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G. B. GRIGGS Division of Natural Sciences, University of California, Santa Cruz, California 95060 L

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G. B. GRIGGS Division of Natural Sciences, University of California, Santa Cruz, California 95060 L G. B. GRIGGS Division of Natural Sciences, University of California, Santa Cruz, California 95060 L. D. KULM Department of Oceanography, Oregon State University, Corvallis, Oregon 97331 Sedimentation in Cascadia Deep-Sea Channel ABSTRACT Cascadia Channel is the most extensive deep-sea tain Foraminifera from neritic, bathyal, and abyssal channel known in the Pacific Ocean and extends depths which have been size-sorted. A sequence of across Cascadia Basin, through Blanco Fracture sedimentary structures occurs in the deposits Zone, and onto Tufts Abyssal Plain. The channel is similar to that found by Bouma in turbidites ex- believed to be more than 2200 km m length and posed on the continent. There is a sharp break in has a gradually decreasing gradient averaging the textural and compositional properties of each 1:1000. Maximum channel relief reaches 300 m on graded bed. The coarser grained, basal zone of each the abyssal plain and 1100 m in the mountains of bed represents deposition from the traction load; the fracture zone. The right (north and west) bank the finer grained, organic-rich, upper portion of is consistently about 30 m higher than the left each graded bed represents deposition from the (south and east). suspension load. Individual turbidity current se- Turbidity currents have deposited thick, olive- quences are thinnest in the upper and thickest in green silt sequences throughout upper and lower the lower channel. Recurrence intervals between Cascadia Channel during Holocene time. The sedi flows range from 400 years in the upper to 1500 ment is derived primarily from the Columbia years along portions of the lower channel. Evi- River and is transported to the channel through dently each flow recorded near shore did not ex- Willapa Canyon. A cyclic alternation of the silt tend its entire length. Turbidity currents have sequences and thin layers of hemipelagic gray clay reached heights of at least 117 m and spread extends at least 650 km along the channel axis. laterally 17 km from the channel axis. Calculated Similar Holocene sequences which are thinner and flow velocities range from 5.8 m/sec along the finer grained, occur on the walls and levees of the upper channel to 3.3 m/sec along the lower portion. upper channel and indicate that turbidity cur- Pleistocene turbidity currents within Cascadia rents have risen high above the channel floor to Basin were much more extensive areally than the deposit their characteristic sediments. A thin Holocene flows, and they deposited sediment surficial covering of Holocene sediment along the which was coarser and cleaner. Pronounced levees middle channel demonstrates the erosional or non- which border the upper channel are due chiefly to depositional nature of the turbidity currents in this Pleistocene overflow. Coarse gravels and ice-rafted area. pebbly clays were also deposited along Cascadia The Holocene turbidity current deposits are Channel during Pleistocene time. graded texturally and compositionally, and con- INTRODUCTION however, the sedimentation picture is not as well understood. Deep-sea channels often tie Submarine canyons, fans, abyssal plains and submarine canyons, deep-sea fans, and abyssal deep-sea channels are all integral parts of an plains together and provide a connecting link enormous system which is continually shaping between the continents and the deep ocean large areas of the earth's submerged surface. floor often hundreds of kilometers away. The erosion, transportation, and deposition of Deep-sea channels have been found in all of large volumes of sediment are the major proc- the world's oceans but have not been studied esses acting in these areas. The first element in in a detailed manner. Most channel studies the system, the submarine canyon, has been have been based on a small number of bathy- studied intensively on continental margins metric profiles and short sediment cores. Re- around the world (Shepard and Dill, 1966). cent acoustic reflection profiling and deep-sea Many of the major deep-sea fans have also photography have provided new information been studied in some detail (Menard, 1960; on channel structure and development (Hamil- Menard and others, 1965; Wilde, 1965; ton, 1967; Laughton, 1968). The determina- Nelson, 1968). The distribution and mor- tion of the sedimentary processes taking place phology of the abyssal plains are well known; in a deep-sea channel and their spatial and Geological Society of America Bulletin, v. 81, p. 1361-1384, 17 figs., May 1970 1361 iruii " O y> o o oC 3.^£ ui? Sn r-t H?. O r-f z D ^ ssr^« ^ r en SfrSlvBiA " ^ g O o- W • r» S 2 a >-• D 2 w !:§• 1 O s D- S o n a £.