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Geological results of a wide angle reflection survey using sonobuoys in the southern Chukchi Sea
Marine Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
GEOLOGICAL
RESULTS OF A WIDE ANGLE
USING SONOBUOYS IN THE SOUTHERN
REFLECTION
CHUKCHI
SURVEY
SEA 1
THOMAS C. JOHNSON AND LLOYD R. BRESLAU Applied Sciences Division, Office of Research and Development, United States Coast Ouard, Washington, D.C. (U.S.A.)
(Received July 24, 1970) (Resubmitted November 2, 1970)
ABSTRACT JOHNSON, T. C. and BRESLAU,L. R., 1971. Geological results of a wide angle reflection survey using sonobuoys in the southern Chukchi Sea. Marine Geol., 10: 281-290. The results of six sonobuoy wide angle reflection profiles in the southern Chukchi Sea show that from 200 to 700 m of unconsolidated sediments with acoustic velocities less than 2 km/sec overlie 1-1.5 km of sandstones and shales with velocities in the range of 2.1-2.8 km/sec. A higher velocity formation of approximately 4 km/sec was found at two of the six stations. These results show that extensive deposition, as well as marine erosion, has played an important role in making the Chukchi Sea floor the fiat feature that it is today. INTRODUCTION One o f the m o s t striking features o f the C h u k c h i Sea floor is its m o n o t o n o u s l y flat t o p o g r a p h y . M i n o r b a t h y m e t r i c features s t a n d out in some areas, b u t for the m o s t p a r t b o t t o m gradients d o not exceed 4 ft./mile (CREAMER a n d MCMANUS, 1966). This unusually flat feature has been studied by m a r i n e geologists for years, a n d varying hypotheses on its origin, b o t h erosional a n d depositional, were p r o p o s e d . HOLMES et al. (1968) presented the first relatively deep ( ~ 1 k m ) subb o t t o m profiles o f the area, a n d they c o n c l u d e d t h a t up to 1 k m o f T e r t i a r y sediments lie u n c o n f o r m a b l y on o l d e r rocks in a s e d i m e n t a r y basin b e n e a t h the C h u k c h i Sea floor. This c o n f i r m e d a p r e d o m i n a n t l y d e p o s i t i o n a l origin for the flat C h u k c h i Sea floor, first p r o p o s e d by HOPKINS (1959). In the late s u m m e r o f 1969 m e m b e r s o f the U.S. G e o l o g i c a l Survey, U.S. C o a s t G u a r d , a n d U.S. C o a s t a n d G e o d e t i c Survey c o n d u c t e d a geophysical survey in the C h u k c h i Sea a b o a r d the U.S. C o a s t G u a r d C u t t e r " S t o r i s " . The e q u i p m e n t used included a 160,000-J arcer c o n t i n u o u s seismic profiling 1 All assertions or opinions contained herein are the private views of the writers and are not to be construed as official or renecting the views of the Commandant or the Coast Guard at large. Marine Geol., 10 (1971) 281-290
282
T. C. JOHNSON AND L. R. BRESLAU
system (CSP) with significant energy in the frequency range of 20-80 Hz, a higher resolution 700-J arcer CSP system with significant energy centered around 1,000 Hz, sonobuoy system, proton magnetometer, 12-kHz echo sounder and a precision navigation system utilizing navigational satellite and LORAN C. This paper concerns the results of the wide angle reflection studies conducted with the high energy CSP and sonobuoy systems in the southern Chukchi Sea. Six sonobuoy records were analyzed; the locations of these six profiles are shown in Fig. 1. Although many more sonobuoy runs were attempted in other parts of the 168 ° T CAPE ~LI Sll,k~RN E I /
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stations. Contour interval is 10 m. Chukchi Sea, including north of Cape Lisburne, no other useful data were obtained; either because of equipment failure or, for reasons not clearly understood, the wide angle reflections were not received over a sufficient range. These sonobuoy records provide measurements of layer thicknesses and acoustic velocities within the subbottom structure over the steamed track, and consequently provide inferences about the kinds of rock from which reflections were received. The results of this investigation further support HOPKINS' (1959) hypothesis of the Cenozoic history of the Bering-Chukchi platform (that the platform was depressed in the Late Tertiary, and that subsequent marine deposition filled in hundreds of meters of hilly relief), and are in agreement with and add to the geophysical data obtained by HOLMESet al. (1968). Marine Geol., 10 (1971) 281 290
WIDE ANGLE REFLECTION SURVEY IN CHUKCHI SEA
283
DATA ACQUISITION AND ANALYSIS
