f l i V . , f , k w y flH--G A/&&&.

Recogn~tion Criteria of Igneous and Metamorphic Rocks on Aerial Photographs of Chichagof and Kruzof Islands

- Southeastern Alaska By J. S. PQMEROY

PROCEDURES AND STUDIES IN PHOTOGEOLOGY

G E O L O G I C A L S U R V E Y B U L L E T I N 1 0 4 3 - E

A n aid in the ident$ccltion of major rock W e s in a comp!ex g.eologic terrane

UNITED STATES WVERNMENT PRINTING OFFICE, WASHINGTON : 1964

UNITED STATES DEPARTMENT OF THE INTERIOR

STEWART L. UDALL, SBCTdorg

Pot glle bg the Saperin-t d Docu-b. U.S. Cwmmme~t Pthti- OiflCc Wsmhhgm. D.C -2

CONTENTS

Abetracf ,,,,, - - - . + . -,, - , . , , -, , , , - - - - - - , - - - - - - -- - -- - -- - - - -- - - - - - - - Iotrodr~ction,,,-,,*~---------~-----------------------~-----~------

Purpase,,,,,,,,,,,--,-,---,--,.---,-------------,--,--,---,,, Gcologia nkctc h - - . - . , . - , , , - - . - -, , - - - - -, , - - - - - - - - -, - ,- - - , --,,- - - - Ceogaphy ,,---, , . , , . - -, - - -, --, - ---, - -, ---------- ,-, , -- +---, - ,

Rccofinition clemonts - - - - , - -. - - - - - - - - - - - - - - - - - - - - - + --- - - - - -- + -,

Chichagof Island- -. ,, ,- - -, ,, . -* - -- - -- --- - -- - --------------- ------ - Summary of geology -,,,,,,,-.---,-,,------------------------- Wcognition oriteria of metamorphic mh on aerial photogrepbs--,-, Fhcognitinu criteria of intrusfvr! M C ~ R on aerial photographs- - -- -,--

KruwtIsland-,, , , ,--,-,-, ,------,-----,-------------------------+ Sumrna~y of geology -,--,---,,---*-----+-,--------~-----+---+--

Recognition criteria of rock t y p e ~ on aerial photographs ------------ Summary and conclusiom-- -,.--.---,,------,---,--,--+------ Source and identifying data of photoplraphs- -,,-,,,,- ,,,, ----. -,-- -,- - Selected bibliography- -----,,--,,----- ,, ----*,-,--d---d-d--

ILLUSTRATIONS

Finurn 28. fndm map showing area discuamd-, - _ -, -- - --. ,, . ,- . , . --. , 29. Tom1 diffmncee and drainage cheracterfatiw d metarnorphfo-

igneous m a n e , Rmt Mountain area, Chichagat Idand, - , 30. Gtructural featurea and rlraina~e chsrwtwiatirri of i~neoua-

metamorphic terrane north of tho head of Hooneh Sound, Chichagof Is land-- - - - - - - - - - - - - - - - - - - - - ,* - - - - - - , - - - - - -

31. Drainage eharacterktiar of igneous-rnettrmorphlc k a n e in Deep Bay-Peril Strait area, Chick@ Iuland .,-,.,--,.

32. Tonsl differenma and structural fcetures of i~naoua-rnata- morphic terrane north of Hoonah h u n d , C h l c h a ~ f bPend-- , - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -~- - : - - - - - - - - -

33. Vegetation and structural features of igmcoua-metsmorpbio terrane natheast of the hsad of Haonah Sound, Chleh~of I~land,,,,-------------~~~---------~----------~-----+

34 , &)rainage cbraacteristice and structural features of igneous- metamorphic twrane near Rust Mountain, Ghiehaml Island-,,---,--_------------------------------2------

35. Vegetation and structural f e a t u r ~ of imeoue-metamorphic tersane in Point MarylSheli kof Bay area, Kruzof Island-,

38. Wrinage chatactwistics of igneoua m a n e northeaslt of Emmom Island, Chichapi Island . . . . . . . . . . . . . . . . . . . . . .

57,aS. Structura~ features. 37. Northenst of Emmons Island, Chicbgof bland-- - - --, 38. Metamorphic &mane on southern Chichagof Island, - -

m

m RGWE 39. Tom1 diffmnccs In igneous-metamorphia k m n e dong

d of northweatan Chic-f Islend,, -, --, , , , , -, ,,, - 105 40. Tonal differences end structural featurea of igneotm-meta-

mmphic terrane along eastern Rruxof Island ,---. ,,,---,, 106 41. Topographic exprmion of volcanic landscape, southmn

Kruzof bland-,,,,,,-------**-----,------------- 107

TABLE

Pam T A ~ L E 1. Summary of recognition miteria of Egneom and metamorphic

rocb on aerial photolysph of Chichagof and Krurcif Ialends,. 108

PROCEDURES AND STUDIES IN PHOTOGEOLOGY

RECOGNITION CRITERIA OF IGNEOUS AND METAMORPHIC ROCKS ON AERIAL PHOTOGRAPHS OF CHICHAGOF AND KRUZOF ISLANDS, *SOUTHEASTERN. ALASKA

Varlons aerlal photographic rwromition elementa ~nch ns photogmphlc tnnp. vegetation, drainage clmracterlstim, stmturnl featurm, and topgrnphir ~ x - prmaton can be osed to aflsist In the semldetaited retnannalssnnw Aetd ~napplnp of a cornpl~x m a of regionally and thermal[v metamorphosed rocks Illtrutld 1 1 ~ wveral tgpw of tgnmus rocks. Gombfnatlons of these recognition ele111ent~

. - are effective in distinguishing the fgneous and metamorphic r w k ~ of C%lchagoP . . and Kmrof Islands. DmInage character Is one of the most reliable criterin.

