Structure and sediment distribution in the western Bering Sea

Marine Geology, 24 (1977) 309--320 ©Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

STRUCTURE AND SEDIMENT DISTRIBUTION BERING SEA*

IN THE WESTERN

PHILIP D. RABINOWITZ and ALAN COOPER

Lamont-Doherty Geological Observatory of Columbia University, Palisades, N. Y. 10964 (U.S.A.) U.S. Geological Survey, Menlo Park, Calif. 94025 (U.S.A.). (Received November 30, 1976; revised and accepted February 14, 1977)

ABSTRACT Rabinowitz, P.D. and Cooper, A., 1977. Structure and sediment distribution in the western Bering Sea. Mar. Geol., 24: 309--320. Eleven seismic reflection profiles across Shirshov Ridge and the adjacent deep-water sedimentary basins (Komandorsky and Aleutian Basins) are presented to illustrate the sediment distribution in the western Bering Sea. A prominent seismic reflecting horizon, Reflector P (Middle-=Late Miocene in age), is observed throughout both the Aleutian and Komandorsky Basins at an approximate subbottom depth of 1 km. This reflector is also present, in places, on the flanks and along the crest of Shirshov Ridge. The thickness of sediments beneath Reflector P is significantly different within the two abyssal basins. In the Aleutian Basin, the total subbottom depth to acoustic basement (basalt?) is about 4 km, while in the Komandorsky Basin the depth is about 2 km. Shirshov Ridge, a Cenozoic volcanic feature that separates the Aleutian and Komandorsky Basins, is an asymmetric bathymetric ridge characterized by thick sediments along its eastern flank and steep scarps on its western side. The southern portion of the ridge has more structural relief that includes several deep, sediment-filled basins along its summit Velocity data from sonobuoy measurements indicate that acoustic basement in the Komandorsky Basin has an average compressional wave velocity of 5.90 km/sec. This value is considerably larger than the velocities measured for acoustic basement in the northwestern Aleutian Basin (about 5.00 km/sec) and in the central Aleutian Basin (5.40--5.57 km/sec). In the northwestern Aleutian Basin, the low-velocity acoustic basement may be volcaniclastic sediments or other indurated sediments that are overlying true basaltic basement. A refracting horizon with sin~ilar velocities (4.6--5.0 km/sec) as acoustic basement dips steeply beneath the Siberian continental margin, reaching a maximum subbottom depth of about 8 km. The thick welt of sediment at the base of the Siberian margin may be the result of sediment loading or tectonic depression prior to Late Cenozoic time. INTRODUCTION During the summer of 1971, a marine geological and geophysical reconnaiss a n c e s u r v e y w a s m a d e b y R V " R o b e r t D. C o n r a d " ( C r u i s e s 1 4 0 5 a n d 1 4 0 6 ) i n t h e w e s t e r n B e r i n g Sea. O v e r 7 0 0 0 k m o f s e i s m i c r e f l e c t i o n d a t a a n d 3 3 *Lamont-Doherty Geological Observatory Contribution No. 2469.

