Acoustic stratigraphy, structure, and history of quaternary deposition in Cascadia Basin

Deep-Sea Research, 1973, Vol. 20, pp. 387 to 396. PergamonPress. Printed in Great Britain.

Acoustic stratigraphy, structure, and history of Quaternary deposition in Cascadia Basin* BOBB CARSON,"

(Received 20 January 1972; in revised form 21 October 1972; accepted i0 November 1972) Abstract--Seismic reflection profiles over northern Cascadia Basin, Juan de Fuca Ridge, and Juan de Fuca Abyssal Plain exhibit a thick sedimentary sequence, consisting largely of turbidites, covering basement. Although most of the section is less than 0.7 × 106 years old, two periods of tectonism and four stratigraphic units can be identified. The two lower units once extended from the continental margin across the basin, covering the north end of Juan de Fuca Ridge and the area west to Explorer Ridge as an abyssal plain. The principal dispersal route at this time, Juan de Fuca Channel, trended roughly north-south on the western side of Cascadia Basin. Block faulting on northern Juan de Fuca Ridge and later, broad upwarping of sediments in the western half of the basin altered this dispersal system, isolating Juan de Fuca Abyssal Plain from subsequent turbidite deposition and shifting the primary thalweg of Cascadia Basin to its present position along Vancouver Valley. Nitinat Fan began to develop during the most recent sedimentary regime, perhaps in response to increased turbidity current activity on the continental margin. The present basin morphology can be attributed largely to topographic control of turbidite dispersal and deposition. The topography, in turn, has been modified by tectonic movements on or near northern Juan de Fuca Ridge.

INTRODUCTION

TURmDITE dispersal and deposition in the deep sea may be controlled by bottom morphology. MENARD (1955), HAMILTON(1967), VAN ANDEL and KOMAR(1969), and others have cited damming and ponding of turbidites behind ridges and within basins as evidence of topographic control. It follows from such studies that entire turbidite dispersal systems and their associated sedimentation patterns adjust to the areal topography of a region. It can be further inferred that such systems may be disrupted and altered by tectonic modification of the bottom morphology, as is observed in Cascadia Basin. The sediments in northern Cascadia Basin (Fig. 1) lie between Juan de Fuca Ridge, a spreading center (VINE, 1966; ISACKS,OLIVER and SYKES, 1968; PITMAN, HI~RRON a n d HEIRTZLER, 1968), and the continental margin off Washington and Vancouver Island, which has been described as a filled trench (HAYES and EWJNG, 1971) and a zone of underthrusting (ATWATER, 1970; SILVER, 1971; SILVERand BARNARD, 1971). As in other areas of inferred subduction (SCHOLL, VON HUENE and RIDLON, 1968; VON HUENE and SHOR, 1969), the turbidites forming the continental rise show little or no deformation related to spreading. Local tectonic activity, however, on northern Juan de Fuca Ridge (MCMANUS, HOLMES, CARSON and BARR, 1972), Sovanco Fracture Zone, and in the basement underlying Cascadia Basin has had a pronounced effect on turbidite dispersal and sedimentation within the basin and in Juan de Fuca Abyssal Plain. *Contribution No. 680 from the Department of Oceanography, University of Washington. tPresent address: Department of Geological Sciences, Lehigh University, Bethlehem, Pa. 18015.

387

388 131 °

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Boaa CARSON 130'

129'

128 °

127 °

126"

125 °

124 °

126"

125"

124 °

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48'

47'

46

131"

130"

]zu-

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127"

