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 "

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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. COL WESTERN WASHINGTON BOG CASCADIA BASIN (Deevey and Flint, 1957) (Armstrong et 01,1965) (Heusser,l965l (Duncan, 1968; Griggs,l969)

5000-

10,000-

15,000

> 20,000

25,000

30,000

35,000 Figure 3. Summary and correlation of late Pleistocene and Holocene glacial chronology and floral and faunal stratigraphy (from Griggs and others, 1970). dances of planktonic Foraminifera and Radio- column (Nelson and others, 1968). Mazama laria supported by radiocarbon dating. Pelagic ash was transported along the ocean bottom, or hemipelagic1 sediments high in planktonic primarily within channels, as turbidity cur- Foraminifera characterize the cooler or glacial rents. The first appearance of the ash in the periods, while Radiolaria-rich sediments are deep-sea environment is a distinct and wide- common in warmer or interglacial periods. The spread horizon which appears to represent a paleoclimatic curve (Fig. 3) shows a rather short interval of time and can be used for abrupt transition from present day interglacial dating and correlating sediments. Mazama ash conditions to glacial conditions about 12,500 occurs in the Holocene sediments from the years ago (Duncan, 1968). This transition can axis, walls, and levees of Cascadia Channel but easily be detected in the sediment cores and is not found on the adjacent abyssal plain. is always marked by pronounced lithologic changes. Several older warming periods, or in- SEDIMENT DISTRIBUTION tervals richer in Radiolaria, have been recog- nized within the Pleistocene section (Duncan, General 1968); two cooling periods have also been The axis of Cascadia Channel and its north- recognized within the Holocene section ern tributaries were characterized primarily (< 12,500 years B.P.: Griggs and others, 1970). by the deposition of alternating sequences of The presence of volcanic ash from the olive-green silt and gray clay during Holocene eruption of Mt. Mazama 6600 years ago has time (Table 1, and Fig. 4). These sequences also been used in developing the stratigraphic also occur on the western wall of the channel and on the levees where they exist. Olive- green silt layers have a range of textural, com- 1 Particle by particle non-turbidite deposition near positional, and structural properties depending the continent with appreciable terrestrial contribution. upon the depositional environment. Layers 1366 GRIGGS AND KULM—SEDIMENTATION, CASCADIA DEEP-SEA CHANNEL

TABLE 1. SEDIMENT TYPES AND CHARACTERISTICS

Sediment type Symbol Color Texture Composition Olive-green silt tail ^^ 5Y3/2 Silty clay Mica, volcanic ash, plant fibers 1 base ^II 5Y3/2 Fine sand- Mica, detrital ^ silt grains, volcanic ash

Gray clay 5Y4/1 Silty clay Radiolaria, diatoms, 5GY4/1 Foraminifera, fecal 5Y4/2 pellets

Terrigenous sand- N-3 Clayey silt Detrital grains, silt 5Y4/1 to coarse mica sand Pebbly clay 5Y4/1 Sand-silt- Detrital grains, clay pebbles Foramimferal ooze 5Y7/2 Sand-silt- Plank tonic 5GY4/1 clay Foraminifera Gravel N-3 Sand to Detrital grains, pebbles shell debris

range in thickness from 3 to more than 400 cm, fraction (Fig. 5). The characteristics of these and consist of two distinct parts (Figs. 5 and sediments and their physiographic distribu- 6). The basal zone is coarse grained, consisting tion indicate deposition by turbidity currents. of sand or silt, and is usually laminated and It is significant that regardless of the core graded; detrital grains and mica constitute the location in the channel or the coarseness of the coarse fraction. Lower contacts are sharp and sediment, samples from the coarse basal zone commonly show grooves cut into the under- of the olive-green silt layers are always tex- lying gray clay which may be filled with fine- turally, structurally, and compositionally dis- to medium-grained sand. tinct from samples from the homogeneous tail Above this coarse basal zone, and sharply (Fig. 6). This is important in the discussion of separated from it, is a homogeneous tail. It the depositional mechanism which follows. is usually much thicker and consists of silty The gray clays, which are rhythmically inter- clay which becomes finer grained upward. bedded with the olive-green silt, contain a Mica and plant fibers compose the coarse biogeneous coarse fraction (Table 1). Radio- fraction. Gray burrows of various sizes and laria predominate in the Holocene clays and shapes are commonly found in the homoge- those of Pleistocene interglacial periods, while neous tail and may penetrate as far as 50 cm planktonic Foraminifera are more abundant downward from the upper surface of the layer in the Pleistocene clays of glacial periods. (Griggs and others, 1969). Benthic Foraminif- Gray clay layers are uniform in thickness era displaced from continental shelf and slope within individual cores (average 2 to 5 cm) depths are also common constituents. and usually have sharp grooved upper contacts Passing upward through an entire olive- from the emplacement of the overlying sand green silt layer, the calcium carbonate de- and silt. Each lower contact is irregular and creases from 1.0 percent by weight in the mottled where organisms have reworked the coarse basal zone to 0.5 percent in the homo- sediment. The biogenic coarse fraction and geneous tail (Fig. 5), while the organic carbon fine grain size of these clays which decreases content increases from 0.5 to 2.5 percent. away from shore, indicate that this material The increase in the organic carbon closely represents the pelagic and hemipelagic depo- parallels that of the plant fibers in the coarse sition in this area. SEDIMENT DISTRIBUTION 1367

TABLE 1. (Continued)

Average organic Average calcium carbon content carbonate content (percent) (percent) Microfauna Age Depositional process 2.3 0.5 Displaced benthic Holocene Turbidity current Foraminifera, diatoms, Radiolaria 0.5 1.0

1.0 Holocene 1.2 Abyssal benthic Holocene and Pelagic and Foraminifera, diatoms, Pleistocene hemipelagic 0.3 Pleistocene 4.0 Radiolaria, planktonic deposition Foraminifera

Displaced benthic Pleistocene Primarily turbidity Foraminifera current

0.4 0.7 Scarce Pleistocene Ice rafting

35.0 Planktonic Pleistocene Pelagic deposition Foraminifera Displaced benthic Pleistocene Turbidity current (?) Foraminifera

