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'WILLIAM R. HE)'IPHILL*
'vVALTER DANILCHIK
U. S. Geological Survey
Arlington, Va., and Denver, Colo.
Geologic Interpretation ofa Gemini PhotoAn orbital photograph of the Salt Range andPotwar Plateau of West Pakistan reveals theregional significance of some geologic features.
(Abstract on page 154)
INTRODUCTION
ASTRO~AUTS COOPER AND CONRAD, duringn their 8-day Gemini V mission in August1965, took mcre than 290 color photographsof cultural and terrain features from orbitalaltitudes over North and South AmericaAfrica, Asia, and Australia. The photograph~were taken with a handheld 70-mm. Hasselblad camera, Model 500, equipped with aZeiss Planar lens of 80-m m focal length andan f:2.8 aperture. The film was An£cochrcmeD-50, ASA-50. Most of the photographs weretaken as "targets of opportunity" presentedthemselves, and as other duties permi tted.Because this was primarily a medical andhardware checkout flight, no attempt wasmade in premission planning to £elect morethan a few specific subject areas or to definelighting conditions under which the photographs would be taken.
One of the orbital photcgraphs obtained onthe Gemini V mission is of the Salt Rangeand Potwar Plateau, a semiarid region ofWest Pakistan (Figure 1) between the Indusand Jhelum Rivers. This photograph (FrontCover) wa£ taken on August 25, 1965, at0436 hours Greenwich Mean Time (0924 hoursLocal Civil Time at longitude 72°E). Thefield of view covers about 7,000 square miles.The photograph was selected for study because the authors have studied adjacent areasin the field. where geologic and geographicfeatures and conditions are similar to thosein the area covered by the photograph. Thepurpose of the study was to as£ess the usefulness of an orbital photograph in providinggeologic information pertinent to the inter-
• Pu~lication authorized by the Director, U. S.GeologICal Survey. Presented at the Annual Convention of the American Society of Photogrammetry in Washington, D. c., March 1967.
pretation of structure and identification ofrock type.
Geographic features and place names annotated on the photograph are taken fromthe U. S. Air Force Operational :\avigationChart G-6 (U. S. Air Force. AeronauticalChart and Information Center, 1963) andfrom Army Map Service Sheets ~ I-42 and 43(U. S. Army Corps of Engineers, Army MapService, 1945, 1956). Geologic features identified from perwnal knowledge of the area areidentified on the photograph by number(Front Cover). The geologic and geographicfeatures that may be delineated. or arejudged interpretable by geologists \\'ith minimal knowledge of the area, are indicated byletter or geologic symbol. Annotation ofstructural features, such as faults and foldsis restricted to the local area on the photo~graph where evidence suggesting the existenceof these features was observed directly.
WILLIAM R. HEMPHILL
150
GEOLOGIC I TTERPRETATIO OF A GEMI NI PHOTO
o 200 ",IL.ES
'...,.'u.''-'-"_...L--,_-,I
FIG. 1. 1ndex map of "Vest Pakistan showing the area covered by Gemini V orbitalphotograph shown on the Front Cover.
151
GEOLOGIC SETTING
The Salt Range forms a steep, generallywest-sou thwest-trending scarp; the a yerageelevation rises abruptly from less than 1,000feet in the plain of the Jhelum River in thesouth to more than 2.500 feet along the crestof the range. In the western part of the photographed area (Fron t Cover) the Sal t Rangecurves northward toward the town of Kalabagh. Sedimentary rocks of Cambrian andPermian ages are exposed near the base ofthe scarp, and rocks of Triassic, Jurassic.and Eocene ages crop ou t near the crest of therange.
The Potwar Plateau north of the Sal tRange is underlain mainly by rocks of Pliocene age that belong to the Siwalik Group.These rocks are gently folded into a broadeast-trending syncline in the central part ofthe photographed area but are relativelyclosely folded and faulted in the north.
