GEOLOGICAL SURVEY RESEARCH 1970 Chapter B

GEOLOGICAL SURVEY PROFESSIONAL PAPER 700-8

Scientific notes and summaries of investigations in geology, hydrology, and related fields

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1970 UNITED STATES DEPARTMENT OF THE INTERIOR WALTER J. HICKEL, Secretary

GEOLOGICAL SURVEY William T. Pecora, Director

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $3.25 CONTENTS GEOLOGIC STUDIES

Petrology and petrography Page Copper in biotite from igneous rocks in southern Arizona as an ore indicator, by T. G. Lovering, J. R. Cooper, Harald Drewes, and G. C. Cone...... B1 Relation of carbon dioxide content of apatite of the Phosphoria Formation to regional facies, by R. A. Gulbrandsen---- 9 Extensive zeolitization associated with hot springs in central Colorado, by W. N. Sharp...... 14 Mafic and ultramafic rocks from a layered pluton at Mount Fairweather, Alaska, by George Plafker and E. M. MacKevett, Jr------21 Authigenic kaolinite in sand of the Wilcox Formation, Jackson Purchase region, Kentucky, by J. D. Sims------27 2 Blueschist and related greenschist facies rocks of the Seward Peninsula, Alaska, by C. L. Sainsbury, R. G. Coleman, and Reuben Kachadoorian ...... 33

Structural geology

Allochthonous Paleozoic blocks in the Tertiary San Luis-Upper Arkansas graben, Colorado, by R. E. Van Alstine- - - - - 43

Geophysics Calculated in situ bulk densities from subsurface gravity observations and density logs, Nevada Test Site and Hot Creek

Valley, Nye County, Nev., by D. L. Healey------_,- A------52 Geologic and gravity evidence for a buried pluton, Little Belt Mountains, central Montana, by I. J. Witkind, M. D. Kleinkopf, and W. R. Keefer ...... 63 Aeromagnetic and gravity features of the western Franciscan and Salinian basement terranes between Cape San Martin and San Luis Obispo, Calif., by W. F. Hanna ---_-----..------66 Reconnaissance geophysical studies of the Trinidad quadrangle, south-central Colorado, by M. D. Kleinkopf, D. L. Peterson, and R. B. Johnson ...... 78 Geochronology Whole-rock Rb-Sr age of the Pikes Peak batholith, Colorado, by C. E. Hedge------.. ------86 Distribution of uranium in uranium-series dated fossil shells and bones shown by fission tracks, by B. J. Szabo, J. R. Dooley, Jr., R. B. Taylor, and J. N. Rosholt ...... 90

Economic geology

Iron deposits of the Estes Creek area, Lawrence County, S. Dak., by R. W. Bayley------.. ------93 High-calcium limestone deposits in Lancaster County, southeastern Pennsylvania, by A. E. Becher and Harold Meisler- - 102 Geology and mineral potential of the Adobe Range, Elko Hills, and adjacent areas, Elko County, Nev., by K. B. Ketner- - 105 Paleontology

Early Permian plants from the Cutler Formation in Monument Valley, Utah, by S. H. Mamay and W. J. Breed------109 Stratigraphic micropaleontology of the type locality of the White Knob Limestone (Mississippian), Custer County, Idaho, byBettySkippandB.L.Mamet ...... - 118 Triassic conodonts from Israel, by J. W. Huddle------124 Middle Pleistocene Leporidae from the San Pedro Valley, Aria., by J. S. Downey ------131 New discoveries of Pleistocene bisons and peccaries in Colorado, by G. E. Lewis------137 Stratigraphy

Geology of new occurrences of Pleistocene bisons and peccaries in Colorado, by G. R. Scott and R. M. Lindvall------141 Clay mineralogy of selected samples from the middle Miocene formations of southern Maryland, by Karl Stefansson and J.P.Owens ...... --150 The Gardiners Clay of eastern Long Island, N. Y.-A reexamination, by J. E. Upson------157 111 IV CONTENTS

Sedimentation Page Settling velocity of grains of quartz and other minerals in sea water versus pure water, by C. I. Winegard- - _ ------B161

Geomorphology The glaciated shelf off northeastern United States, by R. N. Oldale and Elazar Uchupi ......

Analytical methods Determination of cobalt in geologic materials by solvent extraction and atomic absorption spectrometry, by Wayne Mountjoy------A field method for the determination of cold-extractable nickel in stream sediments and soils, by G. A. Nowlan------The fluorimetric method-Its use and precision for determination of uranium in the ash of plants, by Claude Huffman, Jr.,and L. B. Riley------Chemical extraction of an organic material from a uranium ore, by M. L. Jacobs, C. G. Warren, and H. C. Granger- -- A die for pelletizing samples for X-ray fluorescence analysis, by B. P. Fabbi......

HYDROLOGIC STUDIES Ground-water recharge Transmissivity and storage coefficient of aquifers in the Fox Hills Sandstone and the Hell Creek Formation, Mercer and Oliver Counties, N. Dak., by M. G. Croft and E. A. Wesolowski ...... Preliminary analysis of rate of movement of storm runoff through the zone of aeration beneath a recharge basin on Long Island, N.Y.,by G. E.Seaburn------_------,------Ground-water inflow toward Jordan Valley from Utah Valley through valley fill near the Jordan Narrows, Utah, by R. W. Mower------

Ground-water contamination

Waterborne styrene in a crystalline bedrock aquifer in the Gales Ferry area, Ledyard, southeastern Connecticut, by I. G. Grossman ......

Surface water

Meandering of the Arkansas River since 1833 near Bent's Old Fort, Colo., by F. A. Swenson...... Trends in runoff, by P. H. Carrigan, Jr., and E. D. Cobb ......

Relation between ground water and surface water Ground water-surface water relation during periods of overland flow, by J. F. Daniel, L. W. Cable, and R. J. Wolf---- The relationship between surface water and ground water in Ship Creek near Anchorage, Alaska, by J. B. Weeks------Prairie potholes and the water table, by C. E. Sloan ......

Erosion and sedimentation

Hydrologic and biotic effects of grazing versus nongrazing near Grand Junction, Colo., by G. C. Lusby------Sandbar development and movement in an alluvial channel, Rio Grande near Bernardo, N. Mex., by J. K. Culbertson and C. H. Scott------

Geochemistry of water

Spectrochemical determination of microgram quantities of germanium in natural water containing high concentrations of heavy metals, by A. E. Dong ...... --

Hydrologic techniques Evaluation of a method far estimating sediment yield, by L. M. Shown------. Dosage requirements for slug injections of Rhodamine BA and WT dyes, by F. A. Kilpatrick------Comparison of a propeller flowmeter with a hot-film anemometer in measuring turbulence in movable-boundary open- channel flows, by J. P. Bennett and R. S. McQuivey ......

INDEXES

Subject- ...... 263 Author ...... 267 GEOLOGICAL SURVEY RESEARCH 1970

This collection of 46 short papers is the first published chapter of "Geological Survey Research 1970." The papers report on scientific and economic results of current work by mem- bers of the Geologic and Water Resources Divisions of the U.S. Geological Survey. Chapter A, to be published later in the year, will present a summary of significant results of work done in fiscal year 1970, together with lists of investigatio~lsin progress, reports published, cooperating agencies, and Geological Survey offices. "Geological Survey Research 1970" is the eleventh volume of the annual series Geological Survey Research. The ten volumes already published are listed below, with their series desig- nations. Geological Survey Research Prof. Paper 1960------__------400 1961------__------424 1962------__------450 1963------475 1964------501 1965------525 1966------550 575 1968------600 1969------650

GEOLOGICAL SURVEY RESEARCH 1970

MAFlC AND ULTRAMAFIC ROCKS FROM A LAYERED PLUTON AT MOUNT FAIRWEATHER, ALASKA

By GEORGE PLAFKER and E. M. MacKEVETT, JR., Menlo Park, Calif.

Abstract.-Reconnaissance mapping in the Fairweather Range tion of the source pluton was not known, and no ultra- of southeastern Alaska has revealed that a layered mafic and mafic rocks were reported in the float. The belt of ultramafic pluton, the Fairweather pluton, underlies much of the Mount Fairweather area. The mafic rocks, which constitute layered intrusives extends southeastward through most of the pluton, are magnetite- and ilmenite-bearing two- Yakobi Island and western Chichagof Island where pyroxene gabbros and clinopyroxene-olivine gabbros. The ultra- several smaller bodies of gabbroic rock similar in com- mafic rocks consist mainly of sulfide- and chromite-bearing position to those of the Fairweather Range are exposed wehrlite, pyroxenite, and dunite, and locally contain significant (Rossman, 1963, p. F11). Other than at the Pair- concentrations of chromium, cobalt, copper, nickel, and platinum-group elements. The pluton is probably a source for ilmen- weather pluton, ultramafic rocks in the range have been ite, magnetite, platinum, and other heavy minerals that have found only in small isolated nunataks at the Brady been found as placer beach deposits along the adjacent Gulf Glacier nickel-copper prospect (fig. I), which is be- of Alaska coast. lieved to lie near the margin of the Crillon-La Perouse pluton (Cornwall, 1967). During a geochemical sampling program in the SETTING Yakutat quadrangle and adjacent areas in 1968, the authors traced float of mafic and ultramafic rocks in The Fairweather pluton probably underlies an area glacial moraines to a previously undescribed small of more than 15 square miles along the southwest flank layered pluton at Mount Fairweather (fig. 1). The of Mount Fairweather between the trunk stream of discovery is noteworthy because it extends the known Fairweather Glacier and Sea Otter Glacier (fig. 1). area of a belt of layered mafic plutons in the Fair- Its general configuration and its relation to the adja- weather Range some 20 miles northwestward, and be- cent metamorphic and granitic rocks were deduced by cause the pluton contains ultramafic rocks with con- close observation from a helicopter. Our knowledge of centrations of chromite, nickel, copper, and platinum- the lithology of rocks in the pluton and inferences re- group metals. Ultramafic rocks have possible economic garding the distribution of ultramafic rocks within it importance at the one other locality in the Fairweather are based entirely on examination of moraines,of the Range where they have been found. The purpose of glaciers that drain toward the west and southwest from this paper is to outline the general setting of the Fair- the Mount Fairweather area. The combination of weather pluton, as determined from a brief aerial re- rugged terrain and high altitude precluded landings connaissance, and to present the results of petrologic within the pluton (fig. 2). Technical mountaineering and chemical analyses of some samples of float rock capabilities would be required for a ground study. that were derived from the pluton. Virtually all of the pluton, except for the extreme The Fairweather pluton is the most northerly of northwestern end, is within the Glacier Bay National three layered mafic intrusives that lie roughly along Monument. the axis of the northwest-trending Fairweather Range The Fairweather pluton appears to be at least 6 (fig. 1). Its existence in the general vicinity of Mount miles long and 31/2 miles wide, the long axis trending Fairweather was correctly inferred by Rossman (1963, approximately northwestward. It is elongated parallel p. F11) from float of gabbroic rocks found in the to the structural grain of the adjacent foliated country moraines of Fairweather Glacier. However. the loca- rocks, which are mainly steeply dipping amphibolite