The wide angle reflection data were obtained and analyzed in the manner described by LE PICHON et al. (1968). A 160,000-J arcer was used as a sound source and an expendable sonobuoy was used as a receiver. The passive sonobuoy, containing hydrophones and FM radio transmitter, telemetered the seismic returns to the ship where they were received by an FM receiver, passed through a 20-75 Hz band-pass filter, and recorded on a facsimile recorder. Concurrently, the 160,000-J arcer acted as the sound source in the normal incidence CSP system utilizing 20-60 Hz band-pass filtering. Whenever the CSP system revealed continuous, plane layers in the subbottom structure, a sonobuoy was deployed while the ship continued profiling at a constant velocity (usually at a speed of six knots or less). The CSP data were displayed on one graphic recorder, and the sonobuoy data, which were initially received within seconds after sonobuoy deployment, were displayed on a second graphic recorder. Navigational fixes were obtained every ten minutes to a precision of better than 0.15 nautical mile using a direct range-range measuring L O R A N C system in conjunction with navigational satellite. Data reduction and analysis required digitizing the sonobuoy record, determining the topographic correction from the CSP record, and using a modified T2/X 2 solution to arrive at the velocity structure. Further details and an error analysis of this method can be obtained by referring to the paper by LE PICHON et al. (1968). In the surveyed area the combination of shallow water, numerous subbottom reflectors and a powerful seismic source resulted in complicated seismic records containing numerous multiples. Erroneous layer thicknesses could be obtained by the presence of these multiple reflections in the sonobuoy data. The travel timedistance (T/X) values for a multiple would give an erroneous multiple thickness for a given multiple layer, but would give an accurate and precise velocity. Fortunately, the sonobuoy record carried few subbottom reflectors or their multiples to any great distance, and rejecting obvious multiples from the remaining reflectors probably left no multiples to be analyzed as true reflectors, It is difficult to distinguish the directly propagated acoustic arrival from the sea floor reflection when the ship-to-sonobuoy distance is large in comparison to the water depth. Our measurements were all made in shallow water ( < 60 m) so this situation was reached at a relatively short range. This problem turned out to be minor, however, since the seismic velocities calculated for the water layer turned out to agree reasonably well (within 5 ~ ) with sound velocities which would be expected from the temperature and salinity values of Chukchi Sea water.
Marine Geol., 10 (1971) 281-290
284
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Marine Geol., I0 (1971) 281-290
WIDE ANGLE REFLECTION SURVEY IN CHUKCHI SEA
285
DISCUSSION OF RESULTS T h e s e d i m e n t a r y b a s i n first d e s c r i b e d b y HOLMES e t al. (1968) w a s f o u n d t o u n d e r l i e m u c h o f t h e s o u t h e r n C h u k c h i Sea f l o o r , t o l o c a l l y r e a c h d e p t h s o f o v e r TABLE I D A T A OBTAINED FROM THE SIX SONOBUOY STATIONS
Sonobuoy 1 Beginning position: 67 50.6'N 168 27.4'W
Sonobuoy 2 Beginning position: 67 27.5'N 165 00.1"W
Sonobuoy 3 Beginning position: 67 17.4'N 168 17.0"W
Sonobuoy 4 Beginning position 67 16.0"N 168 18.9'W
Sonobuoy 5 Beginning position: 67 32.2"N 169 57.3'W
Sonobuoy 6 Beginning position: 67 56.6'N 167 12.3'W
Layer
Speed of sound (km/sec)
Standard Layer Depthof Total Number deviation thickness layer numberof data (km/sec) (M) (M) of data points points discarded*
1** 2 3 4 5 1** 2 3 4 5 1"* 2 3 4 5 6 1"* 2 3 4 5 1"* 2 3 4 5 6 1"* 2 3 4 5
1.5 1.9 2.8 2.0 2.7 1.5 1.7 2.8 4.0 2.5 1.5 1.8 1.8 2.6 4.l 2.5 1.5 1,5 1.9 1.7 2.3 1.5 1.3(?) 1.9 1.6 2.1 2.3 1.5 1.8 2.8 2.1 2.8
-0.2 0.4 0.2 2.2 -0.1 0.1 0.5 0.2 -0.1 0.2 0.1 0.2 0.6 -0.2 0.0+ 0.1 0.2 -0.1 0.1 0.1 0.1 0.1 -0.1 0.5 0.1 0.4
64 180 213 647 55 46 634 993 117 268 41 366 17 587 134 195 71 93 265 299 565 35 84 124 111 255 475 47 238 226 697 199
64 244 457 1,104 1,159 46 680 1,673 1,790 2,058 41 407 424 1,011 1,145 1,340 71 164 429 728 1,293 35 119 243 354 609 1,084 47 285 511 1,208 1,407
36 10 11 35 35 29 4 23 23 27 33 7 8 16 33 27 32 6 21 31 30 36 5 11 8 16 29 29 5 7 25 18
0 1 2 1 24 0 1 1 3 7 0 1 1 1 1 10 0 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1
* Data points are discarded by the computer if the travel time squared (T 2) turns out to be negative after being reduced by the normal incidence travel time squared and the appropriate slope correction. Layers having several data points so discarded, such as the lowest layer at stations 1, 2 and 3 are questionable. ** Layer 1 is sea water.
Marine GeoL, 10 (1971) 281-290
286
T.C.