310 sonobuoy measurements were obtained in the northern Aleutian Basin, Komandorsky Basin, and Shirshov Ridge areas, using a 40-in. 3 airgun seismic source. In this report, the seismic reflection profiles and the sonobuoy refraction and wide-angle reflection results collected during these cruises are presented. Previous seismic reflection profiler investigations in the area are limited to those on the eastern Bering Sea margin (Scholl et al., 1968), Bowers Ridge (Ludwig et al., 1971b), the interplain channel between Bowers and Shirshov Ridges (Rabinowitz, 1974), Bowers Basin and the eastern Aleutian Basin (Ewing et al., 1965), the preliminary DSDP site surveys (Fornari et al., 1973), and two traverses of the western Bering Sea (Ludwig et al., 1971b). The latter two crossings are included in the present compilation. The previous seismic refraction studies in the Bering Sea include the two-ship refraction measurements in the eastern Aleutian Basin (Shor, 1964) and across the Bowers Basin and Bowers Ridge (Ludwig et al., 1971a), a line of sonobuoy stations extending from the eastern Aleutian Basin across Bowers Basin and into the Komandorsky Basin (Houtz et al., 1970; Ludwig et al., 1971a), high-resolution (upper I km subbottom) sonobuoy stations throughout the Bering Sea Basin (Hamilton et al., 1974) and the anistropy refraction project of Shor and Fornari (1976) in the Komandorsky Basin. The refraction data presented in this report are in areas that have not been covered by these previous investigations. GEOLOGIC SETTING The Bering Sea is a back-arc marginal basin of the Pacific Ocean characterized by an oceanic crustal section and thick overlying sedimentary layers. This deep-water sedimentary basin contains in excess of 1--2 km of Cenozoic sediments, is subdivided by two major submarine volcanic ridges (Bowers and Shirshov Ridges) and is bordered on all sides by structural features of Early Tertiary age or older. Recent comprehensive reviews of the Bering Sea region are given by Gnibidenko (1973) and Scholl et al. (1975). The two basins that are of primary concern to the present investigation, the Aleutian and Komandorsky Basins, are depositionally isolated from one another by Shirshov Ridge (Fig.l). This linear bathymetric feature is presumed to be a pre-Early Miocene ridge of volcanic origin. Scholl et al. (1975) sampled lithified andesitic tuffs and albitized palagonite tuff (K--Ar age of 16.8 m.y.) at the southern end of the ridge and along the west flank of the ridge about 75 km east of DSDP Site 191. The alignment of Shirshov Ridge with onshore Mesozoic volcanic rocks to the north is interpreted as evidence that the ridge was initiated in Early Tertiary time along the continuation of the onshore Late Mesozoic volcanic belts (Scholl et al., 1975). The Aleutian Basin, which contains in excess of 3--4 km of sediment (Shor, 1964; Ludwig et al., 1971a, b; Cooper et al., 1976a, b) has been interpreted as being formed during the Late Cretaceous--Early Tertiary by the entrapment of a piece of oceanic plate behind the newly developing

311 160 °

170 °

180 °

170 °

60 °

55 °

160 °

170 ° E

180°

50 ° 170 °w

Fig.1. Index map of Bering Sea Basin. Numbers along ship's tracks adjacent to filled circles refer to sonobuoy stations. Darkened lines A through K are seismic reflection sections shown in Fig.2. Tracks E and G are from RV " V E M A " Cruise 21 and RV, " R o b e r t D. Conrad" Cruise 12 respectively (Ludwig et al., 1971b). All other track lines are from RV " R o b e r t D. Conrad" Cruise 14. Stars denote locations of DSDP drill holes.

Aleutian arc (Shor, 1964; Ludwig, 1974; Scholl et al., 1975; Cooper et al., 1976c). Sediments within the basin have been cored at DSDP Sites 189 and 190 (Fig.l). A semi-consolidated sequence of turbidites interbedded with diatomaceous oozes of Late Pliocene age is found overlying a more lithified sequence of mudstones and coarser terrigenous debris of Late Miocene age (Creager et al., 1973). The deepest penetration of these t w o drill holes was 850 m and b o t t o m e d in lithified mudstones. Based on interval velocity data and extrapolation of sedimentation rates in the Aleutian Basin and eastern Bering shelf, the age of the oldest sediments overlying acoustic basement is presumed to be Early Tertiary or Cretaceous (Scholl et al., 1968; 1975). Cooper et al., (1976a) have suggested that the observed magnetic anomalies in the Aleutian Basin are part of the Mesozoic sequence and hence assign an Early Cretaceous age for the basement rocks. No basement rocks have been recovered b y deep drilling in the Aleutian Basin. In the K o m a n d o r s k y Basin, tholeiitic basement rocks of Middle Oligocene age (29.6 m.y.; K--Ar dating) have been recovered at DSDP Site 191 (Creager et al., 1973). A 900-m sequence of semi-consolidated Upper Pliocene

