terrain photography from gemini spacecraft final geologic report

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TER R A IN PH O TO G R A PH Y FR O M G EMIN I SPA C EC R A FT:FIN A L G EO LO G IC R EPO R T

Paul D. Lowman, Jr.H e r b e r t A . Tiedemann*

J a n u a r y 1971

G O D D A R D SPA C E FLIG H T C EN TERG r e e n b e l t M a r y l a n d

"Now with Trollinger-Gosney and Associates, Inc. 2150 So. Bellaire St., Denver, Colorado 80222.


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TERRAIN PHOTOGRAPHY FROM GEMINI SPACECRAFT:FINAL GEOLOGIC REPORTPaul D. Lowman, Jr.

Goddard Space Flight CenterHerber t A. Tiedemann

Manned Spacecraft Center*

ABSTRACTThis paper sum mar izes the objectives, methods equipment , and maingeologic results of the Synoptic Terrain Photography Experimeot (SOOS)carriedout by the as tronauts during several Gemini missions in 1965-1966. The objec-tive of the SO05 experiment was to obtain 70mm color photographs of the ear th' ssurface for geologic , geographic, or oceanographic s tudy . The pictures weretaken by the crew s using hand-held cameras with 38 mm, 80 mm , and 250 mmfocal length lenses; approximately 1100 usable for geologic purposes w ere ob-tained, covering various areas between 32"N and 32"S. Geologic applicationsof these photographs reported here include the following. (1) An unmappedQuaternary volcanic field in nor thern Mexico has been discovered, includingover 30 volcanoes and associated basal t flows and pyroclastics. (2) Picturesof northern Baja California indicate that the Agua Blanca fault has not had majorlateral movement as a whole, and is not one of the transform faults along whichthe Gulf of Cali fornia is thought to have opened by s ea floor spreading. (3 ) TheTexas Lineament in southwest New Mexico and southeast Arizona has been shownto be a broad belt of intergrading folds and faults, ra the r than a major wrenchfault. (4) Severa l thousand sq uare miles in North Africa have been found to beeroded primarily by deflation and wind abras ion, suggesting that the importanceof wind erosion in the Sahara is far gr ea te r than in North American and otherdeserts. (5) High areas in dese rt s shown on Gemini photographs are almostinvariably dark er in color than low areas, except for i rrigated far ms , suggest-ing that the dark areas of M a r s are relatively high regions of bedrock and thelight areas lower regions of windblown sand. (6) Speculative thinking on tec-tonic questions of southwest A s i a , India, and North America has been stimu-lated by unexpected views of regional geologic str ucture .The SO05 experiment has demonstrated the potential value of orb ital photographyin natural reso urc e management, regional tectonic studies geologic mapping,

*Now with T rollinger-Gosney a nd Associates, Inc. 2150 So. Bellaire St., Denver, Colorado 80222.

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geomorphology, and geologic education. It has cla rified the problems of orbitalphotogeology, including lithologic determination from space, vegetation and soilcover, t ime requirements for field checking, and resolution limi ts. Most im-portant is the stimulus given to the NASA Ea rth Resou rces Pro gram, to theUSGS Earth Resources Observation Satellite Pro gra m, and to remote sensingin general. It has also provided a demonstration of the scientific value of manin space and of the Apollo lunar landing program (of which the Gemini miss ionsw e r e part).

Note: Dissemination of all NASA earth-orbita l photographs is the responsibilityof:

Technology Application CenterUniversity of New Mexico

P.O. Box 181Albuquerque, New Mexico 87106.

Most Gemini photographs have been prin ted, in color , in the se publicationsEarth photographs fromGemini 111, IV, V(NASA SP-129; $7.00)Earth photographs fromGemini VI through XI1(NASA SP-171, $8.00).

These may be ordered from the U. S. Government Printing Office,Washington, D. C.' 20402.

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C O N TEN TSPage

A B STR A C T . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiINTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . 1TH E SY N O PTIC TER R A IN PH O TO G R A PH Y EX PER IMEN T . . . . . 3

O b j e c t i v e a n d M e t h o d s . . . . . . . . . . . . . . . . . . . . 3C a m e r a s a nd Films . . . . . . . . . . . . . . . . . . . . . . 7P R O B L E M S I N P H O T O G R A P H R E P R O D U C T I O N . . . . . . . . . . 11

O b v e r s e Prints . . . . . . . . . . . . . . . . . . . . . . . . 11Poor C o l o r Balance . . . . . . . . . . . . . . . . . . . . . . 1 2C o n t r a s t L o s s in B l ack-and-Whi t e P r i n t s . . . . . . . . . . . . 12U n k n o w n S c a l e . . . . . . . . . . . . . . . . . . . . . . . . 1 2C ropp i ng . . . . . . . . . . . . . . . . . . . . . . . . . . 12C a r e o f F i l m . . . . . . . . . . . . . . . . . . . . . . . . . 12

G EO LO G IC R ESU LTS O F TH E SO05 E X P E R I M E N T . . . . . . . . . 14Palomas V o l c a n ic F i e l d . . . . . . . . . . . . . . . . . . . . . 14T e c t o n i c s of N o r t h e r n B a j a C a l i f o r n ia . . . . . . . . . . . . . 2 3The Texas L i n e a m e n t . . . . . . . . . . . . . . . . . . . . . 29C o m p a r a t i v e Importance of Wi nd Er os i on in A f r i c a andN o r t h A m e r i c a . . . . . . . . . . . . . . . . . . . . . . 40

50The P h y s i o g r a p h y of M a r s . . . . . . . . . . . . . . . . . . .GEOLOGIC SPECULATIONS . . . . . . . . . . . . . . . . . . . . 55

N a t u r e o f t h e O m a n L i n e. . . . . . . . . . . . . . . . . . . . 55D i s c o r d a n t C o a s t l i n e s . . . . . . . . . . . . . . . . . . . . . 60U ni quene s s of the B as i n - and-R ange Prov i nce . . . . . . . . . . 62SUMMARY AND CONCLUSION . . . . . . . . . . . . . . . . . . 64R E F E R E N C E S . . . . . . . . . . . . . . . . . . . . . . . . . . 69

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TERRAIN PHOTOGRAPHY FROM GEMINI SPACECRAFT:PRELIMINARY GEOLOGIC RESULTS

INTRODUCTIONDuring the ten manned orbita l flights of the Gemini Prog ram, the astronautstook over 2400 70mm color photographs for various purposes. Among thesewere pictur es of the eart h's surf ace taken as pa rt of the Synoptic Te rr ain Pho-tography Experiment (SOOS), whose objective was to obtain photographs of se -lected areas fo r geologic, geographic, and oceanographic study. The purposeof this paper is to summarize the objectives, methods, equipment, and maingeologic results of the (5005) experiment. A iurt her purpose is to discussthe advantages and disadvantages of orbit al photography for geologic purposesin the light of experience from the Gemini Program .The success of the terrain photography experiment was due to the work of manypeople and organizations. First among these are the ast ronauts (named inTable I) who took the pictures, frequently under difficult circumstances; itshould be made clea r that these men were cooperating scientists a s well asengineering test pilots. Other Manned Spacecraft Center personnel who madevital contributions to the photography were R. D. Mercer and R. W . Underwood,experiment monitors , the members of the Experiments Section, Flight CrewSupport Division, and the Photographic Technology Laboratory. Valuable sup-port and encouragement were provided by D r s . John A. O'Keefe (Goddard SpaceFlight Center) and Jocelyn G i l l (National Aeronautics and Space Administration).Field work in Baja California and Chihuahua was done in cooperation with hgs.G. P. Salas, F. Guerra Pena, and F. Garcia Castaiieda, of the Consejo deRecursos Naturales No Renovables , Mexico, who are conducting ;tz1 independentinvestigation with the Gemini photographs. Finally, w e have benefited fromgeologic discussions with many colleagues, especially W. A. Fischer, S. J.Gawarecki, C. R. Warren, and W. R. Hemphill (U. S. Geological Survey),R. L. Stevenson (Bureau of Commercial Fisheries) , H. W. Blodget and J. M.Mead, (Goddard Space Flight Center ), E. Sherkarchi (U . S. Bureau of Mines),M. Abdel-Gawad (North American Rockwell), C. C . Reeves (Texas Technolog-ical College), P. E. Damon (University of Arizona), and J. W. Salisbury(Ai r Forc e Cambridge Research Laboratory)eThe Gemini terrain photography experiment was the outgrowth of a similar ex-per iment carr ie d on the MA-8 and MA-9 Mercury flights in 1962 and 1963(Lowman, 1964), in which a number of geologically usable pictures (chiefly ofTibet) we re obtained. Ea rl ie r, sev era l hundred 70 mm color high obliques, in-cluding many of North Afr ica, had been retu rned by the unmanned MA-4 flight;

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although not as good as later pictures these were successfully used by Morrisonand Chown (1964) in the first regional application of orbital photography. Valua-ble intensive analyses of sounding rocket photography we re done by Merifield(1964). Reviews of the geologic application of orbi ta l photography have beencompiled by Lowman (1967, 1969) and Garcia (1966). An ear ly version of thisreport was published in "Short Course Lecture Notes, Remote Sensing" (Reeves,et al., 1968).A number of geologic studies using the Gemini and Apollo photography a re inprogress as this paper is written; these include investigations of the tectonicsof the Red Sea by M. Abdel-Gawad, the tectonics of Saudi Arabia by H. W.Blodget , sedimentation phenomena by F a J. Wobber, geology of northern BajaCalifornia by G. P. Salas, G. Guerra Pefia, and F. Garcia Casteneda, andst ructur e of the Delaware Basin by R. V. Trollinger.