Fig. 1. Topographic features of northern Cascadia Basin and adjacent areas. Contours in fathoms.* The sub-area outlines the region examined by MCMANuS, HOLMES,CARSONand BARR (1972). METHODS Seismic reflection profiles were obtained along 1650 km of track lines on R. V. Thomas G. Thompson cruises TT-48 and TT-53 (Fig. 2). The sound source was a 656 cm 3 Bolt air gun operating at approximately 2000 psi. The acoustic signals were fed through a variable band-pass filter, set to 40 to 80 Hz, and recorded on a modified Edo PBR 333 recorder, at a 5-second scale. L O R A N A navigation was employed on TT-48; on TT-53, navigation was by satellite. Selected profiles are presented in Figs. 3 to 7. Depths and layer thicknesses are given in two-way travel time, rather than distance, as the velocity of sound in the sediments is unknown and probably variable (MCMANUS, HOLMES, CARSON and ]3ARR. 1972). TOPOGRAPHY From its northern margin at Vancouver Gap, Cascadia Basin (Fig. I) widens generally to the south, bounded on the west by Juan de Fuca Ridge, and on the east by the Vancouver Island-Washington-Oregon continental slope. The northern portion of the basin is dominated by Nitinat Fan, an asymmetric feature with its apex located toward the north at 47°55'N, 126°25'W. Cascadia Channel, the major valley crossing *1 f a t h o m - 1.8288 m.

Acoustic stratigraphy, structure, and history of Quaternary deposition 131"

130"

129"

128"

127"

126"

125"

389 124"

1Jl" 130" 129" 12~" 12T 12U" 12b" 124" Fig. 2. POSitiOnof seismic profile lines taken during R. V. ThomasG. Thompsoncruises 48 and 53. Numbered profiles are illustrated in Figs. 3-7. Shading indicates regions where basement reflector is not recorded. Buried ridgesand basementdiscontinuities with relief in excessof 0.25sec are designated by solid bars. the fan, forms the seaward extension of Juan de Fuca and Nitinat canyons. The western margin of the fan is defined by Vancouver Valley, a deep-sea channel which trends roughly north-south across the middle of the basin. West of Vancouver Valley, the sediments form a gently undulating plain which is traversed, from north to south, by Juan de Fuca Channel. This broad channel originates near the north end of Juan de Fuca Ridge. North of the ridge Cascadia Basin extends as far west as 130°20'W. In this area, Revere Channel trends southwestward from Vancouver Gap to its apparent termination north of Sovanco Ridge. Sovanco Fracture Zone ancl Juan de Fuca Ridge separate Juan de Fuca Abyssal Plain from northern Cascadia Basin. The plain is small, extending from the fracture zone southwest to about 47°47'N (approximately 148 kin), and at least as far west as 130°55'W ('at 48°ll'N). No distinct channels are observed on the plain, and the bathymetric configuration north of Sovanco Fracture Zone suggests that no channels, at the present time, connect the plain with Cascadia Basin. The north end of Juan de Fuca Ridge is characterized by a tectonically controlled ridge and valley topography, which has been described in detail by MCMANUS, HOLMES, CARSON and BARR 0972). Further south, the ridge morphology is less pronounced, and is onlapped by Cascaclia Basin sediments.

390

BoBs CARSON SEISMIC S T R A T I G R A P H Y AND S T R U C T U R E

Acoustic basement is a strong reflector which outcrops on Juan de Fuca Ridge (as basalt) and can be traced over most of the study area, except where the sediment cover exceeds 1.5 to 2.0 sec (Fig. 2). Along the ridge, basement either forms the bottom topography (Fig. 3) or lies beneath a veneer of sediment (MCMANUS, HOLMES,CARSON and BARR, 1972, Fig. 6, leg 53-34). In either case it is rough, with as much as 2.0 sec relief on the north end of the ridge (MCMANUS, HOLMES, CARSON and BARR, 1972, Fig. 10). This decreases to less than 0.8 sec further to the south (Fig. 3). The basement highs are, apparently, discontinuous hills trending northeast-southwest and are often bounded by faults with similar strikes. West of Juan de Fuca Ridge basement lies approximately 0.7 sec beneath the surface of northeastern Juan de Fuca Abyssal Plain. Beneath Cascadia Basin, basement deepens toward the continental margin until it can no longer be traced under the thick wedge of sediments landward of Vancouver Valley (Fig. 2). Its relief in this region is generally less than 0.2 sec (Figs. 3, 5) although isolated ridges or sharp discontinuities may attain relief of 0.7 sec (Figs. 5, 6). Some of these highs apparently existed prior to burial, as evidenced by the lack of deformation in the overlying sediments (Fig. 5), while others have been emplaced, or at least undergone considerable vertical movement after deposition (Fig. 6). Most of these basement irregularities, far removed from the present ridge axis, lie near Vancouver Valley (Fig. 2), and have apparently exerted some control over the position of the valley. The highs are probably not continuous over great distances (note the lack of ridges on legs 48-2 and -6, Fig. 2). Turbidite units

