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II DEPARTMENT OF THE INTERIOR UNITED STATES GEOLOGICAL SURVEY

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MONTANA •

LOCATION OF AREA

AEROMAGNETIC MAP OF THE BIG SNOWIES WILDERNESS AND CONTIGUOUS RARE II STUDY AREAS, FERGUS, GOLDEN VALLEY, AND WHEATLAND COUNTIES, MONTANA

Bv

Carl L. Long

1981

109°00 47°00'

T. 13 N.

' ··, T. 10 N

Geology from Lindsey (1980)

Studies Related to Wilderness

The Wilderness Act (Public Law 88-577, Septembe'r 3, 1964) and related Acts require the U.S. Geological Survey and the U.S. Bureau of Mines to survey certain areas on Federal lands to determine their mineral resource potential. Results must be made available to the public and be submitted to the Administration and the Congress. These maps and reports pre'sent the results of a geological and mineral survey of the Big Snowies Wilderness and contiguous RARE II study areas.

Qal

EXPLANATION

LIST OF MAP UNITS

ALLUVIUM (HOLOCENE)

Qls LANDSLIDES (QUATERNARY)

Qp PEDIMENT DEPOSITS (QUATERNARY)

Khc HELL CREEK FORMATION (UPPER CRETACEOUS)

Km MONTANA GROUP (UPPER CRETACEOUS)--

Kc

Kk

Jme.

Pa

Hbs

Mme

Hl

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f;f

Ym

Exclusive of Hell Creek and Telegraph Formations

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TELEGRAPH CREEK FORMATION (UPPER CRETACEOUS) AND COLORADO GROUP (UPPER AND LOWER

CRETACEOUS), UNDIVIDED

KOOTENAI FORMATION (LOWER CRETACEOUS)

MORRISON FORMATION (UPPER JURASSIC) AND ELLIS GROUP (UPPER AND MIDDLE JURASSIC)

AHSDEN GROUP (PENNSYLVANIAN)

BIG SNOWY GROUP (UPPER MISSISSIPPIAN)

MIS S ION CANYON LIMESTONE (UPPER AND LOWER MISS ,ISSIPPIAN)

I LODGEPOLE LIMESTONE (LOWER HISSISS1PPIAN)

JEFFERSON FORMATION (UPPER AND t!IDDLE DEVONIAN)

UPPER PJ RT SNO\IY RANGE FORHATION (LOWER ORDOVICIAN AND UPPER CAMBRIAN?)

UPPER A~ D MIDDLE CAHBRIAN FORMATIONS, UNDIVIDED

FLATHEAD SANDSTONE (MIDDLE CAl1BRIAN)

NEIILAND I FORMATION (PROTEROZOIC Y)

CONTACT

FAULT--dashed where approximately located; dotted where concealed. Ball and bar on downthrown side

STRUCTURE SYMBOLS--Showing plunge. Dashed where app1roximately located

Anticline

Sy~cline

STRIKE j AND DIP OF BEDDING

DRY HOLE

--58l0D--

CJ MAGNET I C CONTOUR--Contour interv_al 20 and 100 gammas,

wi th 10-gamma supplemental cont.ours (dashed). Hachures indicate areas of lower magnet i c i ntensity

I FL IGHTI LINE

BOUNDARY OF BIG SNOWIES WILDERNESS AREA

BOUNDAf Y OF CONTIGUOUS RARE II STUDY AREAS

INTRODUCTION

Aeromagnetic surveys were conducted in the Big Snowies Wilderness study area, Montana, to supplement geological and geochemical studies of .the area (Lindsey, 1980). The aeromagnetic survey was flown by Aer o Service Corporation under contract to the U.S. Geologi ~al Survey, Denver, Colorado and covers primarily the wilderness study area. These magnetic data provide information on structural relationships and subsurface geology, which, in turn, directly aff ects! assessments of oil, gas, and mineral resource potential of the Big Snowies Wilderness and contiguous RARE II study area .

Data Collection

The magnetic survey was flown in 1978 at a constar:it barometric elevation of. 2743 m (9,000 ft). Flight I lines were oriented north-south and spaced approximately 1.6 km (1 mi) apart.

The data have been compiled and projected on a Universal Transverse Mercator projection at a map scale 1of 1: 62,500, using contour intervals of 20 and 100 gammas, and 10-gamma supplemental intervals. The regionfl magnetic gradient removed was the IGRF (1 975>1978.67) with a bias of. 58 1 513 gammas at the survey j center, and the total field constant was added to the residual values. The map was then reduced to

' 1 : 100,000-scale for direct comparison with geology and gravity maps.

Dominant Features

P1art of a large magnetic high coincides with the Wheatland syncline in the southwestern two thirds of the study area. This high is separated from another magnetic high near the eastern edge by a low saddle. These magnetic anomalies are parts of a larger high and low magnetic feature that is apparent on a larger regional map by Zietz and others (1971) (fig. 1). The anoma :il ies cover a large area, part of what Sonneberg (1956)1 called the Central Montana uplift or Central Montana platform.