JOHNSON AND L. R. BRESLAU
3 kin, and to contain rocks with gentle folds with numerous high angle faults (GRAN~Z et al., 1970). GRANTZ et al. (1970) have named this basin the Hope Basin. The locations of the sonobuoy stations were such that all of the velocity measurements were made in the Hope Basin sediments and rocks. The graphs o f T 2 vs X 2 for the six sonobuoy stations (Fig.2) show how closely straight lines can be fitted to the data. The various parameters obtained from these data are listed in Table I. Most of the velocities lie within a range of from 1.6 to 2.8 km/sec, the major exception to this being values of 4.0 and 4.1 km/sec measured in one unit at stations 2 and 3, respectively. Although the standard deviations of velocities are quite sizable in some instances, the values of the velocities themselves are reasonable from the standpoint of what might be expected for rocks and sediments, and they are internally consistent from station to station. Some of the data are noticeably weak; specifically, those for the lowest layer of stations 1, 2 and 3. A considerable number of the data points were discarded by the computer in each of these three layers (see footnote of Table I). Consequently these layers have been neglected in the interpretation of our results, but are listed in Table 1 in the event that results of future geophysical investigations in this area can be compared with ours. Neglecting these values is of no important geological consequence at station 1, but at stations 2 and 3 it is the determinating factor as to whether the high velocity unit is a thin layer or the top of a thick sequence of high velocity material. Fig.3 is a histogram of seismic wave velocities for various kinds of rocks, which enables a crude geological interpretation of the velocity data that we have
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obtained. Using this figure we have interpreted all layers with velocities under 2.0 km/sec as being unconsolidated sediments, and those layers with velocities between 2.0 and 3.0 km/sec as being sandstone and shale. The high velocity layers ( ~ 4.0 km/sec) could be limestone, a more consolidated sandstone or shale, or igneous or metamorphic rock. Marine Geol., 10 (1971) 281-290
WIDE ANGLE REFLECTION SURVEY IN CHUKCH! SEA
287
Fig.4 shows the results of sonobuoy stations 2, 3 and 4 as well as the subbottom structure obtained along trackline A-B (in Fig. 1) from the CSP. Along this trackline up to 700 m of what we have inferred to be unconsolidated sediments ( < 2.0 km/sec) overlie what is probably sandstone and shale (2.0-3.0 km/sec) varying in thickness from 500 to 1000 m. The thick sandstone and shale units lie above a strong, key reflector. This reflector, which shall be referred to as reflector A for brevity, can be traced discontinuously along the entire trackline, and it coincides with the high velocity layer at stations 2 and 3. Due to the fault between stations 3 and 4, reflector A was too deep at station 4 to be measured by our sonobuoy survey. The structure along the other tracklines is in many respects the same. Reflector A can be traced over much of the area down to a depth of two seconds round trip travel time, and numerous high angle faults are present. The reader is referred to the paper by GRANTZ et al. (1970) for illustrations and further discussion of the structure along these tracklines. Our velocity measurements did not extend below reflector A at the stations along the other tracklines (stations 1, 5 and 6), and no velocities greater than 3 km/sec were measured. However, the velocities measured above reflector A were similar to those measured above it along line A-B. That is, 20(0400 m of relatively low velocity ( < 2 km/sec) material (unconsolidated sediments ?) overlying 700-1200 meters of higher velocity (2-3 km/sec) material (sandstone and shale ?). Presumably, velocities of approximately 4 km/sec would have been measured below reflector A, but this remains to be shown by future work. Any attempt to assign ages to these units or correlate them with onshore units is pure speculation, but two interesting points are to be noted. Fig.1 shows that strata of the Brooks Range geanticline or the Colville geosyncline should lie under the unconsolidated deposits in the southern Chukchi Sea. The Brooks Range strata are Paleozoic carbonates, metamorphics and intrusives, all rather high-velocity rock types, and the rocks of the Colville geosyncline are lower velocity, Mesozoic sandstones and shales (MILLERet al., 1957). Perhaps the high velocity formation which we have found to coincide with reflector A at stations 2 and 3, and perhaps reflector A wherever it is found, marks the top of the seaward extension of the Paleozoic rocks. Future work which should be done to check on this would be to see if reflector A does indeed coincide with the top of a formation having an acoustic velocity of approximately 4 km/sec throughout the southern Chukchi Sea, and to see if a large velocity reversal does exist below a hundred meters or so of the 4 km/sec material, as our data very weakly suggest. If the latter is true, it is unlikely that these are Brooks Range rocks. The presence of thick sequences of unconsolidated deposits which vary greatly in thickness, and yet have a relatively level upper horizon, is strong evidence for deposition being a prime leveling process on the Chukchi Sea floor. The age of these deposits, however, is another matter of speculation. Presumably these Marine GeoL, 10 (1971) 281-290
