Paper G 32

APPLICABILITY OF ERTS-I TO LINEAMENT AND PHOTOGEOLOGIC MAPPING IN MONTANA -- PRELIMINAI~YREPORT

Robert M. Weidman, David D. Alt, Raymond E. Flood, Jr., Katharine T. Hc.drey, and Linda K. Wackwitz, U3iversit,# of Montana, Missoula, Montana; Richard B. Berg, Willis M. Johns, Montana t'ureau of Mines and Geology, Butte, Montana

ABSTRACT

P lineament map prepared fr~ma mosaic of western Montana shows about 85 lines not represented on the state geologic map, including elements of a northeast-trending set through central western Montana wkich merit ground truth checkSng and consiaeration in regional struc- tural analysis. Experimental fold an~lotationresulted in a signif isant local correction to the state geolcgic vap. Photogeologic mapping studies produced only 1imi ted success in identification of rock types, but they did result in the precise delinedtion of a late Cretaceous or early Tertiary volcanic, field (Adel Mountaiq field) arid the mapping of a connection between two granitic bodies shovn on the state map. Imag- ery was used successfully to map clay pans asssciated with bentonite beds in gently dipping Bearpaw Shale. It is already dpparent that ERTS imagery should be used to facilitate preparation cf a much needed statewide tectonic map and that satellite imagery moping, aided by grou~dcalibration, provides an economical means to discover and cor- rect errors in the state geologic map.

TECTONIC MAPFING STUD1ES

A 1ineame;tt map for western Montana !Fig. i)was drawn as an over- lay to a Band 7 mosaic laid directly from 9-inch prints supplied by the NASA Data Processing Facility. Most of the mapped lineaments ap- pear to be topographic expressior,; of fracture (scarps and straight canvons); a few are inclined, resistant strata. They range in length from 8 to 150 km. Lineaments were checked against the geol~gicmap of Montana (1: 5G0,000 scale, koss and others, 1955) and against newly- compi led test site geologi c maps representing approximately one quarter of the total area covered. More than 50 of them appear to be hitherto unrecognized possible faults which wi11 be investigated. Plow some 130 1ineaments annotatea, 1ines of northeast trend are much more coAlnon than was anticipated and those of northwzst trend much less so. Shadw enhancement aids significantly in recognition of 1ineaments trending In a1 1 directions except northwest-southeast.

Most of the importtnt noml faults in the area are represented on the lineament map. In the area east and north of Flathead Lake these

(;*, - u - . , .'.,7 ;;,; -. 33; te b,irch~$edf:", :. ;, ,> ., ... . -,I I -. , ,..I 3r.d bdi\l3t2 Aven!!? Falls, SIi 57198 tend to appear as scarps. In southwster.n Montana they mark tne bound- aries beween mountain blocks and Tertiary intermontane basins. Faults bounding the northeastern and northwestern sides of the Ruby Range, the west side of the Tobacco Root Mountains, and the western flanks of the Madison, Gal latin and Bridger Ranges are especial ly good examples. Lineaments mapped in southwestern Montana from ERTS imagery compare well with faults mapped by Burfeind (1967) with the aid of extensive geopiysi cal work.

Low angle faults, such as the Lewis ou?rthrust. are not readily apparent. Higher angle reverse-slip faults might be inferred from the arcuate pattern of para1 lel ridges in the Sawtacth Range structural salient about 75 miles west of Great Falls.

Short. and medi um 1ength 1 inemtents of northeast trend are prominent i7 a broad zone southeast of a line connectirtg Missoula and Great Fa1 1s; they appear to re?rec,ent a significant fracture irer~dr~ui hitherto ap- preciated. One suck lineament defines the stcaiaht, southeast-dipping flank of the Little Felt upl~ft. Anothor, followed by the valley of the Little Boulder River, trends toward the stroog!y miner?l izoa cni nina district at Eutte.

