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Geological interpretations based on deep-two single channel and multichannel seismic data from the Bermuda Rise
Marine Geology, 96 ( 1991 ) 279-293 Elsevier Science Publishers B.V., Amsterdam
279
Geological interpretations based on deep-tow single channel and multichannel seismic data from the Bermuda Rise F.A. Bowles, J.F. Gettrust and M. Rowe N O A R L , Stennis Space Center, M S 39529-5004, U.S.A.
(Received June 23, 1989; revision accepted May 16, 1990)
ABSTRACT Bowles, F.A., Gettrust, J.F. and Rowe, M., 1991. Geological interpretations based on deep-tow single channel and multichannel seismic data from the Bermuda Rise. Mar. Geol,, 96: 279-293. Multichannel seismic data taken with a deep-tow system approximately 170 n mile south-southwest of Bermuda reveal the upper 300 m of sediment in detail unachievable by conventional single-channel and multichannel seismic reflection methods. These data show the upper sediments to be largely undisturbed although faulting and basement uplift are evident. The acoustic reflectivity of the sediments and nature of the reflectors are consistent with DSDP data from the region. Horizons A v, A', and A c of the Horizon A-complex are observed, in addition to numerous reflecting horizons associated with porcellanitic chert, limestone, and cemented claystone layers of Ac. Seismic velocity inversions observed generally beneath A c are thought to result from sediment loading and buildup of excess pore pressures in the sediments below Horizon A v. Upward migration of these fluids may account for localized zones of little or no subbottom acoustic reflectivity. Seafloor structural details resolved include erosional channels or slump scars, mud waves, and possible furrows.
Introduction In 1984 d a t a were collected s o u t h - s o u t h w e s t o f B e r m u d a (Figs.1 a n d 2) in w a t e r d e p t h s o f a b o u t 5000 m using the D e e p - T o w e d A r r a y G e o p h y s i c a l System ( D T A G S ) d e v e l o p e d by the N a v a l O c e a n Research and Development Activity (NORDA). D T A G S is a m u l t i c h a n n e l seismic system consisting o f a 2 5 0 - 6 5 0 H z b a n d w i d t h source a n d a 24channel (42 m g r o u p spacing) a r r a y . Both the source a n d a r r a y are t o w e d n e a r the seafloor (within a few h u n d r e d meters) in o r d e r to o b t a i n h i g h - r e s o l u t i o n i n f o r m a t i o n a b o u t the structure a n d elastic p r o p e r t i e s o f d e e p - o c e a n sediments. D T A G S spatial r e s o l u t i o n is on the o r d e r o f 15 m h o r i z o n t a l l y a n d 5 m vertically. V e l o c i t y - d e p t h functions derived f r o m the m u l t i c h a n n e l seismic d a t a have been r e p o r t e d by G e t t r u s t et al. (1988), in a d d i t i o n to i n f o r m a t i o n a b o u t the D T A G S system, d a t a processing, a n d survey m o d e . In this p a p e r , we present geological i n t e r p r e t a t i o n s b a s e d on analysis o f a larger set o f these d a t a . 0025-3227/91/$03.50
The seismic s t r a t i g r a p h y o f the s e d i m e n t c o l u m n in the general a r e a surveyed by D T A G S has been discussed by T u c h o l k e (1979) a n d T u c h o l k e a n d M o u n t a i n (1979). C o n v e n t i o n a l single channel seismic reflection profiles, in general, show the sediments a b o v e volcanic b a s e m e n t to be acoustically t r a n s p a r e n t , b u t d i v i d e d into u p p e r a n d lower units by an a c o u s t i c a l l y l a m i n a t e d zone ( H o r i z o n A - c o m p l e x ) t h a t is laterally c o n t i n u o u s (Fig.3). O f the m a j o r reflecting h o r i z o n s within this zone, D T A G S has resolved reflectors A v, A t, a n d A ¢, in a d d i t i o n to several o t h e r reflectors b e n e a t h A c ( G e t t r u s t et al., 1988).
Seismic stratigraphy Horizon A v
C o m p a r i s o n o f Fig.3 with Fig.4 shows that D T A G S achieves a significant gain in resolution over c o n v e n t i o n a l surface-tow seismic systems. In p a r t i c u l a r , the i n d i v i d u a l reflecting h o r i z o n s o f the
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F i g . l . L o c a t i o n o f D T A G S survey t r a c k (dashed line) together with the l o c a t i o n s o f D S D P sites 386 a n d 387 a n d the i s l a n d o f Bermudm
laminated zone appear in considerable detail. Seismic reflection profiles presented by Bowles (1980), for example, reveal Horizon A V(associated with volcaniclastic turbidites) as a thick, intensely reflective layer with little visible internal structure. DTAGS profiles, however, show Horizon A ~ to be highly irregular, exhibiting significant interface roughness and variation in layer thickness (Fig.4B). Abrupt changes in the depth of the layer up to about 0.01 s (7 m) occur over horizontal distances as short as 15 m (Fig.5). There does not appear to be, however, any apparent regularity in wavelength or amplitude to the roughness. Generally, variations in layer thickness occur over relatively short lateral distances (several tens of meters): Measurements of layer thickness were made at several places on the profiles where A ~
showed well-defined top and bottom boundaries. Using an interval velocity of 2079 m/s (Gettrust et al., 1988), the layer thickness was found to vary between 40-110 m. This is less than the 161.2-171.2 m reported at site 386; however, the increased thickness at 386 can be attributed to the fact that the site was a seafloor depression (McCave, 1979). Some of the variation in thickness (in the DTAGS profiles) may be due to acoustic scattering by the rough surface of A v causing poor acoustic penetration (thus giving the impression of thinning). Sediments recovered at D S D P Site 386 show that Horizon A V consists of calcareous turbidites (upper 8.7 m) overlying volcaniclastic turbidites (lower 161.7-171.2 m). Each subunit is characterized by multiple turbidite layers defined b y sharp
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Fig.2. DTAGS survey track together with intersecting ship's tracks along which single-channel seismic reflection data were collected. Locations of specific profiles presented in text are indicated by heavy lines and indexed by figure number. Stippled area indicales extent of Horizon A v.