312 turbidites and presumably Upper Miocene lithified mudstone is found overlying the basement. The age of the tholeiite basalt and the generally thin cover of sediment in the basin suggest that the K o m a n d o r s k y Basin is a younger feature than the Aleutian Basin. The northwestern and northeastern continental margins bordering the Komandorsky and Aleutian Basins are underlain b y a series of structures that are subparallel with the margin. These structures contain mildly deformed eugeosynclinal rocks of Jurassic and Cretaceous age and are overlain b y varying thicknesses of Cenozoic sedimentary rocks (Scholl et al., 1976; Marlow et al., 1976). The expansive Bering shelf, which extends from the Shirshov Ridge to the Aleutian Ridge appears to be draped b y u n d e f o r m e d terrigenous and diatomaceous rocks (Main Layered Sequence, Scholl et al., 1968) similar to the Upper Pliocene and younger rocks found at DSDP Site 190. These well-layered rocks extend over the continental slope and appear to continue beneath the floor of the Aleutian Basin. REFLECTION PROFILES A compilation of line drawings made from eleven seismic reflection profiles across the western Bering Sea Basin is shown in Fig.2a, b. F o r clarity, the profiles have been aligned from north to south along the topographic axis of Shirshov Ridge. A complementary set of reflection profiles crossing the junction between Bowers and Shirshov Ridges as well as the main part of Bowers Ridge are available in earlier publications of Rabinowitz (1974) and Ludwig et al. (1971b), respectively. As noted by other investigators (Ewing et al., 1965; Scholl et al., 1968; Ludwig et al., 1971b), there is a major change in the acoustic properties of the sediments at a s u b b o t t o m depth of a b o u t 1 km (1 sec two-way travel time) in the Aleutian Basin. At this depth, there is a transition from a laminated sequence of reflectors to an acoustically homogeneous unit that has few traceable reflecting horizons. Ewing et al., (1965) refer to this horizon as Layer P; in our profiles, we adopt the same notation. Two traceable reflection horizons are observed within the upper 1 km of the sediment section (profiles A--H; Fig.2a, b) in the Aleutian Basin. Drilling at DSDP Site 190 (near profiles H and I; Fig.2b) indicates that the upper horizon (Horizon U} is an internal reflecting horizon within a well-bedded unit of diatomaceous silty clay and interbedded turbidites. The lower horizon, Layer P, coincides with the b o u n d a r y b e t w e e n the Late Miocene and younger diatomaceous sediments (v = 1.6 km/sec; DSDP Sites 189, 190) and the underlying mudstones of Middle Miocene and older age (v = 2.0--2.4 km/sec; DSDP Site 189). Although these two reflecting horizons appear to be nearly continuous in the reflection profiles, the interval velocities obtained from our s o n o b u o y data are quite variable within the upper part of the sediment section. Ludwig et al. (1971a) note a similar variation in the s o n o b u o y interval velocities along a line of refraction stations crossing the southwestern Aleutian Basin near DSDP Site 190.

313 Reflector P is also observed a b o u t 1 km beneath the sea floor of the K o m a n d o r s k y Basin. This reflector separates an overlying opaque sequence from a homogeneous acoustically transparent layer below the Aleutian Basin. The thickness of sediments overlying Reflector P is relatively uniform throughout most of the western Bering Sea even though the thickness of the total section is variable. The depth to the sea floor is similar in the basins on either side of Shirshov Ridge. However, the depth to basement is substantially greater in the Aleutian Basin. In the Komandorsky Basin, basement is observed at ~ 6.0--6.5 sec of two-way travel time; in the Aleutian Basin basement is at ~7.0--7.5 sec of two-way travel time. The sediments underlying Reflector P are thicker in the Aleutian Basin. The northern part of Shirshov Ridge (profiles A--F; Fig.2a) is characterized by an asymetric bathymetric shape with thick sediments draped over the eastern flank and steep basement scarps on the western flank. Farther south (profiles G--I; Fig.2b) the ridge has more rugged basement relief and is characterized by numerous large summit basins. A reflection horizon, perhaps Reflector P, is present beneath the flanks and the summit basins. Below this horizon, sediment thicknesses exceeding 1.5 sec of two-way travel time are observed in some perched summit basins. The ridge is much narrower in the southernmost profiles (profiles J and K; Fig. 2b), and significant sediment thicknesses are n o t observed over the crest of the ridge. SONOBUOY MEASUREMENTS The refraction and wide-angle reflection measurements were obtained using the airgun--sonobuoy m e t h o d described b y LePichon et al., (1968). Velocities and layer thicknesses are given for the Aleutian and K o m a n d o r s k y Basins (Figs.3, 4) and are summarized in Table I. Interval velocities are also given for the sediments in Fig.2a, b; refraction velocities are given for the layers in Fig.2a, b only if they were tangent to the wide-angle reflection. Refraction velocities for the acoustic basement in the Aleutian Basin tend to be higher in the central and southwestern parts of the basin (5.40-5.75 km/sec) and lower in the northwestern area (4.70--4.95 km/sec). At stations 60 and 61, in the central Aleutian Basin, refraction velocities of 5.40 and 5.75 km/sec are measured for the acoustic basement. A basement refraction velocity of 5.45 km/sec is found in the southwestern Aleutian Basin adjacent to Shirshov Ridge (station 95). This velocity is similar to those observed by Ludwig et al. (1971a) in nearby stations. In the northwest Aleutian Basin (stations 76 and 82), lower refraction velocities of 4.70 and 4.95 km/sec are associated with acoustic basement. Similar low velocities (4.60--5.00 km/sec) are observed in nearby stations 67--69 and 71--72; however, acoustic basement is n o t observed on the seismic reflection profiler records. At two stations in the northwest Aleutian Basin (stations 74 and 75), refraction velocities of 5.45 and 5.75 km/sec are observed beneath the acoustic basement. These velocities are similar to those measured for acoustic basement in the central Aleutian Basin. However, the refractions are n o t