Channel Axis Holocene depositional rates here are as high as 125/cm/1000 yrs. Along the upper channel axis, the olive-green Although the channel is narrow and tightly layers vary in thickness but tend to thicken confined within the Blanco Fracture Zone, and coarsen deeper in the section as the Holocene depositional rates have been very Pleistocene is approached (Fig. 4, core 6705-5). high. The silt sequences are the thickest cored, The interbedded gray clays are commonly averaging about 2 m (Fig. 7, core 6609-19). absent or very thin closer to shore as a result The coarse basal zones are only 1 to 2 cm thick, of the higher frequency of turbidity currents and the remainder of each unit consists of the here. Over-all Holocene sedimentation rates fine-grained tail. This core was taken in a (based on CM, Mazama ash and faunal recent structural depression along the channel breaks) reach 71 cm/1000 yrs along the upper axis at the western end of the fracture zone. channel, while hemipelagic rates vary from 3 The depression drops the channel axis 120 m to over 14 cm/1000 yrs. below the floor on either side and appears to The same sediment sequence occurs along be acting as a sediment trap (Duncan, 1968). the lower channel (Fig. 7), but here the cycles The presence of Mt. Mazama ash in the de- are exceedingly uniform in thickness which posits both up- and down-channel from the indicates similarity in turbidity current flow depression dates its formation at some time conditions. The coarse basal zones of the silt during the last 6600 years. Depositional rates layers show a general thinning down-channel. here probably exceed 400 cm/1000 yrs. The A lack of recognizable hemipelagic sediment absence of recent turbidity current activity at the surface of these cores and the absence in the channel on Tufts Abyssal Plain is in- of extensive burrowing in the surficial layer dicated by a thicker than normal hemipelagic compared to the extensive reworking in lower clay layer (6 to 10 cm) at the top of the cores layers indicate that the last flow was recent. from that area (Fig. 7, core 6509-25A). The presence of Mt. Mazama ash in the sedi- Sedimentation within the middle channel is ment and a radiocarbon date of 4645 years anomalous in relation to the remainder of the B.P. at depth (Fig. 7, core 6509-27) also in- channel. Turbidity currents, which deposited dicate recent turbidity current activity. the characteristic sequences to the north and 1368 GRIGGS AND KULM-SEDIMENTATION, CASCADIA DEEP-SEA CHANNEL UPPER AND MIDDLE CASCADIA CHANNEL O X IOO% 5O IOOK SO000' 10O%

_ e/oG£wc CORE OftsM/c CALCIUM CLAY GRAINS MATERIAL LOG FRAGMENTS CARBON CARBONATE Figure 5. Vertical variation in texture and composi- tion of typical olive-green silt sequence from axis of Cascadia Channel.

Figure 4. Lithology of cores from upper and middle Coarse sand and gravel were found along the Cascadia Channel. Vertical exaggeration of profiles 20x; middle channel, as well as along the channel axis core lithologies explained in Table 1. Hoi = Holocene, some 750 km from shore in Blanco Fracture LP = late Pleistocene. Zone, (Fig. 4, core 6609-30; see also Fig. 8). The gravels generally have a low silt and clay south, left little sediment here. Beneath the content (<10 percent) but are moderately to thin covering of Holocene olive-green silt and poorly sorted (Table 1). Individual pebbles are gray clay occurs a Pleistocene section of diverse primarily equidimensional in shape and round- lithology; it includes pebbly clays, laminated ed to well rounded. Sedimentary, igneous, clay-silt sequences, gravels, and terrigenous and metamorphic rocks are present. Small sand-silt units (Fig. 4, cores 6705-13 and numbers of benthic Foraminifera occur in the 6609-30; see also Fig. 8). gravel deposits; 80 percent of the benthic The pebbly clays extend for at least 75 km fauna is displaced from shallower water, and along the middle channel axis and are very 25 percent originated on the inner shelf. distinct (Griggs and Kulm, 1969). Although Pleistocene sand and silt from the channel mainly massive and structureless, they may axis is coarser grained and better sorted than contain zones without pebbles and thin sand the Holocene olive-green silts and also lacks or silt layers (Table 1). A complete range of their green color. The sand and silt may occur grain sizes from fine clay to clasts up to 7 cm as thin laminae or as layers up to 150 cm thick. in length occurs within these sediments. The The thick units may be laminated, cross- pebbles may be faceted or striated and consist laminated, graded, and have sharp basal con- of igneous, metamorphic, and sedimentary tacts. They contain displaced Foraminifera rocks, which have been traced to continental and distorted clay clasts. These textural and source areas (Griggs and Kulm, 1969). The structural characteristics are all suggestive of interbedding of laminated and gravelly clays, turbidity current origin. the silt and sand layers, the great distance required for transportation, and the proximity Channel Walls to a glaciated region, suggest ice rafting as a The sedimentary sequence on the walls of depositional mechanism. Cascadia Channel demonstrates that Holocene SEDIMENT DISTRIBUTION 1369

SAND 02 04 SKEWNESS BIO.-AUTH.

DETRITAL 0.5 10 1.5 2D PERCENT ORGANIC CARBON Figure 6. Textural and compositional characteristics of olive-green silt sequences. Note difference between coarse basal zone (•) and homogeneous tail (o). LOWER CASCADIA CHANNEL turbidity currents have risen above the channel floor to deposit their sediment. Two cores taken low on the west wall contain up to 21 alternating sequences of green silt and gray clay (Fig. 9, cores 6509-15, 6509-17). Silt sequences are much thinner here than on the channel floor. Higher on the western wall, only hemipelagic clay has been deposited during Holocene time (core 6609-25, 127 m above the floor). This core also contains a section of pebbly clay identical to that cored to the north. Sediments cored on the opposite wall (east) contain none of the typical channel turbidity current deposits in the Holocene section (cores 6705-16, 6705-17). This time interval is represented by only 75 to 150 cm of hemipelagic clay in the two cores taken 8 and 37 m above the channel bottom. Farther to the south, where the channel enters the Blanco Fracture Zone, the sediments on the northern wall show that turbidity currents have risen 54 m above the floor to deposit the typical olive-green silt cycles (Fig. 9, core 6705-21). These thicker units, in contrast to those on the wall farther north, are probably due to the welling up of large amounts of material funneled into this opening during the Figure 7. Lithology of cores from lower Cascadia passage of a turbidity current. The flow must Channel (see caption of Fig. 5 for core explanation). 1370 GRIGGS AND KULM—SEDIMENTATION, CASCADIA DEEP-SEA CHANNEL

