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(DSDP) Results: Far North Pacific, And 37. GEOLOGIC SYNTHESIS OF LEG 19 (DSDP) RESULTS; FAR NORTH PACIFIC, AND ALEUTIAN RIDGE, AND BERING SEA David W. Scholl, U. S. Geological Survey, Office of Marine Geology, Menlo Park, California Joe S. Creager, Department of Oceanography, University of Washington, Seattle, Washington1 INTRODUCTION global climates, pine forests grew only in high latitudes, for example, the uplands flanking the Cook Inlet area of Alaska Elsewhere in this volume (see Parts HI and IV), fairly (see Wolfe, and Evitt, this volume, also Wolfe, Hopkins and detailed treatment of a variety of sedimentological, petro- Leopold, 1966; Wolfe and Hopkins, 1967; and Wolfe, logical, paleontological, and tectonic subjects have been 1971). Deposition of turbidites ceased sometime in the presented. In this paper we synthesize these discussions and middle Oligocene, just prior to the accumulation of other available data in a general evaluation of the significant nannoplankton ooze. Again, the low speciation of the tectonic and sedimentological histories of the far north microflora in this ooze indicates a high-latitude depositional Pacific and the neighboring Bering Sea. This history extends environment. Pelagic clay, barren of age-diagnostic fossils, back to the Late Cretaceous for the Pacific basin, but only accumulated over the chalk until about 12 to 13 m.y. ago, to about the middle Miocene in the Bering Sea. in the middle Miocene, when diatom ooze becan to The stratigraphic columns of ten of the eleven Leg 19 accumulate. Although the introduction of volcanic ash drilling sites are shown on Figure 1. When arranged from began in the late Miocene and ice-rafted debris in the right to left, in the order of drilling, the columns construct Pliocene, accumulation of diatomaceous debris has con- a generalized east-to-west geologic cross section of the tinued into the Holocene at a rate averaging about 16 North Pacific and adjacent Bering Sea. m/m.y. MAJOR TECTONIC RESULTS Relative Plate Motion Far North Pacific Prior to the drilling of Site 183 two schools of thought Although many thoughts have been expressed about the prevailed about the age of the buried turbidite sequence of geologic history of the far north Pacific, and the Gulf of the Aleutian Abyssal Plain. One concluded that the Alaska in particular, (Hurley, 1960; Hamilton, 1967; sequence was most likely of pre-middle Tertiary age Pitman and Hayes, 1968; Grow and Atwater, 1970; Hays, (Hamilton, 1967), an age derived by dividing the thickness 1970; Hayes and Pitman, 1970; Mammerickx, 1970; of the overlying pelagic blanket by a typical sedimentation Opdyke and Foster, 1970; Peter and others, 1970; Jones, rate for pelagic debris. The other considered this "typical" 1971; Jones and others, 1971; Jackson and others, 1972; rate as far too low and believed the plain to be of Miocene and Hamilton, in press). The geologic information obtained and younger age (Opdyke and Foster, 1970; Jones and at DSDP Sites 183 and 192 provides the only pre-Neogene others, 1971). The buried turbidite sequence of the control for this vast region (Figure 2). In the Gulf of Aleutian Abyssal Plain in the vicinity of Sea Map Channel is Alaska, Neogene turbidite and pelagic units were also entirely of Paleogene age, confirming Hamilton's opinion. sampled during DSDP Leg 18 (von Huene, Kulm and Because Site 183 is located near the apex (northern or others, 1971). upslope corner) of the plain, the direction toward which several fossil deep-sea channels appear to converge, it is Aleutian Abyssal Plain and Site 183 likely that turbidite deposition would have ceased here Site 183 is located near Sea Map Channel (Figure 2; about the same time or perhaps later than elsewhere (i.e., Grim, 1969; Grim and Naugler, 1969) close to the apex of southward) over the buried plain. However, some of the the Aleutian Abyssal Plain. Deposition apparently began more easterly channels may have remained active longer here in the late early Eocene with the accumulation of (Figure 2), as suggested by Mammerickx (1970). The end of nannoplankton ooze over an alkali olivine basalt, which all turbidite sedimentation over the existing area of the may have been implaced in association with the growth of Aleutian Abyssal Plain should probably not be placed later relatively nearby Derickson or Sirius seamounts (see than the late Oligocene, or between 22 and 30 m.y. ago Natland, and Stewart and others, this volume). The (Berggren, in press). Initiation of turbidite deposition population-rich but species-limited microflora of this chalk would appear to be late early Eocene at the earliest, about implies a high northern latitude, equivalent or perhaps 50 m.y. ago. slightly less than that of the site (52.5°; see Worsley, this The existence of a fossil abyssal plain of Paleogene age volume). By late early Eocene time, sandy and silty immediately south of Alaska is important evidence bearing turbidite layers began to accumulate over the plain. These on the amount of relative motion that could have occurred terrigenous deposits contain a pollen assemblage that is between the Pacific and American plates during the dominated by conifers. In the Eocene, a time of warm Cenozoic Era (Pitman and Hayes, 1968; Atwater, 1970; Grow and Atwater, 1970; Hayes and Pitman, 1970.) As Hamilton (1967), Mammerickx (1970), Jones and others Contribution No. 694. (1971) and Hamilton (in press) have emphasized, the 897 D. W. SCHOLL, J. S. CREAGER N I 2= Su, 3? ST Z3 >? 