314

tangent to basement reflections and therefore the higher-velocity layer must lie below acoustic basement observed on the seismic reflection profiler records. Many profiles in the Aleutian Basin have refraction velocities typical of oceanic Layer 3 (stations 59--63, 69, 73; mean velocity 6.60 km/sec). There is a large scatter in the data and the values range from 6.1 km/sec (station 62) to 7.1 km/sec (station 73). In the Komandorsky Basin, seismic refraction velocities associated with acoustic basement are considerably higher (range 5.65 to 6.35 km/sec; mean velocity of 5.90 km/sec) than the basement velocities observed in the Aleutian Basin (mean velocities of 5.0

SHIRSHOV RIDGE

A

0

B o

C >

D o

E e7

I

(a)

[ IO0

KM

50

KI~

I

I(WHERE

SO.BUOYS

OBTAINED)

315

SHIRSHOV RIDGE W

E

G

H

0 U I

d

IOO

, I WHERE

KM

I 50 KM SONOBUOYS OSTAINED]

~

(b) Fig.2a, b. Line drawings of seismic reflection profiler records aligned with respect to crest of Shirshov Ridge. The locations of the profiles and sonobuoy station are given in Fig.1. The vertical scale is in seconds of two-way reflection time (1 sec is approximately equal to 750 m in water). The horizontal scale varies with ship's speed (8--10 knots); where sonobuoy profiles are designated, the horizontal scale is expanded by approximately a factor of three. Velocities are from Table I. " U " and "P" are prominent reflection horizons. (Notations from Ewing et al., 1965.) a n d 5.5 k m / s e c ) . In t h e n o r t h w e s t e r n p a r t o f t h e A l e u t i a n Basin, t h e ref r a c t i n g h o r i z o n (v = 4 . 5 - - 5 . 0 k m / s e c ) p r e s u m e d t o b e a c o u s t i c b a s e m e n t descends rapidly beneath the continental margin (sonobuoys 67--70 and 7 1 - - 7 3 ) . A p p r o x i m a t e l y 8 k m o f s e d i m e n t a r y units are o b s e r v e d a b o v e t h e 4.6 a n d 5.0 k m / s e c r e f r a c t i o n h o r i z o n at t h e b a s e o f t h e c o n t i n e n t a l slope ( s o n o b u o y s 69 a n d 71). T h e r e a p p e a r s t o b e a s y s t e m a t i c increase in t h e velocities r e c o r d e d f o r the higher-velocity sediments (>2.90 km/sec) from the center of the Aleutian

316

65

65

64

62

61

60

59

0

i ALEUTIAN

NNW 1.99

zo_o 2.9T

201

1.9/

~o

225

BASIN

SSE

190 _ _

1.67

i.a7

.__

(3 2o) ~'(3"5°)*'" ~6

,.~

2~._.