TERRIGENOUS SAND-SILT LAMINATED SILTY CLAY PEBBLY CLAY

Figure 8. Holocene and Pleistocene sediment facies along the axis of Cascadia Channel. Vertical lines indicate core locations. make a right angle turn here where the channel the channel floor; the east bank is lower, only floor is relatively narrow. 59 m above the channel floor, and shows no overflow (core 6705-12). The asymmetry, or Channel Banks and Levees tilt, of a turbidity current that was first In order to study cross-channel sediment suggested by Menard (1955) is verified here. variation and to determine if turbidity current Coring profiles made farther to the south (3 and spillover had occurred, 4 coring profiles were 4), where levees are absent and where channel taken across the channel along its length (Fig. relief ranges from 238 to 286 m, demonstrate 10). The northernmost profile (1) shows a that Holocene turbidity currents did not top well-developed levee to the west, rising 28 m the banks in this area. The Holocene sediment above the plain and extending 6.5 km laterally. on the banks consists only of hemipelagic clay. Cores from both sides, however, 6705-7 and Pleistocene sections on the levees and banks 6705-9), 86 and 77 m above the channel consist of gray clay interbedded with terri- bottom respectively, contain the characteristic genous sand and silt. Sediments east of the alternating sequences of olive-green silt and channel contain a greater proportion of the gray clay, and illustrate Holocene turbidity coarse material than do those to the west. Only current overflow. The silts are only 5 to 10 cm 6 to 11 percent of the levee heights can be thick and frequently have very thin (1 to 2 cm) accounted for by Holocene turbidity current coarse basal zones and contain Mazama ash. deposits, which indicates that these features The next profile to the south (2) shows that were formed mainly by Pleistocene sedimenta- turbidity currents overflowed onto the western tion. The underlying sediment, which probably plain but were not recorded on the east. The represents Pleistocene overflow, ranges from levee to the west (core 6705-14) is 68 m above thin silty laminae to 60-cm-thick beds of SEDIMENT DISTRIBUTION 1371

CASCADIA CHANNEL WALLS medium-grained sand; it generally is coarser 65O9-I5 6705-17 and cleaner than the Holocene olive-green silts. The range in grain size and thickness of the | HOL I Pleistocene coarse layers, compared to the uni- formity of the Holocene silt sequences, suggests that the competency and load of the Pleisto- cene turbidity currents must have been quite variable. Vancouver Sea Valley, Astoria Fan Tributaries, and Cascadia Abyssal Plain During the Pleistocene, large amounts of sand were transported through Vancouver Sea Valley and the channels on outer Astoria Fan (Figs. 8 and 11). The Pleistocene deposits con- sist of beds of fine sand (as much as 150 cm thick) interbedded with thin hemipelagic gray clays. Turbidity currents have been inactive in these channels during the Holocene, and depo- sition of hemipelagic clay has continued with- out interruption. The southernmost core from Vancouver Sea Valley (Fig. 11, core 6609-27) contains 8 thin sequences of olive-green silt which have spilled over from Cascadia Channel during Holocene time. The flows must have been at least 80 m high and spread laterally Figure 9. Lithology of cores from walls of Cascadia 15 km to reach this point from the floor of Channel (see caption of Fig. 5 for core explanation). Cascadia Channel.

CASCADIA CHANNEL BANKS AND LEVEES VANCOUVER SEA VALLEY AND ASTORIA FAN TRIBUTARIES 6705-7 6705-9 65O9-28 65O9-29

Figure 10. Lithology of cores from banks and levees of Cascadia Channel (see caption of Fig. 5 for core Figure 11. Lithology of cores from Vancouver Sea explanation). Valley and Astoria Fan tributaries. 1372 GRIGGS AND KULM—SEDIMENTATION, CASCADIA DEEP-SEA CHANNEL

On the adjacent abyssal plain, turbidity from Cascadia Channel (Table 2). In contrast, currents have been largely inactive in the the interbedded hemipelagic clays contain only Holocene and hemipelagic clays compose the deep forms. The abundance of shallow water sections. Terrigenous sand and silt interbedded Foraminifera (up to 41 percent of the fauna) with gray clay characterize the Pleistocene suggests that the sediment had its origin in (Duncan, 1968). water depths less than 100 m. The mixture of this fauna with forms from intermediate and DEPOSITIONAL PROCESSES greater depths indicates that the transporting turbidity current accumulated and mixed Holocene Turbidity Currents: Empirical Data sediment as it flowed to its abyssal depositional The need for an accurate picture of modern site. turbidity current characteristics and properties A sorting of the fauna has also been recog- in the marine environment is obvious. Most nized in addition to the mixing within the studies to date have been concerned with sub- olive-green silts. Several of the smaller species marine canyons and fans or abyssal plains where increase in abundance upward, and the average sediments are discontinuous and where correla- size of certain individual species decreases up- tion is virtually impossible. Cascadia Channel is ward. It is apparent that a size sorting of the an extensive and confined route for turbidity Foraminifera, as well as a faunal mixing results currents. Its location close to shore, near the from the action of a turbidity current in this site where large volumes of sediment from the area. Columbia River are deposited, makes it an Textural and Structural Variations. The in- excellent environment for modern turbidity ternal sequence of structures in turbidites has current analysis. been described in detail and related to flow Previously mentioned evidence for the regimes and depositional environments (Kue- turbidity current origin of the Holocene olive- nen, 1953; Bouma, 1962; Walker, 1965, 1967). green silts in the channel system includes Most graded sequences from Cascadia Channel grading of grain size and composition, erosional are different from those previously described basal contacts, interbedding with hemipelagic (Fig. 12). The channel deposits are finer clays, and most important, the physiographic grained (silty) and the lower 4 intervals (coarse occurrence of the sediments. These and other basal zone) usually constitute only 10 to 20 data are elaborated upon here in order to percent of the bed, while the pelitic interval determine more precisely the nature of this (homogeneous tail) is very thick. In contrast to medium of transportation and deposition. The studies of lithified units (Bouma, 1962), the above mentioned characteristics, as well as the pelagic sediment can easily be distinguished extremely poor sorting and high clay content from the pelitic interval, or fine-grained tail, of these sediments, rule out deposition by by differences in color (see Table 1). normal bottom currents. Any reworking by Four structural sequences characterize about bottom currents has probably been minor. 90 percent of the turbidity current deposits Foraminiferal Distribution. Benthic Fora- examined from the channel axis (Fig. 12). In minifera from shallow, intermediate, and deep addition to these, however, several cores from water occur within individual olive-green silts the upper and middle channel display thin TABLE 2. FORAMINIFERAL SPECIES FROM A HOLOCENE TURBIDITY CURRENT DEPOSIT FROM CASCADIA CHANNEL