basal contacts and by size grading (Tucholke, 1979). Interestingly, the D T A G S profiles do not indicate a regular, laminated internal structure, but instead, one that appears more chaotic. This effect is enhanced by lateral variations in reflectivity that give A V a patchy appearance. These observations suggest considerable lateral and vertical variability in the physical make-up of Horizon A Vthat would not be discernible from a single drillhole nor from conventional seismic profiles. Gettrust et al. (1988) report an interval velocity of 2079 m/s for A V A t (as will be discussed later, reliable velocity estimates cannot be obtained for only AV); however, velocities derived at specific shotpoints along the track show wide variability. Examination of a more extended data set shows a mean internal velocity of 2045_+ 500 m/s. The range of internal velocities is indicative of variable internal structure which, in turn, is consistent with the depositional/ sedimentological scenario described below. Horizon A V represents the material from Ber-
281
muda after it was eroded to wave base during middle Eocene to late Oligocene time, with evidence of continuing volcanism into the middle Oligocene (Tucholke, 1979). Seismic reflection data show that Horizon A v is a relict archipelagic apron locally distributed around Bermuda (Bowles, 1980). Thus, debris was shed from Bermuda and distributed radially downslope by a succession of turbidity currents, debris flows, and probably slumps. The middle to outer apron very likely consists, then, of overlapping and coalescing fanlike deposits characterized by a variety of erosional and depositional features such as cut and fill channel structures, levees, discontinuous and deformed layers, sediment lenses, disconformable bedding, etc. The total effect of these features would be to form a deposit around Bermuda that is characterized by rough interfaces,lateral changes in thickness and textural variability (both laterally and vertically), and variable acoustic reflectivity. Such characteristics are consistent with the nature of A v as observed in the D T A G S profiles. Horizon A'
Horizon A t, which occurs approximately 0.05 s below Horizon A V (Fig.4B), has been correlated with the top of a middle Eocene sequence of finegrained siliceous turbidites (Tucholke, 1979). In contrast to Horizon A v, A t appears as a thin, relatively faint layer that disappears and reappears intermittently along the survey track. This patchy, weakly reflective character could be attributed, in part, to acoustic masking by Horizon A V. However, Horizon A t exhibits these same characteristics in areas where A V is absent, indicating that the acoustic appearance of A t is primarily a function of its physical make-up. Strong impedance (density x velocity) contrasts, required to produce a sharp reflector, result primarily from sharp lithological changes because density and velocity are dependent on sediment texture and composition. The low reflectivity of Horizon A' indicates a less distinct lithological change between it and the overlying sediments than, for example, the case of Horizon A". Although the turbidites of Horizon A t are thick, ranging from 30 cm to about 3 m, the bulk of the
Fig,3. Conventional surface-tow, single-channel seismic reflection profile passing through DTAGS survey area. Note the lack of definition within the highly reflective A " A ~ zone
turbidite material consists of a homogeneous silty clay (78% clay). The average thickness of the coarse-grained basal unit is only about 10 cm and, unlike the terrigenous sands and silts that characterize abyssal plain turbidites, are dominated by siliceous fossils (Tucholke et al., 1979). Thus, the poor reflectivity of Horizon A t is largely due to the predominantly fine-grained, structureless character of the layers that make up this stratigraphic unit. The intermittent reflectivity of Horizon A t may be a response to lateral changes in physical character such as texture or roughness. Alternatively, the reflectivity of A t may be linked to the migration of pore fluids. This possibility will be presented in the section discussing velocity inversions.
Horizon A ~
Approximately 0.06 s beneath Horizon A ~begins a sequence of closely spaced parallel reflectors with a thickness of at least 0.11 s (Fig.4). This marks the first time that the layering immediately beneath Horizon A c has been seen in such detail. The evenbedded, rhythmic appearance of the reflectors is striking and, where the sequence is not masked by Horizon A v, ten reflectors of varying reflectivity are resolved (Fig.6). Such regularity arouses suspicion that some of the reflectors seen in Fig,6 may, in fact, be multiples. With regard to this, it is important to note that the lithology of the sediments at sites 386 and 387 supports a highly
GEOLOGY OF THE BERMUDA RISE FROM DEEP-TOW SEISMIC DATA
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Fig.4. DTAGS single-channel profile showing detailed acoustic stratigraphy of the upper 300-350 m of sediment. Note the patches of weak, diffuse reflectivity (fadeouts) within the strongly reflective At-A 3 sequence. Arrow in upper profile (A) shows location of expanded seismographsshown in Fig. 13. Vertical arrow in lower profile (B) shows location of fault suggested by Gettrust et al. (1988).
layered sequence below Horizon A c which should be characterized by both strongly and weakly reflective horizons, as observed. Multiples can be ruled out in the case of the strong reflectors seen in Fig.6 because they all have different interval velocities. Unfortunately, we cannot be as certain in the case of the weaker reflectors because accurate velocity determinations were not possible. We note, however, that in a stacked profile corresponding to Fig.6, all the weak reflectors as well as the strong reflectors are visible. Usually, the stacking procedure will suppress multiples. This strongly indicates, therefore, that the reflectors seen in Fig.6 correspond to real impedance contrasts within the sediments. Gettrust et al. (1988) have correlated the top of the sequence with Horizon A c which, in turn, has been correlated (Tucholke et al., 1979) with the top
of lower to middle Eocene siliceous claystones, radiolarian mudstones, and interbedded porcellanite (immature cherts). These deposits, like those associated with Horizon A', have been interpreted as predominantly turbidites, although the sedimentary structures typical of turbidites are no longer readily (megascopically) apparent. The fact that these turbidite sequences are visible at all is very likely due to the presence of the porcellanite layers. These layers correlate positively with the turbidites in that they result from the conversion of siliceous microfossils, making up the basal unit of each turbidite, into porcellanite (Reich and von Rad, 1979). This conversion appears to be related to depth of burial (i.e., temperature, pressure). Thus, porcellanite has not developed in the siliceous turbidites above A c which, consequently, show no internal layering.