STRUCTURAL FEATURES
Delineations of faults are largely based on
sharp linear truncations of outcrop patternand color. The fault zone northeast of Kalabagh, for example, shows clearly on the orbital photograph (Front Cover); studies in thefield and on aerial photographs corroborateits existence and strongly suggest that thepredominant movement was horizontal, theeastern block, comprising the Salt Range andPotwar Plateau, moving southward. If moreof the mountainous region west of the IndusRiver could be seen, it is believed that thisrelationship would also be strongly suggestedon the orbi tal photograph. Field studies haveidentified numerous east-trending high-anglereverse faults north of the Indus and SoanRivers that are not clearly expressed on theorbital photograph. These faults, individually, are less significant from a regionalviewpoint than the strike-slip fault clearlyshown northeast of Kalabagh.
Close inspection of the photograph indicates that many of the annotated linear features are expressed by alignments of shortstream segments, particularly in the region
152 PHOTOGRAMMETRIC ENGINEERING
FRONT COVER. Gemini V Photograph of the Salt Range and Potwar Plateau in north-central \restPakistan (Magazine 4, frame 19, orbit 55, exposed at 0924 hours Local Civil Time on August 25,1965).Geographic features and place names are from U. S. Air Force Operational Navigation Chart G-6 (U. S.Air Force, Aeronautical Chart and Information Center, 1963) and from Army Map Service Sheets 1\'1-42and 43 (U. S. Army Corps of Engineers, Army Map Service, 1945, 1956).
Geologic features, identified mainly on the basis of personal knowledge of the area, are labelled on thephotograph by number and are described as follows:
1. Reddish-brown or generally dark-toned areas underlain mainly by clastic rocks of Cambrian age.Outcrop is limited to the eastern Salt Range.
2. Mostly limestone of the Zaluch Group, Permian age. Mainly detrital material along base of the southflank of the Salt Range where bedrock areas of the Zaluch Group are not readily distinguishable onthe photograph from younger rocks. Ligh t-brown or white band on the photograph is remarkablypersistent and may be traced northward nearly to the Indus River.
3. Limestone of Eocene age and Mesozoic sedimentary rocks.4. Light-toned band may be an outcrop of limestone of Eocene age.5. Mainly red and brown sandstone and siltstone near the base of the Siwalik Group of late Tertiary
age.6. Light tone may be evidence of "water-logging," a process common to the Indus basin where leakage
from irrigation canals saturate adjacent areas; subsequent evaporation leaves behind a saline concentrate which renders previously arable land unfit for cultivation.
Geologic and geographic features that could be delineated and/or interpreted with minimal knowledge of the area are indicated by map symbol or letter as follows:
Fault. Queried where uncertain.
? ----(
' .." .. • ••••••• I ••• '. I •••
Linear feature. Alignments of short stream segments which are probably related to fractures in the underlying bedrock; distinction between some linear features and faults is arbitrary.
Folds. Showing trace of axial plane and bearing and plunge of axis. Queried where attitude of limbs andplunge of axis are not clear.
-- __ 1 _Inferred strike and dip of beds. Based on outcrop pattern and on topography as interpreted from streamposition and drainage pattern.
- ---Transportation route. Roads and railroads; annotated only in places where evidence of their existence maybe observed.
a Alluvial deposit. Flood plain of the Jhelum River.
b Fan deposit.
c Older alluvium (?). Possibly flat-lying terrace and pediment deposits occupying interstream areas nowundergoing dissection.
d Bedding trace.
e Drainage feature. May indicate more luxuriant vegetation or cultivation adjacent to streams where localponding by either natural or artificial means has increased moisture in the soil relative to surrounding interstream areas.
f Cultural feature. May be aircraft emergency landing strip.
north and northeast of N amlllal Lake. Suchalign men ts are com monly developed parallelto fractures in underlying bedrock, and although no exhaustive (or conclusive) studyof fracture orientation has been made in this
area, this explanation seems to be reasonable.These linear features undoubtedly would beless apparent on conventional aerial photographs because of the limited areal coverageof single prints and the abundance of di~-
GEOLOGIC INTERPRETATiO T OF A GEMfNI PHOTO 153
tracting detail resolved at larger scale. Somelinear features in other parts of the photograph show up as abrupt linear breaks in coloror outcrop pattern. Dist.inction between theselinear features and those mapped as faults isarbitrary.