2.24

--

2~2

2.84

3.05

2.95

.... /"

(3.45)

15.5013

2.10

4

---

6

5.56

3.92

? -'// 3.50 ii

2

150

,.7~, 2.69

2-~z..--" - -

--

-

/2"2272" --- ii//11/

(5.40)

(5.201"

( 6 . 5 5)

(6.40)

--

(4.20)

///./..

/ (6.10)""-

/"

( 5 . 0 0 ) "~ •//

/

•. . . . .

I0

(6.95)

12

(6.40)

KM 0

70

69

68

67

71

72

73

75

76

74

83

82

95

i

2

4 -

179

193

1198

2~

.......

(3.? 7)

--

I.4--9 200

1,64 LB_3 2,13

-... 2=~ ._.. ~

(385)

( 3 . 8 5 ) ~' ( 3 . 6 5 }

1.58 ~'74

251

236

-__

(4.00)

1.91

--__2.34 2.87

(4.00)-~.. 2~3

--'-

1.64

L80

1.72 1.68

3.75

"777272".. 1

6

(4.70)

2.59

227272"..

(4.95)~

l 1

2D4!

2.42 ..1/"r2:'~7

(495)* •

(4.95)* /,/

8

i '

180

-2,47

2.05 --

I"77"/7. 2.64 ,'7/777 (4.95)

1.68

2.70 ~ 22777 (5.45)

--/'i5.45) /"

I0 //

(5.00)

12

8:~

/

(6.80)

. (5.00)

(5.75)

(4.80)

// (4.80)

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(4.60)

(7.10t

( *

) UNREVERSED REFRACTIONMEASUREMENT ASSUMED VELOCITY BASEMENT ON SEISMIC REFLECTION RECORD. (

Fig.3. V e l o c i t y sections in the A l e u t i a n Basin for s o n o b u o y m e a s u r e m e n t s . All velocities and t h i c k n e s s e s tabulated in Table I.

Basin to the northwest cbrner of the basin (sonobuoys 59--62, 67, 69, 70). A similar systematic change is not observed in the Komandorsky Basin. Rather, another layer (4.10--4.30 km/sec) is present beneath the 2.96--3.12 km/sec layer in the northern Komandorsky Basin (compare stations 77--80 in the northern Komandorsky Basin with stations 87--92 in the central basin). DISCUSSION

The observation that Reflector P is present throughout both the Aleutian and Komandorsky Basins suggests that it represents a lithologic boundary that was caused by a major change in the depositional regime of the western Bering Sea Basin. Its age, estimated from DSDP drilling, coincides closely with the Middle--Late Miocene t.ime for uplift on the Aleutian Ridge and Middle Miocene--early Pliocene dates for deformation along the Siberian margin (Scholl et al., 1975). The thickness of the sedimentary layers overlying

317 KOMANDORSKY (KAMCHATKA)

77

BASIN

79

78

80

KM

--

0

2 _ 1.75

2.2---6 162 1.66 - - ;~10

1.72

4 (3.00)* (4.50)

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"'(4.,oi*

6

(4. I O) .,z/ill/

•/ ( 5 . 7 0 )

8

(5.65)

KM 0-2--

94

93

84

92

91

90

89

88

2101

2.0,5 . . . . . . --.I.61 -2.96 &12 " " -

87

tl.80)* (2.55)

(3.70) (5.50) 1.49

4--

1.87

1.65

2-~.. . . . . ~ o 3J6

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(5.85)

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(300)~

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1.51

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1,52

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(7.5o)

8-(

) UNREVERSED ASSUMED

////H

BASEMENT

REFRACTION

MEASUREMEN3

VELOCITY ON S E I S M I C

REFLECTION

RECORDS

Fig.4. Velocity sections in the Komandorsky Basin from sonobuoy measurements. All velocities and thicknesses tabulated in Table I.