3 D S si is : " ---: i ^" S II Eg 192 191 190 189 188 187 186 185 184 183 I • St 1*? • •« t m • • P **» «r« • ° • • •• SH •««> ?riß ••• 100* •** • »« o Io 0 \ **• ° / •»* ««» / *#* • Φ • •• H. MIOC 200 *** *** ç•? :? *-L. MIOC. *•• \ °_- °° \ \ // /1 \ j S U. OLIG. *** //I \ ö-Z z°£ \ « *»* LOWER 300 c \ '_' *** = / j o ° o / 1 OLIGOCENE o / —_- ^*— j —•Q- \ ~ / j ^^ \ ^ — •t* —'ö- ••• / J 0 _°~ ° \ ~y_-_ i / j? / / o j / ~— ~^ & / ••• O} / — * / U.EOCENE O \ — 400 •^ / o o •«« - o _°— ~-^ / ^ / 0 0 / \ •*• / M-U EOCENE j / s \ L. EOCENE a a ^£ / \ / o o # / \ So o °o -M o •** \ Hi / \ /|o • *» \ .# / —i 'I ~ J> \ / s \ . °. T7. -~j• ^ / \ / ° J "* 600 \ K-r- // \ »\ / ------ l\ ~e_ / / 700 / *• •y V\v o _~r \ / ? v / Λ %\ \β \ 800 *•_• 1 "\ ga\\ / J- V\ 900 > ' U-L EOCENE 1000 MAESTRICHTIAf 160°E 180° 160°U EXPLANATION / C/ VA S \ \ K EZI Diatomaceous Ooze i = ^r /? 60°N Egg Diatomaceous Mud 60° N j Clay Silt and Sand I1!!, *"l ,} ALEUTIAf 1 | UMNAK ff 1 1. I1 BASIN I5H Conglomerate / fi 'l||| PLATEAU /Cji il LV 7 I.1--:' 1 J| ,?) BOWERS J Si 85 Z Chalk ^/i ' i ^ 1 l ' . •|'l'lij• 89 50° N Mudstone 50°N II!!'•M *'"'•' hi' .'"•> .'.'wil •' Basalt • • • • •+• m ALEUTIAN TRENCH \ . •#• Ash Horizons et 1 ) ; ALEUTIAN • Ice-Rafted Exotics ABYSSAL PLAIN Figure 1. Stratigraphic columns often DSDP drilling sites, 183-192, far north Pacific and Bering Sea. geomorphology and geometry of the terrigenous sedi- Cenozoic. The evidence includes the following: (1) the mentary bodies underlying the Aleutian Abyssal Plain are mineralogy of the Eocene turbidites indicates a plutonic indicative of a northern or Alaskan source region, the Cook and metamorphic source terrain including blue amphiboles; Inlet area being the apparently obvious drainage route (R.J. Stewart, personn. commun.), this is also true of the (Figure 2). As workable models of relative motion between early Tertiary deposits of the Cook Inlet area of Alaska the Pacific and American plates (Atwater, 1970; and Grow (Kirschner and Lyon, in press), (2) the turbidite sequence and Atwater, 1970) allow for several thousand kilometers lies stratigraphically between early Tertiary chalk beds that of relative motion during the Cenozoic, an Alaskan source are characterized by a population-rich but species-poor terrain could be more apparent than real. Models of microflora, a factor indicating a latitude of deposition continuous plate motion during the Cenozoic imply that equivalent to that of Site 183 (see Worsley, this volume), the turbidites of the Aleutian Abyssal Plain may have been and (3) the turbidite deposits contain a terrestrial micro- deposited in the early Tertiary off the Washington or flora dominated by Pinaceae pollen, a circumstance that Oregon coast. almost compels an Alaskan source area because rocks of Although arguments to the contrary can be presented, equivalent age south of Alaska are characterized by sub- (see below) the bulk of the findings at Site 183 appear to tropical or tropical floras (see Wolfe, this volume.) favor a model of limited, and therefore discontinuous, When combined, the paleontologic findings at Site 183, motion between the Pacific and American plates during the and the geomorphic and geometric arguments of Hamilton 898 GEOLOGIC SYNTHESIS OF LEG 19 RESULTS Figure 2. Location map for DSDP Sites 183 and 192, far north Pacific. (1967), Mammerickx (1970) and Jones and others (1971), underthrusting of the Pacific Plate beneath Alaska and the provide strong support for either little or no motion, an formation of the Aleutian Trench, a circumstance that idea favored by Peter and others (1970), or for discon- could explain the cessation of turbidite deposition on the tinuous plate motion, a model favored by Pitman and Aleutian Abyssal Plain. However, the Site 183 data, and the Hayes (1968) and Hayes and Pitman (1970). This conclu- measured dislocation across the San Andreas Fault during sion has been reached independently by Hamilton (in the last 20 m.y., can be interpreted to mean that less than press). None of the data are inconsistent with relative plate about 500 km of underthrusting has taken place since the motions of about 500 km or perhaps slightly more during early Miocene. If this is true, then during the Neogene, the the Cenozoic the amount required to accommodate spread- average rate of relative motion between the Pacific and the ing adjacent to the Juan de Fuca Ridge and opening of the American plates may have been as little as 2 cm/y.
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