U.S. GEOL. SURVEY PROF. PAPER 700-B, PAGES B21-B26 B21 B22 PETROLOGY AND PETROGRAPHY

FIGURE1.-Index map showing the approximate lacation of the Fairweather pluton and other layered mafic plutms in the Fairwe"atherRange. Orillon-La Perause and Astralabe-De Langle plutone after Rossman (1963). PLAFKER AND MACREVETT B23

FIGURE2.-Aedal view olf the westem part d the Fairneather plutnn (location shown on fig. 1).Dashed line indicates the inferred slo~lthwestcontact. The cmspic~~ouslybanded racks in the foreground are probably inltertofnguing metavcllcanic and metasedimentary country rocks. Photograph by Auskin Post. and mica schist cut by granitic stocks and sills. Dark, Tlie layered igneous rocks on the southwest flank of layered nlafic and ultramafic rocks are exposed in sheer RIol~ntFairweather appear to dip northeast at a mod- cirque walls and knife-edged argtes along the south- erate angle. The nortlleastern contact of the pluton is west margin of the pluton from an altitude between largely concealed beneath the extensive snow and ice '7,000 and 10,000 feet, to the general vicinity of the cover on the highest part of the mountain; the contact summit, which rises 15,300 feet above sea level (fig. 2). shown on figure 1 is inferred from the distribution of B24 PETROLOGY AND PETROGRAPHY schistose country rock at lower elevations. Massive, stricted to moraines on the south side of Sea Otter blocky, light-colored rocks in part border the layered Glacier and on the unnamed glacier between Sea Otter rocks on the southwest in a zone as much as 2 miles and Fairweather Glaciers; gabbroic rocks are abundant wide, and locally seem to crosscut the darker layered in these moraines and in the lateral moraine along the rocks. It was not possible to tell from the air whether north side of Fairweather Glacier (fig. 1). The the light-gray unit represented a felsic granitic intru- distribution of ultramafic float suggests that its source sive or a relatively nonlayered leucocratic gabbro. is in the northern part of the pluton in the general area However, the general scarcity of felsic rocks in the due west of Mount Fairweather. moraines of glaciers draining this part of the mountain Samples of float from the Fairweather pluton are favor the latter alternative, and the light-gray zone composed primarily of virtually unaltered plagioclase, was tentatively mapped as part of the pluton. clinopyroxene, olivine, and orthopyroxene. Accessory No data are available on the age of the pluton or of constituents include sulfides, spinel-group minerals, the adjacent foliated rocks. Rossman (1963, p. F10) ilmenite, hornblende, and traces of rutile and apatite. correlated the schistose rocks of the Fairweather Detailed compositional studies have not been made, but Range with units of Mesozoic aie on Chichagof Island. the optical properties suggest that the plagioclase is Samples of gabbro from the compositionally and struc- mostly labradorite (An,,-,o), the clinopyroxene is turally similar Crillon-La Perouse pluton, which have probably augite, the olivine has a forsterite content of been submitted for radiometric dating (D. A. Brew, about 80 percent, and the orthopyroxene is magnesian oral commun., April 1969), may provide information hypersthene (Ens5). Augite characteristically is twin- on the time of intrusion of the layered rocks. ned and exhibits schiller structure; the hypersthene contains rare exsolution lamellae and blebs of clinopy- DESCRIPTION' OF THE ROCKS roxene. The rock compositions vary from anorthosite The compositional layering, textures, and mineral- or leucocratic gabbr; to pyroxeiite, wehrlite, and ogy of the Fairweather pluton are broadly comparable dunite. Chemical analyses of samples of the various to those of the layered igneous rocks elsewhere in the rock types are presented in table 1. Fairweather Range (Rossman, 1957) and in many Gabbroic rocks are by far the most abundant in the well-known localities throughout the world such as the moraines, and probably constitute the great bulk of the Skaergaard, Stillwater, and Bushveld Complexes pluton. They are leucocratic to melanocratic rocks in (Wager and Brown, 1968). Such rocks are generally which the layering results from variations in the pro- considered to result from fractional crystallization and portions of plagioclase and ferromagnesian minerals. crystal settling in a magma originally of basaltic com- Individual layers in float on the moraines range from position. a fraction of an inch to several feet in thickness. The Ultramafic rocks of the Fairweather pluton are re- rocks that were collected are fine- to medium-grained

TABLE1.-Chemical analyses, in weight percent, of six rock samples from the Fairweather pluton [Samples analyzed by methods similar to those described in U.8. Oeol. Survey Bull. 1144-A, supplemented by atomic absorption. Analysts, Lowell Art&, O. W. Chloe, P. L. Elmore, John Glenn, James Kelsey, and H. Smith]

Rock type- -.-....-...... ------...... Dunite Wehrlite Two-pyroxene gabbm Clinopyroxene Dunite olivine aabbro (sheared) Lab No ..-...--.....-.-.------..-----MI08 MI06 MI06 MI06 MlOe MI06 872W 873W 874W 876W 876W 877W Field No---....--.--.------.------68 APr 68 APr 68 APr 68 APr 88 APr 68 APr 101 A1 100 C2 101 A2 101 A3 101 A4 101 A6 PLAFKER AND MACKEVETT B25 two-pyroxene gabbro and clinopyroxene-olivine gab- fides and minor scattered crystals of chromite and bro with xenomorphic to hypidiomorphic granular tex- spinel from a few microns to 1 rnrn in size. The sulfide tures. Plagioclase in crystals up to 3.5 mm in length minerals, which occur in irregular scatibred masises constitutes 10-60 percent of the gabbros analyzed. Ruti- and microveinlets, are cubainite intergrown wihh ahalco- larted plagioclase makes up an wkirnsvted 85 percent of pyrite and pyrrhotite. Some pentlandite is intergrown a variant that had a decidedly purplish cast in the with chalcopyrite or cubanite, or, more rarely, it occurs hand specimen, and the composition borders on anor- in isolated masses. thosite. The ferromagnesian minerals are as much as 2.5 mm in size. SPECTROGRAPHIC ANALYSES The ratio of clinopyroxene to orthopyroxene in the Seven isolated float samples were analyzed for total two-pyroxene gabbro is variable, and either mineral metals by semiquantitative spectrographic methods, may predominate in a given rock. Irregular masses of and of these, five ultramafic rocks and one gabbro were magnetite and ilmenite(1) in grains 1.5 mm or smaller analyzed for platinum-group elements by quantitative constitute as much as 10 percent of one sample (68 APr spectrographic methods (table 2). The rocks analyzed 101 A2). The iron ores occur both interstitially and include all the lithologic types described in the pre- enclosed within the pyroxenes. The clinopyroxene- ceding section, and some of them had the highest con- olivine gabbro (68 APr 101 A4) is composed mainly of tent of disseminated opaque minerals found in the clinopyroxene and olivine in grains as much as 2.5 mm float. With the exception of the pyroxenite (68 APr across ; the grains contain about 10 percent plagioclase 100 Cl), chemical analyses are given for these same and a few percent interstitial hypersthene. Opaque rocks in table 2. minerals make up as much as 5 percent of the rock. The analyses indicate concentrations of titanium (2 The predominant ore mineral is interstitial chalcopy- percent) and vanadium (2,000 ppm) in the magnetite- rite in grains of less than 0.5 mm. Other accessory and ilmenite-bearing two-pyroxene gabbro that are not minerals in the gabbros are generally less than 0.5 mrn unusual for rocks of this type. As much as 5,000 pprn in size and include brown hornblende, green spinel, chromium, 5,000 pprn copper, and 5,000 pprn nickel are and rare cubanite, pyrrhotite, magnetite, chromite( I), present in the richest samples of sulfide- and chromite- and pentlandite ( ? ) . bearing pyroxenite and wehrlite, as much as 5,000 pprn The float of the ultramafic rock consists of black, chromium, and 3,000 pprn nickel, in the chromite-bear- greenish-black, and olivine-green crystal cumulates ing dunite. Noteworthy amounts of cobalt and platinum- with a faintly layered structure. One sample of dunite group elements were also found in the ultramafic rocks (68 APr 101 Al) consists of more than 90 percent and, to a lesser extent, in the gabbros. The largest fresh euhedral to anhedral olivine in grains ranging amounts of these elements, which were in the sheared from 0.2 to 4.5 mm in size. Some of the grains exhibit dunite (68 APr 101 A5), totaled 200 pprn cobalt and twinning and protoclastic textures. The remainder of the rock is composed of euhedral to subhedral chromite 0.184 pprn palladium, 0.171 pprn platinum, and detect- crystals 0.1 to 0.5 rnm across, a few crystals of pent- able rhodium (0.004 pprn). landite less than 0.05 mm in size, and minor amounts Disseminated opaque minerals occur in all the float of interstitial clinopyroxene and hornblende. One collected from the Fairweather pluton. These minerals sheared dunite specimen (68 APr 101 A5) was cut by are mainly magnetite, ilmenite, sulfides, and chromite, cross-fiber serpentine in closely spaced veinlets less which undoubtedly account for the anomalous metal than 0.02 mm wide that make up as much as 20 percent content of these rocks. Combined magnetite and ilmen- of the rock. The essential mineral of a pyroxenite ite constitute as much as 10 percent by volume of sample (68 APr 100 C1) is anhedral clinopyroxene in some gabbros. The wehrlite and p~roxenitecontain grains 0.5 to 3.0 mm across with about 25 percent anhe- up to 15 percent sulfides with minor chromite, and dral to subhedral hypersthene that is less than 1.0 mm some dunites contain a few percent of chromite with in size. A sample of wehrlite (68 APr 100 C2) consists minor sulfides. The sulfides identified in polished sec- of about equal amounts of subhedral and anhedral tion include cubanite, chalcopyrite, p~rrhotite, and olivine in grains as much as 2.5 mrn across and an- pentlandite. No chromitites or massive sulfides were hedral, partially poikilitic clinopyroxene as much as found. The nickel and copper content of the highest 5.0 mrn. The clinopyroxene is commonly altered along grade rocks sampled is about 0.5 percent each. This grain boundaries and cracks to a mixture of fibrous may be compared with a content of 1.5 percent nickel green actinolite and antigorite. The pyroxenite and and 2 percent copper in selected samples of ultramafic wehrlite contain as much as 15 percent interstitial sul- rocks from the Brady Glacier prospect which is cur- B26 PETROLOGY AND PETROGRAPHY