Shallow forms Intermediate forms Deep forms ( <200 m) (200 to 2000 m) (> 2400m) Eggerella advena Eolivina argenlea Bulimtna roslrala Buliminella B. pacijica Gyroidina gemma elegantissima Elphidium tumidum B. spissa G. so Ida nii Buccella frigida Buliminella exilis Melonis pompiliotdes tenuata B. lenerrima Epistominella pacifica Uvigerina senticosa pacifica Loxostomum pseudobeyrichi Uvigerina peregrina DEPOSITIONAL PROCESSES 1373

Complete Turbidite Sequence (Bouma, 1962) Cascadia Channel Turbidity Current Sequences Ta-e Tn. 27% Tb-e 7% Tde 46% 9% pelitic interval

upper interval of parallel lamination

interval of current ripple lamination

lower interval of parallel lamination

graded interval

Figure 12. Complete turbidite sequence (Bouma, 1962) and turbidity current sequences from Cascadia Channel. Percentage occurrence of Cascadia Channel sequences are indicated.

olive-green clayey layers beneath the coarse ments found by Walker (1967) is not apparent basal zone. The coarse fraction of these clayey in the Holocene sediments along the axis of layers is high in mica and plant fibers, com- Cascadia Channel. One difficulty lies in the pared to the greater abundance of detritals in designation of intervals, such as those which the overlying material. A similar interval has are both graded and laminated. A complete been found in calcareous turbidites from sequence (Ta.e) has never been encountered in Germany and was designated a "pre-phase" by the channel. The most complete development Meischner (1964). In the channel sediments, of structures (Tb-c) occurs most commonly this interval is easily distinguished from the along the middle channel rather than along the hemipelagic clay by its color. The authors upper portion as might be expected. The believe that it represents deposition from the sequence occurring most often (Tde) is found faster moving body of a turbidity current, throughout the 650 km of the channel which which at this point was moving more rapidly has been studied. Ta e occurs most commonly than the head with its concentration of coarse in the upper channel but also is found far from material. shore. The lower intervals disappear, proceed- Clastsof gray clay occur within the Holocene ing laterally away from the channel axis, and olive-green silts from the channel, as well as in the sequences either consist of Tae or Te on the the Pleistocene sands of the adjacent environ- walls and levees. Both intervals are thinner ments. The Pleistocene turbidity currents with here than on the channel floor. their coarser-grained sediments were probably These deep-sea channel environments at more capable of erosion than the finer grained great distances from shore and with reduced Holocene flows, resulting in the accumulation gradients are strikingly different from the and incorporation of more clay fragments in ancient restricted basins with their steeper these deposits. The clasts, the irregular grooved gradients, coarser material, and proximity to basal contacts of the graded beds, and the dis- shorelines (Kuenen, 1964). As a result, lower placed benthic Foraminifera are the only sedi- flow velocities, finer grained sediment, and the mentological evidence for erosion by the turbid- lack of structures associated with the upper ity currents presently active in the channel. flow regime (a-d of Walker, 1965) characterize The clear-cut relationship between turbidite the turbidity currents within Cascadia Chan- structures and proximal and distal environ- nel. 1374 GRIGGS AND KULM-SEDIMENTATION, CASCADIA DEEP-SEA CHANNEL

Some hydraulic properties of the turbidity and tail of each graded unit are very poorly currents in Cascadia Channel can be inter- sorted as a result of the high clay content preted from the textural and compositional throughout (average: base = 15 percent clay, characteristics of the sedimentary sequences. tail =58 percent clay). The differences between the coarse basal zone The generalization that turbidites in proxi- (a-d) and the fine homogeneous tail (e) of a mal environments are thick and those in distal typical graded unit have been discussed (Fig. environments are thin (Walker, 1967) is re- 6). The coarser grained, more positively versed along the axis of Cascadia Channel skewed, basal portion (a-d) of each graded bed (Figs. 13 and 14). During the Holocene, the represents deposition from the traction carpet. thickest beds were deposited along the lower The structures that are present (graded bed- portion of the channel, and thinner beds closer ding, parallel lamination, convolute lamina- to shore. On the other hand, the coarse basal tion) are the result of velocity changes and layer (a-d) is thickest close to shore and thinnest perhaps some simultaneous turbidity current farther away (Fig. 14). The deposition of most reworking. In contrast, the finer grained, of the coarse material from the traction load organic-rich upper portion (e) of each bed probably occurs along the upper channel, represents deposition from the suspension load resulting in thicker basal zones. Farther down- or sediment cloud. The gradual, but regular, channel, there is an increased amount of increase in clay content upward in the tail deposition from suspension, resulting in a seems to substantiate the slow settlement of thicker tail (e). The maximum grain size this material from suspension. Both the base present, with few exceptions, in both the proximal and distal areas is fine sand. How- TRIBUTARY CHANNELS ever, the amount of sand in the coarse basal zone decreases with the distance from shore (Fig. 14).

200 fc > 100 oeiO- o 80 i 5-

20 4O 60 80 _10D0 200 THICKNESS OF BEDS (cm)

CASCADIA CHANNEL HEAD TO BLANCO FRACTURE ZONE 200 -WO 600 8OO KILOMETERS FROM HEAD OF WILLAPA CANYON

ID- S'

0 20 tO 6O 80 IOO 2OO

CASCADIA CHANNEL BLANCO FRACTURE ZONE TO TUFTS ABYSSAL PLAIN KIUDMETERS FROM HEAD OF WILLAPA CANYON Figure 14. (Top) Average thickness of entire tur- 0 20 4O 6O 80 100 3OO 500 bidity current sequence and coarse basal zone in in- dividual cores along axis of Cascadia Channel. (Bottom) Figure 13. Histograms of thicknesses of individual Average sand content of coarse basal zone of turbidity Holocene turbidity current sequences along Cascadia current sequence in individual cores along axis of Channel axis. Cascadia Channel. DEPOSITIONAL PROCESSES 1375