Fig.5. Enlargementof right side of Fig.4B showing the rough sedimentar~interfaceand variable thickness oi Horizon A~',mr,~,de,airsof layering as defined by Genrust et al. (1988) The source area for both the A' and A ~ turbidites is thought to be the North American continental margin. High-density, high-velocity turbidity currents which reach the outer (distal) portions of basins are predominantly fine-grained and broadfronted, moving as sheet flow rather than within specific channels. Such currents are capable of depositing great quantities of largely homogeneous, fine-grained sediment over wide areas. The lateral continuity and thin, even-bedded nature of the A c A 3 deposits (relative to A t) revealed by the DTAGS profiles, supports the interpretation that they are indeed distal turbidites. Post-Horizon
A v
In general, the sediments overlying Horizon A' (and A t) appear acoustically transparent (Figs.3 and 4). Only two weak reflectors, occurring at approximately 0.06 s (98 m) and 0.08 s (132 m) below the seafloor (Fig.4) can be traced laterally with any consistency. The low reflectivity of this unit is, once again, indicative of subtle lithotogical
changes (therefore, weak acoustic impedance contrasts) between the sedimentary layers making up this unit. These observations are consistent with results from sites 386 and 387 which show these sediments to be a nannofossil ooze/clay combination. D T A G S profiles indicate, however, that roughly the top 25 m of sediment are more reflective than the underlying sediments (Fig.4). Examination of the 3.5 kHz profiles (Fig.7) similarly show the upper sediments to be reflective, strongly laminated, and also gently undulating. The contact with the underlying non-laminated Itransparent) sediments is usually sharp. The gentle undulations of these surficial deposits and the nature of the internal bedding suggest mudwave bedforms and, therefore, that deposition is current controlled. Embley et al. (1980) mapped similar sediments and bedforms approximately 170 n mile southwest of the D T A G S survey area. The scale of the mudwaves (20 30 m high, 4 6 km apart) and the appearance of the layering strongly resembles the profile shown in Fig.7. Typically, the seafloor in the survey area returns
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Fig.6. Enlargement of left side of Fig.4B showing layer resolution achieved by DTAGS.
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strong, coherent echoes. Figure 8, however, shows an area of local hyperbolic echoes and reduced surface reflectivity. The roughness elements responsible for these acoustic effects are too small to be resolved by either the surface echosounder or by DTAGS. These effects may, however, be caused by elongate, parallel furrows cut into the sediment• Examples of furrowed bottoms shown by Embley
et al. (1980) are strikingly similar to Fig.8. In addition, hyperbolic eciaoes are widely distributed across the Blake Bahama Outer Ridge (Bryan and Markl, 1966) where they are also caused by long, linear furrows superimposed on mud waves (Hollister et al., 1974; Tucholke, 1979: Flood and Hollister, 1980). The depositional asymmetry exhibited by the
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Fig.8. DTAGS CSP profile. Note the change in bottom character from smooth, high reflectivity on the right t~ ~,:.~i~h. diffuse reflectivity on the left. Bottom furrows may account fi~r this change
post-Horizon A v sediments, in general, indicates that deposition of the entire unit is bottom-current controlled (Bowles, 1980); Laine and Hollister, 1981). In the absence of actual sediment samples, one can presume from the higher impedance of the surficial deposits that they may be texturally coarser than the underlying acoustically nonlaminated deposits. Laine and Hollister (1981) indicate that the acoustically laminated deposits on the eastern and northern Bermuda Rise are Pleistocene in age and reflect the deposition of coarser grained (glacial) sediments during the Pleistocene than during the Neogene and Paleogene. Their reasoning seems applicable, as well, for the acoustically laminated sediments south-southwest of Bermuda. The Pleistocene, however, is noted as having periods of intensified bottomwater circulation. As an alternative possibility, the increased impedance and layering may reflect winnowing of fine-grained sediment due to higher bottom-current velocities during glacial intervals compared with interglacial intervals. Sediment thickness
Acoustic penetration by DTAGS (Fig.4) appears to be limited to about 350 m. Although this is probably near the limit of penetration, it is evident in Fig.3 that the AV-A 3 complex of reflectors rests on predominantly acoustically transparent sedi-
ments. Therefore, it is not possible to determine from the data just how far into the bottom DTAGS was able to penetrate. Whatever the depth of acoustic penetration may be, basement along the survey track is generally too deeply buried to be resolved by DTAGS. Some profiles show areas of diffuse scattering and hyperbolae indicating what are probably shallow buried basement peaks (Fig.9). Scattering of acoustic energy by the slopes and/or rough surfaces of these peaks prevents their surfaces from clearly being resolved. Although total sediment thickness cannot be determined continuously along the survey track, observations can be made concerning interlayer thicknesses. Most of the thickness variation within the upper 300 m of sediment, for example, takes place in the post-A ~ sediments. A general thickening of this interval toward the east-northeast reflects the influence of Bermuda as a source of sediment. Superimposed upon this trend are smaller, more random variations which reflect the current-controlled deposition of this sedimentary unit. Below A' the intervals between reflectors show little tendency to change in thickness. The overall thickness of the entire At-A 3 sequence, however, is slightly greater along the western DTAGS track lines. A westward thickening of the turbidite sequence is consistent with the concept of an influx of turbidite sediment from the Atlantic shelf of
GEOLOGY OF THE BERMUDA RISE FROM DEEP-TOWSEISMICDATA
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Fig.9. DTAGS profile. Areas of diffuse scattering and hyperbolae (arrows) indicate presence of shallow buried basement structures.