THE SYNOPTIC TERRAIN PHOTOGRAPHY EXPERIMENTObjective and MethodsThe original objective of the Synoptic Te rr ai n Photography Experiment (S005)w as to obtain high-quality , small-scale color photographs of selected land are asfor geologic study (Lowman, 1966). However, the striking quality and coverageof the pictu res f ro m Gemini 111, IV, and V demonstrated the value of such pho-tography in other fields. Accordingly, the scope of the experiment was expandedfrom Gemini VI-A on to include ar eas of geographic and oceanographic int ere stas well. Meteorological photography was covered by the Synoptic Weather Pho-tography Experiment (S006), with the same came ras and films. Principal in-vestigators for this experiment were K. M. Nagler and s. D. Soules of theEnvironmental Science Services Administration. It should be st res se d that theSO05 and SO06 experiments were only two of a total of fifty-two car rie d on theten manned Gemini flights ( Foste r and Smistad, 1968). Furth ermore, the ex-periments were all subsidiary to the main objectives of the Gemini Program,which we re essentially to develop the capability for long, complex space mis-sions (Mueller, 1967).The areas fo r ter ra in photography we re picked before each mission (Fig. 1)onthe bas is of previous coverage, availability (latitude, daylight and probableweather) scientific value, and specific request s fr om cooperating agencies(chiefly the U . S. Geological Survey, Navy Oceanographic Office, and Bureauof Commercial Fish erie s). The area list and prior itie s varied from one flightto another, but the are as generally de sired were , in order of importance, thefollowing:

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S. W. United States and nor thern Mexico (for coverage of a geologically im-portant area with considerable ground truth available)Northern Red Sea and adjacent land ar eas (for study of the Rift Valley)North Africa (a well-exposed shield area; fo r study of regional fra ctu repatterns, volcanic fields, vegetation zones, and possible impact structures)Ea st Africa (southern pa rt s of the rift valleys)Northern South America (Andean structure)W e s t Pakistan (area under study by U. S. G. S. , with potential mineral de-posits m d considerable ground truth)Southern India (area under study as part of Upper Mantle Project)N. W. Australia (well-exposed shield area; to study regional fractu repatterns and possible impact struc ture s)S. E. Brazil (shield are a with mineral deposits; also , study of continentaldrift)Ocean off major river mouths, such as the Mississippi, Amazon, Congo,Ganges Irrawaddy, and others (effluent patterns and sedimentation).

In addition to th ese , a large number of relatively small, specific areas were in-cluded in the SO05 experiment plan; many of these were oceanic islands requestedby the Navy Oceanographic Office, or specific physiographic features . Thecrews on later m issions were asked to photograph the glitter pattern (sun's re-flection) whenever possible.Because of the many types of photography requ ired of the astronauts during theGemini Progra m, they were intensively train ed in this subject by the Photograph-ic Technology Laboratory. Fo r the te rr ain photography, the crews were givenone or two briefings covering the following subjects.1. Experiment objectives (Fig. 2): These were as stated previously. In

briefings fo r later flights, some time was devoted to summarizing progre ssto date , with examples of good and bad photography from previous missions.2. Areas to be covered: With the aid of flight path maps (Fig. l), the area s

desired were discussed in orde r of pri ority, with as much time as possible

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EXPERIMENTs-005SYNOPTIC TERRAIN PHOTOGRAPHY

PURPOSETo improve and extend t h e techn iqu es of syn opt ic geolog.ic and topographicaer ia l photography.SPACECRAFT SYSTEMS CONFIGURATION1. A t t i t ude c on t r o l - F'ULSE2 . Unstow - 70mm Camera- Ring s ightFilm magazine80mm l e n sPROCEDURES1. Assemble and prepare camera and acc ess ori es f o r photography.2. Record time, sequence o r subject, mode, magazine number and frame numberon the onboard voice re cord er and/or l o g book.3 . P i t c h spa ce c ra f t s t r a i gh t dow n, i f poss ib le , to ob ta in photographs ofs e l e c t e d t e r r a i n .

Control roll as required to keep sun glare from spacecraft window.4.MODE ASE&010,2030405060708091011

VODE B

- S t r i p phot os - one frame every 5 see.LOCATI O NSouthern IndiaLower Baja Ca l i f o r n i aWest Pakis tan & Gulf of CambayAfrican R i f t Valley.Northwest So . AmericaSouth Mexico, Yucatan Penn, Yucatan S t r a i t s4 2 o r 3 framesMiss i s s ipp i DeltaGanges DeltaAmazon DeltaBay o f BengalArabian Sea

Figure 2. Instructions for SO05 experiment i n fligh t pla n for Gemini 12 mission;areas listed are shown on fl ig ht path map (Fig. 1) . Flight plan pre-pared by Fl ight Crew Support Division, Mission Operations Branch,Manned Spacecraft Center.

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being spent on the geologic reasons fo r studying each region. In all brief-ings> it was s tr es se d that good pictures of-any land area would be of valueif planned ar eas could not be covered because of clouds or other reasons .

3. Techniques (Fig. 3): The cre ws wer e requested to take vertical picturesat 5 second interva ls to obtain 25-mile separation between photographs androughly 60% overlap at normal altitudes (generally between 100 and 200statute miles). Recommended time fo r photography w as local noon plus orminus thr ee hours, to avoid having to change camera settings, which weregenerally f 11 at 1/250 second for Ektachrome. Measures to avoid str ayreflections and scatt erin g fro m the windows were a lso discussed.

The crews followed the planned procedures (Fig. 4)as much as possible; thei rskill and perseverance in this is demonstrated by the quali ty and quantity of thepictures returned. Three main problems were encountered. First, fuel orelec trical power rest rict ions (preventing use of the iner tial platform) frequentlyprevented the crew fro m pointing the spacecraft straight down; because of th is ,many of the ter ra in pictures a re high obliques. Second, cloud cover , especiallyover jungles, frequently obscured the prim ary areas. Finally, window obscura-tion by deposi ts fr om boost phase ablation, rocket exhaust during staging, andwindow gasket degassing degraded some pic tures. This problem was especiallysever e on Gemini VII.A detailed discussion of operational difficulties has been presented by Lowman(1969).Camer as and FilmsThree 70mm hand-held cam era s (Fig. 5) were used during the 10 mannedflights fo r operational, weather, and te rrain photography (Thompson, 1967;Underwood, 1968)1. Hasselblad 500-C - This c amera , modified for space use and equipped witha 65-frame capacity magazine by Cine Mechanics, Inc., was the basic cam-

e ra for most ter ra in photography. The lens used for most pictures was thestandard 80m m Zeiss Planar; a number of pictures were taken on GeminiV II with the 250 mm Zeiss So mar telephoto lens.

2. Maurer Space Camera - This camera was developed especially for astro-naut use, and could accommodate a wide varie ty of components, Te rr ai nphotography with the Maurer was done with the 80 mm Scheider Lens.

3. Hasselblad Super Wide Angle - The 90" field of view of the Zeiss Biogonlens made this camera useful for general purpose photography, but a large

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X P E R l f d E h T 5 -5~ Y ~ ~ O P T I ~TE RR A% ) PtiOTOGRAPHY

1 R F R I CA 24:OO 24:ZO 162 A F R I C A 43:10 43:15 283 U.S. 48:50 49:OO 37TAKE PHOTOS BETBlEEN 0900 ti 1500 LOCAL TIMEPROCEDURESU NSTON :HASSELSLAD C f i 4 E R APHOTO-EVENT 1I . iUICATORR I N G S I G H TUV F I L T E RFIU4 BACKVERIFY AUX RECP - OFFP R E P A R E T H E H A S S E L 6 l A D CAMERA & A C C E S S O R I E S FORPHClTOGRAPHYMARK TIME AND RECORD EXP TQ BE ACCOMPLISHED ONONBOARD VOICE RECORDERCONTROL S/C A T T AS R E Q a D T O O B T A I N P H OT O S OFS E L E C T E D T E R R A I NMARK EACH PHOTOGRAPH ON THE VOICE RECORDERCOIiTROL ROLL AS REQQ TO K E E P T H E S/C WINDOWI N THE SHADEIF WEATt l ER PERMITS, THE PASS OVER THE SOUTHWESTERN

A U X RECP - or4A T T C N T L - P U L S E

LL BE USED TO O B T A I N S T R I P PHOIQS (ONEWEATHER DOES NOT PERF4IT ST R I P PHOTGRAPHY OVER

H EVERY 5 S E C , 35 PHOTOS)MPTED OVER EAST ~ F R ~&

Figure 3 . Instructions for SO05 experiment i n crews fli ght booklet forGemini 4; prepared by Flight Crew Support Division, MSC.

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S-5, S-6, & GEN. PHO3'O.

Figure 4. Notes on SO05 and SO06 experiment taken during flightby Gemini 4 crew J. A . McDiv it t and E. H. White, I I .Details of photography recorded by crewwere helpful i nindexing and interpreting pictures.

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number of good te rr ai n photographs were also taken with it despite the short(3 8 mm) focal length. It was used with the s ame magazine a s the Hasselblad500-C.

