger Formation in 

a westerly dipping homocline near Lawson Creek in the northeast part of the Lost 

River Range. These exposures are designated the type locality and consist of 

predominantly medium-reddish-purple to dark-purplish-gray and maroon quartzite, 

impure quartzite, siltstone, and argillite that are predominantly thin bedded but that 

contain local thick beds of light purple quartzite very similar to that of the Swauger. 

The top of the unit is covered by Challis Volcanics and the total thickness is unknown.

The Lawson Creek Formation appears at places to be gradational from the 

Swauger, and although it does have local structural discontinuities at or near the 

basal contact, distinctive lithologic aspects and structural relations identify it as a 

separate stratigraphic unit overlying the Swauger Formation.

INTRODUCTION

The Precambrian rocks of east-central Idaho are extensively exposed in the Lemhi Range and Beaverhead ranges and the Salmon 

River mountains where they were recognized and partially delineated 

during early reconnaissance studies in the region notably those of 

Umbleby (1913, p. 30-32; 1917, p. 23). They were further described 

and subdivided by the subsequently more detailed work of Ross 

(1925, p. 5-8; 1947, p. 1096-1102; 1961, p. 195-201). It was not, 

however, until the detailed work of Ruppel in the Lemhi Range (1968, 

1975a) that the Precambrian units were carefully delineated, their 

structural relations deciphered, and their relations to the Paleozoic 

section described.

A revised sequence of middle Proterozoic strata has been defined in 

the Lemhi Range by Ruppel's work. The lowest unit exposed in eastcentral Idaho, and presumably the oldest, has been correlated with
CONTRIBUTIONS TO STRATIGRAPHY

114° 113°

EXPLANATION

MOUNTAINOUS AREA 

Darker pattern indicates

aas in which

oterozoic rocks occur

40 MILES

FIGURE 1. Index map of east-central Idaho and southwestern Montana, showing 

general distribution of Proterozoic rocks. Modified from Ruppel (1975).

the Yellowjacket Formation as defined by Ross (1934, p. 16). The 

base of the Yellowjacket has not been found, and the upper contact 

wherever seen in east-central Idaho is the Medicine Lodge thrust 

system (Ruppel, 1978). Above the thrust system in the Lemhi Range, 

the remainder of the middle Proterozoic strata comprises the Lemhi 

Group that includes five formations, the Inyo Creek, West Fork, Big 

Creek, Apple Creek, and Gunsight Formations, and at the top the
LAWSON CREEK FORMATION, EAST-CENTRAL IDAHO 3

Swauger Formation whose upper contact is everywhere a thrust 

fault. These units total more than 12,000 m of generally fine-grained 

quartzite, siltstone, and argillite, and nowhere in the Lemhi Range 

have any younger rocks assignable to the middle Proterozoic been 

identified. Locally, as much as 300 m of the Wilbert Formation unconformably overlies the middle Proterozoic units and in places is 

overlain by the Lower Ordovician Summerhouse Formation (Ruppel 

and others, 1975). Although the Wilbert Formation has been tentatively assigned a late or Proterozoic Z age, part of it may be Cambrian.

Figure 1 shows the geography of east-central Idaho and southwest 

Montana and the general distribution of Proterozoic strata.

LOCATION AND REGIONAL RELATIONS

In the northern part of the Lost River Range and adjacent parts of 

the Salmon River Mountains, isolated exposures of several middle 

Proterozoic formations that are identifiable with those described in 

the Lemhi Range project through the widespread Challis Volcanics 

that blanket most of the area (fig. 2). Most notable of these are the 

three major outcrop areas of the Swauger Formation, which have 

similar stratigraphic succession, generally parallel structure, and 

comparable thickness (approximately 3,000 m). These Swauger exposures (A, B, B', B ", and C, fig. 2) almost certainly represent a threefold structural repetition of most, if not all, of the complete Swauger 

stratigraphic section.