The Holocene-late Pleistocene sediments in Cascadia Basin and on Juan de Fuca Abyssal Plain consist of turbidite sand-silts and intercalated lutites (DUNCANand KULM, 1970; CARSON, 1971). The numerous fiat-lying reflectors observed in the seismic profiles (Figs. 3, 5) the thickening of the deposits toward the continent (Fig. 6), and the lack of conformability of these sediments to the basement topography (Fig. 6) indicate that similar deposits comprise the underlying portion of the sedimentary sequence. Four stratigraphic units can be identified from the seismic profiles. These units are most easily delineated in the northern portion of the area (MCMANUS, HOLMES, CARSON and BARR, 1972, Fig. 5) and beneath Nitinat Fan (Fig. 7). In some parts of western Cascadia Basin, Units A (oldest) and D (youngest) cannot be positively identified. Hence, where the individual units cannot be differentiated, Units A and B are grouped (after MCMANUS, HOLMES, CARSON and BARR, 1972) as Lower Turbidites. and Units C and D as Upper Turbidites. 1. Lower Turbidites

A thick, acoustically transparent sequence (Unit A) forms the lowermost section of the sediment column. It is most clearly seen overlying basement (McMANUS0 HOLMES, CARSON and BARR, 1972, Fig. 5) near the northern end of the basin. Its transparent nature has been cited (HAYES and EWIN6, 1971) as evidence that these basal sediments are pelagic. More recent studies (VoN HUENE, KULM, DUNCAN, INGLE, KLING, MUSICH, PIPER, PRATT, SCHRADER,WESERand WISE, 1971 ; MCMANUS,

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Fig. 3. Seismic reflection profiles TT-48, legs 5 and 6. X indicates inferred position of Juan de Fuca Channel in lower turbidites. 1: Acoustic Basement; 2: Lower Turbidites (Units A, B); 3: Lower/Upper Turbidite Contact; 4: Upper Turbidites (Units C, D). Vertical exaggeration: < 20.

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Zig. 4. Seismic reflection profile TT-48, lug 1. 1: Acoustic Bascment; 2: Lower Turbidites Units A. B); 3: Lower Upper Turbidite Contact: 4: Upper Turbidites (Units 127, D). Vertical exaggeration : 20.

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Zig. 5. Seismic reflection profiles TT-48, legs 9 and 10. 1 : Acoustic Basement; 2: Lower Turfidites (Units A, B); 3: Lower Upper Turbidite Contact; 4: Upper Turbidites (Units C, D). Vertical exaggeration : 20.

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Fig. 6. Seismic rettection profile TT-53, leg 59. X indicates position of Juan de Fuca Channel in Unit B. 1: Acoustic Basement; 2: Lower Turbidites (Units A, B); 3: Lower, Upper Turbidite Contact; 4: Upper Turbidites (Units C, D). Vertical exaggeration: 11 (after McMANUS, HOLMES, CA~SOY and BARR, 1972).

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Fig. 6. Seismic rettection profile TT-53, leg 59. X indicates position of Juan de Fuca Channel in Unit B. 1: Acoustic Basement; 2: Lower Turbidites (Units A, B); 3: Lower, Upper Turbidite Contact; 4: Upper Turbidites (Units C, D). Vertical exaggeration: 11 (after McMANUS, HOLMES, CA~SOY and BARR, 1972).