North America (Ewing et al., 1969). The observed thickening is at apparent odds with DSDP results which show a thicker turbidite sequence at site 386 than at site 387. However, it has been noted (McCave, 1979) that the thickness of the turbidites at site 386 is due partly to the fact that the site was a seafloor depression. That the At-A 3 shows little tendency to change in thickness indicates, further, that no major structure (and therefore nearby sediment source) existed prior to the Eocene volcanic events that built the present Bermuda Seamount. Sediment structure
D T A G S data reveal that the upper 300 m of sediment are largely undisturbed, even on a very fine scale. Undulating and upwarping of reflectors are most commonly observed. Although basement is not directly observable, as previously noted, these effects are presumably due to underlying basement structure. In most cases the gentleness of the bowing suggests differential compaction of the sediments rather than basement uplift. Conventional surface-tow seismic systems show that the Horizon A-complex in the Bermuda area has been deformed primarily by differential compaction (Bowles, 1980). The deformation is evident in the quasi-draped appearance of these predominantly turbidite layers. Individual reflectors within the Horizon A-complex, traced horizontally, show only occasional, minor, fault-like discontinuities. Many of these "discontinuities" appear to be acoustic artifacts caused by a change in layer
reflectivity (i.e. fadeout). Some, however, may reflect stratigraphic peculiarities (e.g. overlapping edges of turbidite deposits, pinchout, etc.), In either case, these types of reflector discontinuities are not evident in adjacent reflectors and, therefore, are clearly not faults. There are, however, a few instances where the Horizon A-complex has undergone minor post-depositional faulting (Fig.10) as well as major uplift and faulting (Fig.ll). The absence of Horizon A v in Fig.ll indicates that uplift occurred before the deposition of A v turbidites, but after the deposition of the A t (i.e., early to middle Eocene). Thus, the uplift is probably related to the development of the Bermuda Rise itself. The absence of any significant surface expression of this uplift is indicative of sediment redistribution (smoothing) by bottom currents. Gettrust et al. (1988) suggest that the termination of the A v horizon (Fig.4B) is related to a fault with surface expression. Re-examination of these data suggests that this "fault" is the result of an anomalous instrument depth reading; therefore, the termination of the A v layer shown in Fig.4B is not related to faulting. Control of the deposition of the volcaniclastic turbidites is principally a function of their coarse-grained nature resulting in rapid deposition once the turbidites reach the fiat seafloor at the base of the Bermuda seamount. Figure 12 shows three areas where the continuity of Horizon A v is disturbed, apparently by faulting. Only profile 12C appears to be an example of a true fault as evidenced by small offsets in the layers below. This criterion does not apply to profiles
12A and 12B in which the reflectors beneath A' a.,c actually rising (smoothly) instead of bemg downfaulted. In both profiles it appears, instead, that the upper part of Horizon A' is missing (exposing older material). In profile A. there is also the impression of truncated beds and more downcutting on the left side. These observathms are indicative of erosion and suggest that the features in profiles 12A and 12B represent turbidity current channels or, less likely, slump scars. Both interpretations are consistent with the deposition history of Horizon A'. Bermuda is clearly the predominant volcanic..tectonic feature of the Bermuda Rise. Considering the proximity (60 n mile) of the survey area to the seamount, it is surprising that seismic reflection data through the area reveal little secondary tectonic activity (faulting, piercement, etc.) related to the origin of the seamount. This suggests that most of the basement relief in the area existed prior to the development of the rise. Thus, it appears that the rise formed as a broad, but gentle upwarping that caused little change in the basement structure (with the exception of Bermuda) or deformation of the sediments. The apparent absence of significant disturbance to Horizon A" indicates that Bermuda has been inactive since its development during early/middle Eocene to middle Oligocene time.
P-wave velocity inversion Fig.[0. DTAGS protiles. Arrows poml I~, mintq bedding discontinuities caused by post-depositional faulling
Velocity-depth functions derived from the DTAGS data (Gettrust et al, 1988) using the Dix
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Fig. ll. DTAGS profile. Arrows point to major post-depositional uplift and faulting of Horizon A-complex.
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(1955) depth model indicate P-wave velocity inversions within the upper sediments of the survey area. One hundred and fifty-six inversions were observed and are distributed stratigraphically as follows: 102 occur between A c and A 3, 36 between A ' and A c, and 18 above A v. Nearly 90% of the inversions occur below Horizon A v within the siliceous turbidite deposits; of these, 65% are
located beneath Horizon A c which marks the top of interbedded porcelanitic cherts (Tucholke, 1981). The inversions range in magnitude from roughly 50 m/s to 2000 m/s with a median magnitude of about 680 m/s. Nearly one-half of the inversions have magnitudes below 500 m/s. The magnitudes of the inversions are generally far in excess of any error we believe would be introduced
by the calculation technique. As an estimate of error, we note that our calculations of the compressional wave velocity in water are within t% of the known (CTD) velocity. This is the simplest case, of course, and one can assume a larger error for the subbottom. Error will increase with the computation of interval velocity (choice of stacking velocity) and decreasing signal noise ratio with subbottom penetration. Nevertheless, the large magnitude of most of the mversions indicates that they are indeed caused by true subbottom variability. Summarizing the above paragraph, three aspects concerning the inversions are significant: (1) the inversions occur sporadically along the survey track, i.e., an inversion is not always found at each shot point, (2) the inversions occur principally, but randomly distributed, within the siliceous turbidite layers, and (3) the magnitude of the inversions differ greatly. We emphasize that within a particular seismic interval (e.g., A1-A 2) there is considerable horizontal variability in both the magnitude and location of the inversions. Such variability is not easily explained in light of the apparently uniform deposition that is indicated by the D T A G S profiles. Measured velocities on core samples recovered at sites 386 and 387 show that large fluctuations in sound velocity, typically several hundred m/s, occur below horizon A ~ (i.e. within the calcareous turbidites and cherty claystones). The high velocities are associated with layers of chert, limestone, and cemented claystone and very likely give rise to the strong reflecting horizons (A t, A ~, A 1 , etc.) seen in the D T A G S profiles. One could assume that the inversions we measured are associated with these high-velocity layers alternating with low-velocity layers, i.e. the inversions are lithological in nature. The high-velocity layers, however, are generally only a few tens of centimeters in thickness and cannot be resolved by DTAGS. We cannot measure the velocity of these layers, we can only measure the velocity of the intervals between these marker horizons. It is within these intervals, which are on the order of 20-50 m in thickness, that the inversions occur. Moreover, it is important to note that although we measure large inversions between A' and A ~, velocity measurements on core samples