Field inv&ilp;ationst inwlvhg .regional reconnaissance mapping - were conducted on Chichagof, Ramnof, and Kruzof Islands during

. . thesummer of 196P ( T ~ n e y and others, 1963). During the field sensorr . - the mthor hrvd the usefulness of vertir~l nerial photographs in

studying ha .~lagicrrlly- complex temne. Bmuse little has pre- xiously been .witten about photuintsrpretive aspects of specific

- .igneous-nsetnmorphictmranes, itist.he pu- of this paper to sum- marize the photointepretive features of the igneous and rnet~morphic hmne of Chichagof and Kruzof Is l~nds (dig. 28). Areas on Chichagof Islmd which w e n mapped by other workers (Rossmm, 3959; Lathram and others, 1959; Loney and othem, 1968; Red nnd Coats, 1941) are not d i s c u d here.

8 f

88 PROCEDURES m~ EFTUDIES m PHOTOGEOLOGY

Rmks ranging in age from SiIurian ( 8 ) and Devonircn { P ) to Quater- nary crop out on Chichagof and Rmaof Islands, Plutonic mks are abundant and constitute a b u t 80 percent of tlie arm discussed. Mast of the layered mks are older than the intrusive Wies, and although some of them are rnetamorphod only slightly, most of them show the effects of either contact, mg-ional, or dynnmic metnmorphism. In many placas the rock type is n result of more than one form of metamorphism.

Th8 area is complicntd structurally by faulting and folding, and some of the layered rock units have been isdinally folded.

During the Pleit3tocene Epoch, an ice sheet from the north and ~iortlieast covered all but the highest elevntions of Chichagof and Kruzof Islands. Aerial photographs indicate that the ice cover reached a pinirnum elevation ht~veen 2,400 and 2,7IK) feet over a large part of Chichagof Island and mudr less, b low 1,900 feet, on Krumf Island. Gladem no longer exist in the area, but the ubiquitous cirques and U-shaped valleys re evidence that glacintion hns occurred.

The area is topopphimlly represenbtive of southeastern Alnskn. Mountains rise diwtly from the major inland ~vakxlvays, and the only low mgians are the ~l luvinl valleys which are best developed at the heads of bays. S o m ~ peaks rim to elevations of slightly less than 4,000 feet.

An annual rainfall of more thnn 85 inches is n major climntic fen- ture. During the winter the climnta is usunlly mdemte nlong the shoreline, nltl~ough a lnrge amount of snow falls in the higher coun- try. Summew nre usunlly moist and cod; the daily temperature during thm summer seldom exceeds 60"415~.

R~mlocknnd sprum nmthedominnnt, treegrowth. Brush, l~sually dder, de~ilsclub, nnd berry bushes, is thick find irnpenetmble in some places in the low amns. The elevntdion of the timberline variw but is usually between 2,W and 2,600 feet.

In studying the aerial photograpl~s of an igneous and metnmorpllic termme, diflerent recognition elements-photograph tone, veg~tntion, topographic expression, drainage and erosional charncterist i cs, no well ns the stn~cturnI fentnm of the rocks sl~cll RS foliation snd jointing- can be ~ m d . Some recognit ion elements am mom va111nb1e than others, jzlthough nny pnrticl~lrw recognition element is more effective when

90 PROCEDURES AND STUDIES IN PHWI'OQEOLOOY

other criteria am used with it. Complete interpretation is prevented in many amas by an insufficient number of criteria.

B D Y OF GEOLOGY

The central and eastern parts of Chichltgof Island are dominated by several pIutonic bodies composed principally of granodiorite, tonalite, diorite, and gabbm but also containing srndl amounts of syenite, monzonih, ~dameellih, trondhjemite, and norite ( h e y and others, 1968). Th- pIutonic bodies have intruded and metamor- phmd many of the Pnleozdc nnd Mmzoic rocks. k r g e areas of mixed migmabitic rocks lie adjacent to several plutonic bodies. Most of the intrusive rocks are probably the same nge ws those of the Const Range batholith (Cretaceous), but a few may be older. Roof pend- ants consist principally of intensely folded hornfels, marble, wm- phihlite, and scllist. A be1 t of graywacke, nrgillite, conglomernte, l immne, and marble crops out along the nodleast edge of the a m . To the west is a thick section of nonplutonic &mapped as seveml distinct units-which range in age from Permian(?) and Trimsic( P ) to Early Cmtnceous. These mi&, some of which are nearly mono- Jithologic, consist of greenstone, rnnrble, greenschist, phyllite, and grmywacke.

Marble is one of the major metamorphic rock types in the region. 13emuse marble is lighter in tone on the photographs thm any other metamorphic rock, it can be distinguished from other robk types wherever it,-is fairly homogeneous (fig. 29) ; marble is also generally lighter in tone than intrusive rock. Depending on the male of the photographs or the rnagnificntion of tlie lens of the stereoscope, karst features may be observed. In a, few places where mnssive nonfolinted rnsrbb m u m , it cnn be distinguished from other metamorphic rocks which usually hnve conspicuous folintion trnces. Also, joints appear to be less developed in marble than in greenschist, phyllite., and hornfels.

