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Turbidites of the Aleutian Abyssal Plain: Mineralogy, Provenance, and Constraints for Cenozoic Motion of the Pacific Plate

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Turbidites of the Aleutian Abyssal Plain: Mineralogy, Provenance, and Constraints for Cenozoic Motion of the Pacific Plate Turbidites of the Aleutian abyssal plain: Mineralogy, provenance, and constraints for Cenozoic motion of the Pacific plate RICHARD J. STEWART Department of Geological Sciences, University of Washington, Seattle, Washington 98195 ABSTRACT ment most likely was not derived from as Mammerickx, 1970; Jones and others, far south as the Oregon-Washington conti- 1971) and led Hamilton (1973) and Scholl The Aleutian abyssal plain is a fossil nental margin, where coeval sedimentary and Creager (1973) to independently abyssal plain of Paleogene age in the west- deposits are dominantly volcaniclastic. suggest models requiring little or modest ern Gulf of Alaska. The plain is a large, This work lends some support to earlier northwestward motion of the North Pacific southward-thinning turbidite apron now suggestions that the fan-shaped turbidite floor in Tertiary time. cut off from sediment sources by the Aleu- body originated on the continental margin The discordance in these tectonic syn- tian Trench. Turbidite sedimentation in the Gulf of Alaska, and it supports mod- theses is significant. The large-motion mod- ceased about 30 m.y. ago, and the apron is els of little or modest motion of the Pacific els would separate the plain from a proba- now buried under a thick blanket of pelagic plate relative to North America. The ble eastern source in mid-Tertiary time and deposits. Turbidites of the plain were mineralogy alone cannot refute more am- carry it to its present position by northward recovered at site 183 of the Deep Sea Drill- bitious motion models, but when combined migration of the Pacific plate relative to ing Project on the northern edge of the ap- with previously published evidence on size North America (Atwater, 1970; Atwater ron. The heavy-mineral fraction of sand- of the plain, sediment thicknesses, and nan- and Molnar, 1973). Prior to separation, the sized samples is mostly amphibole and epi- nofossil species diversity, the data seriously turbidites may have recorded the sweep of dote with minor pyroxene, garnet, and constrain models requiring large-scale the Pacific plate along western North sphene. The light-mineral fraction is mostly northwestward motion of the Pacific plate America, with the initial deposits possibly quartzose debris and feldspars. Subordinate in post-Eocene time. Key words: marine derived from as far south as the western lithic fragments consist of roughly equal geology, tectonics, sedimentary petrology, United States and Canada. For the little or amounts of metamorphic, plutonic, sedimentation, subduction. modest motion mechanisms, the most sedimentary, and volcanic grains. The sand reasonable sediment source is southern or compositions are arkoses in many INTRODUCTION central Alaska, perhaps in the region of sandstone classifications, although if fine Kodiak Island (Hamilton, 1967; Mam- silt is included with clay as matrix, the sand This paper describes the mineralogy of merickx, 1970). Downwarping of the Aleu- deposits are feldspathic or lithofeldspathic sand-sized material from turbidites of the tian Trench in mid-Tertiary time may have graywacke. The sands are apparently first- Aleutian abyssal plain, recovered at Deep isolated the plain from northern sediment cycle products of deep dissection into a Sea Drilling Project site 183 south of sources (Hurley, 1960; Hamilton, 1967; plutonic terrane, and they contrast sharply Kodiak Island, Alaska (Fig. 1). The age, Pitman and Hayes, 1968; von Huene and with arc-derived volcanic sandstones of source terrane, and latitude of deposition of Shor, 1969), or a major drainage reorienta- similar age common on the adjacent North the Aleutian abyssal plain are critical refer- tioji may have starved the plain (Scholl and American continental margin. The turbidite ence marks for establishing the Tertiary tec- Creager, 1973). sands are stratigraphically remarkably con- tonic history of the North Pacific. Atwater Because plate tectonics models suggest stant in composition, which indicates deri- (1970) and Atwater and Molnar (1973) de- several diverse source terranes, provenance vation from virtually the same terrane scribed evidence from magnetic anomaly data on the turbidites of the Aleutian abys- through a time span approaching 20 m.y. data indicating large-scale motion of the sal plain provide additional new informa- Comparison of Aleutian plain data with North Pacific and presented models suggest- tion on the general problem of Tertiary the compositions of coeval sedimentary ing that sediment of the Aleutian abyssal plate motion in the North Pacific. The fol- rocks from the northeast Pacific margin plain was rafted northwestward thousands lowing sections describe the mineralogy of shows that the Kodiak shelf area includes of kilometres to its present position south of the turbidites and attempt to locate a possible proximal equivalents of the more the Aleutian Trench. The plate reconstruc- provenance based