&••§' > ' 9 D > D M W n g 3- &. 5 fr 5.2 § 3 S Z prl I-§.5" z QD f-SfJ ""j:O- tfot po I! >T^ <» _ S . STRATIGRAPHY 1363 Pacific Ocean, and was first studied by Menard but along the remainder of its course may be (1955) and later by Hurley (1960) in more due to structural features, or differential dep- detail. Royse (1964), Carlson (1968), and osition on the adjacent plain. These levees Nelson (1968) have studied submarine canyons rise as high as 28 m above the abyssal plain and and fans on the continental margin which con- extend laterally up to 7.5 km away from the nect with or are adjacent to Cascadia Channel. channel edge. Cross sections of the channel The sedimentation and stratigraphy of Cas- are usually U-shaped with the width of the cadia Basin and the adjacent areas have floor varying from about 650 to 4500 m; a recently been studied by Duncan (1968), and V-shaped profile develops in the Blanco they serve as a foundation for this study. Fracture Zone, due to the impinging sea- mounts. Along the middle channel, a terrace SUBMARINE PHYSIOGRAPHY occurs on the east side 30 to 50 m above the The northeastern Pacific Ocean has the most floor (Fig. 9, Profile 2). The terrace has the extensive system of abyssal plains in the entire appearance of a previous channel bottom Pacific Ocean basin. Menard (1955) has attrib- which has undergone subsequent downcutting. uted this to the accessibility of these areas The channel widens as it crosses southern to turbidity currents, as contrasted with those Cascadia Basin, and is about twice as wide on areas which are inaccessible due to physio- Tufts Abyssal Plain. The walls slope from less graphic barriers such as seamounts or trenches. than 1 to more than 21 degrees, with the Cascadia Abyssal Plain partially fills a basin western wall almost always being steeper. A which lies off the coasts of Oregon and Wash- number of tributaries, of which Vancouver Sea ington. It is bordered on the west by the Valley is the largest, enter Cascadia Channel submarine ridges and seamounts of the Juan along its course and give rise to an extensive de Fuca Ridge, and on the south by the submarine drainage system. Blanco Fracture Zone (Fig. 1). The continental margin, which consists of the continental shelf COLLECTION AND ANALYSIS OF and slope and two coalescing deep-sea fans DATA forms the northern and eastern borders of the The sediment samples used in this study plain. (Fig. 2) were collected using a modified Cascadia Deep-Sea Channel originates be- Ewing piston core and a multiple corer tween Nitinat and Astoria fans where Willapa (Fowler and Kulm, 1966) as a trip weight. Sea Channel and a second large unnamed Textural analyses were run using standard channel merge (Fig. 2). It trends south across sieve, hydrometer, and pipette techniques Cascadia Abyssal Plain, passes through the (Krumbein and Pettijohn, 1938). Coarse Blanco Fracture Zone and proceeds onto fraction counts were made and the components Tufts Abyssal Plain. The total length is were divided into three main groups, detrital believed to be over 2200 km (Hurley, 1960), grains, platy grains (mica, plant fibers, volcanic and the gradient decreases from 1:625 at the glass), and biological remains (Foraminifera, base of the slope to 1:4000 on Tufts Abyssal Radiolaria, diatoms, and so on) which are Plain. The gradient is interrupted a number important in distinguishing sediment types. of times by short sections of reversed slope Organic carbon and calcium carbonate con- with maximum relief of 2 to 8 m. Slumping of tents were determined, using the method de- sediment from the channel walls may be the scribed by Curl (1962). Clay mineral identi- cause of these humps or high areas along the fications were made from X-ray traces of channel axis. Near the western end of the magnesium ion-saturated, ethylene glycol- Blanco Fracture Zone, the channel plunges treated clays ( <2/i size fraction). The semi- 130 m into a recently depressed basin. quantitative method suggested by Biscaye Channel relief varies from 40 to 300 m on (1965) was employed to detect areal and the abyssal plain, but increases to 1100 m in temporal variations in the clay-mineral compo- the mountains of the fracture zone (for bathy- sition. metric profiles, see Figs. 4, 7, 9 and 10). The right (north and west) bank is consistently STRATIGRAPHY about 30 m higher than the left (south and A paleoclimatic curve has recently been east) bank. This difference in height is ex- developed for Cascadia Basin sediments (Dun- plained by the development of levees along can, 1968; Griggs and others, 1970). This the right side of the upper channel (Fig. 4), curve is based primarily on the relative abun- 126° 124° 44° H 44° 128° - - - - ,26° Figure 2. Bathymetry and piston core location map for Cascadia Channel and its tributaries. SEDIMENT DISTRIBUTION 1365 GLACIAL CHRONOLOGY AVERAGE JULY TEMP FAUNAL STRATIGRAPHY SWEDEN ALASKA WASH.-BRIT.
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