Mnrbls usu J ly supports a stand of forest that is denmr than that most cllaractsristic of intrr~sive terranas. On eastern Chichagof Islnnd n wide belt of mxrly homogeneous marble crops out along Chitham Stmilt; a thick stand of forest, obscures the wee between the sllomline and the f e w ridge ccfests above timberline w l ~ ~ r e the marble is e x p ~ d . Elsewhere in soutlieastern Alaska, marble sup- ports a denser stand of evergreen forest than do other metamorphic

Wnnnl ED,-Twal dlffemcea and dralnnge charnctprlmtlcs of metamorphic-lgneonn rerrnne, Rllut Momtn1n uren. C h i ~ h ~ c n t Irlnnd. Llaht-ton4 marble ( A ) can ~ f l ~ 1 I . v he dttteri-ntlnfrd frnm ndjnwnt m~tnmnrphlc r r k ~ . Ertenslve gultylng r a nt D la d l ~ m n ~ t l r of tcrrnne un<lmrlaln hy p l ~ ~ t n n l r rock. 4:rwnstone (C) hau k n tntrudpd 11s gr#knmllorIt~ whlrh r m p nllt nlong ninontaln flnnk and along rldm at D. A M a rpnwr of * m n n t o n ~ mauks thr gmnndiorlt~ In plarpn. K O ~ P drrlfnage In gteenstoov t ~ t E nnA r w o l r r ~ t r l r h u l t nfnrp a t F.

nnd intrusive mh (Candon, 1961, p. 9, 14, 19; Ray, 1960, p, 135). In the emtern part of the island, the smnll number of rills along the ridge slopes indicntm a low surface-drainage- density in the marble, where them is much greater subterr~nesn drainage through mlution chnnnels.

Fine-pined rnafio igneous d s and metamorphic derivntives of pelitic sedimentay rmks such as phyllite usually llave a fins-textured, or high, drainage denai ty (fig. 30), but the drainage density of eonrser p i n e d derivativ~s such RS gnei85e5 or quartmfeldsprsthic schi&s and arnphibolites is often much lless (fig. 31). When discitwing gully d~nmcteristics nssoeiated with the erosion of different rnaterida, Lueder (1059, p. 69) writes about one typa of gully which develops in very smnll grained cohesive materials that are low in porosity. Tllese gullies hnve l o n ~ nnd wide channels, a modemte to l a r p number of branches, nnd a fnirly uniform p d i e n t . Th i s type of gully istypiml of many of the fine-grained metamorphic rocks.

Prncticnlly all the obsemed drainap of both metamorphic and ipmus terranes is imposed upon a thin regolith. Research workers at Pllrdue University (1958, p. 1%) found that chcharacteristics of the r n ~ jor soil textural p u p become nondistinctive and also that combinations are found when the soil has a "strong" or well-developed profile. Conversely, a thin soil muntle would have definite gully characteristics. On the photographs, most metamorphic rocks show not only a can-

spicuous foliation but nlso nn equally well deveIopd joint system (figs. 32, 33). These "checkerbonrd" or grid patterns usually indi- cate an area of metamorphic racks. Gneisses or quartzofeldsp~thie scl~ists, however, do not have these characteristics in this region.

The metamorphic roch of the nverap rroof-pendant area emnot be divided on the photographs. Owing to the complexity of these mks, field mapping of any particuI~r lithology, wen nt a semide- tailed wnle, is often practically impossible. In these ~tpens, particu- larly, any metamorphic unit tends to h thin and of short latersl extent. On southwestern Chichagof Island whem the units are neaily

monolithologic, few criteria are nvailnble to distinguish the metamor- ' plric rocks (other than marble), with but one exception. In mas- sive pwn9tone foljntion is commonly obse~ire or nonexistent both on the pliotopnphs (figs. 29,M) and in the field ; c o n ~ e m l g , ndjncent bodies of p n s c b i s t xnd phyllite llnve a pronounced folintion. In places the phyllite reproduces w lighter photographic tone than the (.reendhiat but this disti~~ction is probably due to the abundant h. siliceous Inminne which nre associated with a particular phyllita unit.

FrGuRR .?O.-Ytr~ict~~rt~l fpnf~~res nntl r ln~ innp chr~r:lctcriutlc* of I ~ I ~ ~ ~ ~ ~ ~ I u - ~ P ~ I I ~ I ~ o T ~ ~ I ~ c trrrnnc riorth of UIP head Of Aool~nh Solrnrl. C I I J C I I R ~ ~ ~ ts111nrl. I'lutonic rock ( A ) ls ~nlrhro 1111ll cllnrlte. I{ is rnninly Rri~grnincrl rnctumowblc rock. Strlkc. untl (Ill, 04 tl~lil~tinn In tht. ~nct~~nltrr~tlile mcks I P tIjqmrent: ILISO frilctures n$ at C. h'ate dlff~renee In tlrnlnngc dvnxity n t D r~nd 6. The firlvr lexturcxl ~lrrlinr~gcb lit U than nt 1 l n c l i e ~ t c ~ less permeable rock at D. In v i ~ i n i t ~ of D. n thln cnvcr o f ~Injrt~ ~ n x l l mrlxkn t111. olltcrnps.