TABLE2.-Spectrographio analyses of selected float tcltramalic and mafic roclcs from the Fairtoeather ptuton [Asterisk indicates quantitative analysis A. F. Dorrzapf and Marian Schnepfe analysts' all other analyses are semiquantitative, Chris Heropoulos analyst. Q major mu- stituents greater than 10 percent; N, not'detected. Other elements looked for b6t not fouhd: As, Au, B, Be, Bi, Cd, La, Mo, Nb, Pb, Sb, Sn, Te, U: W, Zn, Zr,'~e,Qe, Hf, In, Li, Re, Ta, Th, TI, Eu]

Rock type Dunite Dunite Wehrlite Clinopyroxene Two-pyroxene Two-pyroxene Pyroxenite (sheared) olivine gabbro gabbro gabbro Field No ...... 68 APr 101 A1 68 APr 101 A5 68 APr 100 C2 68 APr 101 A4 68 APr 101 A3 68 APr 101 A2 68 APr 100 C1 Laboratory No.. .-...... -.-...-.. .------.... M4949 M4949 M4949 M4949 M106892W M106877W M106873W M106876W M106875W M106874W - -- Weight percent

Par& per million

1 Average of two analyses. 2 Average of three analyses.

rently being actively explored by the Newmont Min- Smith, J. G., Mineral resources of Glacier Bay National ing Co. (Cornwall, 1967, p. 153, table 15). Monument: U.S. Geol. Survey opendle report, July 14, 1967, p. 153-158. The Fairweather pluton is probably a major source Rossman, D. L., 1957, Ilmenite-bearing beach sands near Lituya for magnetite, ilmenite, platinum, and other heavy Bay, Alaska: U.S. Geol. Survey open-file report, June 29, minerals that occur in association with placer gold in 1957, 10 p., 1 pl., 2 tables. 1963, Geology and petrology of two stocks of layered beach deposits along the adjacent Gulf of Alaska coast gabbro in the Fairweather Range, Alaska: U.S. Geol. Sur- (Rossman, 1957, 1963; Thomas and Berryhill, 1962). vey Bull. 1121-F, p. F1-F50. Thomas, B. I., and Berryhill, R. V., 1962, Reconnaissance stud- REFERENCES ies of Alaskan beach sands, eastern Gulf of Alaska: U.S. Bur. Mines Rept. Inv. 5986, 40 p. Cornwall, H. R., 1967, Brady Glacier prospect, in MacKevett, Wager, L. R., and Brown, G. M., 1968, Layered igneous rocks: E. M., Jr., Brew, D. A., Hawley, C. C., Huff, L. C., and Edinburgh and London, Oliver and Boyd, Ltd., 588 p. GPOLOGICAL SURVEY RESEARCH 1970

BLUESCHIST AND RELATED GREENSCHIST FACIES ROCKS OF THE SEWARD PENINSULA, ALASKA

By C. L. SAINSBURY; R. G. COLEMAN, and REUBEN KACHADOORIAN, Denver, Colo.; Menlo Park, Calif.

Abstract.-Blueschist facies rocks of Precambrian age are the geology of Seward Peninsula has been imperfectly exposed in thrust slices over an area of more than 10,000 square known. Since 1960, mapping by the senior author, miles of the Seaward Peninsula, Alaska. Rocks with mineral assisted in 1967-68 by Kachadoorian, has clarified the assemblages characteristic of high-grade blueschist facies and retrograde blueschist facies occur juxtaposed against low-rank stratigraphy and structural setting. The stratigraphy metamorphic pelitic and carbonate rocks, possibly because of is merely summarized here and is depicted in broad thrust faults. From west to east over the Seward Peninsula, outline on figure 1. progressive regional metamorphism of Cretaceous ( ?) age and extensive overthrust faulting of pre-mid-Cretaceous age have Older Precambrian rocks complicated the metamorphic assemblages. Field evidence proves that the blueschist rocks represent metavolcanic rocks and Rocks of Precambrian age form the cores of the metamorphosed mafic rocks of late Precambrian age. Mineral Higluaik and Bendeleben Mountains and crop out over assemblages are strongly influenced by the original chemical wide areas elsewhere on the Seward Peninsula. The composition of the rocks. oldest rocks, exposed in the Kigluaik arch which trends east along the axis of the range, consist of plagioclase- orthoclas~-quartz-biotite-hornblende Glaucophane-bearing rocks have long been known in paragneisses with the south-central Seward Peninsula (Smith, 1910; local beds of calc-silicate rock. These paragneisses Moffit, 1913) ; mapping by the senior author from 1960 grade upward into marble gneisses with forsterite, to 1968 has shown that similar rocks are exposed over monticellite, and muscovite, which are formed by much of the central and eastern Seward Peninsula. In chemical reconstitution of the impurities. Because of view of the intense current interest in such rocks and intense flowage and deformation, the marble gneiss problems of interpretation of their origin (Essene and varies greatly in thickness; where best exposed in the others, 1965 ; Ernst, 1963 ; Coleman, 1967 ; Miyashiro, Kigluaik arch, it locally exceeds 700 feet, but it thins 1961), this preliminary report describes the general rapidly by tectonic squeezing. The marble gneiss is field relations of the Seward Peninsula rocks, sum- transitional with the overlying biotite-orthoclase- marizes the tectonic setting, and briefly describes the plagioclase-quartz paragneiss; this paragneiss is sev- mineralogy and metamorphic classification of some of eral hundred feet thick and is interbedded with thin the rocks collected at widely scattered localities. Be- layers of calc-silicate rocks. These upper gneisses, cause of the importance of assigning the blueschist which have been dated by Carl Hedge (oral commun., rocks to the Precambrian - blueschist rocks are un- 1969) by the whole-rock rubidium-strontium method common in Precambrian rocks-the discussion of the as 750 m.y. (million ears), the probable age of the stratigraphy and age relations of rocks of the Seward metamorphism, grade upward into andalusite-garnet Peninsula is rather complete. The Precambrian rocks schists, locally gneissic, and thence into biotite-garnet are similar to those of the Soviet Arctic (Rabkin and schists and intercalated marble and calc-silicate rocks. Ravich, 1961). Rocks equivalent to. these upper schists extend con- REGIONAL GEOLOGY tinuously from the east end of the Kigluaik Mountains Except for small areas near Nome, which were (fig. 1) and form the bedrock of the western and mapped in detail by Smith (1910) and Moffit (1913), central Bendeleben Mountains. These rocks were in-

U.S. GEOL. SURVEY PROF. PAPER 700-B, PAGES B33-B42 B33 PETROLOGY AND PETROGRAPHY

EXPLANATION [; i :?; ... .. Unconsolidated deposits > Slate, graphitic siltite, and quartz- mica schist

Volcanic rocks Nome GG~ Garnet-glaucophane-epidote-chlorite schists and schistose marble Granitic intmsives L+ ++I Kigluaik Group Graywacke, conglomerate, and andesite High-rank metamorphics, including biotite-andalusite schists, orthogneiss and paragneiss, calc-silicate rocks, and marble n

Undifferentiated- rocks of Paleozoic and Precambrian age Limestone, dolomite, and marble ]i Includes local areas of older rocks ' 2 Contact a Approximately located *8 Sample locality and number