Flow Periodicity. Average recurrence inter- Previously discussed stratigraphic markers val, or time period between turbidity currents, (Mazama ash, faunal changes, and radiocarbon can be calculated if the age of some horizon in dates) have been used to determine the average the core is known. Published data on flow Holocene flow periodicity within Cascadia periodicity is scarce (Table 3) and with 2 Channel. The uniform thickness of the hemi- exceptions (McBride, 1962, and Lovell, 1969), pelagic clay layers between each graded unit represent only Holocene events. Values record- (on the average, 75 to 100 percent of the clay ed in nearshore submarine canyons are on the layers in a single core vary in thickness by a order of several years, whereas in deeper basins, factor of 2 or less) implies that time intervals the time interval between flows seems to be between flows were of a similar duration. In much longer, 400 to 500 years. It is apparent addition, the uniform thickness of the cycles that relatively few flows which pass through suggests that the turbidity currents were of the submarine canyons have sufficient volume similar size and material. A certain amount of or velocity to reach the more distant basins or time would probably be necessary for a suffi- abyssal plains. cient volume of material to accumulate in the

TABLE 3. RECURRENCE INTERVALS FOR HOLOCENE TURBIDITY CURRENTS

Time Location Method or Evidence Period Reference Colorado River-Lake Mead Visual observation lyr Gould (1951) Congo Submarine Canyon Cable breaks 2 yrs Heezen and others, (1964) Magdalena River Delta Cable breaks 2 yrs Menard (1964) Continental Borderland Basins Feeding Canyons Estimate 1 to 10 yrs Gorsline and Emery (1959) Center of Basins Carbon-14 400 yrs Martinsburg Fm (Appala- Estimate 500 yrs McBride (1960) chians-Ordovician) Tyee Fm (Eocene)* Estimate 500 to Lovell (1969) 1000 yrs Tongue of the Oceans Carbon-14 500 to Rusnak and (No age given) 10,000 yrs Nesteroff (1964) Cascadia Channel System Time Location Core Method or Evidence Period Tributary Channel 6705-2 Mazama ash 410 yrs Willapa channel 6705-5 Mazama ash 440 Tributary channel 6705-6 Mazama ash 470 Upper channel 6508-K1 Mazama ash 510 Lower channel 6609-24 Mazama ash 510 Carbon-14 mot Channel wall 6509-15 Mazama ash 510 Pleistocene-Holocene 740f boundary Lower channel 6509-27 Carbon-14 580 Channel levee 6705-14 Mazama ash 660 Channel levee 6705-7 Mazama ash 825 Channel levee 6705-9 Mazama ash 825 Tributary channel 6705-10 Pleistocene-Holocene 1040 boundary Lower channel 6509-25A Carbon-14 1500 Mazama ash Lithified intervals of Ordovician and Eocene age t See Figure 15 for time discrepancy 1376 GRIGGS AND KULM—SEDIMENTATION, CASCADIA DEEP-SEA CHANNEL source area to cause the instability, which 6609-24 6509-15 would result in a turbidity current capable of r I flowing the length of Cascadia Channel. These 22 i r s Ave, Tr =5IOyrs. factors suggest that the average recurrence in- x/2 iMazamq_j' ggQO B P tervals given in Table 3 are probably quite % = Ash -^ representative. tt M Ave. Tr ' 74Oyrs. As in the submarine canyons, the time in- Holocene v tt ~ Pleistocene l2'500 BP terval between flows is also somewhat shorter %^ in the upper channel environments (Table 3, 1 cores 6705-2, 6705-5,6705-6); all of the turbid- 4ne. Tr - 5/Oyrs. ity currents which passed through these areas x/ 6509- 25 A apparently did not reach the middle and lower ^ channel. From the middle channel to the - Blanco Fracture Zone, the periodicity of the P flows in post-Mazama time (last 6600 years) is very uniform; they vary from 500 to 600 years (Table 3). Channel levees and that portion of ^ the channel on Tufts Abyssal Plain have ^ 1Mazama ' /]>•«. Tr 'ISOOyrs. longer recurrence intervals (825 to 1500 years), Ash ~ — 6600B.R 1 as would be expected of environments which \ C| ^-9670B.P were not reached by every flow. Intervals •^ 4 # Ave. Tr =ll!Oyrs. ^ \ between flows in the tributary channel from ^Mi Nitinat Fan (6705-10) are also long (1040 /7 \ years). This may be due to the great distances - C|4—i-ll,04OB. R from the sediment source. From 12,500 to 6600 years ago, turbidity Tr = Turbidity Current Recurrence Interval currents were less frequent than in the post- Figure 15. Turbidity current recurrence intervals Mazama time, at least along the lower channel for dated sections of selected channel axis (cores 6609- (Fig. 15). Recurrence intervals of 740 to 1500 24, 6509-25A) and wall (6509-15 cores). Core 6509-25A years are characteristic of this earlier time. was taken west of Blanco Fracture Zone and has been One explanation might lie in the closer con- isolated from late Holocene flows. nection between the Columbia River and Astoria Canyon during late Pleistocene and lateral, longitudinal, and vertical extent of early Holocene time (Carlson, 1968). Most of these flows (Table 4, and Fig. 16). Turbidity the Columbia River sediment load was prob- currents at least 117 m high have passed ably transported through Astoria Canyon at through the upper channel in Holocene time this time. With the Holocene rise in sea level, (Fig. 16, Profile A). Along the middle portion the marine transport of the Columbia River (Profiles B to E), the flow heights decreased sediment discharge began to approach its to about 80 m. Levee development, which in- present path (Gross and Nelson, 1966), that is, dicates overflow, is characteristic of the upper moving northwest across the continental shelf 200 km of the channel. The width of the toward the head of Willapa Canyon. More levees and their vertical sequence of Holocene sediment was probably supplied to Willapa turbidity current deposits demonstrate that in- Canyon, and subsequently, more turbidity dividual flows extended laterally up to 13 km currents passed through Cascadia Channel from the channel axis. A single core from during the later part of the Holocene. Addi- lower Vancouver Sea Valley near the junction tional evidence supporting this sediment shift with Cascadia Channel contains thin alterna- is the present sediment filling taking place in ting layers of olive-green silt and gray clay Astoria Canyon (Carlson, 1968). (Fig. 11, 6609-27). The absence of these characteristic Holocene deposits farther north Holocene Turbidity Currents: Flow in this valley indicates that the sediment over- Reconstruction flowed from Cascadia Channel. The flows must Flow Dimensions. The physiographic dis- have been at least 80 m high and 17 km wide tribution of known turbidity current deposits at this point. in Cascadia Channel and the occurrence of To the south, Cascadia Channel is deeply submarine levees can be used to determine the incised into the abyssal plain; the absence of DEPOSITIONAL PROCESSES 1377