LAWSON CREEK FORMATION

DEFINITION AND GENERAL CHARACTER

At four localities in two structurally separated tectonic blocks in 

the northern part of the Lost River Range (Mclntyre and Hobbs, 

1978), the Swauger Formation is overlain with apparent conformity 

by a sequence of rocks that are sufficiently distinctive to be considered a new formation. These different rocks are herein named the 

Lawson Creek Formation, a new formation in the Proterozoic sequence of east-central Idaho. The largest of the Lawson Creek Formation exposures in the Lost River Range extends nearly 4 km 

(kilometers) southeast along the east foot of the Lost River Range 

from the place in the southwestern corner of the May quadrangle 

where Lawson Creek flows into the broad Pahsimeroi Valley. These 

exposures, mostly in sections 4 and 9, T. 14 N., R. 21 E., are 

designated the type locality.
CONTRIBUTIONS TO STRATIGRAPHY

«°45' W°15'

«°30'

0 3 MILES 

EXPLANATION

Lawson Creek Formation 

Swauger Formation

Apple Creek Formation

.45

CONTACT

STRIKE AND DIP OF BEDS

FIGURE 2. Distribution of Lawson Creek and Swauger Formations, east-central 

Idaho. Geology by S. W. Hobbs. Letters indicate localities described in text.

The Swauger Formation, on which the Lawson Creek Formation 

rests, is basically similar to that described in the Lemhi Range (Ruppel, 1975, p. 12-14). It is composed of distinctive pale-purple, lightpink, or pale-purple-red, vitreous quartzite that is generally quite 

pure. Beds range from 0.15 to 1.8 m thick and many are prominently
LAWSON CREEK FORMATION, EAST-CENTRAL IDAHO 5

cross laminated. Partings or thin beds of greenish-gray to nearly 

black siltite or argillite separate the quartzite beds at places, but 

these are distinctly subordinate. Some bedding planes are ripple 

marked. Quartz grains that make up from 80 to 97 percent of the 

quartzite beds are 0.1-0.75 mm (millimeter), mostly about 0.5 mm, in 

diameter and are generally well sorted in any one layer. Many grains 

show optically oriented overgrowths of quartz, and some boundaries 

are sutured or form a mosaic pattern. Strain shadows are always 

present and in some places developed a pseudoparallel twinning in 

the quartz grains. Scattered grains of quartzite are usually present 

but form only a few percent at most, and grains of intermixed, finegrained quartz and sericite are probably pseudomorphic after 

feldspar, which never exceeds a few percent. Well-rounded grains of 

zircon and tourmaline together with a very sparse matrix of finegrained quartz, sericite, and hematite dust make up the remaining 2 

or 3 percent of the rock.

In contrast to the Swauger, the Lawson Creek Formation is thinner 

bedded, more heterogeneous in composition, generally finer grained, 

and more vividly colored. As herein defined, the Lawson Creek Formation is a sequence of medium-reddish-purple to dark-purplishgray and maroon quartzite, impure quartzite, siltstone, and argillite 

that is predominantly thin bedded but that contains local thick 

beds of lighter purple quartzite and zones of intermixed medium to 

thick bedded quartzite, siltstone, and thinly bedded silty argillite. 

The quartzite beds within it are generally less pure than those in 

the Swauger and are locally highly feldspathic; the entire section 

shows an abundance of mud chip breccia, ripple marks, and 

micaceous partings.

LITHOLOGY AND CONTACTS

Figure 3 is a generalized stratigraphic section of the Lawson Creek 

Formation as exposed in the hills south of Lawson Creek (locality B", 

fig. 2) where approximately 1,300 m have been measured in a nearly 

uninterrupted section from the contact with the Swauger to the uppermost exposure where it is covered by Challis Volcanics. Although 

the basal contact is covered by surficial debris and the strata adjacent to the contact are sheared and silicified, the general structural 

and lithologic compatability both here and at the other localities indicate a strictly conformable and generally gradational change from 

the Swauger into the Lawson Creek Formation. Two other places in 

the measured section a covered interval about 660 m above the base 

and a silicified breccia zone at 720 m above the base may represent 

minor faults but the beds are not significantly offset.
CONTRIBUTIONS TO STRATIGRAPHY

f

ii 

fl

Ii

*!

i
Medium-pinkish- to purplish-brown, fine- to medium-grained, thick bedded, cross-laminated 

massive, toss laminated beds. Local mud chips________________

Dark-purpish-gray, fine- to medium-grained, platy quartzite with much feldspar. No argilfite pa

similar rock types______________________________

 

: Dark-purple, platy quartzite and siHstone with some beds of massive, cross-lamineted, pink q

base. Some fimonite speckling

Mostly covered but float and scattered small exposures confirm general fithology. Dark-purp