Although standard camera settings, determined by flight testing by the Photo-graphic Technology Laboratory before the missions , w ere generally used, aNASA-modified Honeywell Pentax 1"/21" narrow-angle spot meter was used onsome flights to determine exposures. This was especially useful in situationsinvolving a wide brightness range, such as photographing other spacecraft dur-ing rendezvous.Haze fi lt er s (Haze 50 or 6 3 ) , cutting off light below 3400A, were fit ted to theHasselblads when they were used with ordinary color film. When used withEktachrome Infrared, an infrared fi lt er cutting off light below 5000 angstromswas fitted. N o filters were used for terrain photography with the Maurer 70mmcamera. Polarizing filters were not used because the spacecraft windows werestr es sed , producing stra in polarization.Most of the te rr ain pictures we re taken with Ektachrome S. 0. 217 or S. 0 . 368on a 2 . 5 mil Est ar base. One magazine of Anscochrome D-50 w a s used onGemini V, and one of Ektachrome Infrared, Type 8443, on Gemini VII.PROBLEMS I N PHOTOGRAPH REPRODUCTIONIn the course of handling and studying many thousands of o rbita l photographs,we have encountered several persistent problems in process ing and reproduc-tion. Since the original film from orbita l flights can be examined only underspecial conditions , all user s must deal with multi-generation reproductions;thus any geologist who uses the Gemini o r Apollo photography will encounterthe same problems. The following discussion is intended to help avoid them .We a r e indebted fo r advice on these points to R. W. Underwood (MSC) and G.Ponder.Obverse Prints-Standard darkroo m technique fo r making paper print s is tohave the negative emulsion facing the paper emulsion (i.e. , negative facing downin the enlarge r). Because of repeated copying and the use of internegatives (nec-ess ary to make color prints fr om transparen cies), routine adherence to the e-mulsion-to-emulsion convention will frequently resu lt in obverse ("floppedT1)prints, which are mirr or images of the actual scene. The way to avoid thi s is ,first, to warn the processing laboratory about the problem, and, second, togive the darkroom technician a correct print as a guide. A measu re now takenby the Manned Spacecraft Center (starting with Apollo 6) is the pre-flight num-bering of frame s in such a way that when the numbers on the 70 mm film can beread without a mi rro r, the film is viewed properly.

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Poor Color Balance-The flight films from Gemini and Apollo missions a r e devel-oped with extreme ca re ; fo r example, full-time chemists monitor the processingsolutions, and water is drawn only from specified wells. However, the colorbalance of paper prin ts ordered by the ultimate us er s may be very poor unlesscertain precautions are taken. First among the se , again, is close personal at-tention by the us er to the printing order and proc ess. Second, the darkroomtechnician should be given a print with proper color balance as a guide. Third,the pre-flight te st fram es of a standard gray scalg9'l?whicheach ro ll of Geminicolor film w as star ted (Fig. 6 ) , should be printed as a guide.Contras t Loss in Black-and-white Prints-Black-and-white negatives made fromcolor f ilm generally have less contr ast than the original, and consequently black-and-white prints of Gemini photos frequently have low contrast. This problem isserious if the photos a r e to again be reproduced for publications. It can beavoided par tly by using high-contrast printing paper , but an even more usefultechnique is to make one or more successive generations of black-and-whitenegatives. Each generation has mor e cont rast than the previous one, and ifdone carefully, little detail is lost.Unknown Scale-Orbital photographs frequently appear in the literature with state-ments such as !'The scale is 1:2,000,000,l y in which it is not made clear that thiswas the scale of the original image. This confuses the reader; the eas iest way toprevent this is to put a graphic scale on each photograph, as commonly done withphotomicrographs.Cropping-Darkroom technicians frequently mask a small part of the pictu re edgewhen printing photographs, especially in 8" x 10 " form at. Although this does noha rm in ordinary photography, the loss of even a few square millimeters from anorbi tal print means loss of information fro m as much as several square miles onthe ground. Theref ore, great stress must be placed on the avoidance of any crop-ping whatsoever; a black border should be left around all prin ts even if it lookslike sloppy darkroom technique. Fur ther more, since the later Gemini film s andal l Apollo 70 mm films have pre-flight fra me numbers on the film margin, it maybe helpful to print the ent ire width of the film (including sprocket holes) .Care of Film-Users who obtain Gemini or Apollo &-tographs on 70 mm roll filmshould be extremely careful to protect the film against dirt and scratches.Scratching is inevitable if the film is exarnimd by rolling across a standard lighttable, as is necessa ry if it is not cut. On the other hand, cutting the film intoindividual f ra me s makes it hard to keep tra ck of the magazines, and tends to ob-scur e the synoptic nature of the coverage on series of photos. A compromisethat we have found helpful ib'to cut the film into 10-frame lengths, which a r e thenkept in flat translucent 6olyethelyene sleeves. Single fra me s which ar e to be in-tensively studied should be copied individually and momited in glass sli des forprotection.

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Figure 6 , Black-and-white print of test frames exposed by Photographic TechnologyLaboratory, MSC, a t beg inning of e a c h 7 0 m m magazine for control andcalibra t ion purposes. Chart includes color patch es, gray sc ale , resolutiontarge ts, and reg ister marks. Test frames for some late r missions giv e colo rtemperature of light source used i n photograph, Users of 7 0 m m photog-raphy should provide test frames for eac h magazine to processors i n makingduplic ates or prints e

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To summarize > we s tr es s that orbita l photographs a re in many ways a new me-dium despite their similarit y to air photos, showing strange things from s trangeviewpoints, and they can not be reproduced successfully by routine procedures.Time spent by the us er in cooperating with the processing laboratory will payoff in quality of pr in ts , ease of interpretation, and economy.GEOLOGIC RESULTS OF THE SO05 EXPERIMENTPalomas Volcanic FieldPre lim ina ry examination of the Gemini TV photographs covering northernChihuahua (Figs. 7, 8, 9, and 10) revealed a volcanic field over 200 squaremiles in area , southwest of P alomas, not shown on any geologic maps (e.g. ,Geologic Map of North America, 1965). Fu rthe r inquiry failed to find any men-tion of this field in the geologic lit erature of Mexico or the United States; there-fore, a 5-day reconnaissance w as made in May, 1967, with the following findings.There are more than 30 cinder cones or vents in the volcanic field, some of themmultiple. Most of the cones are between 100 and 400 feet high, and a re generallybreached (Fig. 11). Some if not most ar e composite , consist ing of interbeddedflows and agglomerates. The age of the field is unknown, although it is evidentlyQuaternary. The flows correspond morphologically to Colton's Stage 2 of theSan Francis co volcanic field in Arizona: the original flow fronts have been ob-literated by erosion, but the approximate outline of the flow is still identifiable,especially from the ai r. The surfaces ar e generally smooth (Fig. 12) althoughremnants of the original ridges and possibly spa tter cones ar e occasionally vis-ible. The cones correspond to Colton's Stage 3 , in that they are preserve d butdeeply gullied. In some examples, mos t of the upper la ye rs of ejecta have beenremoved (Fig. 1 3 ) . Colton considered flows in Stage 2 in the San Franciscofield to be more than 50,000 years old, although the differences in altitude andclimate between that area and northern Chihuahua make it risky t o extrapolatesuch estimate s. It seem s safe to say that the Palomas volcanoes a re no youngerthan seve ral thousand ye ar s, and that the field as a whole is several scor e thou-sand yea rs old.Petrographic examination of samples fr om se ve ra l cones and flows in the north-ea stern half of the volcanic field reveals only one rock type, occu rring as flows >agglom erates, dikes, squeezeups, and pyroclastics. This is an olivine basaltcomposed of unzoned plagioclase (An5,, ) , an unzoned magnesian olivine (partlyalte red to iddingsite), clinopyroxene, g lass , and magnetite. Inclusions of clino-pyroxene and olivine are locally abundant, and a few small dunite inclusions havebeen found. Some of the bombs found had granite core s.

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Figure 7. Gemini 4 photograph S-65-34689; see Fig e 8 (facing) for coverage.Note: view is obl ique to south, causing considerable foreshortening;compare with Fig a 9.

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Gemini IV Photograph S-65-3Approximate scale at center;Tilt is to South.IO Miles

Probable faults-----+. Fold axes, generalizedMaars: 0 Kilbourne Hole@ Hunt's Hole

Figure 8 . Structural sketch map of Fig. 7.

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Figure 9. Gemini 4 photograph S-64-34687; see Fig. 10 (facing) for coverage.

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April, 1968GEOLOGIC SKETCH MAPP.D. Lowman, Jr. Gemini IV Photograph S-65-34687 (unrectified) I-- IOmiles ---Iklt center of photo;E-W distance)Southern New Mexico, Northern ChihuahuaLepandQal- Quaternary alluvium Ts - Tertiary sedimentary rocks

Kv - Cretaceous volcanic rocksQP -Qb - " I' baralt(*marksvolcano) Ki - " " intrusive rocksQp[ - " 1' playa deposits Ks - " " sedimentary racksTv - Tertiary volcanic rocks (Tvl. latite) Ps - Paleozoic sedimentary rocks

pC - Precambrian rocks (undifferentiated)

pediment deposits,I ,I

Lithology from Dane and Bachmon (19641;most of area not field-checked.Figure 10. Geologic sketch map of Fig. 9.