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deposits have been in the process of being laid down at least since the beginning of subsidence of the Chukchi platform. Based upon various paleontological, morphological and stratigraphic evidence, HOPKINS (1959) concluded that this submergence began near the end of the Pliocene, approximately one million years ago. HOLMESet al. (1968) stated that uplift of the margins and subsidence of the Chukchi Sea floor occurred since the Mesozoic. Since high angle faults, and particularly the thrust fault in Fig.4, cut through much of the unconsolidated deposits in the Hope Basin, accurate age determinations of these sediments would indicate how recently tectonic activity has occurred in the area. CONCLUSIONS
This wide angle reflection reconnaissance of the Hope Basin has shown that 200-700 m of material with an acoustic velocity of less than 2 km/sec underlie the thin veneer of Holocene sediments which have been found by previous investigators (e.g., MOORE, 1964) in the southern Chukchi Sea. Presumably these are Tertiary and younger unconsolidated deposits. Below these are thick sequences (of 1,000 m or more) of sedimentary rocks with acoustic velocities in the range of 2-3 km/sec, presumably sandstones and shales. A strong, key reflector underlies all of these deposits throughout much of the Hope Basin, and at the two locations that acoustic velocities were measured below this reflector, they were found to be 4.0 and 4.1 km/sec. This reflector could be the upper boundary of the seaward extension of the Paleozoic Brooks Range structure found on land to the east. The thickness, and variation in thickness, of the low velocity ( < 2 km/sec), unconsolidated deposits support the implication of both HOPKINS (1959) and HOLMES et al. (1968) that the flat, featureless topography of this area is due largely to extensive deposition associated with the submergence of the Chukchi platform. ACKNOWLEDGEMENTS
This study was possible only because of the help of many people. The officers and men of the U.S.C.G.C. "Storis", under the command of Cdr. J. H. Byrd, Jr., are to be thanked for their assistance in carrying on operations under difficult conditions. Members of the U.S. Geological Survey were helpful in stand-
Fig.4. Top: Structural trend along trackline A-B (see Fig.l). Seismic velocities for the various layers at stations 2, 3, and 4 are in km/sec. Vertical exaggeration is approximately 10:1. The letter A refers to reflector A. Bottom: Original CSP record for the right-hand portion of the line drawing above, which is labeled X-Y. This shows the complexity of the CSP records obtained in this region, and the reflector referred to as reflector A. Vertical exaggeration is approximately 3.5:1. Marine Geol., 10 (1971) 281-290
290
Y. C. JOHNSON AND L. R. BRESLAU
ing watches, keeping the i n s t r u m e n t a t i o n operating, a n d providing the CSP records which were essential to our study. We are indebted to Sydney Knott, Frederick Hess, Hartley Hoskins and Edward Laine, all of the Woods Hole Oceanographic Institution, for their outstanding support. They provided s o n o b u o y instrumentation, aided in the data reduction, provided c o m p u t e r programs and were always available to discuss m a n y phases of the work. REFERENCES
BIRCH, F., SCHA1RER,J. F. and SPICER, H. C., 1942. Handbook of physical constants. Geol. Sot.. Am. Spec. Papers, 36:325 pp. CREAGER, J. S. and MCMANUS, D. A., 1966, Geology of the southeastern Chukchi Sea. In: N. J. WILIMOVSKYand J. N. WOLFE,(Editors), Environment of Cape Thompson Region, Alaska. U.S. Atomic Energy Commission, Clearing house for Federal Scientific and Technical Information, Springfield, Va., pp.755-786. GRANT, F. S. and WEST,G. F., 1965. Interpretation Theory in Applied Geophysics. McGraw-Hill, New York, N.Y., 584 pp. GRANTZ,A. WOLF,S. C., BRESLAU,L., JOHNSON,T. C. and HANNA,W. F., 1970. Reconnaissance geology of the Chukchi Sea as determined by acoustic and magnetic profiling. In: W. L. AOK1NSON and M. M. BROSGE, Editors, Proceedings of the Geological Seminar on the North Slope of Alaska. Am. Assoc. Petrol. Geologists, Pacific Section, Los Angeles, Calif., pp F.1-F.28. HOLMES,M. L., CREAGER,J. S. and MCMANUS,D. A., 1968. Structure and history of the Chukchi Basin (abstract). Geol. Soc. Am., Program Ann. Meeting, Mexico City, 1968 p.141. HOPKINS, D. M., 1959, Cenozoic history of the Bering land bridge. Science, 129:1519-1528. LE PICHON, X., EWING,J. and Houxz, R. E., 1968. Deep-sea sediment velocity determination made while reflection profiling. J. Geophys. Res., 73: 2597-2614. MILLER, D. J., PAYNE,T. J. and GRYC, G., 1957. Geology of possible petroleum provinces in Alaska. U.S. Geol. Surv. Rept., Open File 421,239 pp. MOORE, D. G., 1964. Acoustic reflection reconnaissance of continental shelves: eastern Bering and Chukchi Seas. In: R. L. MILLER (Editor), Papers in Marine Geology. Macmillan, New York, N.Y., pp.319-362.
Marine Geol., 10 (1971) 281-290
GEOLOGICAL
RESULTS OF A WIDE ANGLE
USING SONOBUOYS IN THE SOUTHERN
REFLECTION
CHUKCHI
SURVEY
SEA 1
THOMAS C. JOHNSON AND LLOYD R. BRESLAU Applied Sciences Division, Office of Research and Development, United States Coast Ouard, Washington, D.C. (U.S.A.)