S3 y^s^'.^L 75 on bedding planes. Mud-chip breccia scattered throughout. Widely spaced, thin- to mediu

weathers out as prominent ribs

7? Thick-bedded, platy-weathering, highly feldspathic, fine-grained quartzite interbedded with pla

on feldspar-rich layers. Some well-developed mud-chip breccia

27 Cross-laminated, pink quartzite in beds 1 m thick interbedded'at 3- to 4-m intervals in platy, th

Medium-thick-bedded to thick-bedded, moderatery feldspathic, medium grained, medium purpli

bedded and toss fektepathic near top_____________________

.. Sequence of thin- to medium-bedded, fine-grained, feldspathic, medium-dark-purple quartzite 

weathering sandy siHstone of similar totor. Feldspar sparse to very abundant___

~ 

: Very dark purple, micaceous siftstone. Weathers to thin plates and small chips. Some purplis

purplish-gray quartnta Quartzite weathers out es prominent ribs_________

Dark-purplish-gray to medium-purple, medium-bedded, fine- to medium-grained, locally feldsp

dark maroon stftstone. Maroon argifirte films on some bedding planes. Locally well-develop

Interlayed fight-pink or pinkish-purpla, medium- to thick-bedded quartzite, very similar to that 

jcn platy-weathering, medium- to fine-grained, impure quartzite. Intel-layers of the two types ran

different proportions. Some thick-bedded purple quartzite is cross-laminated. Some dark-pu

mud-chip breccia

___ Covered 

Swauger 

Formation >iy :..-: : : Ught-grayishfpink to pale-purpla, medium-grained, hematite, medium- to thick-bedded quartzit

FIGURE 3. Incomplete columnar section and description of the L
8 CONTRIBUTIONS TO STRATIGRAPHY

The Lawson Creek Formation can be divided into three informal 

units, as shown in figure 3. These are (1) a basal transitional zone 

from the Swauger Formation about 250 m thick; (2) a middle interbedded quartzite and silty argillite unit 453 m thick, and (3) an upper thin-bedded and laminated argillite unit that is at least 577 m 

thick. The transitional zone is predominantly quartzite that is similar 

in many ways to the Swauger but generally is thinner bedded, dark 

reddish purple, less pure, and less well indurated. Thin interbeds of 

silty quartzite and siltstone become more abundant toward the top. 

Although the member is predominantly dark colored and generally 

thinner bedded, thick beds of pure, light-pink quartzite identical to 

those in the Swauger Formation are scattered through the section. 

The middle unit of interbedded quartzite and silty argillite, although 

not uniform throughout, is in general a sequence of thin-bedded darkpurple and purplish-gray, silty argillite and fine-grained, platyweathering, sandy siltstone layers, interbedded with medium- to 

thick-bedded and somewhat more resistant medium-reddish purple to 

light-pink impure to pure quartzite. In places the middle unit includes widely spaced thick beds of cross-laminated, light-pinkishpurple quartzite that are identical to the predominant lithology of the 

Swauger Formation. The quartzites and the silty and argillitic layers 

are interbedded in different proportions and at different spacings. In 

some places medium to thick interbeds are even spaced; elsewhere 

quartzite ribs 0.5 to 1 m thick are separated by several meters of thin 

bedded silty argillite. Many of the fine-grained, platy, impure quartzite beds are highly felspathic and have maroon argillite films on the 

bedding planes, micacous partings, and mud-chip breccias that have 

been formed by the disruption of the maroon argillite. The upper unit 

of thin-bedded and laminated argillite is composed of thin-bedded, 

dark-purplish-gray siltstone, argillite and very fine grained darkpurplish sandstone or quartzite with a few widely spaced crosslaminated, medium-thick beds of medium-grained quartzite. In 

general, the upper exposed part of the upper member becomes more 

argillaceous and contains much dark-purplish-gray well-laminated 

phyllite. Several beds, 1-3 m thick, of a light-gray-green, siliceous 

mudstone or porcellanite occur in the upper 200 m of the member. 

Mud-chip breccia and micaceous partings are abundant, and ripple 

marks and thin, cross-laminated, impure sandstone beds are present 

but are not common.