I n some parts of the photographed area,attitude of the strata may be inferred fromoutcrop pattern and topography as indicatedby stream pattern. Strike and dip symbolsindicating a gentle northward dip have beenannotated near the crest of the Salt Rangeand in the southern part of the PotwarPlateau. North of the Soan River, beds aremore tightly folded, as is indicated by beddingtraces and stream pattern. Although severalfold axes may be clearly seen, direction ofdip along the lim bs of the fold can be easilymisinterpreted because of the small scale ofthe orbital photograph. Anticlinal axes canbe differentiated from synclinal axes in thisregion by observation of minute outcropdetail in the area of the fold axis with the aidof a magnifying glass. Tn terpretation of bedding attitude is also aided by the knowledgethat in this region anticlines are normallyclosed narrow structures, whereas synclinesare commonly open folds several miles across.
LITI-lOLOGIC INFORMATION
Surficial deposits such as alluvial fans,meander scars, and alluvial deposits alongthe Jhelum River may be readily identifiedon the orbital photograph (Front Cover) bythe distinctive shape and posi tion of these features with respect to present drainage systems. In the Potwar Plateau the predominantsedimentary rock type is revealed by numerous open folds, continuous linearity of outcrop pattern, and fold axes that are normallymore typical of terrain underlain by sedimentary rocks than of terrain underlain bymetamorphic or igneous rocks. Conceivably,some of the dark-toned irregularly shapedareas (Symbol 1 on Front Cover) in thesoutheastern part of the photograph couldbe interpreted as underlain by intrusiverocks; however, field studies have identifiedsedimentary rocks of Cambrian age in theseareas. Dark-toned features in the interstream areas in the east-central part of thephotograph (Symbol c on Front Cover) couldbe interpreted as flat-lying lava flows nowbeing dissected. Although these features havenot been visited in the field, they may beterrace and pediment deposits whose darktone is due to desert varnish deposited onboulders and cobbles that are known tocompose the surface of many older alluvial
deposits in this region. The dark tone couldalso be due to scrub thorn bush coveringhillslopes between alluvium-filled intermitten t streams.
Although specific rock types cannot beidentified unequivocally from the photographwithout supporting information obtained inground studies, it may be pointed out thatoutcrop pattern and tone strongly suggest theregional con ti nui ty of some of the stra tao Thebeds exposed at 5, Front Cover (identified onthe basis of field evidence as LO\\"er Siwalik),are an example where distinctive tone andoutcrop pattern permits tracing of the unitfor more than 50 miles. Light-toned beds atthe base of the southern and II'estern SaltRange (identified on the basis of field evidence as detritus and bedrock of the ZaluchGroup of Permian age) also exemplify theuse of small-scale photos in tracing the regional continuity of some outcrops.
CO:\'CLU5IO:\'S AND UGGESTIO:-,TS
FOR FUTURE \VORK
The regional significance of some geologicfeatures may be more readily recognized andin terpreted from a single photograph takenfrom orbital altitude than on conventionalaerial photographs of larger scale wherehundreds of prints would be required to viewthe same area. The Gemini V photograph ofthe Sal t Range- Potwar Plateau region of \VestPakistan covel's 7,000 square miles; withmini mal previous geologic and geographicknowledge of the area, structural featuressuch as folds and faults may be delineated onthe basis of rock-outcrop pattern, color ortone, and drainage pattern. Some stratigraphic relationships may also be recognized,bu t iden ti fication of speci fic rock type requires additional ground information andgenerally is less conclusive or reliable thanin terpreta tion of structure.
N early all orbi tal photographs to date havebeen taken with conventional color films.Color rendition of certain features known orbelieved to be brightly colored on many ofthese photographs is poor, partly becauseexcessive atmospheric scattering of shorterwavelengths in the visible spectrum impartsa blue cast to the color xposure; as a consequence, image contrast and color saturationat longer wavelengths in the green and redare reduced. For example, although red andbrownish red are diagnostic of rocks of theSiwalik Group that underlie the PotwarPlateau, these colors are poorly expressed onthe Gemini V photograph of this region.