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GI-0P)VU2UVQx

VLln-.-t

.

o vc .-3%5 2c u l n

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The Palomas volcanoes are cle arl y relate d to those mapped by Balk (1962) in theTres Hermanas Mountains just ac ro ss the United States bord er, and probably tothe Quaternary basalts in a larger area of southern New Mexico. It would appearthat they, and the volcanoes of the W e s t Potrillo Mountains, represent the lateststages of a long period of Cenozoic volcanic activity, although Griswold (1961)cautions agains t confusing Te rt ia ry and Quaternary basalt s in Luna County. Theuniformity of the P a l o m s basalt and the absence of evidence of reaction or as-similation suggests that it was generated (presumably in the mantle) and eruptedrapidly, with little modification. There is no obvious structu ral control in thelocation of the Palomas volcanoes, unlike the W e s t Potrillo Mountains, althoughthey lie in a region dominated by northwest-trending folds and faults (Fig. 8).Although probably rela ted to the Pot rillo Mountains rat he r than to Palom as vol-canic field, another volcanic feature not shown on existing maps of the area wasnoticed on the Gemini N photographs. This is a shallow depression occupied bya playa, about two miles in the longest dimension, located about six miles south-west of the Potrillo M a a r (Fig. 14) (itself reported for the first time by Reevesand De Hon only in 1965). Although the depression could not be visited, picturestaken on a reconnaissance flight at 2500 feet altitude show it to be rimmed on thenortheast by lava flows, indicating its volcanic nature . The depression is muchmor e like the Pot rillo, Hunt's and Kilbourne Maars than the volcanoes of theW e s t Potrillo Mountains, and it seem s possible that it too is a maar. However,C. C . Reeves (personal communication) has recently made an aerial reconais-sance of the feature, and does not consider it one.Tectonics of Northern Baia CaliforniaThe first few pictu res of the overlapping s tr ip of 39 taken over North Americaby McDivitt and White on Gemini N have been unusually valuable f or the studyof northern Baja California. There have been few published papers of th is area;chief among these are the comprehensive memoir on Baja California by Beal(1948), the initial report on the Agua Blanca fault by Allen, et al. (1960), andregional investigations by Krause (1965) and Allen, et al. (1965). Salas, et al.,(1967) have produced a 1:250,000 scale geologic map and a comprehensive re-port f ro m the Gemini N photographs. We present here a more specializedthough preliminary analysis of the str uc tur e shown by the first two photographsof the Gemini N series.A frac ture map has been constructed from 10-1/Z1' by 10-1/2" black-and-whiteunrectified prints (Figs. 15 and 16). Although a thorough discussion of the tec-tonics of nor thern Baja Cal ifornia would be beyond the scope of this paper, a fewmajor inferences can be made.

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c

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Figure 15 . Black-and-white prints of Gemini 4 photographs S-65-34671 (left)and S-65-34672 (right), showing northern Baja Cal ifornia,Me xico . See Fig. 16 (facing) for locution.

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INDEX NAP 1tooNi bNi

xIf J

MAJOR FRACTURESNorthan BOJOColifwnia, YaxtwGamin, IIL Phdagrophr S-65-34671, 3-65-34672-0 10mtlsrAppmxinoh Wle of c d r 01 map,p t m l c q r ~nol r6Utfad

pobabk fwlt (dated when cmcdabd OT nterrsdl--, I ignnwr inlruslm.--, PD Lamno1968~ ~ ~~

Figure 16. S tru ctu ra l s k etch map of Fig. 15.

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1. The Agua Blanca fault is but one of a series of roughly paral lel faul ts trend-ing about N75"W in a zone 20 to 30 miles wide. The next most prominent,mapped for the first time by Salas, et al. , and named the Portrerillo Fault,is best displayed about 15 miles north of Canon Dolores. There are a fewminor northwest-trending fractures south of Paso San Matias, but the AguaBlanca fault appears to be the southernmost of the long faults with thistrend. Preliminary study of Apollo 6 photographs indicates that south ofthe Agua Blanca fault , the regional structure trends (chiefly sedimentarystrata) are about N30"W.

2. The Po rt re ri ll o fault shows no evidence on the photograph of l ater al dis-placement. The elliptical features at the approximate middle of the fault isa granitic intrusion, exposed north of the fault and covered by the San Telmoformation to the south, which is not measurably displaced horizontally; theoutcrop pattern points to essentially ver tica l displacement. Similar evi-dence of vertical rather than horizontal movement is seen in other faultedintrusions northeast of Valle de San Rafael.

3 . There is no noticeable horizontal displacement of the eas te rn mountain fron tat Paso San Matias, at the southeast end of the Agua Blanca fau lt, and it isquestionable (see also Salas, e t al. 1967) if the fault extends to the south-east past th is point. This observation is consistent with the conclusion ofAllen, et al. (1960) that both recen t activity and total displacement decre aseto the east. It suggest s, however, th at, despite the clea r evidence of re-cent right- lateral displacement found by Allen, et al., there has not beenmajor horizontal movement along the fault as a whole. Fur thermore, whenthe apparently vertical movement on the Portrerillo and other faults to thenorth is taken into account, it appears tha t strike-s lip faulting may be arelatively minor and late char acte rist ic of the tectonics of this a rea .

4. There is a major NE-SW fracture system north of the Agua Blanca fault.Although not as strongly ex pres sed physiographically as the Agua Blancafault and its companions, it is widespread and distinct, and evidently of thesame general age as the Agua Blanca sy stem since the faults cut sev era l ofthe igneous intrusions. These tw o fault directions form a surprisingly sys-tematic pattern whose meaning is not clear, although many apparently sim-ilar examples are found in the geologic literature (e. g. Anderson, 1951,p. 32). They are evidently not conjugate she ar sy stem s, since the dis-placement is primarily vertical on both and the angle between them isabout 100" as against the 45" to 60" angle that would be expected. Shearingunder east west compression a lso appears unlikely because of the orienta-tion of the right-lateral San Jacinto fault; als o, the northwest-trendingfaults ar e normal. There is no obvious relation to the theoretical wrench-fault directions tabulated by Moody and H i l l (1956); even neglecting the lack

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of evidence for major horizontal displacement; the Agua Blanca fault cannot qualify as a second order wrench complementary to the an Jacinto fault(which represents the San Andreas system in this area) because the AguaBlanca horizontal movement is right-la teral. The orientation and sense ofthe Agua Blanca fault would dictate a northeast-trending first-order wrenchto which it would be complementary, but there is no evidence at all for sucha fault direction here or in southern California.

Although a detailed interpretation of the fracture pattern revealed by the Geminiphotographs can not be made without extensive field work and study of the re-gional struc ture , a tentative suggestion can be made in the light of recent stud-ies by Larson, et al . (1968), and Moore and Buffington (1968). These authorspresent evidence that the Gulf of California has opened, within the last 4 millionyears, at the south end by sea-floor spreading along a se ri es of northwest-trending transform faults , the general process being related to the intersectionof the Eas t Pacific Rise with the North American continent. If the Agua Blancafault developed first as a fault with vert ical displacement, and has only recentlymoved horizontally, it may represent the beginning of dilation of the northernGulf of California. Northward migration of the dilation process would be con-sistent with the lack of horizontal displacement along the Por tre ril lo fault, whichmight eventually turn into a wrench fault. The Agua Blanca fault would, underthis hypothesis, be considered an incipient transform fault. This would be partlyin accord with the interpretation presented by Moore and Buffington; however,there is no evidence northeast of Ensenada of a terminating rift, nor any strongevidence that the Agua Blanca fault continues southwest to the Gulf ofCalifornia.The Texas LineamentThe Texas Lineament is a hypothetical shear zone of regional extent reachingfrom west Texas to southern California. Its nature and indeed its existence arecontroversial: Mayo (1958) considers it one of the most important controls ofeconomic mineral deposits in the southwest, yet several authoritative trea tmentsof regional geology (e. g. , Eardley, 1962; Anderson, 1966) do not even mentionit. A number of Gemini and Apollo photographs provide excellent coverage ofthe supposed Lineament, and they have therefore been used to study the problem.The following discussion should be considered a progress report, since it ishoped to extend the investigation to the ent ire length of the Lineament with otherphotographsThe problem of the Texas Lineament can be summarized only briefly here. Theterm was first proposed by Ransome (1915) for an east-west-trending structuralzone noticed earlier by H i l l (1902) ea st and west of For t Stockton, Texas. Baker(1934) tabulated a list of major geologic contrasts betweenthe ar eas north and

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south of the zone, such as facies changes, great differences in thickness of theCretaceous section, and abundance of volcanics. He considered the Lineamentto extend from Point Conception, California to the easte rn tip of Brazi l, and tobe "probably the greates t single st ruc tural line of the Western Hemisphere.Albritton and Smith (1956) reviewed the problem, and proposed that the type lo-cality for the Texas Lineament should be the 55-mile segment including the Hil l -side fault running from Van Horne northwest through Sierra Blanca (A-B,Fig. 1 7 ) and beyond (Fig. 18). They compiled further evidence for the Line-ament as a major geologic boundary, but found litt le evidence fo r transcurrentfaulting.Moody and H i l l (1956), in thei r well-known paper on wrench-fault tectonics, pro-posed that the Lineament was controlled by a left-lateral wrench fault, partly onthe basis of left-lateral movement along the Hillside fault, which they consideredan element of the Lineament. (Moody later (1966) suggested that the Lineamentmight be a relatively broad belt of tectonism.) Mayo (1958) described the Line-ament as a "belt of transverse structures" more than 150 miles wide in southernArizona and extending from trans-Pecos Texas to the Transver se Ranges ofsouthern California. H e presented evidence that the Lineament is one of the mostimportant st ruc tur es localizing or e deposits in the southwest United States, andrecommended its intersections with other transverse zones as prime sites forinvestigation. Griswold (1961) sta ted , in his report on the geology and mineraldeposits of Luna County, New Mexico, that the Lineament "most certainly" goesthrough that county, probably between the Florida and Tres Hermanas Mountainsin a northwest direction (Figures 19, 20). Muehlberger (1965) integrated theTexas Lineament into a regional scheme for explaining the relations between theclearly related but now disconnected Appalachian, Ouachita, and MarathonPaleozoic fold belts. He proposed that the Marathon belt had been severed fromthe Ouachita belt by some 200 to 250 miles of r ight- lateral movement on theTexas Lineament during the late Paleozoic (with an analogous movement betweenthe Ouachita and Appalachian belts on a para llel Lineament to the north). Per-haps the broadest view of the nature of the Lineament in recent literature is thatof Schmitt (1966). Calling it a "zone of struc tura l chaos of at least subcontinentalextent , he proposed that it might be a zone of left-lateral wrench-faulting alongwhich continental drift had taken place, and summarized evidence that it mightextend to the Mid-Atlantic Ridge at the Equator. Citing Schmitt's views, Guilbertand Sumner (1968) endorsed the idea that the Texas zone, a 'kontinental crustanalog" of the great marine fracture systems, was probably a transform faultsystem in Laramide time. They pointed out its possible role in localizing theporphyry copper deposits, and recommended further exploration along the Line-ament in west Texas. King (1969, Fig. 14) labeled the Lineament a %on-faulted transverse zone" extending from west Texas to the Garlock fault, imply-ing that the latt er may be related to the Murray fractu re zone, though he s tatedthat the Lineament is not a fault.