TABLE 4. DIMENSIONS OF TURBIDITY CURRENTS IN CASCADIA CHANNEL INTERPRETED FROM SEDIMENT DISTRIBUTION AND LEVEE DEVELOPMENT

Distance from Flow Lateral Width of Lateral Total head of Willapa height extent to channel extent to width of Canyon (km) (m) west (km) floor (km) east (km) flow (km) 131 117 6.0 2.0 8.0 138 99 7.5 2.5 3.0 13.0 153 108 3.0 2.5 5.5 185 86 6.5 2.5 9.0 201 82 1.5 2.0 3.5 237 80 4.0 2.0 6.0 248 82 1.5 2.0 3.5 324 80 15.0 2.0 17.0 337 <37 0.6 0.6 357 > 4 1.5 1.5 367 > 8 2.5 2.5 381 1.0 1.0 415 1.1 1.1 460 1.2 1.2 483 54 5.5 2.5 8.0 735 2.7 2.7 olive-green silts on the banks and the lack of le- vees demonstrate that no overflow has occurred (Profiles F-G). Holocene turbidity current deposits on the western wall show that the flows were at least 4 m high here. Hemipelagic sed- iments on the eastern wall demonstrate that the flows did not rise more than 8 m above the floor on this side of the channel. As the sed- iments were funneled into Blanco Fracture Zone, they were deposited as high as 54 m above the floor (Profile H). The flows con- tinued through the fracture zone and onto Tufts Plain for an unknown distance. Maximum known dimensions of Holocene turbidity cur- rents in Cascadia Channel are believed to be: Length, 650 km; width, 17 km; height, 117 m. Velocity Determinations. Turbidity cur- rent velocities have been determined in other areas (Table 5) by actual measurement and by the timing of submarine cable breaks (Gould, 1951; Heezen, 1963; Menard, 1964). Within Cascadia Channel, estimates of flow velocities have been made by use of a modified Chezy equation, which yields average velocities of uniform flow. Hurley (1964), who assumed values for the density difference between the flow and sea water and for the channel rough- ness, arrived at average velocities of 5.2 to 7.8 m/sec for the entire channel. Middleton (1966a, 1966b, 1967) however, after numerous Figure 16. Reconstruction of a typical Holocene laboratory experiments, states that the uniform turbidity current in Cascadia Channel. Flow dimen- theory (Chezy equation) is not completely sions derived from levee and sediment distribution. applicable to turbidity currents. Bank overflow Vertical lines indicate core locations. Vertical exaggera- (that is, height of flow) will be determined by tion of profiles 20x. 1378 GRIGGS AND KULM-SEDIMENTATION, CASCADIA DEEP-SEA CHANNEL

TABLE 5. HOLOCENE TURBIDITY CURRENT VELOCITIES

Velocity Location Method (m/sec) Reference Laboratory flume Visual measurement 0.20 to 0.36 Middleton (1966a) Colorado River, Lake Mead Visual observation 0.25 Gould (1951) Orleansville, Algeria Cable breaks 2.6 to 20.6 Heezen (1963) Grand Banks, Newfoundland Cable breaks 19.1 Menard (1964) Monterey Canyon Chezy formula 1.0 to 8.5 Wilde (1965) Cascadia Channel Chezy formula 5.2 to 7.8 Hurley (1964) Cascadia Channel* Distance from head of Willapa Maximum Velocity Canyon (km) height (m) (m/sec) 131 Upper channel 117 5.8 153 Middle channel 108 5.7 237 Middle channel 80 4.7 337 Lower channel 37 3.3 483 Lower channel 54 3.8 * Using formula from Middleton, 1966a

the thickness of the head, not by the thickness would take about two days to travel the of uniform flow. The velocity of a turbidity entire length (735 km) of the surveyed portion current head on a slope up to about 1:25 is of Cascadia Channel. adequately expressed by Keulegan's (1957, 1958) formula Pleistocene Turbidity Currents The turbidity current deposits of the V = 0.75 V(Ap/p) gd2 Pleistocene are distinctly different from those where V = velocity of the head, Ap is the of the Holocene (Table 6). The older sediment difference between the density of the current is generally coarser and cleaner. In contrast to (p) and that of the overlying water, d% is the the Holocene sequences, lamination, cross- thickness of the head and g is the acceleration lamination and clay clasts are much more due to gravity. The density difference measured abundant in the Pleistocene, while the fine- for Lake Mead turbidity currents (Gould, grained "tails" (t interval) are not as thick. 1951) and commonly used in flow calculations Plant fibers and organic carbon content of the (Hurley, 1964; Wilde, 1965) is 0.05 gms/cc. Pleistocene coarse layers are very low. Burrow- The thickness of the head can be determined ing, which is very common in the organic-rich from the sediment distribution and levee Holocene deposits, is much less abundant in the dimensions, which record the approximate Pleistocene sediments. These differences sug- height of individual flows. gest that Pleistocene turbidity currents had Velocities determined from the Keulegan a much greater traction load and contained formula decrease from a high of 5.8 m/sec much less suspended material than the Holocene in the upper channel to a low of 3.3 m/sec flows. Consequently, more erosion and a more along the lower channel (Table 5). These are complete development of sedimentary struc- somewhat lower than the values determined by tures characteristic of the lower flow regime Hurley, and considerably less than the max- occurred. The Pleistocene flows were also much imum values obtained from cable breaks off more extensive as indicated by their deposits the Grand Banks and Orleansville (Menard, in Vancouver Sea Valley, the channels on outer 1964; Heezen, 1963). A turbidity current with Astoria Fan, and on the abyssal plain, where velocities of 3.3 to 5.8 m/sec would probably only hemipelagic clays were deposited in be capable of erosion, but perhaps the fine- Holocene time. Pleistocene overflow along grained nature of the sediments composing the upper and middle Cascadia Channel appears to Holocene flows curtailed erosion. A turbidity have been almost entirely responsible for the current with an average velocity of 4 m/sec development of levees in those areas. SEDIMENT SOURCE 1379