THE LAWSON CREEK FORMATION AT OTHER LOCALITIES

The type locality at Lawson Creek is the southernmost and most 

complete of a string of three exposures of the formation along the 

east side of the Lost River Range (fig. 2). All of these exposures are
LAWSON CREEK FORMATION, EAST-CENTRAL IDAHO 9

parts of a single west-dipping homocline of Swauger Formation 

overlain by Lawson Creek Formation. At Anderson Spring, about 

5 km northwest of Lawson Creek, the same basal contact relations 

and lithologies that have been described for the type locality are 

repeated, but only the lower 400 m of the Lawson Creek Formation 

is exposed. Farther to the northwest, between Shep and Shotgun 

Creeks, the contact of the Lawson Creek Formation with the underlying Swauger is fully exposed and undisturbed, but the transition 

zone is either missing or much compressed. At this locality, thickbedded Swauger quartzite is conformably overlain in sharp contact 

with a sequence more than 400 m thick of alternating layers 1 to 2 m 

thick of quartzite and siltstone. The quartzite is medium gray to 

purplish gray, partly mottled or speckled, thick to medium bedded, 

and somewhat impure. The siltstone is dark purplish gray, thin 

bedded, platy weathering and laminated and contains some irregular 

stringers of brown, sandy siltstone. Siltstone and argillite layers increase upward in the section and quartzite beds decrease. The rocks 

include sparse mud-chip breccias and locally have well-developed ripple marks and mud cracks, and cross lamination in the thick beds of 

quartzite.

At locality A (fig. 2) south of Morgan Creek, the Lawson Creek Formation appears to rest upon the thick section of Swauger strata that 

is well exposed on both sides of Morgan Creek. The formation and the 

basal contact are in sections 16 and 21, T. 15 N., R. 19 E., in the westcentral part of the Challis 15-minute quadrangle (Mclntyre and 

Hobbs, 1978) where they are crossed by the old access road to upper 

Morgan Creek. Here less than 100 m of strata are exposed between 

the contact with the Swauger Formation and the cover of Challis 

Volcanics, and the prevalence of thin beds of mixed lithologies, darkred-purple color, and impure quartzite show that these rocks are the 

lower part of the transition zone of the Lawson Creek Formation. A 

zone of steep dips, near-vertical fractures, and quartz veining along 

the contact with the Swauger Formation indicates some structural 

adjustment at or near the contact but probably no significant movement. However, about 3 km southeast along the contact in Fuller 

Gulch, the strata both above and below the contact are strongly 

disturbed, probably as a result of thrusting from the southwest.

At locality X (fig. 2), north of Darling Creek which is in the Twin 

Peaks quadrangle and less than 2 km from the western border of the 

Challis quadrangle, several hundred meters of strata that probably 

are part of the Lawson Creek Formation are exposed in a 2-km-long 

northwest-trending window eroded through the Covering Challis 

Volcanics. The correlation with the Lawson Creek is uncertain, 

however, because this thin and isolated sequence of rocks is structurally complex, and the presumably underlying Swauger Formation 

is not exposed.
10 CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGIC HISTORY OF THE AREA AND PROBLEMS OF 

CORRELATION

The limited known distribution of the Lawson Creek Formation is a 

result of the complex geologic history of east-central Idaho, which is 

slowly being unraveled through the efforts of many workers. A report 

by Ruppel (1978) on the Medicine Lodge thrust system, east-central 

Idaho and southwest Montana, summarized the state of knowledge 

on this area. Great thicknesses of Precambrian rocks were eroded in 

late Precambrian and early Paleozoic time, and almost all of the rocks 

of the region are cut by great flat thrusts and are allochthonous. The 

Paleozoic rocks of the region overlie the Precambrian rocks with 

strong angular unconformity, and in places the Middle Ordovician 

Kinnikinic Quartzite was deposited on the Gunsight Formation. In 

these places, all of the Swauger Formation as well as the Lawson 

Creek and Wilbert Formations are missing as a result of late Precambrian and early Paleozoic deformation and erosion.

Thrust faults related to the Antler and Sevier orogenies 

transported almost all of the rocks of the region many kilometers east 

of their original depositional basins and interleaved them in a complex of imbricate thrust plates. And steep faults of Cenozoic age further complicate the understanding of stratigraphic relations.

The Lawson Creek Formation includes the only known rocks conformable above the Swauger that have survived the tectonic events 

and erosional destruction. As far as I know, these rocks are preserved 

only in the small area in and around the north end of the Lost River 

Range. The formation is a distinctive sequence of rocks, however, and 

is unlike any of the Precambrian rocks lower in the section. These different and lithologically varied rocks indicate a major change in 

depositional conditions, from the long-continued stability that the 

thick, homogeneous Swauger rocks suggest, to a more varied and 

unstable depositional environment. Recognition of the Lawson Creek 

as a new formation, above the Swauger Formation, therefore adds 

significant new information on the post-Swauger Precambrian 

depositional history of this region.