(Received July 24, 1970) (Resubmitted November 2, 1970)
ABSTRACT JOHNSON, T. C. and BRESLAU,L. R., 1971. Geological results of a wide angle reflection survey using sonobuoys in the southern Chukchi Sea. Marine Geol., 10: 281-290. The results of six sonobuoy wide angle reflection profiles in the southern Chukchi Sea show that from 200 to 700 m of unconsolidated sediments with acoustic velocities less than 2 km/sec overlie 1-1.5 km of sandstones and shales with velocities in the range of 2.1-2.8 km/sec. A higher velocity formation of approximately 4 km/sec was found at two of the six stations. These results show that extensive deposition, as well as marine erosion, has played an important role in making the Chukchi Sea floor the fiat feature that it is today. INTRODUCTION One o f the m o s t striking features o f the C h u k c h i Sea floor is its m o n o t o n o u s l y flat t o p o g r a p h y . M i n o r b a t h y m e t r i c features s t a n d out in some areas, b u t for the m o s t p a r t b o t t o m gradients d o not exceed 4 ft./mile (CREAMER a n d MCMANUS, 1966). This unusually flat feature has been studied by m a r i n e geologists for years, a n d varying hypotheses on its origin, b o t h erosional a n d depositional, were p r o p o s e d . HOLMES et al. (1968) presented the first relatively deep ( ~ 1 k m ) subb o t t o m profiles o f the area, a n d they c o n c l u d e d t h a t up to 1 k m o f T e r t i a r y sediments lie u n c o n f o r m a b l y on o l d e r rocks in a s e d i m e n t a r y basin b e n e a t h the C h u k c h i Sea floor. This c o n f i r m e d a p r e d o m i n a n t l y d e p o s i t i o n a l origin for the flat C h u k c h i Sea floor, first p r o p o s e d by HOPKINS (1959). In the late s u m m e r o f 1969 m e m b e r s o f the U.S. G e o l o g i c a l Survey, U.S. C o a s t G u a r d , a n d U.S. C o a s t a n d G e o d e t i c Survey c o n d u c t e d a geophysical survey in the C h u k c h i Sea a b o a r d the U.S. C o a s t G u a r d C u t t e r " S t o r i s " . The e q u i p m e n t used included a 160,000-J arcer c o n t i n u o u s seismic profiling 1 All assertions or opinions contained herein are the private views of the writers and are not to be construed as official or renecting the views of the Commandant or the Coast Guard at large. Marine Geol., 10 (1971) 281-290
282
T. C. JOHNSON AND L. R. BRESLAU
system (CSP) with significant energy in the frequency range of 20-80 Hz, a higher resolution 700-J arcer CSP system with significant energy centered around 1,000 Hz, sonobuoy system, proton magnetometer, 12-kHz echo sounder and a precision navigation system utilizing navigational satellite and LORAN C. This paper concerns the results of the wide angle reflection studies conducted with the high energy CSP and sonobuoy systems in the southern Chukchi Sea. Six sonobuoy records were analyzed; the locations of these six profiles are shown in Fig. 1. Although many more sonobuoy runs were attempted in other parts of the 168 ° T CAPE ~LI Sll,k~RN E I /
~Q° • \
/
CHUKCHI
SE~
/
164 '~
c°t~°~cE G E O s y ~ ¢ I I ~
~
4 ~
/',
' S
66
j ~
SEWARD PENINSULA
Fig.l. Area of study with trend of geological structure onshore to the east. Dashed line represents ship's trackline. Numbered arrows designate location and direction of sonobuoy
stations. Contour interval is 10 m. Chukchi Sea, including north of Cape Lisburne, no other useful data were obtained; either because of equipment failure or, for reasons not clearly understood, the wide angle reflections were not received over a sufficient range. These sonobuoy records provide measurements of layer thicknesses and acoustic velocities within the subbottom structure over the steamed track, and consequently provide inferences about the kinds of rock from which reflections were received. The results of this investigation further support HOPKINS' (1959) hypothesis of the Cenozoic history of the Bering-Chukchi platform (that the platform was depressed in the Late Tertiary, and that subsequent marine deposition filled in hundreds of meters of hilly relief), and are in agreement with and add to the geophysical data obtained by HOLMESet al. (1968). Marine Geol., 10 (1971) 281 290
WIDE ANGLE REFLECTION SURVEY IN CHUKCHI SEA
283
DATA ACQUISITION AND ANALYSIS