TABLE 6. COMPARISON BETWEEN PLEISTOCENE AND HOLOCENE TURBIDITY CURRENT DEPOSITS IN CASCADIA BASIN

Property Pleistocene Holocene Maximum thickness of 150cm 225 cm an individual deposit Maximum grain size gravel tine sand Average sand-silt-clay 50%-45%-5% 5%~55%-40% content of entire deposit Sorting moderate to poor poor to extremely poor Composition detrital grains, mica detrital grains, mica, plant fibers Sedimentary structures irregular basal contacts, irregular basal contact, parallel and cross graded bedding, parallel lamination, distorted and cross lamination, clay clasts, graded extensive burrowing beading Locations all abyssal plain, fan, Cascadia Channel and canyon, and channel northern tributaries, environments Astoria and Willapa Canyons, upper Astoria Fan

SEDIMENT SOURCE cene. The abrupt change in clay mineral com- position at the Pleistocene-Holocene boundary Clay Mineralogy was noted by Duncan (1968). The Columbia River was presumably the source for the The mineralogy of the clay fraction ( <2ju) Pleistocene as well as the Holocene clay; of the olive-green silts is remarkably uniform Duncan and others (1970) have shown that along the entire length of Cascadia Channel. changes in the relative contributions from the The most abundant constituent is montmorillo- different sub-basins of the Columbia River nite, which constitutes an average of 52 percent were responsible for the stratigraphic changes of the clay fraction; illite and chlorite consti- in clay mineral composition. tute 23 and 25 percent, respectively (Table 7). Significant differences in clay mineralogy These clays originate in the Columbia River occur between the various sub-basins (Knebel drainage and represent the first of three clay and others, 1968). The Snake River presently mineral groups which radiate outward from carries several times as much suspended material the river mouth (Duncan, 1968, Duncan and as the upper Columbia River and greatly in- others, 1970). fluences the relative percentages of the clay The Holocene gray clays, which represent minerals in sediments of the lower Columbia hemipelagic sedimentation in Cascadia Basin, River. The clay mineral composition of the are widely distributed. Their clay mineral sediments in the lower Columbia Basins, the composition is almost identical to that of the Snake River, and the Holocene sediments of olive-green silts, but they have a slightly lower Cascadia Channel is almost identical (Table 7). chlorite content. The Pleistocene gray clays The arid to semi-arid climate, plateau or foot- also have a uniform composition; montmorillo- hill topography, poor water circulation, and nite constitutes only 37 percent of the clay meager accumulation of organic matter and fraction, while illite and chlorite make up 34 acids in the lower Columbia and Snake River and 29 percent, respectively (Table 7). The sub-basins give rise to clays high in montmoril- consistent composition of these clays over a lonite (Knebel and others, 1968). Clays of the large area suggests that hemipelagic sediments upper Columbia sub-basin are characterized by were uniformly distributed over Cascadia low montmorillonite and high illite contents, Basin during the Pleistocene due to a large identical to the Pleistocene clays of Cascadia Columbia River discharge. Basin (Table 7). The cool sub-humid to humid Illite and chlorite are more abundant and climate, bold and rugged topography, active montmorillonite is less abundant in the Pleisto- water circulation, and an accumulation of cene gray clays compared to those of the Holo- organic litter and acids in this sub-basin have 1380 GRIGGS AND KULM-SEDIMENTATION, CASCADIA DEEP-SEA CHANNEL

TABLE 7. COMPARATIVE CLAY MINERALOGY OF CASCADIA BASIN AND COLUMBIA RIVER SUB-BASIN SEDIMENTS Physiographic Environment Mean Values Montmorillonite Illite Chlorite + Kaolinite (percent) (percent) (percent) Cascadia Channel (Holocene clays) 52 23 25 (no kaolinite) Lower Columbia River Bonneville Dam* 55 24 21 The Dalles Dam* 57 27 16 Snake River* 55 28 17 Cascadia Basin (Pleistocene gray clays) 37 34 29 (no kaolinite) Upper Columbia River Basin* 35 33 32 * Data from Knebel and others (1968) produced a different clay mineral composition extensive sediment deposition on the outer (Knebel and others, 1968). shelf, which in turn produced a distinct pattern These data strongly suggest that the Snake of sediment dispersal in the deep sea. Sediments River Basin is presently the major contributor transported by turbidity currents passed to the suspended load of the Columbia River, through Astoria and Willapa Canyons to the whereas during the Pleistocene, the upper deep-sea floor and moved south along Astoria Columbia River was of major importance. The Channel to Blanco Valley (Duncan, 1968) and presence of ice dams and lakes, and the periodic along Cascadia Channel onto Tufts Abyssal flooding in the channel scabland area of the Plain. Coarse material was also deposited on upper Columbia River during the Pleistocene Astoria Fan, both in channel and interchannel (Bretz and others, 1956) would have produced areas (Nelson, 1968). The Pleistocene turbidity a greater Pleistocene contribution from this currents were much larger and more competent area. than those of the Holocene. The coarser material and greater percentages of sand and Sand Mineralogy silt in the sediments deposited during the The Holocene and Pleistocene sands of Pleistocene are evidence of this activity. Cascadia Channel and the adjacent abyssal plain are characterized by a Columbia River Holocene heavy mineral assemblage (Duncan, 1968). In response to the world-wide melting of When compared to the discharge of other glaciers at the end of the Pleistocene epoch, drainage areas of the Pacific northwest, it is numerous changes occurred which greatly af- obvious that the load of this river and its tribu- fected sediment dispersal and depositional taries should dominate the terrigenous sedi- processes. The rising sea covered the continen- ments of the adjoining marine environment. tal shelf, drowned river mouths, and decreased Most of the Tertiary andesites and Pliocene stream gradients. Far less material is at present to Recent basalts of the Cascades and the vast transported to the abyssal environment. The Miocene tholeiitic basalts of the Columbia lower supply of terrestrially derived material Plateau region occur in the drainage system to the continental shelf restricts the generation of the Columbia River (Waters, 1955). These of turbidity currents which are capable of rocks provide a detrital mineral assemblage transporting coarse-grained sediment to the characterized by the dominance of clinopy- deep-sea. This decrease in terrigenous sand-silt roxene over hornblende. deposition has occurred in Astoria Canyon and on upper Astoria Fan since the Pleistocene SEDIMENT DISPERSAL (Carlson, 1968; Nelson, 1968). Channels which cross the lower fan and join Cascadia Channel Pleistocene have received only hemipelagic sediments The Pleistocene climate created a set of con- since the Pleistocene. ditions on the continent different from those of The distribution of Mt. Mazama ash and Holocene time. Extensive continental glacia- radiocarbon dating demonstrate that Cascadia tion and the lower sea-level stand resulted in Channel and its northern tributaries have been SEDIMENT DISPERSAL 1381 active during Holocene time. An important 1967) and by investigations of the radionu- factor here is the direction of sediment move- clides associated with sediments from the ment from the Columbia River mouth. A net Columbia River (Gross, 1966; Gross and Nel- north and westward transport across the shelf son, 1966). Much of the material appears to (Fig. 17) is indicated by textural studies of con- come to rest at the head of Willapa Canyon. tinental shelf sediments (Gross and others, The continuity in physiography, gradient, and