As pointed out by Ruppel (1975, p. 18), any definitive correlation of 

the middle Proterozoic strata in east central Idaho with the better 

known Belt Supergroup units of comparable age in west central Montana is premature. In a tentative correlation table (table 1), Ruppel 

suggested that the Swauger Formation could most logically be 

equated to some part of the Missoula Group. If so, the Lawson Creek 

Formation as a stratigraphically conformable extension of this part 

of the section is also a logical correlative part of the upper Missoula 

group.
LAWSON CREEK FORMATION, EAST-CENTRAL IDAHO 11

TABLE 1. Tentative regional correlation of Proterozoic rocks in Idaho and western 

Montana (modified from Ruppel, 1975).

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East-central Idaho

Belt miogeosyncline

Lawson Creek Formation

Swauger Formation

Gunsight Formation

Apple Creek Formation

Big Creek Formation

West Fork Formation

Inyo Creek Formation

Yellowjacket Formation

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Western Montana-northern Idaho

Belt Basin

Missoula Group

Helena and Wallace Formations

Ravalli Group

Prichard Formation

Paleomagnetic sampling by Donald P. Elston and Stephen L. 

Bressler has been carried out in strata of the lower Swauger Formation exposed near Patterson in the Lemhi Range, the middle(?) 

Swauger exposed along the Salmon River near May, the uppermost 

Swauger exposed at Lawson Creek, and the lower Lawson Creek Formation exposed at Anderson Spring. Elston and Bressler (oral commun., 1979) reported that preliminary paleomagnetic directions have 

both normal and reversed polarities that are statistically identical to 

directions reported for the Missoula Group of the Belt Supergroup 

(Elston and Bressler, 1979). Because only normal polarity directions
12 CONTRIBUTIONS TO STRATIGRAPHY

have been found for Belt strata underlying the Missoula Group, the 

mixed polarity in the Swauger as well as the paleomagnetic directions indicate a correlation with the Missoula Group. On the basis of 

polarity zonation and directions, the upper Swauger Quartzite and 

Lawson Creek Formation appear to correlate best with the upper 

Mount Shields Formation, Bonner Quartzite and lower McNamara 

Formation. In addition, certain stratigraphic details of the Lawson 

Creek more closely match those found in the Mount Shields and 

McNamara Formations than those of any earlier units, and they suggest a more proximate or possibly interconnected depositional 

environment during this latest middle Proterozoic time span.

REFERENCES

Elston, D. P. and Bressler, S. L., 1979, Paleomagnetic poles and polarity zonation

from the Middle Proterozoic Belt Supergroup, Montana and Idaho. Journal of

Geophysical Research [in press]. 

Mclntyre, D. H., and Hobbs, S. W., 1978, Geologic map of the Challis quadrangle,

Custer County, Idaho: U.S. Geological Survey Open-File Report 78-1059, scale

1:62,500. 

Ross, C. P., 1925, The copper deposits near Salmon, Idaho: U.S. Geological Survey

Bulletin 774, 44 p. 

___1934, Geology and ore deposits of the Casto quadrangle, Idaho: U.S. Geological

Survey Bulletin 854,135 p [1935]. 

___1947, Geology of the Borah Peak quadrangle, Idaho: Geological Society of

America Bulletin, v. 58, no. 12, pt. 1, p. 1085-1160. 

.1961, Geology of the southern part of the Lemhi Range, Idaho: U.S. Geological

Survey Bulletin 1081-F, p. 189-260. 

Ruppel, E. T., 1968, Geologic map of the Leadore quadrangle, Lemhi County, Idaho:

U.S. Geological Survey Geologic Quadrangle Map GQ-733, scale 1:62,500. 

____1975, Precambrian Y sedimentary rocks in east-central Idaho, Chapter A of

Precambrian and Lower Ordovician rocks in east-central Idaho: U.S. Geological

Survey Professional Paper 889-A, 23 p. 

.1978, Medicine Lodge thrust system, east-central Idaho and southwest Montana: U.S. Geological Survey Professional Paper 1031, 23 p.

Ruppel, E. T., Ross, R. J., Jr., and Schleicher, David, 1975, Precambrian Z and lower 

Ordovician rocks in east-central Idaho, Chapter B of Precambrian and Lower 

Ordovician rocks in east-central Idaho: U.S. Geological Survey Professional 

Paper 889-B, 12 p.

Umpleby, J. B., 1913, Geology and ore deposits of Lemhi County, Idaho: U.S. Geological Survey Bulletin 528,182 p.

___1917, Geology and ore deposits of the MacKay region, Idaho: U.S. Geological 

Survey Professional Paper 97,129 p.

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