Further experimentation may demonstrate
154 PHOTOGRAMMETRIC ENGINEERING
the usefulness of color infrared film (alsoknown as camouflage detection film from itsuse by the military during World War II)in providing high color con trast betweennatural features photographed from orbitalaltitude. The long-wavelength sensitivityof color infrared extends to about 8500A, and,properly filtered, the film is insensitive toblue light (Tarkington and Sorem, 1963). Itis believed that color infrared photographyfrom orbital altitude would combine broadbandpass sensitivity with atmospheric penetration qualities that are superior to conventional color film. Although color rendition isdistorted on color infrared photographsthat is, features are imaged in colors otherthan their true natural colors-this qualityis not judged a shortcoming for interpretation of many geologic and terrain features,particularly as the improvement of atmospheric penetration would also improve colorcontrast.
The limited number of color infrared photographs obtained on Gemini VII over theGulf Coast of the United States and Brazilare difficult to evaluate because of adverselighting conditions, filtering problems, andinadequate exposure time.* In further orbitalphotography experiments with color infraredfilm, it would be desirable to photographareas where bedrock is well exposed, such asarid regions in the southwestern UnitedStates, the west coast of Sou th America,Saudi Arabia, and \Vest Pakistan.
Use of hand-held cameras will continueon Apollo flights. Brinkmann (1966) describessome fea tures of precision 70-mm. cameras ofadvanced design for use by astronauts in aspace environment. There undoubtedly willbe future opportunities to obtain orbitalphotographs of the Salt Range-Potwar Plateau region. Stereo coverage would greatlyfacilitate geologic interpretations, particularlyof structural features of regional significance.Where hand-held cameras are used, photo-
* Richard Underwood, Manned Spacecraft Center, Houston, Texas, personal communication.
graphs could be exposed in accordance withthe principles of convergent photographyintroduced several years ago (Brucklacher,1958; Theis, 1958) as a procedure to increaseparallactic image displacement of featuresand effectively increase the vertical scalewith respect to the horizontal scale of aerialphotographs used in precision photogrammetric mapping. This approach would facilitate detection of relief differences that aresmall in comparison with orbital altitude.
I t is believed that in some areas the expression of topographic relief on orbital photographs could be improved or observed moreclearly by deliberately photographing theterrain at low sun angle. In the PotwarPlateau for example, rocks of the SiwalikGroup commonly crop out as a series ofparallel hogback and cuesta ridges, generallyless than 200 or 300 feet high. Shadow expression of relief that would be obtained on photographs taken at low sun angle would greatlyassist in the recognition of outcrop patternand structure. Shadow expression would beparticularly valuable where topography iswell adjusted to structure and where stereocoverage is not available.
REFERENCES
Brinkmann, J. R., 1966, Space photography:Space/Aeronautics, v. 45, no. 2, p. 72-81.
Brucklacher, W. A., 1958, Wide-angle convergentphotography with angles of convergences of 27°or 40°: Photogrammetric Engineering, v. 24, no.5, p. 786-789.
Tarkington, R. G., and Sorem, A. L., 1963, Colorand false-color films for aerial photography:Photogrammetric Engineering, v. 29, no. 1, p.88-95.
Theis, J. B., 1958, Increased base: height ratio:Photogrammetric Engineering, v. 24, no. 1, p.127-132.
U. S. Air Force, Aeronautical Chart and Information Center, 1963, Operation navigation chart:St. Louis, Mo.
U. S. Army Corps of Engineers, Army Map Service, 1956, Kabul, Sheet NI-42: Washington,D. C.
U. S. Army Corps of Engineers, Army Map Service, 1945, Kashmir, Sheet NI 43: Washington,D. C.
ABSTRACT: Study of the Gemini V photograph of the Salt Range and PotwarPlateau, West Pakistan, indicates that small-scale orbital photographs permitrecognition of the region(~l continuity of some geologic features, particularlyfaults and folds that could be easily overlooked on conventional air photographs oflarger scale. Some stratigraphic relationships can also be recognized on theorbital photograph, but with only minimal p'revious geologic knowledge of thearea, these interpretations are less conclusive or reliable than the interpretationof structure. It is suggested that improved atmospheric penetration could beachieved through the use of color infrared film. Photographic expression oftopography could also be improved by deliberately photographing some areasduring periods of low sun angle.