Horizon P is relatively uniform and does not change over the thick sediment wedge observed in the northern Aleutian--Komandorsky Basins. This wedge and sediment at the base of the Siberian margin may be a result of sediment loading or a tectonic depression in pre-Middle Miocene time. The nearly uniform draping of the sediments above Horizon P reflects the high concentration of pelagic (diatomaceous) materials in the sediments of the central basin. The high pelagic c o n t e n t can also account for all the sediments in the perched basins of Shirshov Ridge. The sedimentary layers bordering the margins of the northwest Bering Sea Basin are substantially thicker in the Aleutian Basin than in the Komandorsky Basin. The thickness of the low-velocity (~1.6--3.0 km/sec) sediments is approximately the same near the continental margin in both basins; however, the thickness of the layers >13.0 km/sec is nearly twice as thick in the Aleutian Basin. Two possible explanations can be given for this difference. The sedimentation rates may have been different on either side of a pre-

1.50 1.67 1.79 1.90 1.91 2.01 1.99 1.83 1.49 1.93 1.79 2.31 1.58 1.64 1.64 1.91 1.80 1.75 1.72 1.62 1.66 2.05 1.80 1 . 8 0" 1.82 1.83 1.52 1.51 2.01 1.65 1.49 1.87 1.68

2.04 1.81 1.71 1.87 2.25 2.80 2.00 2.13 2.00 2.44 1.98 (4.00) 1.74 1.72 2.04 2.59 2.34 2.05 3.00* 2.26 2.10 2.64 2.47 ( 2. 55) 2.60 2.24 1.61 2.05 2.94 3.00* 2.34 2.30 2.70*

3.16 (5.45)

(5.90)

(3.70) 3.00* 3.00* 3.12 2.96

2.10 2.24 2.69 2.42 2.26 3.50* 2.97 2.36 2.66 ( 3. 85) (3.77) (4.80) 4.00* 1.68 2.42 4.95* 2.87 3.00* (3.80) 3.00* 3.00* (4.95)

3.20*

(5.50) ( 5. 57 ) ( 6. 35 ) 6.00* (6 .00)

(4.1 0) 4.10"

( 4. 60 ) 2.23 4.95* (5.75) ( 4. 70) (4.30)

2.84 2.62 3.56 3.92 3.50* ( 4. 20) 3.20* ( 3. 65) 3.85* (5 .00)

Us

(5.85)

(7.50)

(5.65) (5.70)

3.75 (5.45)

(3.45) (4.80) (5.00) (6.80)

2.95 3.05 (5.75) 5.50" 5.00*

U6

Vs

4.95*

(6.40)

(6.10)

(7.10)

(6.55)

(6.95)

(6.40)

5.20*

(5.40)

V~ 3.73 3.76 3.75 3.68 3.49 3.42 3.23 3.47 3.44 3.32 3.09 3.24 3.32 3.43 3.23 3.20 3.11 2.87 2.94 3.04 2.98 3.39 3.20 0.15 3.64 2.63 3.65 3.57 3.56 3.57 3.38 3.45 3.74

0.28 0.56 0.39 0.46 0.51 1.01 0.57 0.39 0.53 0.51 0.59 1.31 0.81 0.48 0.35 1.11 0.52 0.60 1.13 0.33 0.72 1.19 0.69 0.93 0.54 0.66 0.33 0.44" 1.04 0.83 0.60 0.40 0.69

h2 0.56 0.46 0.73 1.48 1.28 0.67 0.78 0.63 0.52 0.73 0.71 6.49 0.33 0.33 0.46 1.61 0.38 0.55 0.56 0.49 0.34 1.36 0.98 0.44 1.40 0.69 0.28 0.42 1.03 1.14 0.60 0.63 0.55

h3

h~

U4

U2

U3

T h i c k n e s s (ki n)

Velocity (km/sec)

( ) Unreversed r ef r actio n v elo cit y. * A s su m ed velocity. All o t h e r velocities are interval velocities f rom w i de angle r e f l e c t i o n data.