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Figure 17. Gemini 4 photograph S-65-34697. Obliqueview to southwest over westTexas, showing type locali of Texas Lineament, asdefined by AlbrittonBlanca (B). Sharp line i s traceof Texas and Pacific Railways and U.S. 80 - Interstate 10. Rocks southof Hillside fault near Van Horn are nearly vertical north-striking Pre-cambrian metasediments, visible as aligned ridges, dragged to west alongfault; rocks to north of faul t are gently-dipping Cretaceous sediments.Other landmarks include: Rio Grande (C); Sierra Madre Orientale (D);Davis Mountains (E); Delaware Mountains (F); E l Capi tan (GuadalupeMountains) (G) e

and Smith (1956), the Hi1Yside fault between Van Horn (A) and SierraA-B distance about 65 miles (104km).

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Figure 18. Gemini 4 photograph S-65-34695. Oblique view to southwest over RioGrande val ley, showing supposed location of Texas Lineament betweenSierra Blanca (A) and E l Paso (E). Adjoins area of previous photograph.No topographic expression of Lineament visible, w it h possible expres-sion of scarp of Diablo Plateau. Landmarks include: Rio Grande (C);Sierra Juarez, Chihuahua (D); Franklin Mountains, Texas (F); DiabloPlateau (G); E l Capitan (B); Cornudas Mountains (Tertiary intrusions) (H) .Sierra Blanca to E l Paso distance 80 miles (129 km) a

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Frankl in

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Study of Gemini, Viking, and Apollo 6 photographs leads to the following tentativeconclusions about the existence, location, and nature of the Texas Lineament,1. There is no single la rge fault of Cenozoic age in the Texas direction (N60"W)(the term used by Moody and H i l l ) west of E l Paso in New Mexico or south-

eastern Arizona. Referring to Figs. 19 and 20, this is inferred from thefollowing evidence.There is no northwest alignment of the volcanoes in the West Potri lloMountains. These, and the three maar s shown, appear to be controlledinstead by the southwest-trending fau lts of the Sier ra de la s Uvas(Reeves and D e Hon, 1965; Kottlowski, 1960), or by north-trending foldaxes (Merifield, 1964). N. M. Short (personal communication) haspointed out that wrench faults, being primarily shear ra ther than exten-sional features do not commonly localize volcanoes. However sincelocalization of igneous intrusions is cited as evidence fo r the existenceof the Texas Lineament, it is necessary to search for comparable con-tr ol of volcanic cen ter s where feasible .There are no obvious fea tures typical of recent faulting, such as vege-tation lines, aligned or offset streams , or fault scarps, with a N60"Wtrend between El Paso and the Victorio Mountains, although Baker (1934)ascribed alignment of spr ings in trans-Pecos Texas to the Texas Line-ament. (H e also noted that the Yates oil field was on it.)An oblique Viking 12 photograph (Fig. 21) looking along the supposedcourse of the Lineament through cent ral Arizona toward the Mojaveshows no aligned mountain passe s, st ream segments, ri ft s, or volcaniccenters, although the viewing angle would emphasize such aligned fea-tures if they were present . This conclusion is most tentative, pendingstudy of verti cal orbital photographs along the supposed trace of theLineament.

2. The "Texas direction" (Moody and H i l l , 1956) is a real tectonic fea ture , butconsists of a broad band of folds and dip-slip faults related to the Mexicanfold belt now comprising the Sier ra Madre Oriental and formerly (in theMesozoic) extending much far ther northwest (Anderson, 1966). This con-clusion is based on Gemini 4 photographs (Figs. 7 and 8) of northernChihuahua which indicate that folds of the northwestern Si er ra MadreOriental grade smoothly into the fault t rends of the Basin and Range Provinceboth sharing a N60"W direction in this area (Lowman, McDivitt, and White,1967). The reason for this simi larity of trend is probably that, as impliedby Jones (1963), the Basin and Range faults in this area para llel pre-existingfold axes. The folded sediments are frequently covered by Tertiary or

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Figure 21. Viking 12 obl iqueview (infrared f i lm) from 120 miles altitude over WhiteSands, Ne w Mexico, to southwest, showing supposed location of TexasLineament interpreted as a single fau lt, according to Moody and H i l l(1956) and Griswold (1961). Lineament passes near Victorio Mountains(A), Phoenix, Arizona (B), and across Colorado River. Other landmarksinclude: Pyramid Mountains (D); Gi la River (E); Mogollon Mountains(F); Willc ox Playa (G); Pinacate volcanic field (H); Gu lf of California(I). Distance from Victorio Mts. (A) to Phoenix (B) about 260 miles(420 km). Foreground overlaps Fig. 19; le ft center overlaps Gemini 4coverage.

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3.

Quaternary volcanics, and it appears that the extent of volcanics t o thesoutheast coincides roughly with the extent of block-faulting (faults and vol-canics being relatively minor in the Sierra Madre Oriental). If verified,th is would support the suggestions of Mackin (1960) and Eardley (1963) thatmagma generation and vulcanism are genetically related to uplift and sub-sidence (see also Damon's (1968) discussion of the correlation betweenmagmatism and Basin and Range orogeny).The regional extent and width of the northwest-trending zone, as outlined byauthors such as Guilbert and Surnner (1968), are illustrated by a Geminiphotograph of southern Arizona (Fig. 22). The apparent dominance of north-west trends over those of any other direction, in particu lar northeast trendsillustrated by Badgley (1965), is somewhat surprising .The type locality Texas Lineament, as redefined by Albritton and Smith(1956) to mean the belt of northwest-trending faults south of the Diablo Pla-teau in west Texas, is a relatively minor feature satellitic to the Sie rraMadre Oriental fold belt. This is inferred from, first, the evidence justcited that the Texas direction west of El Paso is structurally the extensionof the Si er ra Madre, and second, f rom Gemini 4 photographs such as Fig.ITshowing the regional dominance of the fold belt.

graphs also show no obvious physiographic connection between the type lo-cality of the Lineament and the E l Paso area. They do, however, supportthe charac terization by Albritton and Smith of the Lineament as "the bound-ar y between two geologic provinces,mobile Sier ra Madre.

The Gemini photo-

the cratonic Diablo Plateau and the

4. The regional fold and fault directions in a large area centered on El Pasoa re a virgation, or branching, rather than the result of major horizontaldislocation by wrench faulting along the Texas Lineament. The divergencetakes place around El Paso, with one branch, including the Franklin-Organ-San Andres Mountains, turning north and the other, including the SierraJua rez and the East Potri llo Mountains, turning northwest. This concept isin agreement with Eardley's (1962, p. 399) interpretation of the Rio Grandedepression and the Tularosa Valley as fold-controlled zones of block-faulting.This inferred virgation is based solely on inspection of the Gemini photo-graphs. A field check might be made by comparing the stratigraphy andstructure of the Franklin-Organ-San Andres Mountains with the ir possibleMexican extension, the Si er ra de San Ignacio, and of the Sie rr a Juarez withthe Sie rr as de Guadalupe and del Presidio. Although the Franklin-San Andresranges are shown on the Geologic Map of North America (Goddard, 1965)as Paleozoic and Precambrian rocks , and the Mexican ranges as Cretaceous,the discrepancies in geologic maps revealed s o *farby orbita l photographs

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Figure 22 . Gemini 4 photograph S-65-34681 . Oblique view to south over southernArizona and northern Sonora, showing pronounced northwest-trendingregional fracture pattern considered part of the Texas zone. E-W dis-tance at north edge of photo about 70 miles (112 km) . This and adjoin-ing Gemini 4 photos are discussed by Tit ley (1968) and Haynes (1968).Landmarks include: Tucson (A); Rincon Mountains (B); Whetstone Moun-tains (C); Santa Rita Mountains (D); Huachuca Mountains (E); Mule Moun-tains with Bisbee copper mines visible (G); Wilcox Playa (H) a