FIGURE1.--Generalized geologic map of the Seward Peninsula, Alaska, showing sample lwalities. Geology modified by C. L. Sainsbury from Dutro and Payne (1957). Geology will 'be modified further when current work has ibeen completed. SAINSBURY, OOLEMAN, AND KACHADOORIAN B35 cluded in the Kigluaik Group by Moffit (1913), and composition of the slates of the York region is always this name is retained in this report. apparent. Although the normal contact with the Nome All the metamorphic rocks described above are in- Group rocks has not been observed, the disparity in truded by biotite-orthoclase-quartz orthogneiss, which degree of deformation between Nome Group rocks and forms extensive masses in the Kigluaik Mountains, and the slates of the York region suggests that the slates by gneissic biotite granite, which is cataclastic in are unconformable above the Nome Group rocks. An texture and which is associated with numerous coarse- unconformable relation is suggested, too, by the fact grained pegmatites. All these intrusive rocks are be- that Nome Group rocks are intruded by intensely de- lieved to {be of Precambrian age, principally because formed mafic dikes, as well as by the gabbros which they are not known to have intruded younger Pre- alone intrude the slates. cambrian rocks. Latest Precambrian rocks Younger Precambrian rocks In the York Mountains of the western Seward Pen- Younger rocks which represent metamorphosed insula, detailed mapping by Sainsbury (1965, 196913) volcanic rocks, including tuff aceous sediments and has demonstrated that the slates of the York region are mafic intrusives probably related to the volcanics, and overlain by more than 2,300 feet of thin-bedded argil- a thick sequence of graphitic pelitic rocks crop out laceous and dolomitic limestone, which is completely widely over the Seward Peninsula both in thrust slices unmetamorphosed and utterly devoid of fossils. The and as wide expanses of bedrock that exceed hundreds contact between slate and limestone in at least one of square miles in area. The older metavolcanic rocks locality appears to be gradational and conformable; are of principal interest to this report, for they include elsewhere over wide areas it is definitely a thrust-fault the blueschist facies rocks; they have been called the contact. These limestones are not delineated separately Nome Group by Brooks, Richardson, and Collier on the geologic map (fig. 1). At other places on the (1901), by Collier (1902), by Moffit (1913), and by Seward Peninsula, a thick sequence of thin- to medium- Smibh (1910). Albhough Dhe Nome Group was generally bedded limestones, argillaceous limestone, and dolo- considered to be of Paleozoic age, certain puzzling mitic limestone devoid of fossils overlies the thin- features of it (principally the wide disparity in meta- bedded dolomitic limestone. All these carbonate rocks morphic rank in rocks close to each other) have led are clearly of pre-Ordovician age (Sainsbury, 1965). some workers (for example, Moffit, 1913, p. 23) to Because they are not intruded by any of the numerous believe that rock of Precambrian age may be included gabbros that intrude the slate of the York region, they therein. are unquestionably younger than the slates and are, The younger graphitic rocks have been called the therefore, probably of latest Precambrian age. slate of the York region on the western Seward Pen- insula by Collier (1902) and by Knopf (1908), and the Paleozoic rocks Kuzitrin Series in the Kigluaik Mountains area by Detailed and reconnaissance geologic mapping in the Brooks, Richardson, and Collier (1901) ; they are Teller, Bendeleben, and Solomon 1:250,000 quadran- probably correlative with the Hurrah SIate and Puck- gles has shown that carbonate rocks of tremendous mummie Schist (Smith, 1910) in the Solomon and thickness were deposited on the Seward Peninsula Casadepaga areas east of Nome. Although several vari- without a major lithologic change from Early Ordo- ants are well established, these graphitic or slaty rocks vician at least tihrough Mimimimian tim. The paleo- wherever seen exhibit the common characteristic of stratigraphy has been deciphered from disconnected being composed principally of silt-size quartz grains in exposures in thrust plates at widely scattered localities a graphitic matrix. The percentages of quartz, and hence is incomplete. Nevertheless, abundant fossil graphite, carbonate, and less common albite, chlorite, collections have firmly established that the Ordovician, and muscovite vary from place to place, but in general Silurian, Devonian, and Mississippian Systems are rep- appearance the rocks are remarkably similar. In con- resented by aarbonate rocks throughout 'the Sewad trast to the older Nome Group rocks, which everywhere Peninsula. It can be stated emphatically that no fos- are intensely deformed into recumbent isoclinal folds, siliferons noncarbonate rocks of early or middle Paleo- sheared, and completely recrystallized, the paphitic zoic age are known on the Seward Peninsula and, slates not deformed by thrust faulting commonly ex- moreover, that the known stratigraphy allows no ap- hibit a clearly definable clastic texture with angular preciable time for deposition of noncarbonate rocks quartz grains of silt size. Tectonism, thermal meta- from Ordovician through Mississippian time. No rocks morphism, and hydrothermal alteration quickly obliter- of late Paleozoic age are known on the Seward ate the sedimentary texture. Nevertheless, the siliceous Peninsula. B36 PETROLOGY AND PETROGRAPHY

Post- Paleozoic rocks Mountains were not in normal stratigraphic position Throughout most of the Seward Peninsula, rocks of above the older rocks (Collier, 1902, p. 18). post-Paleozoic age consist mainly of intrusive rocks, In the York Mountains of the western Seward which comprise biotite granites of middle to Late Peninsula, the thrusts mainly are horizontal or dip Cretaceous age and granitic and mafic dikes (includ- gently south, and the Paleozoic rocks are completely ing lamprophyres) of Late Cretaceous to early Ter- unmetamorphosed. Eastward from the York Moun- tiary age. The geologic map of Alaska (Dutro and tains, the thrusting becomes more complex, thrusts are Payne, 1957) shows rocks of Triassic age on the Sew- folded, and a weak regional metamorphism begins ard Peninsula; this age assignment is erroneous, for near the American River (fig. 1) and becomes pro- these rocks are the slates of the York region, which gressively stronger eastward and southward. East of are of pre-Ordovician age. During the Tertiary, small the Rougarok River, imbricate thrust sheets are in- local basins were filled, giving rise to coal-bearing tricately folded, and the Paleozoic carbonate rocks are beds; and during the Quaternary, extensive lava fields recrystallized to sugary-textured marble in which bed- were formed in the Imuruk Basin and lowlands. East ding is largely obliterated. In this area, thrust sheets of the Seward Peninsula, volcanic rocks and eugeo- involve the Precambrian rocks as well as the Paleozoic, synclinal graywackes of Jurassic and Cretaceous age fold axes generally trend northward, and thrusting filled the subsiding Yukon-Koyukuk basin (Miller and was eastward. Mapping in 1969 by Sainsbury has others, 1959). The Mesozoic rocks extended west only shown that the York Mountains thrust sheets, where to the Darby Mountains, east of the main Seward thrusting was northward, are succeeded to the west by Peninsula areas where blueschist facies rocks were thrust sheets with intensely folded limestones of Mis- studied by the senior author. Some Afesozoic volcanic sissippian age. The Mississippian rocks are thrust rocks occur west of the Tubutulik River, where blue- northeastward. Seemingly, then, two distinct cycles of schist rocks occur. thrusting are represented ; the earlier was characterized by intense folding and eastward transport, the younger Summary of age relations was characterized by imbricate thrusting without The above discussion shows clearly that the rocks major folding and by northward transport. of the Nome Group, which contain the glaucophane- Because the thrust sheets have been intruded by bearing rocks to be discussed, are intensely deformed granite stocks of middle to Late Cretaceous age, the and underlie unmetamorphosed pre-Ordovician lime- thrusting is earlier than middle Cretaceous. As the stones. The field relations and stratigraphy allow but thrust belt is probably buried by the Cretaceous rocks one conclusion : the Nome Group rocks are considerably of the Yukon-Koyukuk basin, the thrusting is ap- older than the slates of the York region, which them- parently of pre-mid-Cretaceous age, for the Jurassic selves are older than the pre-Ordovician limestones. and Lower Cretaceous rocks are intimately folded. No Hence, it is concluded that the blueschist facies rocks upper Paleozoic or lower Mesozoic rocks being known are of Precambrian age. in the thrust belt, the thrusting cannot be dated more precisely than post-Mississippian and pre-Late Cre- STRUCTURE taceous, although it most likely is of Early Cretaceous age. The regional thermal metamorphism impressed Throughout the Seward Peninsula, the geologic upon the thrust sheets in the eastern Seward Peninsula structure is dominated by thrust sheets. Thrusting was is probably related to the intrusion of large batholiths first inferred by Collier (1902, p. 18), and proved in and stocks of granite and related syenites, which have the Nome area by Moffit (1913) and Smith (1910). De- tailed mapping by Sainsbury (1965, 1969a, b) in the been dated as of mid-Cretaceous age (Miller and York Mountains showed the great extent of the thrust- others, 1966). Biotite separated from Precambrian ing and demonstrated the existence of imbricate thrust schists in the Kougarok River area, where Paleozoic faults as the dominant structure of the western Sew- limestones of thrust sheets are metamorphosed to ard Peninsula. Reconnaissance mapping to the east for marble, gave a "re-set" age date of 100 m.y., on the 150 miles and low-level aerial reconnaissance and spot basis of the potassium-argon method (J. D. Obrado- ground ehscks as far etas Norton Bay have demon- vich, oral commun., 1969). Apparently, regional meta- strated that rocks of the entire Seward Peninsula are morphism and intrusion of granitic batholiths were involved in thrust sheets of tremendous extent and related and occurred generally in the Cretaceous. This complexity. This thrust belt has been named the Collier regional thermal metamorphism probably caused the thrust belt (Sainsbury, 1969b), to honor A. J. Collier, marked retrograde metamorphism of the blueschist who first recognized that the limestones of the York facies rocks. SAINSBURY, OLEMAN, AND KACHADOORIAN B37