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Acoustic stratigraphy, structure, and history of Quaternary deposition

391

HOLMES, CARSON and BARR, 1972), however, indicate that Unit A is composed largely of turbidites. A series of strong, thin, nearly parallel reflectors (unit B) overlies the transparent sequence (Fig. 5). On leg 30 (MCMANUS, HOLMES,CARSON and BARR, 1972, Fig. 5), both have been upwarped by basement uplift. Indeed, these deposits have been deformed by tectonic activity over much of the area, and are characterized by folding, and small-scale faulting. The Lower Turbidites are present over much of northernmost Cascadia Basin, northern Juan de Fuca Ridge, and Juan de Fuca Abyssal Plain. Only the northeastern end of Sovanco Ridge, Northeast High (Fig. 4), and the southern ends of Middle and West ridges are devoid of these reflectors. Beneath Nitinat Fan (Figs. 5, 7) they are thick, undisturbed, and nearly horizontal, while west of Vancouver Valley (Figs. 3, 6) they are thin (< 0.5 sec) and there is evidence of post-depositional deformation. Near the southern boundary of the area (Fig. 5), the Lower Turbidites may be present but, due to the lack of folding or faulting, are difficult to identify. Unit A is more restricted areally than Unit B, lensing out against basement south of Vancouver Gap (MCMANUS, HOLMES, CARSON and BARR, 1972, Fig. 5) and becoming very thin or absent in western Cascadia Basin south of about 48°N (Fig. 3). The upper portion of Unit A, however, does extend into the north end of Middle Valley (MCMANUS, HOLMES, CARSON and BARR, 1972) which lies on the central magnetic anomaly associated with Juan de Fuca Ridge. This fact allows a maximum age of 0.7 × 106 yr to be placed on the upper portion of Unit A and all subsequent deposits. Unit B, which is more easily recognized than Unit A, extends across the northern end of the basin and on to Juan de Fuca Abyssal Plain. It can also be distinguished farther south in western Cascadia Basin (Fig. 3), and traced beneath Nitinat Fan (Fig. 7). Like Unit A, Unit B dips and thickens landward (Fig. 6) east of Vancouver Valley, suggesting relative, broad downwarping parallel to the continental slope. Channel deposits are rarely observed in the Lower Turbidites. Only Juan de Fuca Channel (Fig. 6, X; and, possibly, Fig. 3, leg 48-5, X) and an un-named valley southwest of Sovanco Fracture Zone can be found in Unit B. Juan de Fuca Channel has migrated westward from its position in the Lower Turbidites to its present location (Fig. 6, X), indicating that it was a major north-south dispersal route for turbidites through much of Units B- and C-time. The lack of channel reflectors in other parts of the basin suggests that Juan de Fuca Channel may have been, in fact, the dominant north-south dispersal system. Alternatively, lack of similar structures elsewhere in the basin may be an artifact of the extensive deformation exhibited by the Lower Turbidites. Subsequent to deposition of Unit B, tectonic activity along the crest of Juan de Fuca Ridge (MCMANUS, HOLMES, CARSON and BARR, 1972) and, apparently, also on its flanks (Figs. 3, leg 48-5 ; 6), gave rise to faulting of the basement and vertical movements resulting in the rough topography of northern Juan de Fuca Ridge and Sovanco Fracture Zone. This activity deformed the Lower Turbidites and produced a new topographic configuration upon which younger turbidites were deposited. 2. Upper Turbidites The Upper Turbidites (Units C and D) are generally horizontal beds which blanket northern Cascadia Basin and Juan de Fuca Abyssal Plain. The major portion of these