Frco~m 31.-Dralnnpe chnractcriatics of igr~~n~~~-rnetnrnnrpI~le trrrnne 111 r)wp Bay-Peril Strnit rtren. Chlrl>acof It;l;~nrl. T ~ P c l c ~ p l y incinr-rl drulnnm a y w t t : ~ ~ ~ dt.r.t.lol~ed on sfolres of the grnnodforltc hill at .i contrrrsts n i th f l ~ e tlm1n:rge s > s t ~ n l #>1 1 1 i I 1 s1ol1t.x 11nd1.rla1n by rnmrdlunl- trl L U I I I . ~ - ~ ~ O ~ F I I q l l U t l d r l f t ~ l d ~ l l l l t ~ schis t ( C ~ P ~ X P } ~ t f T{. 13~111 ribck tyjws. t'n]b~w1ally the 1:1ttrr. fr11tli113 II 11)w tlrnl~inm= db-nc1t.r. hilt the terrane unrterlnln by qrrort&ufelrlnpntliic schtst nppenrs to lnck dpe~vlp incised gullies.

RECOGNIRON CRITERIA, ROCKS ON AERIAL PHOTOGRAPES 95

Frr:URw aB.--JTnnltl r l l l T ~ r m m nnd strllrtrirnl r l in rnr t~ r la t lm rrr lgnpn~in-m~tnmnrphic IPrrnnn n o r t l ~ of rho hmd of Hnnnnh Snlrntl. C h l r h a F P Iu111rid. A, p b b r o : n, mntlr l~ ; F. Iiitrrh~t1rl1-11 Ilornfrls, wrhl~ t. 1cmlrt1lhn2lt~. nnrl mlnor trrn rhlr : U, l o l ln - tlon : nnd I.:. J n l n l ~ . . I l t l ~ o u c h t h ~ lrmr rrT f h ~ mnrhlp mjrt)ullrfn ( R J E R 11rJ11~r ~ I I I ! ~ fhnt r r t t lw riihlrro (.\I. 1111. tunv nf t111. rewtntlnrr C R qll~rki-r (F). M.irh11, us i~nl ty mhnwr I P F R ~ \ l r l l + d e v ~ l n ~ ~ r ~ I J O I U ~ R tlrltn do lrlr>qt othrr n~vt~! l l~twphic rot k.. Xotr ~hvck~.rhonr#l I grltl) ]lirttl.rrl I I ~ Z'.

96 PROCEDURES AND STUDIES IN P H ~ O O E O I A M Y

FICI'TI~: 32.-Vwtation enrl r t m r t u r ~ l l I I~~~~ILTPP of i m m ~ ~ c m ~ t n m ~ r p h l c terrane n o r t h p ~ t of tllr h ~ n t l O T tfnnnnh S n l ~ n ~ t , 4 'hIrh~~ot Imland. Onlrhrolr tprranp ( A ) I8 CnmpBMtlV~1S R ~ R ~ R P I J wnnfled e r a at ~ I ~ ~ ~ a t l n n u n u luw nR R M I fwt. I ~ J R ~ P s muntm I B J 11 u n d ~ r l n l n moutly hy hornteln, C+ wprcarntn Jnlntu, nnd D l a t h ~ *frIkp or the follatlotl In the metnmorphtr rocks.

RECOGNITTON CRITGBIA, ROCKS ON AERIAL P H O T O G W E 8 97

.,. - d ' LmL-- ",* ti?:&

I-, 'I, , ,, ,

~i t )lick anmet anunrphowcl gsaywncke Illlit border; t lle met runorpl~ic ~ w k s to t k ~ ~ r - m t , h~rf on thr photop-aphs t h ~ paywacke cannot l ~ e [list in~uishei l from the rn~tnmorphic rocks.

RECOQETfTION CRITERIA OF INTRUSIVE ROCIS ON AERIAL PEOTOGRAPHS

.\win1 p h n t o ~ ~ : ~ ~ l ~ s rlrp of little vz~lue in di f fere~~t iat ing intrusive rnck t y p s . .Is sho~t-n l ~ y Iirlfl ril:hpping, indirirlnnl plntona on Chi- c~h:i,rrof Island nrp 11su:11ly w r y cnr~lplex un~rl map include diverse rock

98 PROCEDURE8 AND STUDLEB IN PHOTOGEOLOG'P

types. Also, migmntitic arms cannot be m p i z e d on the m&1 photographs.

Tonal variations, for example, between a granodiorite or tonalite and a gabbro outcrop are either nonexistent or very weakly reproduced on the same photograph. Although tonal. di~tinctians would logically resnlt from the predominance of light- or dark-colored minerds in the rock type md from the tone of the weathered surface, only ocxa- siomlly is this criterion reliable. Likewise, tonal variations between intrusive rock types mapped by the author in other amas of south- eastern Almka are not well repduced on the aerial photographe.

Vegetational differences are sometimm more useful than tonal contrast in distinguishing plutonic rock types. Residual soils de- veloped on silicic rocka are usually fertile and support a p t variety of vegetation. Mafic rocks and their residual soils, however, contain tt 1- well balanced. mpply of nuhiants and support a more re- stricted type of vephtion Soils derived from ultramafic rocks are infartile and frequently lack vegetation (Eardley, 1942, p. 159- 160). The silicic plutonic rocks of t,hhe eastern half of Chichagof Island

m w d l as the gmncdiorite of Kruzof Island (fig. 35) tend to support a dense forest. In mntrast, the gabbmic area northeast and north- west of the head of Hoonah Sound has a low timberline and a manty brush growth. (Sm figare 33.) Admittedly, some terranea under- lain by silicic plutonic rock do not support a dense stand of vegeta- tion ; for example, in some areas of central and western Chichagof Island, a smnt tree growth is typical. Local climatic factors and the e t h t of glaciation on mil cover and drainage possibIy cause these di Rerenoes.