Routes of Holocene sediment transport \

Shelf movement of Columbia River sediment discharge

Postulated accumulation site for sediment

Figure 17. Holocene dispersal pattern and postulated accumulation site for Columbia River sediment discharge (supplemented by data from Duncan, 1968, and Gross and Nelson, 1966). 1382 GRIGGS AND KULM—SEDIMENTATION, CASCADIA DEEP-SEA CHANNEL sedimentation between Cascadia Channel and groups of organisms within the sediments has the Willapa Canyon-Channel systems indicates led to the development of a combination strati- that the Holocene olive-green silts originated graphic column and paleoclimatic curve. in Willapa Canyon. The earliest late Pleistocene event recorded The present day Columbia River sediment in the sediments was the deposition of ice- discharge is 12.2 million m3/yr. The bottom rafted material derived from the Cordilleran load is 1.4 million m3, and the suspended load glacier complex. The Columbia River, how- amounts to 10.8 millionm3 (Lockett, 1965; U.S. ever, was the major sediment source during Army Engineers, 1962). Carlson (1968) com- Pleistocene time. Most of the material brought puted a littoral current contribution of about to the ocean was sand, silt, or clay, but gravel 2.4 million m3/yr. Most of the suspended ma- was also transported down the river and terial (silt and clay) probably moves offshore through Cascadia Channel. With sea level with the plume of the river which extends to lowered approximately 130 m, the course of the southwest during peak discharge (late the lower Columbia was probably such that it spring and early summer) and to the north emptied almost directly into Willapa Canyon, and west for the remainder of the year (Oster- providing a continual route for sediment dis- berg and others, 1965). The sand, which con- persal across Cascadia Basin. Large volumes stitutes much of the bottom load, is at present of coarse material were deposited by the tur- trapped in the Columbia estuary (Lockett, bidity currents active in all of the channel, fan, 1965). If only 25 percent of the combined and abyssal plain environments. Levees were bottom load and littoral drift, or 10 percent constructed during this period along the north- of the suspended load (1 million m3/yr), or west side of upper Cascadia Channel by turbid- some combination totaling this volume, eventu- ity current overflow. ally comes to rest in the vicinity of Willapa About 12,500 years ago, the Holocene began Canyon, 550 million m3/yr of sediment would as the glaciers started to retreat and sea level accumulate in the average recurrence interval began to rise. The suspended load of the Colum- (550 years) between turbidity currents in bia River continued to fan out and settle over Cascadia Channel. Considering only that por- Cascadia Basin; much of the bottom load was tion of the channel studied and assuming uni- trapped in shallow water. With the exception formity of sediments between cores, a volume of Cascadia Channel and its northern tributar- of 525 million m3 of sediment would result from ies, which are almost directly connected to an average flow. There appears to be sufficient continental sediment sources, only hemipelagic silt and clay available from the Columbia clay has been deposited in Cascadia Basin dur- River discharge to supply the postulated tur- ing the Holocene. Much of the Columbia bidity currents with sediment. The high seis- River traction load has been moving north micity of this area (Berg and Baker, 1963) and during this period, and coming to rest in the the atmospheric conditions of the northeast vicinity of the head of Willapa Canyon. Peri- Pacific produce periodic earthquakes or severe odically the accumulating sediment mass be- storms capable of generating turbidity currents comes unstable and, with a disruption, begins from a sediment mass on the upper continental to move as a turbidity current, down Willapa slope. Canyon and through Cascadia Channel. The flows are very uniform and have extended over 650 km along the channel axis. CONCLUSIONS: GEOLOGIC HISTORY Sediments deposited by these flows are finer Cascadia Deep-Sea Channel and its tributar- grained, more poorly sorted, and have a higher ies form an extensive submarine drainage organic carbon content than their Pleistocene system in the northeast Pacific Ocean with a counterparts. Along the upper channel, flows combined length of over 2200 km. The nature more than 100 m high topped the banks, de- of the sediments deposited within the channel positing sediment as far as 17 km laterally and basin environments has been strongly away from the channel. Longitudinal changes influenced by climatic conditions. Hemipelagic in the thickness, grain size, and structures of the sediments high in planktonic Foraminifera channel deposits indicate that the traction load characterize the glacial periods of the Pleisto- of the turbidity currents was deposited pri- cene, while Radiolaria-rich sediments charac- marily along the upper and middle channel, terize the interglacial periods and the Holo- while deposition from suspension became more cene. The relative abundances of these two important along the lower channel. REFERENCES CITED 1383

Turbidity currents have been passing R. Duncan. Appreciation is also extended to through the channel at approximately 500 year the staff members and the students of the De- intervals. However, a structural depression partment of Oceanography, Oregon State formed about 4000 to 5000 years ago within the University, including the personnel of the Blanco Fracture Zone has prevented the recent R/V Yaquina who helped with the collection flows from extending onto Tufts Abyssal Plain. of the data at sea. This research was made possi- Calculated flow velocities of 3.3 to 5.8 m/sec ble through the financial support of the Na- indicate that an average turbidity current tional Science Foundation (Grants GP-5076 would pass through the surveyed portion of the and GA-1246) and the Office of Naval Re- channel in about 2 days. search [Contract Nonr 1286 (10)]. A graduate fellowship from the National Science Founda- ACKNOWLEDGMENTS tion is gratefully acknowledged by G. B. The writers are grateful for helpful discus- Griggs. sions with Dr. Gerald A. Fowler and Dr. fohn

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