392

BoaB CARSON

deposits (Unit C) consists of parallel horizons which vary from weak (Fig. 4) to strong (Fig. 3, leg 48-5) reflectors. Beneath Nitinat Fan these deposits take on a hummocky, discontinuous appearance (Fig. 7, legs 48-3, -4). The uppermost horizon (Unit D) is associated with the development of Revere Channel, Vancouver Valley, and, possibly, Cascadia Channel, Nitinat Valley, and upper Nitinat Fan. These obvious channel structures are not related to similar features in Unit C and, therefore, it is inferred that Unit D is a distinct stratigraphic sequence. In most interftuve regions, Units C and D are indistinguishable and are considered together as Upper Turbidites. At the northern end of Cascadia Basin, the Unit C turbidites have onlapped the upwarped portions of the Lower Turbidites southeast of Paul Revere Ridge and on Sovanco Ridge (MCMANUS, HOLMES,CARSON and BARR, 1972, Fig. 5). The unconformable, Unit C-Lower Turbidite contact is also evident where the younger deposits abut the bases of East, Middle, and West ridges, and beneath Nitinat Fan (Fig. 5). Locally, the accumulation of these deposits can be extensive, as in Middle Valley where they constitute the major portion (> 2.0 sec) of the sediment fill (MCMANUS. HOLMES, CARSONand BARR, 1972). On Juan de Fuca Abyssal Plain, the Upper Turbidites form a thin (0. 12-0.15 sec) veneer over the thicker (up to 0.60 sec) Lower Turbidites. The lack of channel development, the 'ponding' of Revere Channel north of Sovanco Ridge, and the contrasting heavy mineralogy (CARSON, 1971) of Cascadia Basin and Juan de Fuca Abyssal Plain, suggest that Unit D is not present on the plain, but this cannot be ascertained from the profiles. In that portion of northern Cascadia Basin south of 48 °30'N, the Upper Turbidites blanket the basin from the continental slope to Juan de Fuca Ridge (Figs. 3, 5, 6, and 7). Like the Lower Turbidites, the upper deposits increase in thickness landward, achieving their maximum thickness (1.05 sec) beneath Nitinat Fan. This wedge-like accumulation reflects not only increased deposition (and subsidence) near the continent, but also uplift west of Vancouver Valley. In this latter region, the Unit C deposits have been broadly upwarped and elevated (Fig. 6) relative to the deposits landward of Vancouver Valley. Perhaps in partial response to this upwarping, the youngest turbidites (upper Unit C-Unit D) have accumulated less rapidly in the western half of the basin than elsewhere. Beneath Nitinat Fan, the Upper Turbidites consist of two sequences of strong reflectors (Fig. 7, legs 48-4, -7). The lower horizon roughly parallels Unit B, while the upper sequence bifurcates from the lower beneath the upper fan, and rises to intersect Cascadia Channel. Lying above this upper horizon is a lens of transparent material which forms upper Nitinat Fan. This lens, which is apparently associated with the development of Cascadia Channel, may represent Unit D on Nitinat Fan, although no positive correlation can be made with Unit D deposits elsewhere. DEPOSITIONAL

PATTERNS

AND

HISTORY

The early depositional history in the area is difficult to recount in any detail, due to deformation of the sediments by tectonic activity. Nevertheless, a broad reconstruction can be given. UnitA probably represents the uppermost portion of the sedimentary sequence which filled the Tertiary trench (ATWATER, 1970) Off western North America. The Unit A deposits spread, as a continuous plain, from the continental