An area, however, of ultramafie rocka along the eastern shoreline of nearby Ramnof Island has a vegetation-rock-typa relation un- affected by other influences. The boundary of the Red Bluff Bay ehromitx+bearing body as mapped in detail by Guild and Ralsley (1942) coincides with the vegetational boundary. Although it is near sea level, the ultramafic area has almost no vegetation. Robin- son and others (1935) a t e that large quantitiw of chromium and nickel in soils cause infertility.

Surface drainage patterns in the intrusive rock of Chichngof Island nre less dense than those in mmt metamorphic rocks. (See figurn 30, tXi, 34.36.) Such wide spacing of streams results in a come-textuxd drainage system. The channels tend to be curvilinear although a few are nearly straight where slopes are steep. As previousIy mentioned (p. 92), the fine-grained metamorpllic rocks have fine-textured or high drainage densitim. Ray and Fischer (1980) discuuss the drainage density of different rock t y p a .

FIcmF: 33.-Y~petntlon and strurtnral fpahtrcs of Igneous-mctnmnrphic terranr lo Point Jlnry-Shelilrof D~rp nrea, 6r1ltoP Isrand. Densely woodrd nrex along rnriflc Orenn ;kt A l u undcrlgl~~ by grrroo#llorltc. Along ~ I I P nwt ecbaat o f Cl~~cl~irgof Irli~rld \\.here g'ritvn.:ickr on8 scblst rroll vlrt l # % ~ s t I ~ n ~ e v~zctat ion nnd A lower ti111br.rllnc 1, lylbicill. The l l r l ~ t tone of ~runod~or l t e ( A ) :tlol~g shorellnc contrnslr with di~rker tone of n~rtngrk~ywltrk~ ( R ) . Lioenr fr:tfurv C re\rrPxcnts the foHntton of the metn- gra) \!*nckc. Lille~u fenture D is indfeatlvr of n conR1)lcuons r~gtonnl jolnt aet !n d ~ e plutonic rock.

FIOoaR 36.-Drnina~e chor~cterbtIcs of lrenons terrnnc oorthenrt nf E m m o ~ s Islnnd. Cblchhngof 1rl;lnB. Deeply Inclew1 (Irc~lnng~> nyntprn bnn tlerdoped on Loiinlite. ,Rapid cllulntegmtion of rock nccuunrrc f o r Ilnht-colored bile of Illll~Ide rnrintw. Thls chrrrnctrrlntic In usually relinble In diiTereollatlns plutonic from mrtamorpble rock In ureus lnrgelg covered hy vegetation.

One of the kt criteria for distinguishing ~lutonic from meta- morphic rocks is the recognition of the steeply incised heads of hillside ravines chetracteristie ef many tkrens of intrusive rock. (See figs. 29,31,34,35,3(5.) This criterion is helpful and reliable in areas where outcrop are sparse and where rjdges are covered by trees or brush. Lueder (1959, p. 69) discusses one type of gully which develops in non- cohesive material that has appreciable grain size rind porosity ; it is chsracbrized by * * cornparntively dmrt length; simple and direct plan with few or no short simple bmnches, steep, uniform grndient wit11 shorp knickpoint at head end, and n descriptive short sfeep psl-1 or nick." Such s dmription fits mast anma underlain by si licn-rich to itltermedinte plutonic rocks on Chichngof Idand.

ResidmI soils developed from mks of gabbmic composition some- times will have gullies of the type mentioned by Lueder. Gnbbroic rock, however, even though it is usually coarse-pined, more often produces a mmewhat c l~yey and impermeable soil, the mil composi- tion depending on the amount of hornblende or other m d c minernls in the parent rock. Charncteristics of the drainage system will likely include features of gullies developed both in silicic intmive mks and in peli tic and other fine-pined metamorphic mh.

A large part of Iha plutonic mck on Chichagof Idand i~ directionless or nonfoliatd. Occasionally, however, an intrusive mass i a foliated, and tram of the foliation are sometimm distinct in the photograph if the area is large enough. This feature can be confused with the conspicuous steep joint set that prevails in some plutons (fig. 87). Joint systems are more common and as easiIy distinguished in areas partly covered by spama vegetation as they are in areas of pootE outcrop. In the higher areas of Chichagof Island, igneous and metamorphic

m k s can often be distinguished by the topographic expmion of the ridges. Plutonic ridge crests are somewhat rounded or much 1- rugged than the chamcteristic sharp-c&d serrate ridgee of metamorphic m k s such as marble, phyllite, p n m h i s t , and hornfels. Gneimic ridges are much less mggd than those rid- underlain by most metamorphic rocks.

Dikes intruding metamorphic rmk are nsually discernible on the photographs (fig. 38). As they am more resistant than the country rock in this area, the dikes d o n a l l y have some relief.

KRUZOP X W D

Roclcs ranging in age from Triassic and (or) Jurassic ta k n t crop out on Krurmf Island. T h e oldest mks , which constitute only

102 PROCEDURES AND STUDIES IN PHOTOGEOLOGY

a very snlall part of the isl:ulr2, ronsisi of grrenstontb, greenschist, g n ~ y - wacke se~nischist, n~ct:lc!~ert, :~nd plly l l i t P. C;I':LY IVRC:~A, ~ n ~ t : u n o ~ - pllowrl ir l ~~ lnces , o~w=li~c: tlirtl i s tr.;n\sil ion:~l wi t11 these roclrs. A grxnodioritic plnton wns ~rt~pli~rerl :li 't~r tllc deposition of the gr:ty- ~vncke nncl hns c;tused cnnt:~ct n ~ ~ t n n m r ~ ~ l i i s n i in t lw ;~cljnccnt, ~rvlrs, Post-Pleistw~n~ b1ls:tItic flows erupted civet- whnt i s now sor~thrrn Rruzof Isl:~nd.