BLUESCHIST ROCKS eastern Seward P~ninsula'are described b~riefly,and reference is made to probable blueschist rocks discussed The blueschist and related rocks to be described were by Smith (1910). The suite of related rocks (group 6) sampled at widely scattered localities on the Seward Peninsula-all samples, however, came from rocks be- from a small area in t.he Salmon Lake area (fig. 1) was longing to the Nome Group (p. B35). Wherever seen, classified by Sainsbury. the Nome Group rocks consist of greenish schistis Five groups of rocks are recognized : (1) glaucophane and intercalated beds of schistose marble. Locally, schists, (2) retrograde glaucopha.ue schists, (3) meta- dark-green rocks intrude both the limestones and the sedimentary rocks, (4) metagabbros not converted to schists and are themselves locally converted to blue- blueschist rocks, and (5) metagabbros or metavolcallic schist rocks. Where exposures are good, color banding rocks intrusive into Precambrian limestones and belong- and lithologic variations in the Nome Group rocks ing to the blueschist facies. The detailed collection local- suggest that the Nome Group represents old volcanic ities of all the samples to be described are given in rocks, tuffs, and tuffaceous limestones. Locally, as in table 1. the Nome River valley, where samples 68-ASn-566 to Group 1 rocks represent the following mineral 566H (p. B41) were collected, irre,dar masas of assemblages : rock related to the blueschist facies are isolated with- Field No. Description in schistose marble (fig. 2), thus showing unquestion- 67-ASN-264 ------Glaucophane-actinolite-ehlorite-epidote- able contemporaneity of calcareous muds and material garnet-sphene-white mica-quartz. most logically interpreted as tuff or volcanic bombs. 67-ASn-264A ------Glaucophane-epidote-garnet-sphene- Similar relations have been observed in southeastern rutile-chlorite. 67-ASn-503 ------Glaucophane-epidotegarnet-chlorite- Alaska (Sainsbury, 1961, p. 315), where pyritized sphene-white mica-quartz-rutile. volcanic bombs are encased in unaltered limestone. For 67-ASn-595 ------Glaucophane-garnet-sphene-white comparison with the blueschist rocks, altered mafic mica. intrusive rocks from west of the main blueschist belt are These mks are foliated because of orientation of the described. All sample localities are shown on figure 1. platy minerals; garnets are euhedral t'o subhedral por- Petrography phyroblasb (nonhelical) , whiclh contain abundant inclusions of the other minerals that make up the rest of For this preliminary report, Coleman has classified the rock. The blue anlphibole was identified as glauco- rocks collected by Sainsbury in 1967, and determined phane on the basis of the follo~vingoptical constants : to be glaucophane-bearing, into four ,groups that are 2V=3040° ( - ), Z//C less than To, x=colorless, y= correlative with types originally defined by Co'lenlan purple, and z =blue. In sample 67-ASn-264, the actino- and Lee (1963) in the Cazadero area, California. Rocks lite coexists with the glaucophane and is not retrograde. collected by F. H. Moffit from the Solomon area and by The epidote forms anhedral masses, and individual W. C. Mendenhall from the Darby Mountains on the grains- have iron-rich cores as deduced by birefringence and color. Sphene is common, generally forming anhedral masses--euhedra are very rare. The ruhile occurs within the sphene masses and may be retrograding to sphene. Some of the minor chlorite may be retrograde, but much of it is intergrown with glaucophane and probably is in equilibrium with it. The sparse white mica may be phengitic. The small amount of quartz commonly occurs in the pressure shadows behind garnets. Apatilte is a minor accessory; small opaque grains associated with rutile may be ilmenite. The textures (fig. 3) and mineral assemblages are nearly identical with those of the LLhigh-grade"type IV blueschists from California and New Caledonia (Cole- man, 1967). However, these Se~vardPeninsula glauco- B'IQURE ~.-Po~s(P) of rock related to the blueschist facies in phane schists lack omphacite, which is common in type schistose marble of the Nome Group? IV blueschists of California (Coleman and Lee, 1963). B38 PETROLOGY AND PETROGRAPHY

TABLE1.-Local.lties and descriptions of samples referred to in this report

Group Field No. Locality Latitude (N.) Longitude (W.) General description of sampled rock body

West of Kougarok River- 65'26'20" 164°55'30" Frost-riven massive boulders on surface of tundra-covered ridge. _----do------65'26'20" 164°55'30" Schistose frost-riven boulders surrounding 67-ASn-264. - - - - -do------65'24'20" 164'54'30" Frost-riven massive boulders on surfaces of tundra-covered ridge. West headwaters of 65'42'20" 164'59'50" Float boulders in creek bed, surrounded by Kougarok River. tundra-covered hills. West of Kougarok River- 65'30'35" 164'55'50'' Frost-riven boulders on tundra-covered ridgeline. Bed of Kougarok River.. - 65'28' 164°42'30" Small outcrop on east bank of river, massive-appearing blueschist. -----do------_------65'29' 164'43' Outcrop on west bank of River, relatively sheared, pyrrhotite noticeable. North of Grantley 65'19' 166°06'30" Graphitic slate band in chloritic schist, Harbor. float on ridgeline. East of Kougarok River, 65'37'30" 164'33'40" Thrust "xenolith" in marble. top of Harris Dome. West headwaters of 65'44' 164'58' Outcrop of calcareous, lineated slate. Kougarok River, north bank of Washington Creek. West fork of Kougarok 65'44'20'' 164'52' Outcrop of sheared calcareous slate. River, north bank of Washington Creek. Northeast shoulder, 65'33' 167'04' Outcrop, siliceous graphitic bed with Brooks Mountain. numerous quartz pods. West bank, Kanauguk 65'32' 167°31'30" Outcrop in creek bed, graphitic siltite. River. North shore, Imuruk 65'11'40" 165'46'30" Outcrop at beach level, gray-green mafic Basin. dike, sheared. South of Grantley 65'12' 166'12' Frost-riven rubble of mafic intrusive. Harbor. Southeast of Gantley 65°08'30'f 166'22' Altered gabbro body intrusive into lime- Harbor. stone and slate. East bank, Goodhope 65'53' 164'03' Altered gabbro dike intrusive into chloritic River. schist. Headwaters of Nome 64'50'12" 165'12'20'' Sheared gabbro intruding limestone schist. River. Southeast of Salmon 64'52' 165'10' Lenticular body (tuff?) in limestone schist. Lake. Northeast of Salmon 64'57'30'' 164°50'20" Mafic inclusion in limestone schist. Lake. Southeast of Salmon 64'52'04'' 165'10' Well-exposed outcrop of chloritic schist, Lake. faint color banding. 68-ASn-566B -----do------do------do-----Do. 68-ASn-566C -----do------do------do-----Do. 68-ASn-566D -----do------do------doDo. 68-ASn-566E -----do------do------do-----Do. 68-ASn-566F -----do------_------do------do-----Do. 68-ASn-566G -----do------_------do-do Do. 68-ASn-566H ---_-do------do--do Do. (None)- -- 22 Moffit's (1913) Between Nome and ...... Chloritic schist of the Nome Group, es- area. Solomon. pecially intrusives in marble. (None)--- 23 Smith's (1910) Northeast of Solomon --_-___-_-_-__-_------"Greenstones" and chloritic schists of the area. Nome Group. (None) - -- 24 Mendenhall's Tubutulik River, eastern ...... Chloritic schist within or beneath thrust (190l)area. Seward Peninsula. slices of marble.