Northwest trending faults zre difficult to see because of their orientation para1 lel to the i11 umination direction. An exception is the Shroder Creek fault, clearly visibie on ERTS imagery as a linervent extending 17 miles west from Flathead Lake, but mapped with considerable difficulty by LaPcint (1971), wb7 showed that it has undergone one to two miles of right slip.

Of six prominently visible west-trending 1 ineaments crossing the Kootenai River Canyon in extreme northwestern hntana, none are shown as faults on the state geologic map and or,ly the soathernmcst two on the ; :125,090 scale geologic map (Johns, 1970). Two minor s.hocks re- corded by the Libby Dam seismic network in August, 1972 have been refer- red to epicenters a1 ong these 1ineaments (personal comnunication, GiIJ H. Crosby, University of Montana).

Rather detailed mapping of folds in differentially eroded Pale- ozoic and Mesozoic striita is possible for arcas of moderzte relief and scanty forest cover. Study of the area between the Clkhorn and Crazy Mountains indicates that the low sun angle imagery of Novem- ber (1106-17465-7) is much better for such mapping than that taken in August. Ridge patterns often ailow delineation of plunging folds with wave lengths of a few kilometers; resolution of axial traces 1.6 km. apart is possible. Land form indication of dip direction allows identification of fold type at scattered locations. The most successful fold mapping uti1 ized 1: 500,000 scale enlargessnts under overlays and at 2r magnification under the zoom transfer sco9e. Annotation of a plungltig fold pair zuggests a significant correction to the state geoiogic map tor nnse pattern and trend, involving a shift of an axial trace by as much as 2.6 km.

PHOTOGEOLOGIC MAPPING STUDIES

Efforts to develop criteria for photo1 ittiologic recogni t-Ion of rock ty~esand rock un'ts frcn EkTS imagery have met with limited huccess. It is possible, vith some assistance from ground truth infomation, to delin2ate the boundaries of certain major rock bodies, includjng those of Cenozoic bas'n f i11 , upturned Paleozoi c and Mesozoic strata, and the igneous rock bodies discussed below.

Large volzanic piles are distinctive in ESTS imagery because cf tbpi r t~nalqual ity, the?r lack of strongly apparent inte~,nal geologic structure cr structarally control led drainage patterns, and their tendency to bury structures evident in adjacent oldsr rock;. The Adel Mountain vol cani cs , of latest Cretacsous or earl iest Tertiary age, were successfull.y delinedted with only minor dooartures from the outlines sho~non published maps, as were satellits laccoli ths to the north (Fig. 2). Four topographically expressed dikes in a swarm extendiq northward from the main body of volcar~icswere recognized even though the?r thicknesses are well below resclution 1imi ts. Other large volcanic piles in wes :ern Montana, such as the El khorn Mountains and Lowland Creek fie;ds, are less apparent, but the latter was mapped with considerable success. Outlying areas of structurally deformed Elkhorn Mountsins volcanics were not recognized in imagery.

Large granitic bathol iths, such as the Bouyder batholi thy are moderately distinctive in ERTS ililagerj because of their intermediate tonal qual ity, lack of apparent internal geologic structure, and strcng tevdeilcy for control of drainage patterns by fracture sets. Smaller grar~itic intrusions are more readily apparent through their structura? effect on the country rock than chrough any distinctive qual ity in the appearance cf the intrusive r~ckitself. Eff~~tsto delineate the ~~tlines of bathol ithic intrustons of granite in the Pioneer Mountains led to a map (Fig.3) showing both strong correlations and major depart- ures from the outl ines shown on the st~tegeologic map. Both the correlations and the departures are believed to be significant and this area will be examined during the coming field season.