The wide angle reflection data were obtained and analyzed in the manner described by LE PICHON et al. (1968). A 160,000-J arcer was used as a sound source and an expendable sonobuoy was used as a receiver. The passive sonobuoy, containing hydrophones and FM radio transmitter, telemetered the seismic returns to the ship where they were received by an FM receiver, passed through a 20-75 Hz band-pass filter, and recorded on a facsimile recorder. Concurrently, the 160,000-J arcer acted as the sound source in the normal incidence CSP system utilizing 20-60 Hz band-pass filtering. Whenever the CSP system revealed continuous, plane layers in the subbottom structure, a sonobuoy was deployed while the ship continued profiling at a constant velocity (usually at a speed of six knots or less). The CSP data were displayed on one graphic recorder, and the sonobuoy data, which were initially received within seconds after sonobuoy deployment, were displayed on a second graphic recorder. Navigational fixes were obtained every ten minutes to a precision of better than 0.15 nautical mile using a direct range-range measuring L O R A N C system in conjunction with navigational satellite. Data reduction and analysis required digitizing the sonobuoy record, determining the topographic correction from the CSP record, and using a modified T2/X 2 solution to arrive at the velocity structure. Further details and an error analysis of this method can be obtained by referring to the paper by LE PICHON et al. (1968). In the surveyed area the combination of shallow water, numerous subbottom reflectors and a powerful seismic source resulted in complicated seismic records containing numerous multiples. Erroneous layer thicknesses could be obtained by the presence of these multiple reflections in the sonobuoy data. The travel timedistance (T/X) values for a multiple would give an erroneous multiple thickness for a given multiple layer, but would give an accurate and precise velocity. Fortunately, the sonobuoy record carried few subbottom reflectors or their multiples to any great distance, and rejecting obvious multiples from the remaining reflectors probably left no multiples to be analyzed as true reflectors, It is difficult to distinguish the directly propagated acoustic arrival from the sea floor reflection when the ship-to-sonobuoy distance is large in comparison to the water depth. Our measurements were all made in shallow water ( < 60 m) so this situation was reached at a relatively short range. This problem turned out to be minor, however, since the seismic velocities calculated for the water layer turned out to agree reasonably well (within 5 ~ ) with sound velocities which would be expected from the temperature and salinity values of Chukchi Sea water.
Marine Geol., 10 (1971) 281-290
284
T. C. JOHNSON AND L. R. BRESLAU ~
• ONOBUOY
SOUTHERN
m
RESULT8
CH U K C H I
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....
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Marine Geol., I0 (1971) 281-290
WIDE ANGLE REFLECTION SURVEY IN CHUKCHI SEA
285
DISCUSSION OF RESULTS T h e s e d i m e n t a r y b a s i n first d e s c r i b e d b y HOLMES e t al. (1968) w a s f o u n d t o u n d e r l i e m u c h o f t h e s o u t h e r n C h u k c h i Sea f l o o r , t o l o c a l l y r e a c h d e p t h s o f o v e r TABLE I D A T A OBTAINED FROM THE SIX SONOBUOY STATIONS
Sonobuoy 1 Beginning position: 67 50.6'N 168 27.4'W
Sonobuoy 2 Beginning position: 67 27.5'N 165 00.1"W
Sonobuoy 3 Beginning position: 67 17.4'N 168 17.0"W
Sonobuoy 4 Beginning position 67 16.0"N 168 18.9'W
Sonobuoy 5 Beginning position: 67 32.2"N 169 57.3'W
Sonobuoy 6 Beginning position: 67 56.6'N 167 12.3'W
Layer
Speed of sound (km/sec)
Standard Layer Depthof Total Number deviation thickness layer numberof data (km/sec) (M) (M) of data points points discarded*
1** 2 3 4 5 1** 2 3 4 5 1"* 2 3 4 5 6 1"* 2 3 4 5 1"* 2 3 4 5 6 1"* 2 3 4 5
1.5 1.9 2.8 2.0 2.7 1.5 1.7 2.8 4.0 2.5 1.5 1.8 1.8 2.6 4.l 2.5 1.5 1,5 1.9 1.7 2.3 1.5 1.3(?) 1.9 1.6 2.1 2.3 1.5 1.8 2.8 2.1 2.8
-0.2 0.4 0.2 2.2 -0.1 0.1 0.5 0.2 -0.1 0.2 0.1 0.2 0.6 -0.2 0.0+ 0.1 0.2 -0.1 0.1 0.1 0.1 0.1 -0.1 0.5 0.1 0.4
64 180 213 647 55 46 634 993 117 268 41 366 17 587 134 195 71 93 265 299 565 35 84 124 111 255 475 47 238 226 697 199
64 244 457 1,104 1,159 46 680 1,673 1,790 2,058 41 407 424 1,011 1,145 1,340 71 164 429 728 1,293 35 119 243 354 609 1,084 47 285 511 1,208 1,407
36 10 11 35 35 29 4 23 23 27 33 7 8 16 33 27 32 6 21 31 30 36 5 11 8 16 29 29 5 7 25 18
0 1 2 1 24 0 1 1 3 7 0 1 1 1 1 10 0 1 1 1 1 0 1 1 1 1 1 0 1 1 1 1
* Data points are discarded by the computer if the travel time squared (T 2) turns out to be negative after being reduced by the normal incidence travel time squared and the appropriate slope correction. Layers having several data points so discarded, such as the lowest layer at stations 1, 2 and 3 are questionable. ** Layer 1 is sea water.