Irom this interval are very unil\wm. In hght ol ~Jic above considerations, we suggest that flae invclsions may be related to excess pore pressures and present the following hypothesis. The distribution of Horizon A ~ (Fig.2t shows that the western portion of the DTA(;S survey track crosses the outer edges of this volcaniclastic turbidite layer. The appearance of A ~ can be seen in the right side of Fig.4B which, including 4A, shows the profile along which Gettrust et al. (1988) made their velocity-depth determinations. Horizon A ~ represents a series of rapid depositional events. The effect of these events was to rapidly load the sediments beneath A ~ causing increased consolidation and higher (excess) pore pressures relative to the adjacent unloaded sediments (i.e. where A ~ is absent). The increased consolidation is reflected by higher internal velocities in the sediments beneath A ~ (Fig.3; Gettrust et al., 1988). The effect of excess pore pressure is to decrease the effective stress and, therefore, the strength (rigidity) of the sediment. The reduced strength is expressed acoustically by lower seismic velocities where the fluids are primarily trapped. Velocity depth functions show that the inversions occur in the sediments beneath A ~, particularly beneath A ~ which marks the top of interbedded porcelanitic cherts, limestones, and cemented claystones (Tucholke, 1981). This suggests that pore fluids are being squeezed between these layers, resulting in excess pore pressures. Variations in the degree of chert development and limestone/claystone lithification, as well as variations ira the lateral continuity of these layers, could account for variations in pore fluid entrapment and, therefore, the patchy distribution of the inversions. Some vertical migration of fluids probably takes place (through cracks, gaps, etc.), however, one could expect a predominantly lateral migration of" the fluids as they move from beneath A' toward regions of lower overburden pressure. Figure 4 shows that the interval A~-A 3 is characterized by fadeouts, i.e. patches of diffuse reflectivity or even no reflectivity. These areas may be locations where the pore fluids are selectively migrating upward through "breaks" in the entrapping layers. It is important to point out that the fadeout phenomenon is common to seismic reflection
GEOLOGY OF THE BERMUDA RISE FROM DEEP-TOW SEISMIC DATA
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is markedly reduced. This suggests fundamental changes in the physical nature of the reflectors such as density, porewater, etc. Presently, we are examining our data in greater detail as we address the fadeout phenomenon. Horizon A ~ is a coarse-grained, high-velocity layer overlaying finer grained, lower velocity material; thus, it is likely that one would find inversions immediately beneath A ~ even without the buildup of excess pore pressures. However, the roughness at the bottom of A ~ makes it impossible to derive reliable velocity estimates for this layer alone. The A ~ A t interval velocity reported by Gettrust et al. (1988) is an average of the highvelocity A ~ material and low-velocity material directly beneath it (i.e. overlying At). As a result, we are unable to resolve inversions between A ~ and A t"
profiles and that there may be other explanations for the fadeouts that we observe. Figure 13A and 13B shows expanded seismic traces of the fadeout area indicated by the arrow in Fig.4A. Some preliminary observations can be made. Although the signal amplitudes of reflectors A ¢ and the one immediately below diminish sharply in shots 100 102 (Fig.13A), the A G C profiles (Fig.13B) demonstrates the coherency of these reflectors through the fadeout area. This suggests that (1) the actual layers represented by the reflectors are continuous (vis-a-vis, separations) and that (2) the layers have not thinned below some critical threshold thickness of detectability. The strength of reflector A 2 indicates, furthermore, that energy is penetrating the fadeout area. We also observe no diffraction effects associated with the fadeout. Both observations indicated that energy is not being scattered in the vicinity of the fadeout. The above observations indicate that changes in reflector thickness, gaps in the reflectors, or acoustic scatter, do not (in this case) appear to be the cause of the fadeout. It is clear that the impedance of the reflectors within the fadeout area
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The inversions above A ~ are associated with the weak reflectors occurring approximately 0.06 s (98 m) and 0.08 s (132 m) below the seafloor. These inversions are most unusual because they occur above A ~ and within predominantly transparent sediments. As a result, neither excess pore pressure
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nor lithology, as discussed above, seem appropriate reasons for these inversions. Note, however. that the reflectivity (i.e., impedance contrast) o1' these layers is highly variable, indicating that the physical phenomenon (e.g,, density, porosity, etc.) causing the inversions have considerable spatial variability. Silva and Booth (1986)state that there is evidence showing the sediments over large areas of the Bermuda Rise to be underconsolidated at depths below 8 m. Typically, underconsolidated sediments are soft, watery, and lower m rigidity than normally consolidated sediments. Thus, the presence of inversions within the post-A ~ sediments may be related to underconsolidation (Gettrust et al., 1988).
Conclusions (1) The geometry achieved by towing a relatively high frequency seismic source (250-650 Hz) and multichannel receiving system near the seafloor (500-700 m altitude) produces major improvement in resolving power for both structure and compressional velocities within the sediments over conventional surface-tow seismic methods. (2) The upper 300 m of sediment appear largely undisturbed. Although evidence of faulting associated with basement uplift is found, reflecting horizons can generally be traced horizontally with virtually no offset. Gentle undulating of reflectors is most common and is thought to reflect differential compaction over basement relief. These observations indicate that development of the Bermuda Rise caused little change in the basement structure (with the exception of the Bermuda Seamount) or deformation of the sediments, i.e. the rise formed as a broad but gentle upwarping, (3) The acoustic reflectivity and character of the reflectors is consistent with DSDP results from sites 386 and 387. Post-Horizon A" sediments, for example, are characterized by low impedance contrast which is consistent with their clay/ooze nature. The lateral continuity and even bedded, rhythmic appearance of earlier reflecting horizons is indicative of distal turbidite deposits. A slight tendency of these deposits to thicken toward the west suggests a source area along the North American continental margin.