RECOGNITION CRITERIA OF ROCK TYPES ON AERIAL PROTOGRAPITS

Aerial pl~otogrnplls iwc 11scf ul in distinguishing intrasivr mck from the grny w:icke :mrl its met nmorphic ~ q u i vnlen t s. Pronoanc~d ron- trnst in tor10 is fou11Cl along 4llu slioreli~~e ml both the enst : l r ~ r I t h e r r ~ s t sidles of the islnnrl (Jigs. 35, tip, .IrF) where plutonic rock is liglll in t o i i ~ when compr~red to the mertiuan-toned gr:ty~~ncke, and r r : ~ y \ v ; ~ r k ~ I~orn- $013. 11s only z few parts of the r i d ~ e s 3rc nbovc tllc t i r n h r l i n ~ , ton111

Frcmre 37.-StructurnI rPnturcn northenst of Emrnona Islnnd, Cbichngof Island. .\, tlorninnnt jrr I n t pe t 111 f o ~ l ~ ~ l l t e . I * ~ r ~ I b t ~ n t linear features represent dominant Y ~ r t l r ~ i I J01nt br-t In nunfol!;tr#bd tonulite. Thpsp trendg coold be mietnken for In l f~~ t lon In n n~r-tamurphlr (or i ~ n m u a l rock.

1 ~ ' m r ~ p B R . -Rtructuml fcnture* of mctnmorphlc krmne on vorrthrrn rl~lchngnf Ialnntl. A1 t ~ r e d mbhrolc b o d l ~ ~ [ A ) cut 71 wqllrnricln nf rrwn*tone, gre~nrrchiat, und rn~tachert. Note llgllt tone of A. Idinear trpnd a t R represents fotfatlon of the tnvtnmorphlc rmka. A t C thin bPns af 1Eyht-toned metachert 1-rr111rl be mlutukrn for f p l ~ i c ~Il l - . f ight-toned llnmr bodlea nt U lire dlkes that have fllled jolnta in the metamorpblc rwku.

variations cannot be used here. - The metagrsywacb ridges, however, h d to be s h p r crested thm .the less rugged ridges of intrusive rock

- that lie at similar elevations. The photapphs show that the &mdmal:pattarns in the two rock

types are dissimilar. Fairlr to modenttely well defined northeast- .ward-trending joints are common in the ganodiorite. Conspicuous .northwestward-trending bedding and folistion tracw are' found in the pywsckaancfpywacke hornfels.

The contact between -the gmywacka and its metamorphic aquivalent is pdationrtl; rn neither unit .can be distinguished on the photo- L

graphs, nor can a contact be delineated between the gmywacke and the older rocks wwhh are transitional with it. The basaltio flows and their assmiated cones of mathern Kruzof

Island are obvious features on the aerial photographs (fig. 41). The mlandscrspe is characterized both by a, radial.and by a dendritic drain- . age pattern having more dense vegetation growing along the creeka ..and shoreline and by an absence of-structural trends (fig. 40). Al- though the photapphic tone of the basalt- and metagraywacke along t,he shoreline ia similar, clmIy spaced persistent northwest foliation traces in .the m e t a p p a c k e contrast with the featureless and flat- lying basalt.

BUMMARY AND CONCLUSION8

Recognition criteria of the major igneons and metamorphic m k tmes on the raerial phatoppl~s of Chichagof and Kruzof Islands am ~ummarized in :table- 1. The author Gelieves that most of thase rec- oflition elements are applicable elsewhere in southeastern Alaska, and that, a few can bused in almost any &on. While photointer- pretation techniques can mldom be used as a substitution for field mapping, they can p t l y mist m experienced field mapper even in a complex geologic terrane. The ~ ~ t h o r certainly cannot subscribe to any suggestion t h ~ t rocks, pnrticularly tho% of igneous and meta- morphic ari~in, r n i ~ h t be kst differentinted by nsing aerial survey in- stend of field and Iahmtory methods (Lueder, 1959, p. 269,872,56'4).

Aerial phstqrmpbs for the discussed aretl are chiefly valuable for the help.they give in the differentiation of igneous from rnet,nmorphic rocks. Aerial photogrnphs are of limited value when distinguishing the complexly foldd and varied metamorphic Iithologiss from one another .or when identifying int,rusive rock typee. Various m g n i - tion elements .such as. a~itcrop tone, veptntion, topographic expres- . sion, drainage nnd~erosional.char&eristics, and structural features of

- the rocks themselves sire used in the demarcation of the mnjor rock types. Of thes~criteria, drainage chnrncter is one of the more rdinble mgnit ion elements.

FIGURE X 9 . 4 n n n l dlffcrences ln imeous-metamorpblr: t errnn~ nlonz conxt or nnrth\rest~rn Kmzof Ielnnd. tlpht-toned rock (A) I R mano- diorite. Dnrkcr-toned rock (3) i s hornf~ l s . .\long acl l -r .x l~r~s~d phorellnrs: tonal tllstl~ictlrrnz: hpmpen r~lt~tonlc nnrl n~etuniorphlc rack ure usuully npgnrent. Sea foam vl~ucures rock ronp sonten-hnt on left side of p11oto. WIIIP anndy beaches where outcrop nre Ineking as a t C are Lntlfcatlrc of nearby granitic rock.