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indicate that such a field check might be worthwhile. Unfortunately, whilea positive resu lt (similar lithology and stratigraphy) would definitely supportthis interpretation, a negative result could be explained by southeastwardplunge of the Franklin Mountains struc ture , without disproving it.It is concluded that the so-called Texas Lineament is not a single fault o r evena discrete fault zone in New Mexico, Texas, and Mexico. It appears to be in-stead a broad belt of folds and dip-slip faults closely re lated to, and coincidentwith, the northwest a rm of the Mesozoic Mexican geosyncline as delineated byEardley (1962) and Hunt (1963). Since the folding and faulting in this former geo-syncline appear to have resulted chiefly from stresses normal to its trend (i .e.southwest-northeast directed), the concept of the Texas Lineament as a wrenchfault o r shear zone seems njus ti fie d. It should at least not be considered asstruc tural ly and chronologically equivalent o r closely related to the major Pa-cific coast wrench faults.The value of the Texas Lineament concept in searching for mineral o r oil depos-its is open to question in view of its great width and variability. However, if the'!Texas zone" (Schmitt's term) is used as a guide in regional exploration, it wouldappear that efforts should be concentrated in northern Sonora and Chihuahua,rather than in west Texas as suggested by Guilbert and Sumner (1968).Comparative Importance of Wind Erosion in Africa and North AmericaOne of the more obvious benefits of the Gemini photographs is the comparativeview they provide of North American and North African deser ts . In particular,they appear to throw light on the relative importance of wind as an erosive agentin the two regions.It is axiomatic, at least in American textbooks , that wind erosion is of min-o r importance as a major land-sculpturing process. Thornbury (1954), fo r ex-ample, states ; "Wind abrasion may aid in the shaping of the details of majorform s but is itself hardly capable of producing features of great area l extent. 1 1Comparable sta tements a re found in many texts, and probably reflect the influ-ence of Blackwelder's (1934) views (although Blackwelder also introduced to theAmerican literature the te rm yardang for wind-eroded grooves). Holmes (1965),on the other hand, ascr ibed the origin of the southeasterly-trending depressionsof western Egypt to wind erosion. Smith (1963) discussed residual rock knobsand ridges , with examples from North Africa, but evidently considered them tobe of only local occurrence.The Gemini photographs of North Africa indicate that wind erosion has been amajor land-forming process over many thousands of square miles. It can bediscussed under two headings: deflation and abrasion. '

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Deflation, o r removal of loose material by wind, has long been recognized as animportant though localized erosive agent in deser ts (Smith, 1963), and particu-larly in the Sahara. The Gemini photographs are therefore chiefly of interestin presenting examples of at least one large area (Fig. 23) in which it is the dom-inant process. In this region, which is probably near the one shown by Smith,deflation has carved basins on structu ral highs and lows over an ar ea of about5000 square miles. The fact that these part icular basins a re in well-consolida-ted sedimentary rocks of Devonian and Carboniferous age, ra ther than in loosesoil , tends to support the suggestion of Ball (1927) that large depressions of thissort are formed by s tr ea m erosion around the r im and deflation of the alluviumfrom the center.Of considerably more interest is the la rge number of apparent wind abrasion fea-tu re s shown by the Gemini North Africa photos. Perhaps the best of these is theare a surrounding the Tibesti Mountains of northern Chad (Figs. 24 and 25), whichare nearly surrounded by arcuate linear features tens o r scores of miles long.These are at leas t partially depositional, resembling sand str eamers or longi-tudinal dunes localized by various topographic fea tures. However, Grove (1960),in a comprehensive paper on the geomorphology of western Tibesti, mapped thesein pa rt of the area shown in Fig. 25 as erosional remnants of sandstone, shaleand schist, and suggested that they might have been eroded by the prevailingnorth-easterly winds. The example ot similar wind eroded bedrock ridges pre-sented by Smith ( 1963, Fig. 3 ) , also strongly supports this possibility. Theprocess responsible for the Tibesti features appears to be self-perpetuating windabrasion of the bedrock, perhaps along pre-existing joints or faults, and thendeposition of sand in long st ream er s down-wind from the grooves and valleys soformed.Similar fields of what appear to be residual knobs or ridges and dunes are shownon the Gemini photos in several other par ts of the central Sahara (Figs. 26, 27and 28); although it is hard to tell, without low altitude coverage, just how muchof this topography is erosional and how much depositional, the overall effect oferosion has clearly been considerable. Some of the rock ridges in An Nafud, inthe northern Arabian Peninsula, may be partly erosional. Another interes tingexample of wind erosion (Fig. 29) is the Dasht-i-Lut of southeastern Iran, inwhich evaporites and clays have been carved into ridges and valleys scores ofmiles long by the wind (Charles Warren, personal communication).The contrast between the deserts of North Africa and North America covered bythe Gemini photos is remarkable . Examination of Fig, 30 and other photos ofthe southwestern United States and northern Mexico reveals few if any erosionalfeatures that can be attributed to wind. This is also true, with rare exceptionssuch as the Dasht-i-Lut, of other de se rt s in the Arabian Peninsula, southwestAsia, and weste rn South America. (Good coverage of Australia has been too

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Figure 23 Gemini 7 photograph S-65-63785. View to southeast over central Sahara,i n southern Algeria; center of photo at about 25"N, 4"E. Distance frombottom of photo to dark mountains (the Ahaggar) at upper righ t about 250miles (400 km) lrregu lar closed basins in foreground are d efl ati on fea-tureson gent ly folded Paleozoic sediments e Dark mountainsat le ft centerare the Emmidir (Mouydir), part of the bel t of tassilis surrounding theAhaggar massif (at upper r ight above the spacecraft nose). Some de-flation basins are occupied by sebkhas (playas) e Light-toned hills atcenter are sand dunes of an un-named erg.

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Figure 24. Gemini 4 photograph S-65-34778. View to northwest over Chad andLibya; Tibesti massif a t le ft . Prominent volcano (A) at left i s Emi Koussi;concentric structure (B) at center i s i n Devonian sandstones, and suggestedby Lowman, et, al., (1967) to be expression of an unmapped igneous in-trusion. Concentric linear features are composite linear sand dunes andwind-eroded grooves (yardangs?) possibly control led by tension fracturesrelated to the uplift of the Tibesti massif.

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Figure 25. Gemini 1 1 photograph S-66-54527. View to northeast over Libya.Marzuq Sand Sea, 250 miles (400km) wide at bottom under Agena an-tenna. Harui al Aswad (volcanic field) at left (A); Tibesti massif atright (B). Linear features (C) are bel ieved to be composite dunes anderosion grooves similar to those east of Tibesti massif (Fig. 24) e

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Figure 26. Gemini 12 photograph S-66-63474. View to east along Tassili Najjer,a seriesof northward-dipping cuestas bounding the Ahaggar massif on thenorth; center of p icture about 26"N, 6"E. Tiffernine dunes, in foot-shaped depressionabout 80 miles (128km) long, areshown by Raisz (1952)as being 1000 feet (330 meters) high e Linear features a t right are thoughtto be partly wind-eroded knobs and ridges and sand streamers. Bedrockchief ly basalt.

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Figure 27. Gemini 6-A photograph S-65-63158. View to southwest over Nige r.Desert with s i f dunesin foreground is the Tenere; A ir Mountains at cen-ter (about 19"N, 8"E). Dark masses are peralkaline granite intrusions;largest (at center) i s Mont Tamgak, roughly 30 miles (48 km) wide, in -truding metamorphic rocks. Plateau o f Irhaouriten, underlain by sand-stone, i n upper right. Linear features north of Mont Tamgak appeartobe wind-eroded ridgesand sand streamers; note continuity wit h s i f dunesi n foreground. Steep-cliffs around intrusions are suggestive of examplesof w ind erosion pictured by Holmes (1965)

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Figure 28 e Gemini 4 photograph S-65-3478 1 . View to northeast over N i le River insouthern Egypt; center of picture about 23-1/2'N, 33"E. Distance fromarea at bottom of pic ture to north, near Aswan Dam (arrow) about 100miles (160 km) . Linear features trending from upper left to lower rightapparently sand dunes and wind-eroded knobs and ridges of bedrock,mapped by Said (1962) as chief ly Cretaceous Nubian sandstone westofN i l e and Nubian sandstone and Archean crystalline rocks east of Ni le .

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Figure 29. Gemini 5 photograph S-65-45723. View to west over eastern Iran to-ward ci ty Kerman; center of picture about 31"N, 58"E. Linear featuresat le ft center are wind-eroded grooves in salt and soft sediments of theDasht-i-Lut, a large salt desert. Mountains west of the salt desert arecomplexly folded sediments related to the Zagros Mountains to the west.Note fau It (A-B) e

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Figure 30, Gemini 4 photograph S-65-34676. Near-vertica l view (slightly obliqueto south) of northwest Sonora and southwest Arizona (north at top). Areais in Basin and Range Province. Pinacate volcanic f ie ld at far left;Sonoita River (C-C-C)at center, outlined by vegetation (cottonwoodsand similar plants) e Area covered about 70 miles wide.