59 60 61 62 63 64 65 67 68 69 70 7J 72 73 74 75 76 77 78 79 80 82 83 84 87 88 89 90 91 92 93 94 95

Sonobuoy

Results o f s o n o b u o y s tat ions , Cruise 14 of R V " R o b e r t D. C o n r a d "

TABLE I

2.75 4.24 4.76 0.71

1.09 0.45 1.90 1.57 3.90

h~

0.78

1.20 0.45 1.08 1.24 0.56

1.69 1.05

0.42

1.94

1.87 1.67

4.74 0.30 0.70 1.46 2.46 2.51 1.24 1.18

0.43 0.66 0.70 0.57 0.87 4.46 0.72 0.32 0.48 6.61

h4

1.61

1.40 1.63 2.28 1.25 1.81

h6

Lat. (N)

1.95 5 6 ° 3 9 ' 1.86 5 6 o 4 3 ' 57o51 ' 58°02 ' 59058 ' 60°13 ' 60°27 ' 59°41 ' 59°46 ' 59°52 ' 59°57 ' 58027 ' 59016 ' 2.63 5 9 0 0 2 ' 58°52 ' 58056 ' 59000 ' 59°33 , 59030 ' 59026 ' 59°22 ' 58017 ' 58°19 ' 59°17 ' 57o43 , 57o47 ' 57°51 ' 57°56 ' 58000 ' 58°05 ' 58°16 ' 58"07 ' 55°55 '

h7

Location

178°29'W 178°53'W 177°48'E 177°42'E 176°21'E 176°08'E 175°56'E 174°58'E 174°38'E 174°09'E 173°45'E 172°40'E 172°49'E 172°59'E 173°05'E 172°43'E 172°23'E 167°49'E 168°09'E 168°28'E 168°50'E 172°23'E 172°01'E 166°00'E 167°46'E 167°25'E 167°04'E 166°46'E 166°32'E 166°13'E 164°47'E 165°00'E 171°27'E

Long.

0o

319

existing ridge or, alternatively, Shirshov Ridge was developed during a time when larger amounts of sediments were being deposited in the Aleutian Basin. The development of the ridge effectively cut off the sediment source to the Komandorsky Basin. If the higher-velocity sediments are pre-Miocene, the latter explanation may explain why parts of Shirshov Ridge (perched basins and western flanks) have thinner sections of the higher-velocity sediments (below Layer P). Because the depth to the top of the high-velocity sediment layer is relatively uniform, the systematic increase in velocities of this layer approaching the continental margin appears to be related to increased terrigenous and/or volcaniclastic material rather than to sediment compaction. Relatively low velocities (less than 5.00 km/sec) are associated with acoustic basement in the northwest Aleutian Basin (stations 74--76 and 82). Beneath acoustic basement, higher refraction velocities (5.45--5.75 km/sec) are observed that are similar to those observed for acoustic basement in the central Aleutian Basin. These observations suggest that acoustic basement may not be basalt. Rather, the acoustic basement may be volcaniclastic sediments, limestones, etc., similar to rocks found onshore. In either case, the basement rocks of the Aleutian Basin appear to be dipping beneath both the northern part of Shirshov Ridge (stations 74--75) and the Siberian continental margin (stations 71--73; 67--70). In contrast, there is no evidence indicating the basement rocks in the Komandorsky Basin are dipping beneath Shirshov Ridge. Basement velocities in the Aleutian and Komandorsky Basins appear to be significantly different, with the higher velocities recorded in the Komandorsky Basin. Cormier (1975) proposes that younger basalts have flooded and engulfed pre-existing sediments in the Komandorsky Basin. Because the acoustic basement is a strong reflector that can be followed throughout most of the Komandorsky Basin (except in the nothernmost part), the basaltic flooding, if present, must have been regionally extensive. ACKNOWLEDGEMENTS

We wish to thank the officers, crew, and scientists aboard Research Vessel " R o b e r t D. Conrad" for their assistance in gathering the data. We are grateful to Drs. William J. Ludwig, David W. Scholl, Robert Embley, and Steven C. Cande for critically reviewing the manuscript and to Robert E. Houtz for offering valuable advice for the reduction of the sonobuoy records. This work was supported by National Science Foundation Grant GA-27281 and Office of Naval Research Contract N00014-67-A-0108-0004. REFERENCES Cooper, A.K., Scholl, D.W. and Marlow, M.S., 1976a. Mesozoic magnetic lineations in the Bering Sea marginal basin. J. Geophys. Res., 8 (11): p.1916. Cooper, A.K., Bailey, K.A., Marlow, M.S., Scholl, D.W. and Carpenter, C.E., 1976b. Preliminary isopach map of the Bering Sea Basin. U.S.G.S. Open file report.

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