F l c u ~ e 40.-ronnl d l f crenc~s and structural fentnren of t:u~cil~u-~r~ctnrnnrphic Ierrnne .!long eilsteru ICruzoe Isl.ind. Sharp roaal contrast i u emdent between tonnlire ( A ) nnd metnrrnynacke ( J i . ) , .it C, the strike of f n l i n t i t l n In nenrly vertlc;il c r l ~ i t ~ c t ~nrr.l>uorl?hic rocks can be seen. \Vide beach exposuws of Ont-lrlng rocks nrP Recent b~xaI1I.k 1 3 0 ~ ~ (Dl Kote dendrltlc drainare system and concentration of tree plom-th long stream courses.

marine 41.-Topognrphlc expressloo o l vo lc~n lc londsmpe. soulberu Kruzof Jeland. Quateroarg basalt, a s 4 and Inp(lll ehnracserhe w

the lnndacnpe. A, parnsltlc cone and Ita Bow: nod B, b m c b e d cone. Wotc radlnl and dendritic dralnnge system and macen- 0

tradon of regetntlon nloog creeks and alopea of cones.

TABIS 1 .4umnaa~y of ~ecug&#€on OPlfePdd of dgneoua and m e f m r p h i o rocks m mrfal ghotogrughs 01 0MQhago.I on& Xruaoj zsaunds

Topogsphlc erpres- slon

Bldge m s b are W e d ; h Ngeed but with steeper slopes than thorn 01 meteam-

h elcvstlons %%iabg* ~ s - Isod.)

Cones, crntera, MI- vlduel Bows.

o@neraUy aharp- msestea Acarab ridges tor m b b . PhyUfe y e n - sehM,ak h m - Lk less rugged ax- pre;isio~ /or a h u mtamapbk rock zOqzi'- gmroll3 ndand at hlgb ekvatforu, on Cblcba~ol b lad.)

Ve@atlon

V a b h bul clam- manly d em.

Commonly kiw draw than on 8ranltio b tonsHLk T&.

along m k e , abmUneS, d dopes of mnes.

Commonly d w

Cammonly lsY demb tbn on marbb.

Variable but lsas d m than on marble.

Pbobgraphk tone of oataop

~ l g h t ta medhm

Varleble but waaUy raedium.

Medlnm to dark

v q Nbf

Medlmn

Vnhbb but corn- manly medium.

M t m Drabgo cbaracterlstlcs

C a m extared draInsee low dralnug6 deoslty); 6 ssp]y ~ O C M @Nes,

asme as above Ln p h e s , but more commody in- tarmediate between %I- h i c lnbna(ve rocks e d Bne-greload meramor- phlc F W ~ .

RbdRl and deodrltlo pat- t e r ~

Low d m drainege den- slg: Karst [ e s c m In ptawa

Fixw- drafnage (hkh draloage denslly) foc pbyUlld artd o l b s

r o c k E2Z-z- dratll- ege end lower dmlnage denqlty IUr m!.%m or qmrblaldspathio r&.

a d t k rn tomlltk.

Oabbmlo

Qm-y

m o m

B h c h w l leaturea

Jdnts. anld WImI to multtdirecklonsl WLS.

Follatlon, prominent In p b s .

A h n t

~ o k t l a n u ~ l r m*&tc to%m 6xpmwlon wbew rock wnmesalva.

J o l n t , iatr -re$- don.

Follstlcm, poor ex- d o n .

IO&U. pmr erp- slon.

FoUM md%e?$&ag. expre4don m p t In me&.

Sotnta, orvleuy mod- era& to atrong ex- Prm2lor, empt In Web.

Inhuslw

Extruelve

Metamctvhia

Mwbb

0teenat.ona

PhplUtb Omaschbt Ho&k arnph~~mlite Onek.9

RECOGPJlTION CRITERIA, ROCKB ON AERIAL PHOTOG- 109

BOURCE AND IDENTIFYINGl DATA OF PROTOQRAPHGE [Role& wde BEA (Boutheadern Alash)]

Other data: Approximate scale of photographs is 1:40,000. Photo aphs were taken by U.S. Navy during July 1998. Focal gngth of lens of camera was t3 inches, Photographa may be urchased from:

US. Geological 8w)urvey Map Information Office Wmhington, D.G. 20235

T o w hit quadrangle 8:am

Sitka C-0 D-6 B-5 D-6 D-5, w6 C-6 A 4 C-5 C!-5 C-6 B-6 A-5 A-5, A 4 '

SELECTEE) BTBLIOQRAPHY

Exparme~

22-23 3 W 9 90-91 39-40 80-81 75-76 3 W O 39-40

39 70 35 103

57-58

Figore

29-- - -+- - -+- - - - - - - - - - - - - - - 30--------------------_- 31------------------ ,------ 32--------+--------------- 33------------------------ 34- - - - - - - - - - - - - - - - - - - - - - - - 35------------------------ 36,,---------------------- 37---,-------,------------ 88--------,--------------- 39-----------+--- , -------- 40-----,------------------ 41-----------,------------

Xlluni, J. A E., 1981, Photogeological Interpretatloa of arew of &anal meta- morphlsm : [Londonl In&. Mining and Metallurgy Bull. 6 5 , p. 521-642.