The absence of lawsonite and aragonite- in the Alaskan Fleld No. Description rocks indicates that these rocks are not similar to the 67-tLSn373 ------Chlwite-actinolite-white mica-albite- garnet-epidote-sphene- (aragonite?). lower grade 'YP~111 rocks of the Cazadero area, where 67-ASnA3g ------Albite-chlorite-actinoGte-garnet- ~lauco~hane-lawsonite-chlo- epidote-sphene. the mineral assemblage- is " rite& phengite faragonite fjadetic pyroxene-sphene. 67-ASn439 ------AlbitechZorite-actinolI.teepidote- garnet-sphene-glaucophane-calite. Group 2 rocks clearly are retrograde blueschists. These rocks clearly show a strong retrogression to 'pecimens and msemblages are listed below, greenschist facies under conditions; some original with the unusual minerals in italic : foliation remains but is not inherited by the retrograde SAINSBURY, OOLEMm; AWD KACHADOORIAN B39 tion has further modified some of the blueschist rocks. The retrograde metamorphism recorded by the rocks of group 2 cannot as yet be definitely correlated with the Cretaceous thermal event, principally because blueschist rocks have not been found outside the zone of regional thermal metamorphism. However, massive garnet-glaucophane rocks of group 1 and retrograde rocks of group 2 occur within a few miles of one an- other, showing that retrogression was not regional in scope. Group 3 rocks consist of metapelitic rocks equivalent to 611e slate of the York region; these racks are younger than the Nome Group rocks of groups 1and 2. They were examined principally to see if they contained aragonite, for some contain appreciable carbonate. All are from rocks believed to be younger than the Nome Group, which contain the blueschists here described ; afid all represent mildly mekamorphosed pelitic FIGUR~3.-Phobomicrograph of typical coarse-grained garnet- rocks, which were originally composed principally of I glaucophane rock from the Kougarok River drainage; gl, glaucophane; gn, garnet; S, sphene; xgn, garnet with glauco- silt-size quartz grains, minor clay minerals, carbonace- phane in fractures. Plain light. Photomicrograph by Ernest ous matter, and carbonate. The samples studied are Krier, U.S. Geological Survey. glistening black phyllites with veinlets of white quartz. minerals. The apparent original blueschist minerals Two distinct S-planes are visible, an S-1 plane \vhich are glaucophane-epidote-garnet-sphene; of these, glau- represents the main foliation, and an S-2 plane which cophane and garnet have been most altered, glauco- consists of a strong crinkling of the S-1 planes. Indi- phane being virtually absent. The glaucophane relicts vidual samples and their mineral assemblages are as are being replaced and surrounded by a pale-green follows : amphibole, which is set in a matrix of chlorite and Field No. Des~tion i albite. The new amphibole looks like actinolite but 67-ASn-137------Quartz-mica-graphite-chlorite-albite. with detailed study may prove to be common horn- 67-ASn-326------Quartz-mica-carbonate-chlorite- blende. A possible reaction for this retrogression is as graphite. follows : 67-ASn-36@------Quartz-albite-mica-chlorite- carbonate-graphite. I ~~(HzN~zM~~AIzS~~O~~)+6(HCaTA13Si3013) +7(Si02) + 14H20-+ glaucophane + epidote + quartz +water 67-ASn-434------Quartz-albite-chlorit+mica- I carbonate-graphite. 1 50(NaA1Si308)+ ~(H~M~AIzS~~OI~)+ ~(HzC~ZM~SS~~O~~). 62-ASn-652D------. Quartz-albite-white mica-carbonate- albite + chlorite + actinolite i graphite. Although chlorite and actinolite occur in rocks of the first group, they are intergrown with glaucophane Present textures of these rocks are metamorphic and and are not clearly retrograde as in the second group. markedly schistose. The albite (indices less than for Retrograde rocks have been identified at other local- Canada balsam) forms irregular porphyroblasts that ities on the Seward Peninsula, and apparently an contain numerous inclusions of graphite; carbonate initial period of blueschist metamorphism was fol- occurs as irregular porphyroblasts. The mica is mainly lowed by at least one period of static heating or loss of white (phengitic?) and is concentrated along folia; high pressure with concomitant change to greenschist locally, chlorite is interleaved with the white mica. or epidote-amphibolite facies. As vill be shown in the Opaque clots are mostly graphite, but some rutile final part of this report, the dynamic metamorphism needles were observed. that produced the blneschist rocks was probably of These graphitic metapelitic rocks probably belong to Precambrian age; hence, the blueschists have gone the greenschist facies-a. classification which agrees through a major thrust cycle, which produced no wit11 the impression given by field mapping. ,4lthough regional metamorphism, as well as a younger regional in places the rocks are so completely recrystallized that metamorphism in Cretaceous time that made marble quartz grains are much larger and graphite is com- of limestone. In addition, local hydrothermal altera- pletely clotted, temperatures were so low that quartz B40 PETROLOGY AND PETROGRAPHY did not react with calcite to form calc-silicate rocks. calated in the blueschists of the Nome Group. These There is no hint of blueschist-facies metamorphism in rocks and their mineral assemblages are listed below: any of the rocks examined. Field No. Deacrlption Group 4 rocks represent selected specimens of meta- 67-ASn-41------Garnet-quartz-epidote-apatite~sphene- gabbros and related mafic dikes that intruded rocks albite-chlorite-calciteblue-green as young as the slate of tlie York region at many places amphibole. throughout the Sew-ard Peninsula. These mafic dikes 68-8Sn-567------Garnet-white mica-epidote-quartz- calcite-sphene-apatite-magnetite- are compositionally close to volcanic rocks and might hematite. have been expected to become blueschists, if they had 6%ASn-444------_ Garnet-albite-epidot~hlorite-sphene- gone through the blueschist metamorphic event. All quartz-calcite. have gone through the thrust cycles. Selected speci- These rocks are unlxslxal in that they contain garnet, mens were collected progressively from west to east are obviously schistose, and lack glaucophane. The across the Seward Peninsula (fig. 1, table I), from mafic material which occurs in the limestone lacks outside tlie zone of blueschist facies into the Bougarok amphibole entirely, whereas it commonly contains River area where blueschist rocks are common. The wliite mica. Garnets are subhedral to anhedral, and following is a list of these rocks and their mineral most contain numerous inclusions. There is no clear assemblages : evidence that these rocks ever contained glaucophane. F4eld No. Description The limestone shown in figure 3, which surrounds the 61-ASn477------Augite-titaniferous magnetite- material represented by 68-ASn-567, is schistose and leucoxene-labradorite-serpentine- contains rounded quartz grains and wliite mica. The sideritic carbonate. 67-hSn-65------Actinolite-epidote-chlorite-albite- quartz grains have not reacted with the carbonate. The augite ( ?)-white mica. schistosity, complete mineralogical reconstitution, and 67-ASn-l35E_------. Titaniferoufl augiteilmenite-leucoxene- lack of relict igneous textures and minerals show that white mica-chlorite-actinolite this group of rocks, though in part definitely intrusive, epidote. has passed through a regional metamorphism in a 67-ASn-242------Augite-actinolite-epidote-albite- leucoxene-titaniferous magnetite ( ?) - high-stress environment which did not affect the meta- chlorite. gabbros of group 4. Hence, it is concluded that the 68-ASn-136------Augite-actinolite-leucoxene-epidote- bluescllist metamorphism occurred between the intru- chlorite-plagioclase. sion of the two different mafic-dike suites that are These rocks, though containing minerals such as represented by rocks of grouPs 4 and 57 and must, there- epidote, chlorite, and actinolite, which are often found fore, be of late I'rcxambrian age. in the retrograde blueschists, differ markedly from the of variant.i blueschist rocks in several respects: (1) primary angite remains, (2) zoned high-calcic plagioclase remains, (3) A suite of samples (group B, from a ~~~~~-~~p~~~~ opaque iron ores remain and are altering to leucoxene, of Nome Group lolueschist (fig- 4, was (4) the relict igneous texture is clearly visible, and (5) none contain garnets. The textures, shearing, and metamorphic minerals are possibly related to the thrust cycles and to the younger regional static thermal metamorphism; clearly, ho~vever,none of these rocks have been completely reconstituted and made scliistose as has happened to chemically similar blueschist rocks. Rever- theless, the most altered of these rocks resemble the retrograde blueschists somewhat, although they are readily distinguislied in thin section. The rocks of this group intruded the upper Precambrian slates, but not the younger carbonate rocks, and are probably of late or latest Precambrian age. Group 5 rocks represent mafic dikes that cut limestone schists in the Nome Group of Precambriall age,- and altered mafic material occurring as ]ellticnlar beds FIGURE4.-Photograph of outcrop of retrograde bluechist rocks, showing location of closely spaced samples. All sample num. (fig. 2) and isolated clots in limestone schists inter- hers are preceded by CIg-LSn. SAINSBURY, OOLEMAN, AND RACHADOORIAN B41

It should be stated, however, that although some of the felds- studied to determine the mineralogicalu variations with- pathic schists were undoubtedly formed from rocks similar in in a restricted area. Individual samples- and mineral composition to the greenstones, a part of the greenstones are assemblages as determined by Sainsbury are listed later than these schists, for they show but slight evidences of below : having been subjected to the same amount of metamorphism. Field No. Description Smith did not describe individual mineral assem- 68-ASn-566------Garnet-white mica-chlorite-sphene- blages of single rock specimens. He noted, however, that calcite-quartz-blue-green amphibole- most contain garnet, green to blue soda amphibole, epidote-py rite. 68-ASn-WB------Quartlcwhite mica-garnet-epidote- chlorite, sphene, quartz, and albite; this mineralogy sphene-minor blue-green amphibole- relates them to the blueschist facies. The rocks de- apatite. scribed by Smith belong to the Nome Group. 68-ASn-566C------. Quartz-albite-white mica-garnet- Mendenhall ( 1901) collected chloritic rocks in the calcite-sphene-epidote-apatite. Darby RIountains of the southeastern Seward Penin- 68-ASn-566D------Quartz-albite-white mica-hlorite- garnet. sula (area 24, fig. 1). Two thin sections representing 68-ASn-5WE------Calcite-white mica-quartz-epidote- two of these rocks were reexamined in 1969 by T. P. sphene-garnet in calcite-rich layer; Miller (written commun., 1969), who reported that and quartz-calcite-white mica- one rock is a glaucophane schist. The mineral assem- sphene-chlorite-apatite-pyrrhotite in blage reported by Miller is chlorite-albite-epidote- quartz-rich part. 68-ASn-5WF------Quartz-albite-white mica-chlorite- sphene-glaucophane; the rock is foliated. The second garnet-magnetite-bihte-apatite- sample, collected 500 feet away, has relict clinopy- biotite. roxene associated with colorless amphibole. The min- 68-ASn-566G------Quartz-albite-white mica-garnet- eral assemblages in these two rocks are similar to those chlorite-magnetite-biotite-apatite. of groups 1 and 4 of this report and are the basis for 68-ASn-566H------Quartlcalbite-white mica-chlorite- epidote-magnetite-calcite. inference by the senior author that the rock containing clinopyroxene is a later intrusive into blueschist-facies These rocks are clearly retrograde garnebbearing rocks of the Nome Group ; such a relationship is com- rocks, but the mineral assemblages are controlled by mon far to the west. the bulk composition of the rock. Where quartz, albite, or calcite predominates, blue amphibole and sphene CONCLUSIONS are virtually absent. In quartz-albite-rich rocks, garnet All the blueschist rocks herein described are believed is poorly developed, forming a net that surrounds to belong to a metamorphosed sequence of Precambrian quartz and albite grains; in the rock that contains age-the Nome Group. The original rocks probably amphibole, garnet is subhedral. Chlorite is retrograde contained varying amounts of mafic volcanic material from white mica. The development of bits of biotite in that included lavas, tuffs, and dikes related to the vol- 68-ASn-566F suggests that a thermal event has been canism. Numerous interbedded limestones suggest that impressed upon the retrograde rocks. This event is the depositional environment was marine. After depo- probably correlative with the intrusion of granite sition of the Noine Group rocks, an intense regional stocks in the Kigluaik Mountains. dvnamic metamorphism was impressed upon Nome Other areas of blueschist rocks Oroup rocks over thousands of sqiare miles of the Sea- ward Peninsula, creating blueschist-facies rocks. After Blueschist-facies rocks were reported by Moffit (1913, the dynamic metamorphism, a carbona~moussiltite wias p. 32-33) east of Nome at Osborn and Buster Creeks deposited over much of the Seward Peninsula; the (area 22, fig. I), where they are in part intrusive into siltite was then intruded by numerous dikes, sills, and schistose limestone of the Nome Group. The mineral bosses of gabbro. All these rocks are believed to be of assemblage reported by Moffit (1913) is garnet-glauco- late Precambrian age. In latest Precambrian time, im- phane-chlorite-epidote-titanite-albite ; locally, iron pure thin-bedded limestones were deposited over much ores, pyrite, rutile, quartz, and calcite occur. Moffit of the Sem-ard Peninsula and are completely unmeta- noted that the rocks intrusive into limestone are less morphosed in the area from Teller west. During Cam- schistose than the surrounded blueschists. Smith (1910, brian time, the Setvard Peninsula probably was a p. 76-83) discussed glaucophane-bearing rocks in the positive area, for no rocks of this age are known. From Solomon area (area 23, fig. 1); he stated that some are Early Ordovician through Alississippian time, carbon- unquestionably intrusive into limestone (see photo- ate rocks were deposited over the Seward Peninsula. graph on p. 76 of Smith, 1910), and that two ages of After deposition of the Mississippian limestone and mafic intrusives are represented : prior to the injection of granitic rocks in the mid- B42 PETROLOGY AND PETROGRAPIFY