(4) Velocity-depth functions derived i,~m~ lhcsc data (Gettrust et al., 1988) resolve seismic velocit~ reversions that occur principally below l torizo[~ A' and most occurring within a sequence of strong, closely spaced reflectors associated with Horizop, A ~. The lower velocities are thought to result from a buildup of excess pore pressure between layers oF porcellanitic chert, limestone and cememed clays-tone. The excess pressure is, in turn, thought to result from the rapid loading (hence, consolidation) of the sediment column by the deposition of volcaniclastic turbidites (i.e. Horizon A'). Areas of little or no reflectivity in the reflection profiles are interpreted as possible vertical migration sites lk~r the escaping pore fluids. (5) Velocity inversions were resolved within the predominantly transparent sediments overlying the A ~ horizon. The existence of these inversions is inconsistent with present models depicting increasing velocity gradients within the upper (t00-170 m) unconsolidated sediment column and emphasizes the spatial variability of physical properties within these sediments. (6) Although the post-Horizon A ~ sediments are, in general, acoustically transparent, the topmost 25 m or so of sediment are reflective and acoustically laminated. The shaping of these surficial deposits into low-amplitude mudwaves is indicative of the current-controlled deposition that has characterized post-A ~ deposition in general. The greater reflectivity and laminated nature of the surficial deposits is thought to reflect intensified bottom-current velocities during Pleistocene glacial periods.
Acknowledgements The authors thank Dr. Peter Fleischer for critically reviewing the manuscript. This work was supported by the Office of Naval Research, Program Element 61153N through the NORDA Defense Research Sciences Program, and the Office of Naval Technology (Program Element 624 35N). NORDA Contribution Ja 362:042:89.
References Bowles, F.A., 1980. Stratigraphy and sedimentation of the archipelagic apron and adjoining area southeast of Bermuda, Mar. Geol., 37: 267-294.
GEOLOGY OF IHE BFRMUDARISEFROMDEEP-TOWSEISMICDAFA Bryan, G.M. and Markl, R.G., 1966. Microtopography of the Blake-Bahama region. Lamont-Doherty Geol. Obs. Tech. Rep. 8 CO-8-66, 44 pp. Dix, C.H., 1955. Seismic velocities from surface measurements. Geophysics, 20:67 86. Embley, R.W., Hoose, P.J., Lonsdale, P., Mayer, L. amd Tucholke, B.E., 1980. Furrowed mud waves on the western Bermuda Rise. Geol. Sco. Am. Bull., 91: 731-740. Ewing, M.. Worzel, J.k., Beall, A.O., Bergren, W.A., Burky, D., Burk, C.A., Fischer. A.G. and Pessagno, E.A., 1969. Initial Reports of the Deep-Sea Drilling Project, 1. U.S. Gov. Print. Off'., Washington, D.C., 672 pp. Flood, R.D. and Hollister, C.D., 1980. Submersible studies of deep-sea furrows and transverse ripples in cohesive sediments. Mar. Geol., 36: MI M9. Gettrust, J.F., Grimm, M., Madosik, S. and Rowe, M., 1988. Results of a deep-tow multichannel survey on the Bermuda Rise. Geophys. Res. Lett., 15 (12): 1413 1416. Hollister, C.D., Flood, R.D., Johnson, D.A., Lonsdale, P.F. and Southard. J.B., 1974. Abyssal Furrows and hyperbolic echo traces on the Bahama Outer Ridge. Geology, 2: 395 4O{). Laine, E.P. and Hollister, C.D., 1981. Geological effects of the Gulf Stream on the northern Bermuda Rise. Mar. Geol., 39: 277 310. McCave, I.N., 1979. Diagnosis ofturbidites at Sites 386 and 387 by particle-counter size analysis of the silt (2-40 gm) fraction.
293 In: B.E. Tucholke, P.R. Vogt et al., Initial Reports of the Deep-Sea Drilling Project, 43. U.S. Gov. Print. Off., Washington D.C., pp.395 402. Reich, V. and Von Rad, U., 1979. Eocene porcellanites and early Cretaceous cherts from the western North Atlantic Basin. In: B.E. Tucholke, P.R. Vogt et al., Initial Reports of the Deep-Sea Drilling Project, 43. U.S. Gov. Print. Off., Washington D.C., pp.437 448. Silva, A.J. and Booth, J.S.. 1986. Seabed geotechnical properties and seafloor utilization. In: P.R. Vogt and B.E. Tucholke (Editors), The Geology of North America. Vol. M, The Western North Atlantic Region. Geol. Soc. Am., Boulder, Colo., pp.491 506. Tucholke, B.E., 1979. Relationships between acoustic stratigraphy and lithostratigraphy in the western North Atlantic Basin. In: B.E. Yucholke, P.R. Vogt et al., Initial Reports of the Deep-Sea Drilling Project, 43. U.S. Gov. Print. Off., Washington, D.C., pp.827 846. Tucholke, B.E., 1981. Geologic significance of seismic reflectors in the deep western North Atlantic Basin. Soc. Econ. Paleontol. Mineral. Spec. Publ., 32:23 37. Tucholke, B.E. and Mountain, G.S., 1979. Seismic stratigraphy, lithostratigraphy, and paleosedimentation patterns in the North American Basin. In: M. Talwani, W. Hey and WB.F. Ryan (Editors), Deep Drilling results in the Atlantic Ocean: Continental Margins and Paleoenvironment. Am. Geophys. Union, Washington, pp.58 86.
279
Geological interpretations based on deep-tow single channel and multichannel seismic data from the Bermuda Rise F.A. Bowles, J.F. Gettrust and M. Rowe N O A R L , Stennis Space Center, M S 39529-5004, U.S.A.