Colidon, W. H., 1961, Geology of the Craig quadrangle, Alaska : U.8. Geol. Snrvep Bull. 110&B, p. B1-I343 [1982].

Eartllw, A. 3.. 1842, Aerial photographs-their uae and interpretation : New Turk, Harper R Rras., 203 p.

Guild, P. W., and Balsley, 1. R., Jr., 1942, Chmmlte deposits of Red Bluff Bay and vicinity, Baranof Island, Alaska: U.S. Geol. Suweg Bull. m G , p. 171-187.

Lathram, E. H., and others, 1959, Pmgrem map of the geology of the Juneau quadrangle, Alaska: U.S. Geol. Surrey Misc. Geol. Inv. Map 1308, wale 1 : 250,000.

I,une~, R. A., Berg, E. C., Pomeroy, J. S., and Brew, D. A., 1963, Ibmnnais~ance

Roll

135 139 128 139 128 135 135 128 125 135 135 I28 135

geologic map of Chichapf Island and north\vestern Raranof Island. Alaska : U.8. Geol. Sun7ey blisc. Geol. Inv. Map 1-388, 1 p.

I#neT, R. X., Condon, V. H., and Dntru, J. T., Jr., 1M3, Geology of the Fresh- ~ t e r Bag m a , Chi-of Island, Alaeka: U.S. Geol. Survey Bull. 1108-C, C1-m.

Lueder, D, R., 1969, Aerial photographic interprets tion-prlndpbs and appll- rations : Xew Pol-k, AZcGraw-Hill Book Co., 482 p.

JHller, 7'. C., and Miller, C. P., 1061, Photegeology: New Pork, McGram-Hill Book Co., 248 p.

F u r d u ~ Cniverslb, I @ - , A manual on the airphoto Interpretation of soils and rocks for engineering parpow: Purdue Unlv. School of Civil Enginmring and Engbering Mechanic& 206 p

110 PROCEDURES AND STUDIES IN SHOTOQEOLWT

Rag, R. G., 1880, Aerlal photogmph in geologic interpretation and mapping: U.S. Geol. Survey Prot. Paper $78, 250 p.

Ray, R. Q, aud Fiseher, W. A., IsSO, QuanEltanve photog*nphy+t ge~logfc re search tool: Photagramm. Eng., 7. 20, no. 1, p. 148-130.

R e d , J. C., and Coats, R. R., 1941, Geolo~y and are depoaltil of the CMchagni mining district, Alaska: U.B. Geol. Survey Belt MO. I48 p.

Robinson. TV. O., Edgington, Glen, and Byera, H. G., 1&?5. Chemical stadlea of infertile mib derived from rocks high In magnefdum and genemlly hkh in chro~ainm and nickel : U.S. Depb. Amicnltnre Tech. Rull. 471.

Xoasman, D. L., 1959, Geology and ore deposlta of northwestem Chlchamt Island: U.B. GeoL Snmey BuH. 10j&E, p 13S216.

Bmith, H. T. U., 1943, Aerial photographs and their interpretation: ,\em Pork, Appleton-Centnr~-Cmft.~, 372 p.

Tator, B. .A., and others, 1980, Photo hterpretation in geology, Chap. 4 In Manual of PhoXographIc Interpretation: Am. Soc. Photograu~m., p. 188342.

Turner, F. J.. 1952. "Geftigerelie??' illash9M bp "schiat tor" t o m p b y in wnttwl Otago, ?Sew Sealand: Am. Jour. Sci.. T. 250, no. 11, p 802-801.

Twenbofel, W. S.. and S a l n s b u ~ ~ C. L, 1958, Fault patterns In ~outhe~stern Xla~ka : Cfeol. Roc. America Bull., v. 89, no. 11. p. 1431-1442.

Procedures and Studies in Photogeology - --

G E O L O G I C A L S U R V E Y B U L L B T I N 1 0 4 3

This Bulletin was prepared as reparolte chapters A-E

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1964

UWITED STATFX DEP-ENT OF THE WTERIOR

STEWART L. UDALL, sm~tary

-G WLOGICAL SURVEY

T h a m a a 5, Nolan, Dhdw

CONTEXTS

(Cetttrr deal~nate the repnrnte chantern puhllahedl p a p

(A) P b o t ~ l a g I c ~br&urea in gmloglu Interpretation and mapping, hy RIrh~rd G. Rng-,---,,------------------------------------ 1

Applfmtinn of hi~h-r~rder stelm~copic plotting instruments to p h ~ togeolo~lc atudlm by C. L. Pillmom --------,--,--------- 23

0 ) Det~nnjna tlon ai rlun~htitntive geologic data wiW stereometer-tgp Instruments, tjg Willlam R. Heuphi11 . . . . . . . . . . . . . . . . . . . . aR

(U) Imyach ntnpping try phologeologlc nlethods a?r en aid in the location of %wale and channels in the Xonnment Y a l l q area, Arhona, by Irving J. IVitltind, NilLiam R. Hemphill, Uharl~s Id. Plllmm, and RobeC A. Morris ------,-----,-,-------,--,,-+,,,,,,,-,- 67

(E) RecogoItion criteria of Igaeoas and metamorpblc rocks on ae&l phmrapbbll of Chichagor and Kruzoi X~lands, ~ 0 u t h a S t e ~ n Alaska, by a. S- Pomemy---------,-----,-----,---,,-,,-------, 87