Cretaceous, rocks of the entire Seward Peninsula were fields of Seward Peninsula, Alaska, in 1900, p. 1-185 L involved in intensive thrust faulting, which juxtaposed Reconnaissances in the Cape Nome and Norton Bay regions, Precambrian schists and clastic rocks against Paleozoic Alaska, in 1900: U.S. Geol. Survey Spec. Pub., 222 p. Coleman, R. G., 1967, Glaucophane schists from California and carbonate rocks. This juxtaposition led to the puzzling New Caledonia, in Age and nature of the circum-Pacific: age relations that baffled early workers and led them to Tectonophysics, v. 4, no. 4-6, spec. issue, p. 479-498. conclude that the Nome Group rocks were of Pre- Coleman, R. G., and Lee, D. E., 1963, Glaucophane-bearing meta- cambrian and Paleozoic age. After the thrusting, morphic rock types of the Cazadero area, California: Jour. granitic rocks were emplaced and formed isolated Petrology, v. 4, no. 2, p. 2W301. Collier, A. J.. 1902, A reconnaissance of the northwestern por- stocks on the western Seward Peninsula and large tion of Seward Peninsula, Alaska: U.S. Geol. Survey Prof. batholiths on the eastern Seward Peninsula. The heat- Paper 2, 70 p. ing associated with the intrusion of granitic rocks Dutro, J. T., Jr., and Payne, T. G., 1957, Geologic map of probably accounts for the great increase in the meta- Alaska : U.S. Geol. Survey. morphosis of the Paleozoic carbonate of the thrust Ernst, W. G., 1963, Petrogenesis of glaucophane schists: Jour. Petrology, v. 4, no. 1, p. 1-30. sheets from west to east. The west-to-east increase in Essene, E. J., Fyfe, W. S., and Turner, F. J., 1965, Petrogenesis the size of the intrusives-and, concomitantly, in the of Franciscan glaucophane schists and associated meta- amount of heat introduced-probably caused the cor- morphic rocks, California: Beitr. Mineralogie u. Petro- responding great increase in the metamorphism of the graphie, v. 11, no. 7, p. 695-704. Paleozoic ca~rhatesof hhe hhrust sheets. Knopf, Adolph, 1908, Geology of the Seward Peninsula tin deposits, Alaska : U.S. Geol. Survey Bull. 358, 71 p. The blueschist rocks are confined to units older than Mendenhall, W. C., 1901, A reconnaissance in the Norton Bay gabbros of latest Precambrian age. Glaucophane-bear- region, Alaska, in 1900, p. 187-218 in Reconnaissances in the ing rocks occur near retrograde blueschist rocks be- Cape Nome and Norton Bay regions, Alaska, in 1900: U.S. onuse of tectonic transport by bhru~tingland by local Geol. Survey Spec. Pub., 222 p. factors, such as hydrothermal alteration, nearness to Miller, D. J., Payne, T. G., and Gryc, George, 1959, Geology of possible petroleum provinces in Alaska, with an Annotated younger granites, and proximity to thrust faults. The bibliography, by E. H. Cobb: U.S. Geol. Survey Bull. 1904, metamorphic fabric of the blueschist rocks clearly 131 p. shows two periods of deformation, the second of which Miller, T. P., Patton, W. W., Jr., and Lanphere, M. A., 1966, probably corresponds to the thrust cycle. Blueschist Preliminary report on a plutonic belt in west-central Alaska, in Geological Survey Research 1966: U.S. Geol. facies rocks often occur in polymetamorphic terranes Survey Prof. Paper 550-D, p. D158-D162. (Zwart, 1967) or are juxtaposed against lower grade Miyashiro, A., 1961, Evolution of metamorphic belts: Jour. Pe- rocks (Coleman, 1967) in a way that leads to enig- trology, c. 2, no. 3, p. 277-311. matic geologic situations, and the blueschists of the Moat, F. H., 1913, Geology of the Nome and Grand Central Seward Peninsula conform to that general pattern. quadrangles, Alaska : U.S. Geol. Survey Bull. 533, 140 p. Rabkin, M. I., and Ravich, M. G., 1961, The Precambrian of the However, it should be emphasized that the Seward Soviet Arctic, p. 18-30 in Raasch, G. O., ed., Geology of the Peninsula is not part of the circum-Pacific tectonic Arctic, v. 1: Toronto Univ. Press, 732 p. belt which contains the well-studied blueschist rocks Sainsbury, C. L., 1961, Geology of part of the Craig 62quad- of late Mesozoic age, but rather represents a well-pre- rangle and adjoining areas, Prince of Wales Island, south- served area of Precambrian blueschist rocks. eastern Alaska : U.R. Geol. Survey Bull. 1058-H, p. 299-362. This preliminary report records the results of studies 1965, Geology and ore deposits of the central York Mountains, western Seward Peninsula, Alaska : U.S. Geol. made as a very minor part of a regional mapping pro- Survey open-file report, 150 p., and Stanford Univ. Ph.D. gram, and important relaitions may be gldover. thesis. The blueschist-facies terrane here discussed offers geo- 1969a, Geologic map of the Teller B4and southern part logic conditions, such as intense thrusting and later of the Teller C4quadrangles, western Seward Peninsula, progressive mertamorphism, which are not everywhere Alaska: U.S. Geol. Survey Misc. Geol. Inv. Map 1-572. found. It is hoped that this report ~wrilllead to detailed 1969b, Geology and ore deposits of the central York studies by geologists and mineralogists interested in Mountains, western Seward Peninsula, Alaska : U.S. Geol. the formation and destruction of blueschist-facies Survey Bull. 1287, 101 p. [I9701 Smith, P. S., 1910, Geology and mineral resources of the Solomon rocks. and Casadepage quadrangles, Seward Peninsula, Alaska : REFERENCES U.S. Geol. Survey Bull. 433, 234 p. Brooks, A. H., assisted by Richardson, G. B., and Collier, A. J., Zwart, H. J., 1967, The duality of orogenic belts: Geologie en 1901, A reconnaissance of the Cape Nome and adjacent gold Mijnbouw, v. 46, no. 8, p. 283-309. GEOLOGICAL SURVEY RESEAaRCH 1970

THE RELATIONSHIP BETWEEN SURFACE WATER AND GROUND WATER IN SHIP CREEK NEAR ANCHORAGE, ALASKA

By JOHN B. WEEKS, Anchorage, Alaska lVork done in cocrperation with tha city of Anchorage and the Greater Anchorage Area Borou~h

Abstract.-Ship Creek drainage basin is an important re- station, Ship Creek at Elmendorf AFB, is located charge area for the Anchorage artesian aquifer. Discharge rec- downstream from the alluvial fan and %bout 4 miles ords show Ship Creek loses an average of 25 cfs in the recharg- above the mouth of the stream. ' ing area and gains about 22 cfs in the reach below the recharge area. A water-budget analysis demonstrates that of this gain Discharge that for the period of record at least 11 cfs is ground-water return flow from the recharge the mean annual flow over the diversion dam is 150 area. cubic feet per second and that 6 miles downstream at Elmendorf the discharge is 125 cfs. Previous investiga- Ship Creek has its headwaters in the Chugach Moun- tors (Cederstrom and others, 1964; Waller, 1964; Som- tains and discharges into Cook Inlet near downtown mers and Marcher, 1965) recognized that this reach of Anchorage, Alaska. Its stream course traverses 10 miles Ship Creek was a losing reach. A recent investigation of alluvial gravel and glacial outwash deposits in the by the author has show11 that part of this loss returns lowlands and foothills. Since the last glaciation, Ship to the stream below the Elmendorf gaging station. Creek has built an extensive alluvial fan at the foot of A series of seepage measurements was made to deter- the Chugach Mountains (fig. 1). This fan provides mine the relationship between surface water and about one-fourth of the total recharge to the artesian ground water in Ship Creek basin. Concurrent dis- aquifer system which underlies the city of Anchorage. charge measurements were made at the Elmendorf The geology of the Anchorage area has been de- gaging station and at Post Road near the mouth of scribed by Trainer (in Cederstrom and others, 1964) Ship Creek (fig. 1). The measurements were made dur- and Miller and Dobrovolny (1959). As schematically ing periods of steady stage and no local surface runoff. shown in figure 2, the surficial gravels in the reach of The discharge measurements near the mouth were Ship Creek basin below the gaging station at Elmen- made above the tide-affected reach so that drainage dorf Air Base are underlainby a clay bed which from bank storage did not contribute to the measured is exposed at the moutll of shipcreek. This clay discharge. Under these conditions, any increase in dis- stratum forms the confining layer of the underlying charge downstream must be due to ground-waterin- artesian aquifer and the lower boundary of the uncon- flow. Table 1 tabulates the measurement data as well fined aquifer in Ship Creek basin downstream froln us^^ seepage data. the Elmendorf gage. Two of the previously existing seepage runs were T~~ gaging stationsare operated on Ship Creek. disregarded because they were made under conditions Continuous discharge records have been obtained since which did not conform to the above measuring cribria. 1946 at the Fort Richardson diversion dam, and are Climatologic records show that the seepage run on published as Ship Creek near Anchorage. This station April 27, 1959, was made while surface runoff from is located on exposures of metamorphic rock upstream snowmelt was contributing to streamflow. The seepage from the alluvial fan. Six miles downstream, the sec- run on May 16, 1967, was made during a period of ond gaging station has been operated since 1963. This rising stage so that the measured gain was affected by