on coeval sedimentary distal turbidites. Derivation from the vol- tions of Pitman and Hayes (1968) and rocks now exposed on the adjacent conti- caniclastic Mesozoic flysch of the Hayes-and Pitman (1970) indicate that the nental margin. Shumagin-Kodiak shelf is unlikely; more plain was deposited at about its present lo- probably the sediments were derived from cation, and large-scale rafting did not oc- GEOLOGIC SETTING AND primary plutonic sources. The turbidites cur. Peter and others (1970) concluded that DESCRIPTION, DSDP SITE 183 also resemble deposits in the Chugach the Aleutian Trench was formed by crustal Mountains and the younger turbidites of downfault or downbow and argued against Three major abyssal plains blanket the the Alaskan abyssal plain and could con- any northward motion of the abyssal plain. Pacific Ocean floor in the Gulf of Alaska ceivably have been derived from the coast Additional paleontologic, geomorphic, and (Fig. 1). The Tufts and Alaskan abyssal ranges of southeastern Alaska or western geometric evidence suggests little or no rela- plains are Neogene turbidite aprons con- British Columbia. The Aleutian plain sedi- tive motion of the plain (Hamilton, 1967; structed of debris derived from the adjacent Geological Society of America Bulletin, v. 87, p. 793-808, 14 figs., May 1976, Doc. no. 60518. 793 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/5/793/3418824/i0016-7606-87-5-793.pdf by guest on 27 September 2021 794 R. J. STEWART continental margin of British Columbia and and Mammerickx (1970), and their loca- diatomaceous debris, and hemipelagic de- Alaska (Hamilton, 1967; Kulm and others, tions were refined by Hamilton (1973). Of tritus (Fig. 2). The turbidite section consists 1973). The westernmost Aleutian abyssal these, the Aquarius channel is the longest, of olive-gray clay and interbedded size- plain is a fossil or buried turbidite assem- extending about 1,300 km southwestward graded silt and silty sand layers in beds as blage of Paleogene age now cut off from across the plain. Seamap Seachannel, the much as 1 m thick. Approximately 5 m of sediment sources by the Aleutian Trench northernmost of the known major channels nannofossil limestone and varicolored clay (Hurley, 1960; Hamilton, 1967, 1973; (Fig. 1) is unusual because its floor slopes separate the turbidite section from underly- Grim and Naugler, 1969; Mammerickx, both north into the Aleutian Trench and ing basalt basement. The turbidite sequence 1970; Naugler, 1970; Creager and others, southwestward across the abyssal plain contains a very sparse upper, middle, and 1973). (Grim and Naugler, 1969). Grim and upper lower Eocene nannoflora and rare The Aleutian abyssal plain is a fan- Naugler interpreted this anomalous pattern displaced foraminifera (Worsley, 1973; shaped turbidite apron that extends more as resulting from downwarping of the Echols, 1973). Bracketing ages for the tur- than 2,000 km southwestward from the northern part of the channel associated bidites are based on nannofossils recovered Aleutian Trench (Fig. 1). The turbidite with formation of the Aleutian Trench, from a thin middle to lower Oligocene apron thins southward, decreasing in thick- which may have cut off sediment sources chalk bed overlying the sequence and an ness from 400 to 800 m near the trench to for the channel. upper lower Eocene nannofossil limestone about 200 m where the plain terminates Deep Sea Drilling Project site 183 is lo- on which the sequence rests (Worsley, against the abyssal hills north of the Sur- cated about 40 km north of Seamap 1973). veyor Fracture Zone (Jones and others, Seachannel near the northern edge of the 1971; Hamilton, 1973). Four main dis- Aleutian abyssal plain (Fig. 1; see Creager METHODS tributary channels on the plain were discov- and others, 1973, for complete site descrip- ered and charted by Hamilton (1967), tion). At site 183 a 253-m-thick turbidite Grain separates from the turbidite sands Grim and Naugler (1969), Naugler (1970), section is overlain by 248 m of pelagic clay, were prepared following standard tech- 170° 165° 160° 155° 150° 145° 140° 135° 130° 125° 120° 170° 165° 160° 155° 150° 145° 140° 135° 130° 125° 120° Figure 1. Configuration of abyssal plains in Gulf of Alaska and localities discussed in text. Adapted from Chase and others (1971), including modifications of Hamilton (1973). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/87/5/793/3418824/i0016-7606-87-5-793.pdf by guest on 27 September 2021 TURBIDITES OF THE ALEUTIAN ABYSSAL PLAIN 795 niques, mounted in epoxy, and ground to primary detrital modes and to minimize ef- metamorphic sources, rather than surficial standard thin-section thickness. All light- fects of diagenetic alteration. volcanic complexes. mineral grain mounts were stained for potassium feldspar and plagioclase follow- MINERALOGY OF THE TURBIDITES Light Minerals ing the methods of Laniz and others (1964). The grain mounts were analyzed by count- Heavy Minerals The light-mineral composition of the ing 300 to 400 nonopaque, nonmicaceous sand-size fraction is mostly quartzose frag- grains, following the procedures of Dickin- The heavy-mineral assemblage from ments and feldspar; typically less than 30 son (1970).
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