Marine GeoL, 10 (1971) 281-290
286
T.C.
JOHNSON AND L. R. BRESLAU
3 kin, and to contain rocks with gentle folds with numerous high angle faults (GRAN~Z et al., 1970). GRANTZ et al. (1970) have named this basin the Hope Basin. The locations of the sonobuoy stations were such that all of the velocity measurements were made in the Hope Basin sediments and rocks. The graphs o f T 2 vs X 2 for the six sonobuoy stations (Fig.2) show how closely straight lines can be fitted to the data. The various parameters obtained from these data are listed in Table I. Most of the velocities lie within a range of from 1.6 to 2.8 km/sec, the major exception to this being values of 4.0 and 4.1 km/sec measured in one unit at stations 2 and 3, respectively. Although the standard deviations of velocities are quite sizable in some instances, the values of the velocities themselves are reasonable from the standpoint of what might be expected for rocks and sediments, and they are internally consistent from station to station. Some of the data are noticeably weak; specifically, those for the lowest layer of stations 1, 2 and 3. A considerable number of the data points were discarded by the computer in each of these three layers (see footnote of Table I). Consequently these layers have been neglected in the interpretation of our results, but are listed in Table 1 in the event that results of future geophysical investigations in this area can be compared with ours. Neglecting these values is of no important geological consequence at station 1, but at stations 2 and 3 it is the determinating factor as to whether the high velocity unit is a thin layer or the top of a thick sequence of high velocity material. Fig.3 is a histogram of seismic wave velocities for various kinds of rocks, which enables a crude geological interpretation of the velocity data that we have
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obtained. Using this figure we have interpreted all layers with velocities under 2.0 km/sec as being unconsolidated sediments, and those layers with velocities between 2.0 and 3.0 km/sec as being sandstone and shale. The high velocity layers ( ~ 4.0 km/sec) could be limestone, a more consolidated sandstone or shale, or igneous or metamorphic rock. Marine Geol., 10 (1971) 281-290
WIDE ANGLE REFLECTION SURVEY IN CHUKCH! SEA
287
Fig.4 shows the results of sonobuoy stations 2, 3 and 4 as well as the subbottom structure obtained along trackline A-B (in Fig. 1) from the CSP. Along this trackline up to 700 m of what we have inferred to be unconsolidated sediments ( < 2.0 km/sec) overlie what is probably sandstone and shale (2.0-3.0 km/sec) varying in thickness from 500 to 1000 m. The thick sandstone and shale units lie above a strong, key reflector. This reflector, which shall be referred to as reflector A for brevity, can be traced discontinuously along the entire trackline, and it coincides with the high velocity layer at stations 2 and 3. Due to the fault between stations 3 and 4, reflector A was too deep at station 4 to be measured by our sonobuoy survey. The structure along the other tracklines is in many respects the same. Reflector A can be traced over much of the area down to a depth of two seconds round trip travel time, and numerous high angle faults are present. The reader is referred to the paper by GRANTZ et al. (1970) for illustrations and further discussion of the structure along these tracklines. Our velocity measurements did not extend below reflector A at the stations along the other tracklines (stations 1, 5 and 6), and no velocities greater than 3 km/sec were measured. However, the velocities measured above reflector A were similar to those measured above it along line A-B. That is, 20(0400 m of relatively low velocity ( < 2 km/sec) material (unconsolidated sediments ?) overlying 700-1200 meters of higher velocity (2-3 km/sec) material (sandstone and shale ?). Presumably, velocities of approximately 4 km/sec would have been measured below reflector A, but this remains to be shown by future work. Any attempt to assign ages to these units or correlate them with onshore units is pure speculation, but two interesting points are to be noted. Fig.1 shows that strata of the Brooks Range geanticline or the Colville geosyncline should lie under the unconsolidated deposits in the southern Chukchi Sea. The Brooks Range strata are Paleozoic carbonates, metamorphics and intrusives, all rather high-velocity rock types, and the rocks of the Colville geosyncline are lower velocity, Mesozoic sandstones and shales (MILLERet al., 1957). Perhaps the high velocity formation which we have found to coincide with reflector A at stations 2 and 3, and perhaps reflector A wherever it is found, marks the top of the seaward extension of the Paleozoic rocks. Future work which should be done to check on this would be to see if reflector A does indeed coincide with the top of a formation having an acoustic velocity of approximately 4 km/sec throughout the southern Chukchi Sea, and to see if a large velocity reversal does exist below a hundred meters or so of the 4 km/sec material, as our data very weakly suggest. If the latter is true, it is unlikely that these are Brooks Range rocks. The presence of thick sequences of unconsolidated deposits which vary greatly in thickness, and yet have a relatively level upper horizon, is strong evidence for deposition being a prime leveling process on the Chukchi Sea floor. The age of these deposits, however, is another matter of speculation. Presumably these Marine GeoL, 10 (1971) 281-290