USGEOL. SURVEY PROF. PAPER 706B. PAGES B224-B228 B224

WEEKS B225

FI .... .' A A A .. . . . Stream deposits Alluvial fan Gaging station at Gaging station at Measuring site Fort Richardson Elmendorf AFB at Post Road diversion dam

FIOUBE1.-Sketch of the Anchorage area, &wing gaastations and sd-d features in S8hipCheek basin.

channel storage in the reach. If these two seepage runs are disregarded, the average gain in the reach for the iii n:= remaining nine runs is 21.5 cfs and the standard devia- 4- .0 u Bo z kU tion is 1.9 cfs. i4 g 4-;: - ;sE Seepage measurements were not made during the db ME e o summer months, although the major part of the annual runoff occurs during this period. The mean daily discharge during the high-flow period generally ranges from 200 to 600 cfs, and the discharge measurements are only accurate to about 10 percent. For a discharge of 200 cfs, the possible error in the difference between

Metamorphic two measurements is 240 cfs, and for 600 cfs, rt-120 Y cfs. Therefore, ground-water inflow cannot be reliably determined during high flows because the measurement

NOT TO SCALE errors will be nearly as large as, if not larger than, the ground-water contribution to streamflow. FIGURD2.-Idealized geologic cross section along Ship Greek, The seepage data show that Ship Creek gains about showing the general direction (arrows) of ground-water movement. 22 cfs in the reach below the Elmendorf gaging sta- B226 RELATION BETWEEN GROUND WATER AND SURFACE WATER

TABLE1.-Ship Creek seepage measurements made at the Elmendorf 149'55' Air Force Base gage and near Post Road, in cubic feet per second.

Discharge measurements 0 1 2 MILES Date 8eepsge Elmendorf Post Road gm - 1 Ewe 61" Apr. 27, 1959 ------28.3 80. 8 52. 5 15' Apr. 26, 1960 ------20.4 44. 6 24. 2 Oct. 26, 1960 ------172 193 21. 0 Mar. 13, 1961 .------19.2 40. 8 21. 6 Measuring site May 16,1967 ------182 191 9. 0 at Post Road Sept. 12, 1968------75.0 97. 0 22. 0 Nov. 8, 1968------_ 17.6 41. 6 24. 0 Nov. 15, 1968------2. 8 24. 0 21. 2 Jan.3, 1969------2. 3 21. 3 19. 0 Jan. 30, 1969------1. 4 23. 3 21. 9 Mar.26, 1969------+------1. 3 19. 9 18. 6

1 Data collected prior to this study. tion during the low-flow period. Because the ground- water levels are higher in the summer than in the winter, the ground-water contribution to streamflow is likely to be greater during the high-flow period than during the low-flow period. Thus, it can be concluded that the mean annual gain is at least 22 cfs and possibly more. This discharge is supplied is part by FIGURE3.-Sketch ehowing ehange in area of Ship Creek basin ground-water inflow from the recharge area above the where artesian po~tentiom&riclevels were above land surface in 1958 and 1968, as indicated by patterns Elmendorf gage. This can be shown by the following analysis of the unconfined aquifer in the reach of Ship ing this period (standard deviation of 1.9 cfs), and Creek basin below Elmendorf. field observations have disclosed that most of the gain The clay bed underlying Anchorage is exposed along occurs in the first half mile below the Elmendorf gage. the coastline and forms a ground-water dam which Because the present potentiometric surface is below prevents discharge from the unconfined aquifier to stream level in this reach (fig. 3), vertical leakage Cook Inlet except through Ship Creek. Field observa- through the clay cannot contribute to streamflow. The tions have located only minor seeps from the bluffs 11-cfs gain must come from the recharge area above along the coast. The glacial deposits of the Elmendorf the Elmendorf gaging station. Consequently, any esti- moraine (fig. 1) form a barrier to ground-water move- mate of recharge to the aquifer system from the area ment to the north, and a topographic high forms an of Ship Creek basin between the diversion dam and the unconfined ground-water divide to the south. Thus, gage at Elmendorf must account for this return flow. except for vertical leakage through the clay, the only The presence of recent stream deposits along Ship sources of recharge to the unconfined aquifer are in- Creek (fig. 1) suggests that the 11-cfs return flow may filtration of precipitation and ground-water inflow be largely in the form of underflow. That is, about half from the recharge area above the Elmendorf gage. of the 25 cfs that is lost in the upper reach of Ship The average annual precipitation ait Anchorage is Creek actually recharges the aquifer system, and the 15.14 inches for the 25 years of record, and the area remaining half moves through the highly permeable tributary to Ship Creek in the reach below Elmendorf gravels under and adjacent to the stream down- is about 10 square miles. This average precipitation on gradient. the tributary area is equivalent to only about 11 cfs REFERENCES per year. Because direct runoff and evapotranspiration Cederstrom, D. J., Trainer, F. W., and Waller, R. M., 1964, Geol- losses reduce the amount of precipitation available for ogy and ground-water resources of the Anchorage area, recharge, the recharge to the unconfined aquifer must Alaska : U.S. Geol. Survey Water-Supply Paper 1773, 108 p. Miller, R. D., and Dobrovolny, Ernest, 1959, Surficial geology be less than 11 cfs. Therefore, if changes in storage of Anchorage and vicinity, Alaska : U.S. Geol. Survey Bull. are neglected, at least 11 cfs must be ground-water in- 1093, 128 p. flow to account for the 22-cfs gain in Ship Creek. Sommers, D. A., and Marcher, M. V., 1965, Water resources ap- Potentiometric levels in the reach of Ship Creek praisal of the Anchorage area, Alaska: U.S. Geol. Survey basin below Elmendorf have dropped approximately open-file report, 34 p. Waller, R. M., 1964, Hydrology and the effects of increased 25 feet in the last 10 years. The seepage data indicate ground-water pumping in the Anchorage area, Alaska: that no significant change in the gain has occurred dur- U.S. Geol. Survey Water-Supply Paper 1779-D, 36 p. B226 RELATION BETWEEN GROUND WATER AND SURFACE WATER

TABLE1.-Ship Creek seepage measurements made at the Elmendorf 149'55' Air Force Base gage and near Post Road, in cubic feet per second.

1 Data collected prior to this study. tion during the low-flow period. Because the ground- water levels are higher in the summer than in the winter, the ground-water contribution to streamflow is likely to be greater during the high-flow period than during the low-flow period. Thus, it can be concluded that the mean annual gain is at least 22 cfs and possibly more. This discharge is supplied is part by ~GURE3.-Sketch ehowing ehange in area of Ship Creek basin ground-water inflow from the recharge area above the where artesian po~tentiom&riclevels were above land surface in 1958 and 1968, as indicated by patterns Elmendorf gage. This can be shown by the following analysis of the unconfined aquifer in the reach of Ship ing this period (standard deviation of 1.9 cfs), and Creek basin below Elmendorf. field observations have disclosed that most of the gain The clay bed underlying Anchorage is exposed along occurs in the first half mile below the Elmendorf gage. the coastline and forms a ground-water dam which Because the present potentiometric surface is below prevents discharge from the unconfined aquifier to stream level in this reach (fig. 3), vertical leakage Cook Inlet except through Ship Creek. Field observa- through the clay cannot contribute to streamflow. The tions have located only minor seeps from the bluffs 11-cfs gain must come from the recharge area above along the coast. The glacial deposits of the Elmendorf the Elmendorf gaging station. Consequently, any esti- moraine (fig. 1) form a barrier to ground-water move- mate of recharge to the aquifer system from the area ment to the north, and a topographic high forms an of Ship Creek basin between the diversion dam and the unconfined ground-water divide to the south. Thus, gage at Elmendorf must account for this return flow. except for vertical leakage through the clay, the only The presence of recent stream deposits along Ship sources of recharge to the unconfined aquifer are in- Creek (fig. 1) suggests that the 11-cfs return flow may filtration of precipitation and ground-water inflow be largely in the form of underflow. That is, about half from the recharge area above the Elmendorf gage. of the 25 cfs that is lost in the upper reach of Ship The average annual precipitation & Anchorage is Creek actually recharges the aquifer system, and the 15.14 inches for the 25 years of record, and the area remaining half moves through the highly permeable tributary to Ship Creek in the reach below Elmendorf gravels under and adjacent to the stream down- is about 10 square miles. This average precipitation on gradient. the tributary area is equivalent to only about 11 cfs REFERENCES per year. Because direct runoff and evapotranspiration Cederstrom, D. J., Trainer, F. W., and Waller, R. M., 1964, Geol- losses reduce the amount of precipitation available for ogy and ground-water resources of the Anchorage area, recharge, the recharge to the unconfined aquifer must Alaska : U.S. Geol. Survey Water-Supply Paper 1773, 108 p. Miller, R. D., and Dobrovolny, Ernest, 1959, Surficial geology be less than 11 cfs. Therefore, if changes in storage of Anchorage and vicinity, Alaska : U.S. Geol. Survey Bull. are neglected, at least 11 cfs must be ground-water in- 1093, 128 p. flow to account for the 22-cfs gain in Ship Creek. Sommers, D. A., and Marcher, M. V., 1965, Water resources ap- Potentiometric levels in the reach of Ship Creek praisal of the Anchorage area, Alaska: U.S. Geol. Survey basin below Elmendorf have dropped approximately open-file report, 34 p. Waller, R. M., 1964, Hydrology and the effects of increased 25 feet in the last 10 years. The seepage data indicate ground-water pumping in the Anchorage area, Alaska: that no significant change in the gain has occurred dur- U.S. Geol. Survey Water-Supply Paper 1779-D, 36 p.