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limited to permit generalization about wind erosion there.) But it appears thatthe theory that stream erosion is dominant in deserts does not apply to largeparts of the greatest desert of all, the Sahara. (As a check on this statement,note the scarcity of pediments in the photographs of the Sahara. ) Many individualregions probably totaling severa l score thousand square miles, seem to havebeen deeply eroded by wind abrasion and covered by dunes associated with theerosion.A detailed analysis of the reasons for this unique characterist ic othe Saharawould be beyond the scope of this report , but a few should be mentioned. Fir stis the fact that most of North Africa (and all the areas illustrated here) are southof 30" latitude, and lie in the belt of northeast trade winds (locally called theharmattan); the Basin and Range Province, on the other hand, is north of 30" inthe belt of prevailing westerlies (Gentilli, 1968). The physiography of North Af-rica - essentially an immense'shield with a few high massifs such as the Ahaggarand Tibesti - permits these northeast winds to blow without interruption for hun-dreds or thousands of miles, while the north o r northwest trending ranges ofNorth America break the westerly winds. Another reason for the dominance ofaeolian features in North Africa is simply the extremely low rainfall; unlike theNorth American deserts, many parts of the Sahara outside the massifs have rainonly at intervals of years. Wind erosion may thus win by default in North Africa.The Physiography of MarsBecause of the low inclination of the Gemini orbits and the generally clear desertweather, many of the Gemini photographs are of deserts. Since Mars is gener-ally thought to be essentially desert-like, it is of interest to compare the physio-graphy of Mars with that of terrestrial deserts.The main problems of Martian physiography (Loomis, 1965; Michaux, 1967) arethe nature of the light and dark areas (Fig. 31). In particular, it is still notagreed, despite the achievements of Mariner IV, whether the dark areas (maria)are lower (and possibly vegetated) or higher than the light areas. Related to thisis the question of what the canals are; as pointed out by Sagan and Pollack (1966),the ir location, tone, and periodic color changes suggest some connection withthe dark areas.Examination of the Gemini photos of deserts, especially the Sahara (Fig. 25),shows thei r general similarity to sketches of Mars made at rare moments ofgood seeing and, to a lesser degree, to color photographs such as those of Fin-sen (1961). (This similarity does not of course extend to the crater s photo-graphed by Mariner IV * ) The major dark areas of te rres tr ia l deser ts a re , al-most without exception, higher than the light areas; specifically, they are usuallyplateaus or mountains of bedrock surrounded by lighter-colored wind or '

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cIu)e--u)S

0I.-3v).L.I-alS8 6s ct u.o .I-8 8Y ZE 5-

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water-deposited sediments This generalization does not necessari ly hold forsma ller features or are as; for example, in Fig. 32, chains of light-colored seifdunes overlie dark bedrock. Also, the few large lakes in some dese rts a re dark-er than their surroundings, but this possibility can safely be ignored for M a r s .It seem s safe to say that if deser ts on the eart h were viewed at the same resolu-tion with which we view Mars, the dark areas seen would be highlands and thelight areas lower te rrain consisting of less consolidated material . This is inagreement with the interpretat ions of Martian topography by Tombaugh (1966)and Sagan, et al. (1966), in which it is proposed that the dark areas and canalsare plateaus and ridges, and the light areas deserts of wind-blown sand or dust.The foregoing arguments do not by themselves eliminate the theory that the mariaof M a r s ar e low areas in which vegetation is localized. However, the Geminiphotographs reveal no such large featu res of natural origin in terrestrial deserts.In Fig. 3 0 , for example, part of the Sonora desert (relatively lush as desert s go)is shown; although the ephemeral Sonoita River and its tributaries ar e dark be-cause of the vegetation along them, these ar e relatively small. From M a r s , thedark features seen in this a rea would be the mountains. The only large, darklow areas in deser ts ar e those formed by artifica l irrigation, as in the ImperialValley of California and the Ni le Valley of Egypt (Fig. 33).Another problem of Martian physiography is the cause of the reddish color of thebright areas. It is generally agreed that this is due to limonite, but there is dis-agreement on its concentration. From astronomical studies (summarized byMichaux, 1967), several workers have concluded that the material of the lightareas must be largely limonite. However, Van Tas se l and Salisbury (1964) pointout that silicate grains coated with limonite are much more likely on geologicalgrounds. The Gemini photographs of terrestrial deserts appear to support thelatter concept: many deserts are as red or redder than those of M a r s , but al-most invariably consist of quartz or other silica tes with a very small percentageof i ron oxides.The Gemini photographs shed no obvious light on other problems of Martian to-pography, such as the cause of the wave of darkening, which Sagan and Pollack(1967) attribute to seasonal deposition and removal of dust. It will be of in terestto examine repetitive orbi tal photographs of deserts from future missions to seeif there is any terrestrial analogue.In summary, the simplest interpretation of Martian physiography in the light ofthe Gemini photographs is that the dark ar eas and possibly the canals are rela-tively high plateaus o r ridges , and the light areas deserts of eolian silicatesand and dust with a small proportion of iron oxides. The general pattern of theMartian dark areas is strongly suggestive of the basin-and-swell physiographicpat tern of North Africa (e. g. , Holmess 1965, Fig. 763), but this interpretationis little more than speculation.

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Figure 32. Gemini 7 photograph S-65-63786 e View to southeast over northwestAlgeria toward the Tademait Plateau; center of picture about 28"N,1"W. Ridges at le ft are the Tabelbala and Ougarta Ranges, Hercynianfoldsat rightangles to the trend of the Atlas Mountains (Tertiary) e Notelight-colored dunes of the Erg lguidi i n right foreground overlying darkbedrock

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Figure 33. Gemini 4 photograph S-65-34668.N il e delta at top (A); E l Faiyum (B) lower right i s a depression irrigatedby water from the N i le and partly fi ll ed by the Bohirat Quiron (C), alake 148 feet (46 meters) below sea level. Dark areas are irrigation-supported vegetation, light areassandy desert. Cairo (D) at upper right,

Oblique view of northern Egypt

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Study of Mariner 6 and 7 photographs of Mars is underway to test the suggestionsmade here.

GEOLOGIC SPECULATIONSThe foregoing applications of the Gemini photography have been relatventional, being essentially photogeology from extreme es . However, inthe course of indexing and studying the 1100 pictures, a number of intriguingquestions have ar isen , leading to the following speculationseNature of the Oman LineA s shown in Figs. 34 and 35, there is a sharp discordance in the Zagros-MakranRanges of southwest A s i a opposite the Musandam Peninsula. This feature hasbeen known for some years, and has been studied in part icular by Gansser (1955,1964). Gansser has suggested that th is is a rejuvenated tectonic line of strike-slip faults, and is the continuation northward of the Oman Range structure underthe Zagros-Makran Ranges. He further speculates on the possible importanceof overthrusting in this area.The Gernini photos of this area, in particular Fig. 34 provide a remarkablygood view of the regional structure. Of particular interest is the remarkableresemblance, especially when viewed from the west, of the Makran Range to thePine Mountain overthrust and similar structures; at first glance, the W r a nRange looks like an enormous thrust plate that has been displaced to the south.That this is not simply an optical effect is indicated by the strike-slip nature ofthe faults along the north-east coast of the Strait of Hormuz (Gansser, 1955;seealso the British Petroleum Company Geologic m p of Iran, 1:250,000 series,BandarIAbbas and Strait of Hormuz sheets, 1963). Further support is given bythe fact, as shown on Figs. 35 and 36, that the northern limit of marine Mioceneand Pliocene rocks in the Makran is 50 to 100 miles farther south than the expect-ed continuation of this boundary from the Zagros Mountains.Before considering this speculation further, let us consider the physical plausi-bility of such major overthrusting. The Zagros Mountains ar e underlain by theCambrian Hormuz Formation, a salt layer which has given r is e tosa lt plugs of the Persian Gulf (Fig. 35). The Salt Range of India, 1400 miles tothe northeast is also underlain by a Cambrian sal t layer, the Saline Series,which Gansser (1966, p. 26) suggests may have played a par t in overthrusting.In contrast to the Zagros Mountains, there a re practically no salt plugs in theMakran Range o r in the Salt Range; referr ing to the latter, Gansser suggeststhat thrusting may preventConditions for major overthrusting by gravitational gliding in the Makran Range..a more diapiric rise of the underlying salt zone. l t

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Pakis tan) ex t end s farther north than wes t end (across from north en d ofOman Range) ,

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igure 35. Gemini 12 photograph S-66-63846. View to east along Makran Rangefrom about 168 miles (272km) alti tude. Photograph keyed to Fig. 26.Note the dust clouds at top center being blown from the Asianmainlandinto the Gu lf of Oman by northeasterly winds,

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GEOLOGIC SKETCH MAPSOUTHWEST ASIALegend

Based on Gemini XI1 Photograph S-66-634-86+generalized fold trends+ anticline Scale varioble; photo area is 800 milesE-W and 350m iles N-S (ot center).Lithology from geologic maps ofBritish Petroleum, Ltd., and U.S.S salt plug (Hormuz salt) Geolo gical Survey I?D. Lowrnan,Jr. - 1967Figure 36. Geologic sketch map of Fig. 35,

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appear to be favorable including a thick sedimentary section, rapidly deposited,and a stratum that can s erve as a lubricating layer (Hubbert and Rubey, 1959).Finally, the John Murray Expedition (Wiseman and Sewell, 1937), discoveredsubmarine ridges parallel to and some 60 miles south of the Makran Coast thatmight be the toe of the hypothetical thrust plate. It is interesting, in this generalconnection, to recall the proposal by Rich (1951) that the syntaxial bends in themountains of southwest A s i a might be the re su lt of major overthrusting to thesouth,There are, of course, obvious difficulties with the concept of the Makran Rangeas a giant thrust plate. First, the "plate" is not symmetrical, the eastern bound-ary (probably represented by the ranges of Baluchistan) extending much farthernorth. Second, the northwest-trending range just east of the Iran/Pakistan bor-der (Fig. 35), which has no name as a unit does not f i t the overthrust patternvery clearly unless it is a separate thrust plate which has, so to speak, caughtup with the Makran thrust plate in Pakistan. Finally, no such major thrustinghas been recognized by those who have mapped the Makran Range in Iran andPakistan (John Reinemund, personal communication).In summary, it appears that the Oman Line is , as Gansser suggested, a zoneof major wrench faulting and paralle l folds. Whether the ent ire post-Cambriansection has s lid to the south, singly or in slices, remains an intriguing but to-tally unproven possibility. Perhaps the most definitive te st would be drilling ofthe ocean floor south of the Makran coast; if rocks s imilar to the Fars limestoneand other units of the Zagros Mountains were encountered, it would tend to sup-port the theory.Discordant CoastlinesIt was proposed by E. Suess in the last century that there are tectonically speak-ing two types of coastlines: the Atlantic type in which the regional structure istruncated by the coast , and the Pacific type, in which the regional structure isgenerally para llel to the coast. The Indian Ocean is generally thought to be ofthe Atlantic type. It is interesting to see if the Gemini photos support this rela-tively simple classification eSome of the Atlantic coast lines covered are clearly discordant; for example, thePrecambrian structur es of South W e s t Africa strike into the s ea, as shown byBrock (1956). However, there is at least one confusing exception to Suess'classification; as shown in Fig. 37, the Hercynian folds of the Anti-Atlas Moun-tains tend to para llel the coast whi e the Cenozoic folds of the High Atlas aretruncated by it. This does not appear easily explained by continental drift; onewould expect the pre-dri ft (pre-Cretaceous?) structur es to be truncated and the