Small granitic stocks and 1accol ithic intrusions of a1 kal ine rocks are numerous in several parts of the area covered by our imagery. These tend to draw attention to themselves through their darker tonal qual itj, a matter of vegetative cover rather than of rock tone, but other criteria make them distinctive. Laccol iths normally exhibit ci:.cular outl ines and domal form, whether or not their igneous cores are exposed. Upturned edges of sedimentary beds along the flanks of bcth laccoli ths and granitic stocks are clearly evideiit in ERTS imagery. BEARPAW SHALE BENTOhITE STUDv

The possibility of recognizing Bearpaw Shale known to contain bentonite beds of economic potential was irvestigated for an area on the flanks of the Ingomar anticline where ground truth is we1 1 krlokn (Berg, 1970). The area is about 130 km northeast of Billinqs. Using a 1/500,000 scale negative print, an overlay flap (Fig.4) was drawn to show highly reflective zreas of irregular pattern as well as a continuous tonal boundary thought to represent a 1 itllologic contact. The i-esults were compared to a ground truth map and large scale aerial photographs covering most of the bentonite deposits iq the area.

Known bentonite exposures in the area are too small to be recognized directly in the ERTS imgery, but the irre~ul ar reflectivc patches may be a clue to their occurrence. These areas of high reflectivity are surficial clay pans, mud-fi 1led channels and depressions, that develop on the Bearpaw Shale where dips dre less than 3 degrees and bentonite beds well exposed. Further work may show that areas of high reflectivity can be used for recoqnition cf those areas within the Bearpaw Shale where bentonite beds are exposed.

CONCLUSIONS

ERTS imagery is potentialiy a major tool for improving geologic knowledge of Montana. Lineament mapping ha: dl I-eady successful 1y identified previously unmapped faults and emphasized the importance of northeast-trending fractures. Experin~entalanrdtation of folds has demonstrated that imagery may be usec' to correct errors in trend and location of axial traces inferred from the state geologic map. Satellite imagery should be used to expedite preparation of a much needed tectonic map of the entire state at a scale as large as 1:500,000. Use of imagery for tectcnic mapping should result in significant saving in time and money, as well as jdentification of previously unmapped faults. A probable b01i2~would be the extensidn of known faults and the recognition of patterns f?r qhorter 1ineaments important to regional structural analysis.

Photolithologic recognition of at least some major rock units is possible with the help of limited ground truth inform~tion. Delin- eation of their boundaries as expressed in ERTS imagery should lead to significant refinement of the state geologic map. REFERENCES

Berg, R.B., 1970, Bentonl te deposl ts In the Ingomar-Vananda trea, Treasure and Rosebud Counties, Montana: Montana Bureau of MInes ana Geology Spec. Pub. 51, Burfelnd, W.J., 1967, A gravity lnvestlgatlon of the Tobacco Root Mountalns, Jefferson Basln, Rourder bath01 i th, and adjacent areas of southwestern Montana: urlpubl . Ph.D. th,?sls, Dept. of Geology, Indfana Unlv. Johns, W.M., 1970, Geology and mlnersl deposits of Lincoln acd Flathead Count1 es, Montbna: Montana Bureau of MInes and Geology, Bull. 79. LaPoint, D. J.', 1971, Geology and geophysics of thr. southwesitrn Flathead Lake reglon, Montana: unp~S1. Master's thesis, Dept. of Geology, Unlv. of Montana. Ross, C. P., -Andrews, D.A. and Wltklnd, I.J., 1955, Geologic Fap of Montana: U.S. Geological Survey. Hachured 1ines represent scarps. Plain 1 ines show straight canyons.

--LL known fault specifically recognized as imagery lineament

.---'-'o 1ineament not represented on state or test site geologic maps _ _ - - inclined, resistant strata

Fig. 1. Preliminat-y ERTS-1 lineament map of western Montdna, drawn originally at 1:1,000,000 scale on overlay of Band 7 mosaic for late August, 1972. # - Missoula, Bu - Butte, Bc - Bozeman, H - Helena, G - Great Falls, KR - Kootenai River, CF - Clark Fork of Columbia, FR - Flathead River, FL - Flathead Lake, MR - Missouri River, YR - Yellowstone River.

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