(Received June 23, 1989; revision accepted May 16, 1990)
ABSTRACT Bowles, F.A., Gettrust, J.F. and Rowe, M., 1991. Geological interpretations based on deep-tow single channel and multichannel seismic data from the Bermuda Rise. Mar. Geol,, 96: 279-293. Multichannel seismic data taken with a deep-tow system approximately 170 n mile south-southwest of Bermuda reveal the upper 300 m of sediment in detail unachievable by conventional single-channel and multichannel seismic reflection methods. These data show the upper sediments to be largely undisturbed although faulting and basement uplift are evident. The acoustic reflectivity of the sediments and nature of the reflectors are consistent with DSDP data from the region. Horizons A v, A', and A c of the Horizon A-complex are observed, in addition to numerous reflecting horizons associated with porcellanitic chert, limestone, and cemented claystone layers of Ac. Seismic velocity inversions observed generally beneath A c are thought to result from sediment loading and buildup of excess pore pressures in the sediments below Horizon A v. Upward migration of these fluids may account for localized zones of little or no subbottom acoustic reflectivity. Seafloor structural details resolved include erosional channels or slump scars, mud waves, and possible furrows.
Introduction In 1984 d a t a were collected s o u t h - s o u t h w e s t o f B e r m u d a (Figs.1 a n d 2) in w a t e r d e p t h s o f a b o u t 5000 m using the D e e p - T o w e d A r r a y G e o p h y s i c a l System ( D T A G S ) d e v e l o p e d by the N a v a l O c e a n Research and Development Activity (NORDA). D T A G S is a m u l t i c h a n n e l seismic system consisting o f a 2 5 0 - 6 5 0 H z b a n d w i d t h source a n d a 24channel (42 m g r o u p spacing) a r r a y . Both the source a n d a r r a y are t o w e d n e a r the seafloor (within a few h u n d r e d meters) in o r d e r to o b t a i n h i g h - r e s o l u t i o n i n f o r m a t i o n a b o u t the structure a n d elastic p r o p e r t i e s o f d e e p - o c e a n sediments. D T A G S spatial r e s o l u t i o n is on the o r d e r o f 15 m h o r i z o n t a l l y a n d 5 m vertically. V e l o c i t y - d e p t h functions derived f r o m the m u l t i c h a n n e l seismic d a t a have been r e p o r t e d by G e t t r u s t et al. (1988), in a d d i t i o n to i n f o r m a t i o n a b o u t the D T A G S system, d a t a processing, a n d survey m o d e . In this p a p e r , we present geological i n t e r p r e t a t i o n s b a s e d on analysis o f a larger set o f these d a t a . 0025-3227/91/$03.50
The seismic s t r a t i g r a p h y o f the s e d i m e n t c o l u m n in the general a r e a surveyed by D T A G S has been discussed by T u c h o l k e (1979) a n d T u c h o l k e a n d M o u n t a i n (1979). C o n v e n t i o n a l single channel seismic reflection profiles, in general, show the sediments a b o v e volcanic b a s e m e n t to be acoustically t r a n s p a r e n t , b u t d i v i d e d into u p p e r a n d lower units by an a c o u s t i c a l l y l a m i n a t e d zone ( H o r i z o n A - c o m p l e x ) t h a t is laterally c o n t i n u o u s (Fig.3). O f the m a j o r reflecting h o r i z o n s within this zone, D T A G S has resolved reflectors A v, A t, a n d A ¢, in a d d i t i o n to several o t h e r reflectors b e n e a t h A c ( G e t t r u s t et al., 1988).
Seismic stratigraphy Horizon A v
C o m p a r i s o n o f Fig.3 with Fig.4 shows that D T A G S achieves a significant gain in resolution over c o n v e n t i o n a l surface-tow seismic systems. In p a r t i c u l a r , the i n d i v i d u a l reflecting h o r i z o n s o f the
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laminated zone appear in considerable detail. Seismic reflection profiles presented by Bowles (1980), for example, reveal Horizon A V(associated with volcaniclastic turbidites) as a thick, intensely reflective layer with little visible internal structure. DTAGS profiles, however, show Horizon A ~ to be highly irregular, exhibiting significant interface roughness and variation in layer thickness (Fig.4B). Abrupt changes in the depth of the layer up to about 0.01 s (7 m) occur over horizontal distances as short as 15 m (Fig.5). There does not appear to be, however, any apparent regularity in wavelength or amplitude to the roughness. Generally, variations in layer thickness occur over relatively short lateral distances (several tens of meters): Measurements of layer thickness were made at several places on the profiles where A ~
showed well-defined top and bottom boundaries. Using an interval velocity of 2079 m/s (Gettrust et al., 1988), the layer thickness was found to vary between 40-110 m. This is less than the 161.2-171.2 m reported at site 386; however, the increased thickness at 386 can be attributed to the fact that the site was a seafloor depression (McCave, 1979). Some of the variation in thickness (in the DTAGS profiles) may be due to acoustic scattering by the rough surface of A v causing poor acoustic penetration (thus giving the impression of thinning). Sediments recovered at D S D P Site 386 show that Horizon A V consists of calcareous turbidites (upper 8.7 m) overlying volcaniclastic turbidites (lower 161.7-171.2 m). Each subunit is characterized by multiple turbidite layers defined b y sharp
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basal contacts and by size grading (Tucholke, 1979). Interestingly, the D T A G S profiles do not indicate a regular, laminated internal structure, but instead, one that appears more chaotic. This effect is enhanced by lateral variations in reflectivity that give A V a patchy appearance. These observations suggest considerable lateral and vertical variability in the physical make-up of Horizon A Vthat would not be discernible from a single drillhole nor from conventional seismic profiles. Gettrust et al. (1988) report an interval velocity of 2079 m/s for A V A t (as will be discussed later, reliable velocity estimates cannot be obtained for only AV); however, velocities derived at specific shotpoints along the track show wide variability. Examination of a more extended data set shows a mean internal velocity of 2045_+ 500 m/s. The range of internal velocities is indicative of variable internal structure which, in turn, is consistent with the depositional/ sedimentological scenario described below. Horizon A V represents the material from Ber-
281