Acoustic stratigraphy, structure, and history of Quaternary deposition

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margin over the northern end of Cascadia Basin, Juan de Fuca Abyssal Plain, eastern Cascadia Basin (Nitinat Fan), and, perhaps, some parts of western Cascadia Basin (although south of about 48°N, Unit A becomes exceedingly thin, if, indeed, it is present west of Vancouver Valley). Several basement highs (Sovanco Ridge, Northeast High, and Juan de Fuca Ridge south of 48°30'N) were not covered by Unit A deposition. Unit B spread over Unit A in Cascadia Basin and onlapped Juan de Fuca Ridge across the entire study area. Deposition of the Upper Turbidites has not significantly altered the western boundary of sediment fill in Cascadia Basin from that defined by Unit B. While the distribution of the Lower Turbidites implies a general dispersal pattern from northeast to southwest, the only channel observed (within Cascadia Basin) in these deposits, Juan de Fuca Channel, trends nearly north-south and lies near the western margin of the basin. The magnitude of the subsurface valley ( ~ 2 km in width, Fig. 6, X) suggests that Juan de Fuca Channel may have been the major dispersal route across the abyssal plain formed by the Unit B deposits. The existence oF this major channel on the west side of Cascadia Basin, in any case, requires a northern source and/or subsidiary channels feeding into it from the continental margin to the east. Neither possibility can be discounted as tectonic movement on the northern end of Juan de Fuca Ridge has obscured the northern reaches of Juan de Fuca Channel, while the predominantly east-west profiles upon which this study is based cannot delineate like-trending channels which may have fed into Juan de Fuca Channel from the east. A well-developed valley south of Sovanco Fracture Zone in the Lower Turbidites suggests that a major dispersal route fed Juan de Fuca Abyssal Plain during Units Aand B-time. The origin and course of this system, however, are unknown. The deposition of Unit B was followed by a period of tectonic activity (Table 1) which resulted in the fracturing and uplift of basement on Juan de Fuca Ridge, deformation of the Lower Turbidites in the most northern and western portions of Cascadia Basin, and (relative) downwarping of these deposits along the continental margin. These vertical movements appear to have had a major effect on the subsequent turbidite dispersal and depositional patterns within the basin, as evidenced by the major discontinuity in stratification between the Upper and Lower Turbidites. This discontinuity can be traced, near the base of the continental slope (William Barnard, personal communication), to Astoria Fan where it has been dated as middle to late Pleistocene (VoN HUENE, KULM, DUNCAN, INGLE, KLING, Muslcn, PIPEI',, PRATT,SCHRADER,WESERand WlsE, 1971). Extrapolation of Pleistocene sedimentation rates (CARSON,1971) suggests that the age of the discontinuity and, hence, the age of the tectonic activity in the northern portion of the basin, is somewhat less than 0"5 x 106 yr. Uplift of basement near the northern margin of the basin and vertical movement along Sovanco Fracture Zone (MCMANUS, HOLMES,CARSONand BARR, 1972), at this time, may have sharply reduced the supply of terrigenous sediment to Juan de Fuca Abyssal Plain. This hypothesis is supported by the rather thin (0-15 sec) accumulation of Upper Turbidites on the plain. Their presence, however, indicates that some deposits continued to reach the plain. Concurrently, Middle Valley was slowly subsiding (MCMANUS, HOLMES, CARSON

394

BOBB CARSON

Table 1. Age (x 106 yr)

Summary of acoustic stratigraphy and tectonic history.

Stratigraphic unit

Upper Turbidites

< 0'5*

< 0.7?

Lower Turbidites

tl

Structuralboundary

Tectonic activiO'

Broad upwarping in northwestern Cascadia Basin

Middle-Late Pleistocene Block-faulting on northern Discontinuity Juan de Fuca Ridge. Uplift and deformation of sediments in northernmost portions of basin. Relative upwarping in western Cascadia Basin.

(B tA

*Based on extrapolation of core-determined late Pleistocene sedimentation rates (CARSON,1971). ?Based on relationship of sediments to central magnetic anomaly (McMANuS, HOLMES,CARSON and BARR, 1971). and BARR, 1972), thus providing a depositional sink for sediments funneled into it from the north and northeast. Assuming an accumulation of 2-0 seconds of Upper Turbidites in Middle Valley (and a sonic velocity of 2-0 km/sec), more than 1600 km 3 of Upper Turbidites have been deposited in this graben. Some sediments continued to move north-south along Juan de Fuca Channel during Unit C time, as turbidites continued to fill the basin and the channel slowly migrated westward near northern Juan de Fuca Ridge (Fig. 6, X). It is not clear, however, whether the source of the channel, during Unit C time, lay to the north at Vancouver Gap or farther to the east on the continental margin. The latter possibility seems more reasonable as the Middle Valley graben probably consumed much of the sediment transported south from Vancouver Gap. Parallel reflectors beneath Nitinat Fan (Fig. 7) indicate that deposition through much of Unit C time at this site was similar to the accumulation during Unit B time. The essentially horizontal nature of the deposits suggests that no fan was present or that it was poorly developed and much smaller than the modern fan. In late Unit C time, or possibly with the advent of Unit D, however, intense deposition began on Nitinat Fan, resulting in a rapidly thickening sequence of sediment and intersecting reflectors (Fig. 7, legs 4, 7). This structural configuration contrasts with the uniform wedge of nearly parallel deposits observed elsewhere in the basin and beneath the Upper Turbidites on Nitinat Fan. Fan deposition, primarily upbuilding near the continental margin, is evidenced by an inclined reflector which intersects Cascadia Channel, and is presumably, closely related to the establishment of the channel. A similar (upbuilding) constructional pattern associated with a fan valley has been observed by CURRAY and MOORE (1971) on the Bengal Deep-Sea Fan. The development of Nitinat Fan undoubtedly began in response to intense