1 major difficulty in interpreting the aeromagnetic data was that t he scale of the magnetic features is such that there .i s little in the magnetics ~hich I is relevant to the small study area. Little magne t ic relief i~ present in the area; this is probably due to the non-rrw.gnetic character of the thick sedimentary section.

110°30' 47°15'

110°00' -, '~

' I

109°30'

"

MISCELLANEOUS FIELD STUDIES MAP MF-1243B

AEROMAG, BIG SNOWIES WILD. AREA, MONT.

• 110"00 1 09 00

47°00""1'::::"'i:i;:i;--r--r"C"""-.::::::r~-r---,::-----------,;;::=::::::::,~-;

EXPLANAT I ON

Q Q Q

-1000- MAGNETIC CONTOUR--Contour interval 20 gammas. Datum is arbitrary Hachures indicate areas of lower magnetic intensity

---!$!--- AXIS OF BIG SNOWY ANTICLINE

---!•r-- AXIS OF WHEATLAND SYNCLINE

-- ------ APPROXIMATE BOUNDARY OF BIG SNOWH'.S WILDERNESS AND CONTIGUOUS RARE II STUDY AREAS

Figure 1.--Aeromagnetic map (from Zietz, 1971) of the 'Big Snowies Wilderness and contiguons RARE II study areas and vic i n i ty, Montana.

109'CJO'

INTERPRETATION

Magnetic anomalies in the study area are part of a large, circular, anomalous magnetic high to the south mapped by Zietz and others (1971), shown on figure 1 . This anomaly covers part of the Cat Creek­Hinsdale block, which was structurally and topographically low before Late Jurassic time ('Th.omas, 1974) . The block began to rise dur'ing Late Jurassic time, as can be shown by the lack of the Jurassic Rierdon Formation south of the Big Snowy Mountains (Lindsey, 1980). The circular anomaly is probably related to a deep body of igneous rock that may have been intruded into a zone of weakness in the crystalline basement beneath the Cat Creek-Hinsdale block.

A magnetic low along the north side of the wilderness study area is in the same region as the east-west magnetic lineaments mentioned by Zietz a:nd others (1971) and may be indicative of a deeply buried shear zone noted by Smith (1965).

The gentle magnetic gradients in the study area reflect contrasts in rock magnetization at depths estimated to be 15-20 km. The magnetic data give no evidence of a near-surface intrusive or other magnetic rocks near the surface, thus providing verification that the "Big Snowy anticline is a tectonic fold and not a dome above intrusive rock.

SUMMARY

The relatively g~ntle magnetic gradients in this area indicate that no near-surface magnetic intrusions underlie the study area. The hypothesis of a thick section of non-magnetic Paleozoic and MesozoiC sedimentary rocks is supported by aeromagnetic and gravity data and maps; folding produced few if any conduits for intrusive and extrusive igneous activity. Structural and lithologic contacts are more evident from the gravity map (Long, 1981) than from the magnetic map.

REFERENCES CITED

Lindsey, D. A., 1980, Geologic map and sections of the Big Snowies Wilderness and contiguous RARE II study areas, Fergus, Golden Valley, and Wheatland Counties, Montana: U.S. Geological Survey Miscellaneous Field Studies Map MF-1243A.

Long, c. L., 1981, Gravity map and modeled profiles of the Big Snowies Wilderness and contiguous RARE II study areas, Fergus, Golden Valley, and Wheatland Count~es, Montana: U.S. Geological Survey Miscellaneous Field Studies Map MF-1243C.

Smith, J. G., 1965, Fundamental transcurrent faulting in Northern Rocky Mounta.ins: American Association of Petroleum Geologists Bulletin, v. 49, no. 9, P· 1398-1409.

Sonnenberg, F. P., 1956, Tectonic patterns of central Montana, in Judith Mountains, Central Montana: Billings Geological Society 7th Annual Field Conference, Guidebook, P· 73-81.

Thomas, G. E., 1974, Lineament .... block tectonics, Willistmn-Bl ood Creek basin: American Association of PetroleLDD. Geologists Bulletin, v. 58, no. 7, P· 1305-1322.

Zietz, Isidor, Hearn, B. c., Jr., Higgins, M. w., Robinson, G.D . , and Swanson, D. A., 1971, Interpretation of an ae.romagnetic st·rip across the northwestern United States: Geological Society of America Bulletin, v. 82, no. 12, p. 3347-3371.·

108°30'

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BOUNDARY OF LEWIS A.ND CLARK NATIONAL FOREST

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Roundup

• A.REA. OF 81G SNOWIES

WILDERNESS STUDY AREA

. AREA. OF RARE II STUDY AREAS

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INTERIOR-GEOLOGICAL SURVEY, RESTON, V IRGIN_IA-,1961

For sale by Branch of Distribution, U.S. Geological Survey, Box 25286, Federal Center, Denver, CO 80225 INDEX MAP

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