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hotograph S-65-45 Near-vertical view of west coast oforocco, with north are discordantright) are con-tains bounded on south by theto coast, though Ter

the Tertiary-Quaternary fold belt fromat center along coast is su n glitter; cn e (not a storm) e mass at bottom is


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post-drift (Tertiary?) struc tures to parallel the coastline. W e have no explana-tion to offer for the rev ers e arrangement noted her e, but it seems cl ear thatSuess' classification can not be universally applied to the shores of the Atlantic.Another exception to the supposed classification in this cas e of the Indian Ocean,is seen in Fig. 38. Study of this photo and Qth er s, in conjunction with theU. S. G. S. Geologic Map of the Arabian Peninsula (1963), demonstrates that theaxis of the Oman Range turns due south and then southwest along the BatainCoast, rather than str iking out into the Arabian Sea as commonly depicted(e. g. , Carey, 1958). The east end of the Arabian Peninsula would thus be ofthe Pacific ra the r than the Atlantic type. This relation, with other cr it er ia ,has been used elsewhere (Lowman, 1967) as evidence that the Arabian Sea isnot a sphenochasm, as proposed by Carey (1958).It will be worthwhile to examine photographs of coastlines from future orbita lmissions to fur the r check the usefulness of Suess' classification; these prelim-inary studies indicate that it may not be applicable in cert ain regions or abovecertain scales.Uniqueness of the Basin-and-Range ProvinceGemini photos of northern Mexico and the southwestern United States have pro-vided an excellent view of the physiography of a significant portion of theBasin-and-Range Province. Comparison of these with other photos in the35ON-35"S latitude band shows that the Basin-and-Range Province is unique inthat no other continent has such a large area of closely spaced horst-and-grabente rrai n. The Andes Mountains are a relat ively narrow band of chiefly foldedstruc ture; most of Africa is clearly on basin-and-swell; and most other moun-tains in Asia belong to the Te rt ia ry folded belt extending from Indonesia to theAtlantic. Examination of any good physical map of the world, moreover , sug-gests that nothing like the Basin-and-Range Province w i l l be found outside theGemini latitude band.This uniqueness can not be easi ly explained. However, it appears to supportMenard's (1961) suggestion that the Basin-and-Range Province is the re sul t ofthe intersection of the E ast Pacific R ise with the North American continent.There are few other places where an oceanic rise intersects a continent, one ofthem being the Red Sea, whose s truc ture is essentially like that of the Basin-and-Range Province but narrower.This curious relationship, rega rdles s of its cause, furnishes a good example ofthe stimulus to geologic thinking tha t can be provided by the global coverage oforb ita l photography.

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Figure 38. Gemini 4 photogra h S-65-34661 e Vertical view of the east end of theArabian PeninsulaArea shown is about 65 miles (104km) wide; north at top. Light-tonedi s chiefly northward-di pping Tertiary sediments, coverings of the Wawasina and SemaiI igneous complexes that formth e Oman Range (see Fig. 34 for a regional view of thist areas at lower le ft are the Wahibah sands (urug dunes) e

showing concordance of fold trends to coastline

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SUMMARY AN D CONCLUSIONSThe success of the SO05 experiment can not be adequately judged from the num-ber of planned areas actually photographed. Those not covered, or at least notcovered with usable pictures, include the southern part of the African rift val-leys, much of northern South America, southern India, Australia, and Brazil.But balanced against these gaps is photography of unscheduled though geologi-cally important areas, such as South-West Africa, Peru, and Tibet. Overall,it seems safe to say that despite departures fro m the pre-flight plan, the t err ainphotography experiment was highly successful in te rm s of quantity and quality ofcoverage.Perhaps the major general result of the SO05 experiment was a clear idea ofjust what the main geologic uses of orb ital photography will be (see also Lowman,1967) . The following list is a conservative one.1.

2.

3.

4..

5.

Natural resource evaluation, use and conservation: Investigating relationof mineral deposits to geologic st ructur e, evaluation of surf icial deposits(e. g. , sand and gravel) locating unsuspected extensions of mineralizedzones or oil-bearing struc tures fundamental research on nature of regionalor e controls, location of potential mineral or oil-bearing structures for in-tensive ground study, detection of sediment pollution sources and incorrectsurface mining practices.Regional tectonics: Studies of major fracture patterns, structures associ-ated with batholiths, relat ion of regional st-ructure to vulcanism, nature ofdisplacement on wrench faults, intercontinental correlation of structure.Geologic mapping: Reconnaissance mapping of remote areas, correction ofsmall -scale maps, correlation of geology with geophysical surveys map-ping of Quaternary volcanic rocks.Geomorphology: Evolution of dese rt landscapes, origin of pediments, sanddune mapping, soi l mapping, studies of fluvial and coastal erosion and sedi-mentation, world-wide monitoring of glacial advance and retreat,Geologic education: Teaching world geology, tectonics geomorphology,and coordinated earth science courses (e.g. , ESCP), illus trating geologichazards (e.g. , faults) in public press .An equally important result of the SO05 experiment is greater knowledge of f&e

limits of orb ita l photography. The operational rest rict ions , such as orbitalcha rac ter ist ics , cloud cover and daylight availability have been treated else-where (Lowman, 1969) . In addition to these, orbi tal photography has cer tainlimitations as a geologic tool, some of which are shared with aerial photography:

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1. Composition determination: Determination of rock types on photographs hasalways been difficult, and has relied heavily on indirect indicators such ascharacter istic drainage patterns plant cover, and karst topography. Toneo r color are also used, but are frequently ambiguous (von Bandat, 1962).Orbital photography has not eliminated thi s problem; if anything, the lowresolution generally characteristic of orbital photographs aggravates it.Multispectral sensing has considerable promise. However, a e r e are fewif any broad spectral peaks in the photographically accessible par t of thespectrum (0.3 to 0 . 9 microns), so that remote rock identification will prob-ably have to be dane with non-film sensors operating in the 10 micron vicin-ity. Even then s evere problems remain: the speed of the spacecraft, in-complete atmospheric transmiss ion, and resolution. Field checking andsampljng will probably always be required; it should be remembered thatdespite seve ral decades of persevering astrcmomical efforts at remote sen-sing par excellence, the composition of the moon's surface was essentiallywnknown until samples were returned by the Apollo missions .

2. Vegetation and soi l cover: Like aerial photography, orb ita l photography de-pends on reflected radiation from the surface of the earth, and so is limitedin usefulness where the bed rock is covered by vegetation and soil . Even inareas of generally good exposure, the superficia l nature of photography canbe a handicap. Fo r example, in Fig. 7 , a dark tongue of m ater ial can beseen extending s eve ral miles east from the Palomas volcanic field, and wasinterpreted before field checking as a lava flow. But it was later discoveredto be a thin residua l gravel deposit consisting largely of quartz and cher tpebbles with a coating of desert varnish; the photographic tone was thus pro-duced by an oxide coating a few microns thick on a layer of pebbles only afew centimeters thick.

3. Great areal coverage: The immense area per photo provided by orbital al-titudes is a handicap as well as an advantage because of the great amount oftime required for ground checking. To make a reconnaissance of even oneorbita l photograph involves hundreds of miles of travel and at least severalweeks time. Therefore, for rea lly effective use of orbital photographs ingeology, fast transportation is absolutely necessary . The ideal solutionwould be a helicopter; failing that low-altitude airplane reconnaissancecoupled with standard air photos, should be considered essentia l in fieldwork with orb ita l photography. Furthermore, when on the ground the geol-ogist should suppress his love of hiking and drive as close to points of in-terest as he possibly can; otherwise he will spend severa l days covering afew square millimeters of his photograph. Finally, field checking on orbi-tal photographs should be careful planned to include only the most cr it icalpoints; systematic contact walking and fault-chasing, on 1:1,000,000 scalephotographs is hopelessly inefficient

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4. Resolution: The subject of ground resolution on orbital photographs has beendiscussed before (Lowman, 1967 and 1969). However, it should be st res sedhe re that the comparatively low resolution of orbi tal photographs, comparedto aerial photographs, means tha t in addition to loss of topographic detail,the tone o r color of any one point on the photograph may represent a widevariety of rock types. This is especially true in areas of steeply dippingstratified or foliated rocks with varied lithology. This problem intensifiesthe difficulty al ready discussed of determining rock composition from orbitalphotography. A s pointed out elsewhere (Lowman, 1969) very long focallengths can increase resolution t o nearly aerial standards, but only a

terrain photography from gemini spacecraft final geologic report

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