Acoustic stratigraphy, structure, and history of Quaternary deposition

395

turbidity current activity in Juan de Fuca and Nitinat canyons and, perhaps, to further tectonic modification of Cascadia Basin. The cause of accelerated turbidity current activity is a matter of conjecture, but it may be related to the excavation of the glacial trough which connects Juan de Fuca Canyon with the Strait of Juan de Fuca. Unit D deposition was initiated by broad upwarping (perhaps related to continuing tectonism on Juan de Fuca Ridge) of the northern end of western Cascadia Basin (Fig. 6) which led to the abandonment of Juan de Fuca Channel. The relict nature of the channel is substantiated by heavy mineral studies and extremely low Late Pleistocene-Holocene accumulation rates (CARSON, 1971). Uplift in western Cascadia Basin displaced the basin's primary thalweg eastward to its present position along Vancouver Valley. The exact position of Vancouver Valley has apparently been determined by ridges and basement discontinuities (Fig. 2), which have inhibited westward migration of the channel, and by seaward construction of Nitinat Fan. The position of the valley, in turn, has controlled the shape of Nitinat Fan and led to its asymmetric development by forming a barrier to turbidity currents traversing the fan. Hence, turbidity currents moving down the west and northwest flanks of the fan have been arrested and channelized in Vancouver Valley only 70 or 90 km from the apex of the fan, while flows on the southwest flank may extend 260 km before encountering the valley. Vertical movement related to those which broadly upwarped the basin sediments may have also ponded Revere Channel. The termination of this Unit D channel at Sovanco Ridge suggests that density current transport to Juan de Fuca Abyssal Plain was cut off by uplift along the Sovanco Fracture Zone. Such movement, however, cannot be confirmed on the basis of the seismic profiles. CONCLUSIONS Tectonic modification of bottom topography can result in realignment of deep-sea turbidite dispersal systems and accumulation patterns. In northern Cascadia Basin, several distinct depositional systems can be recognized, each apparently initiated or partially controlled by tectonic movements on or near Juan de Fuca Ridge. Localized block-faulting on the northern end of the ridge and uplift along Sovanco Fracture Zone have resulted in diversion or disruption of previous dispersal systems, while broad, regional uplift on the western side of the basin has displaced the thalweg (and, hence, the major dispersal route) of the basin to the east. As a result, the present basin topography represents a combination of tectonically-induced features and both relict and modern depositional morphologies. This topographic configuration has formed over the relatively short period of a few hundred thousand years. Acknowledgments--The author thanks Dr. D. A. MCMANUSand Mr. WILLIAMBARNARDfor their helpful suggestions and discussions throughout this study. This research was supported by the Office of Naval Research, Contract Nonr~77 (37), Project NR 083 012.

REFERENCES ATWATER T. (1970) Implications of plate tectonics for the Cenozoic tectonic evolution of western North America. Bull. geol. Soc. Am., 81, 3513-3536. CARSONB. (1971) Stratigraphy and depositional history of Quaternary sediments in northern Cascadia Basin and Juan de Fuca Abyssal Plain, Northeast Pacific Ocean. Ph.D. Dissert., Univ. of Washington, Seattle, Wash., 249 pp.

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