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deposits have been in the process of being laid down at least since the beginning of subsidence of the Chukchi platform. Based upon various paleontological, morphological and stratigraphic evidence, HOPKINS (1959) concluded that this submergence began near the end of the Pliocene, approximately one million years ago. HOLMESet al. (1968) stated that uplift of the margins and subsidence of the Chukchi Sea floor occurred since the Mesozoic. Since high angle faults, and particularly the thrust fault in Fig.4, cut through much of the unconsolidated deposits in the Hope Basin, accurate age determinations of these sediments would indicate how recently tectonic activity has occurred in the area. CONCLUSIONS
This wide angle reflection reconnaissance of the Hope Basin has shown that 200-700 m of material with an acoustic velocity of less than 2 km/sec underlie the thin veneer of Holocene sediments which have been found by previous investigators (e.g., MOORE, 1964) in the southern Chukchi Sea. Presumably these are Tertiary and younger unconsolidated deposits. Below these are thick sequences (of 1,000 m or more) of sedimentary rocks with acoustic velocities in the range of 2-3 km/sec, presumably sandstones and shales. A strong, key reflector underlies all of these deposits throughout much of the Hope Basin, and at the two locations that acoustic velocities were measured below this reflector, they were found to be 4.0 and 4.1 km/sec. This reflector could be the upper boundary of the seaward extension of the Paleozoic Brooks Range structure found on land to the east. The thickness, and variation in thickness, of the low velocity ( < 2 km/sec), unconsolidated deposits support the implication of both HOPKINS (1959) and HOLMES et al. (1968) that the flat, featureless topography of this area is due largely to extensive deposition associated with the submergence of the Chukchi platform. ACKNOWLEDGEMENTS
This study was possible only because of the help of many people. The officers and men of the U.S.C.G.C. "Storis", under the command of Cdr. J. H. Byrd, Jr., are to be thanked for their assistance in carrying on operations under difficult conditions. Members of the U.S. Geological Survey were helpful in stand-
Fig.4. Top: Structural trend along trackline A-B (see Fig.l). Seismic velocities for the various layers at stations 2, 3, and 4 are in km/sec. Vertical exaggeration is approximately 10:1. The letter A refers to reflector A. Bottom: Original CSP record for the right-hand portion of the line drawing above, which is labeled X-Y. This shows the complexity of the CSP records obtained in this region, and the reflector referred to as reflector A. Vertical exaggeration is approximately 3.5:1. Marine Geol., 10 (1971) 281-290
290
Y. C. JOHNSON AND L. R. BRESLAU
ing watches, keeping the i n s t r u m e n t a t i o n operating, a n d providing the CSP records which were essential to our study. We are indebted to Sydney Knott, Frederick Hess, Hartley Hoskins and Edward Laine, all of the Woods Hole Oceanographic Institution, for their outstanding support. They provided s o n o b u o y instrumentation, aided in the data reduction, provided c o m p u t e r programs and were always available to discuss m a n y phases of the work. REFERENCES
BIRCH, F., SCHA1RER,J. F. and SPICER, H. C., 1942. Handbook of physical constants. Geol. Sot.. Am. Spec. Papers, 36:325 pp. CREAGER, J. S. and MCMANUS, D. A., 1966, Geology of the southeastern Chukchi Sea. In: N. J. WILIMOVSKYand J. N. WOLFE,(Editors), Environment of Cape Thompson Region, Alaska. U.S. Atomic Energy Commission, Clearing house for Federal Scientific and Technical Information, Springfield, Va., pp.755-786. GRANT, F. S. and WEST,G. F., 1965. Interpretation Theory in Applied Geophysics. McGraw-Hill, New York, N.Y., 584 pp. GRANTZ,A. WOLF,S. C., BRESLAU,L., JOHNSON,T. C. and HANNA,W. F., 1970. Reconnaissance geology of the Chukchi Sea as determined by acoustic and magnetic profiling. In: W. L. AOK1NSON and M. M. BROSGE, Editors, Proceedings of the Geological Seminar on the North Slope of Alaska. Am. Assoc. Petrol. Geologists, Pacific Section, Los Angeles, Calif., pp F.1-F.28. HOLMES,M. L., CREAGER,J. S. and MCMANUS,D. A., 1968. Structure and history of the Chukchi Basin (abstract). Geol. Soc. Am., Program Ann. Meeting, Mexico City, 1968 p.141. HOPKINS, D. M., 1959, Cenozoic history of the Bering land bridge. Science, 129:1519-1528. LE PICHON, X., EWING,J. and Houxz, R. E., 1968. Deep-sea sediment velocity determination made while reflection profiling. J. Geophys. Res., 73: 2597-2614. MILLER, D. J., PAYNE,T. J. and GRYC, G., 1957. Geology of possible petroleum provinces in Alaska. U.S. Geol. Surv. Rept., Open File 421,239 pp. MOORE, D. G., 1964. Acoustic reflection reconnaissance of continental shelves: eastern Bering and Chukchi Seas. In: R. L. MILLER (Editor), Papers in Marine Geology. Macmillan, New York, N.Y., pp.319-362.
Marine Geol., 10 (1971) 281-290