muda after it was eroded to wave base during middle Eocene to late Oligocene time, with evidence of continuing volcanism into the middle Oligocene (Tucholke, 1979). Seismic reflection data show that Horizon A v is a relict archipelagic apron locally distributed around Bermuda (Bowles, 1980). Thus, debris was shed from Bermuda and distributed radially downslope by a succession of turbidity currents, debris flows, and probably slumps. The middle to outer apron very likely consists, then, of overlapping and coalescing fanlike deposits characterized by a variety of erosional and depositional features such as cut and fill channel structures, levees, discontinuous and deformed layers, sediment lenses, disconformable bedding, etc. The total effect of these features would be to form a deposit around Bermuda that is characterized by rough interfaces,lateral changes in thickness and textural variability (both laterally and vertically), and variable acoustic reflectivity. Such characteristics are consistent with the nature of A v as observed in the D T A G S profiles. Horizon A'
Horizon A t, which occurs approximately 0.05 s below Horizon A V (Fig.4B), has been correlated with the top of a middle Eocene sequence of finegrained siliceous turbidites (Tucholke, 1979). In contrast to Horizon A v, A t appears as a thin, relatively faint layer that disappears and reappears intermittently along the survey track. This patchy, weakly reflective character could be attributed, in part, to acoustic masking by Horizon A V. However, Horizon A t exhibits these same characteristics in areas where A V is absent, indicating that the acoustic appearance of A t is primarily a function of its physical make-up. Strong impedance (density x velocity) contrasts, required to produce a sharp reflector, result primarily from sharp lithological changes because density and velocity are dependent on sediment texture and composition. The low reflectivity of Horizon A' indicates a less distinct lithological change between it and the overlying sediments than, for example, the case of Horizon A". Although the turbidites of Horizon A t are thick, ranging from 30 cm to about 3 m, the bulk of the
Fig,3. Conventional surface-tow, single-channel seismic reflection profile passing through DTAGS survey area. Note the lack of definition within the highly reflective A " A ~ zone
turbidite material consists of a homogeneous silty clay (78% clay). The average thickness of the coarse-grained basal unit is only about 10 cm and, unlike the terrigenous sands and silts that characterize abyssal plain turbidites, are dominated by siliceous fossils (Tucholke et al., 1979). Thus, the poor reflectivity of Horizon A t is largely due to the predominantly fine-grained, structureless character of the layers that make up this stratigraphic unit. The intermittent reflectivity of Horizon A t may be a response to lateral changes in physical character such as texture or roughness. Alternatively, the reflectivity of A t may be linked to the migration of pore fluids. This possibility will be presented in the section discussing velocity inversions.
Horizon A ~
Approximately 0.06 s beneath Horizon A ~begins a sequence of closely spaced parallel reflectors with a thickness of at least 0.11 s (Fig.4). This marks the first time that the layering immediately beneath Horizon A c has been seen in such detail. The evenbedded, rhythmic appearance of the reflectors is striking and, where the sequence is not masked by Horizon A v, ten reflectors of varying reflectivity are resolved (Fig.6). Such regularity arouses suspicion that some of the reflectors seen in Fig,6 may, in fact, be multiples. With regard to this, it is important to note that the lithology of the sediments at sites 386 and 387 supports a highly
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layered sequence below Horizon A c which should be characterized by both strongly and weakly reflective horizons, as observed. Multiples can be ruled out in the case of the strong reflectors seen in Fig.6 because they all have different interval velocities. Unfortunately, we cannot be as certain in the case of the weaker reflectors because accurate velocity determinations were not possible. We note, however, that in a stacked profile corresponding to Fig.6, all the weak reflectors as well as the strong reflectors are visible. Usually, the stacking procedure will suppress multiples. This strongly indicates, therefore, that the reflectors seen in Fig.6 correspond to real impedance contrasts within the sediments. Gettrust et al. (1988) have correlated the top of the sequence with Horizon A c which, in turn, has been correlated (Tucholke et al., 1979) with the top
of lower to middle Eocene siliceous claystones, radiolarian mudstones, and interbedded porcellanite (immature cherts). These deposits, like those associated with Horizon A', have been interpreted as predominantly turbidites, although the sedimentary structures typical of turbidites are no longer readily (megascopically) apparent. The fact that these turbidite sequences are visible at all is very likely due to the presence of the porcellanite layers. These layers correlate positively with the turbidites in that they result from the conversion of siliceous microfossils, making up the basal unit of each turbidite, into porcellanite (Reich and von Rad, 1979). This conversion appears to be related to depth of burial (i.e., temperature, pressure). Thus, porcellanite has not developed in the siliceous turbidites above A c which, consequently, show no internal layering.
Fig.5. Enlargementof right side of Fig.4B showing the rough sedimentar~interfaceand variable thickness oi Horizon A~',mr,~,de,airsof layering as defined by Genrust et al. (1988) The source area for both the A' and A ~ turbidites is thought to be the North American continental margin. High-density, high-velocity turbidity currents which reach the outer (distal) portions of basins are predominantly fine-grained and broadfronted, moving as sheet flow rather than within specific channels. Such currents are capable of depositing great quantities of largely homogeneous, fine-grained sediment over wide areas. The lateral continuity and thin, even-bedded nature of the A c A 3 deposits (relative to A t) revealed by the DTAGS profiles, supports the interpretation that they are indeed distal turbidites. Post-Horizon
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In general, the sediments overlying Horizon A' (and A t) appear acoustically transparent (Figs.3 and 4). Only two weak reflectors, occurring at approximately 0.06 s (98 m) and 0.08 s (132 m) below the seafloor (Fig.4) can be traced laterally with any consistency. The low reflectivity of this unit is, once again, indicative of subtle lithotogical
changes (therefore, weak acoustic impedance contrasts) between the sedimentary layers making up this unit. These observations are consistent with results from sites 386 and 387 which show these sediments to be a nannofossil ooze/clay combination. D T A G S profiles indicate, however, that roughly the top 25 m of sediment are more reflective than the underlying sediments (Fig.4). Examination of the 3.5 kHz profiles (Fig.7) similarly show the upper sediments to be reflective, strongly laminated, and also gently undulating. The contact with the underlying non-laminated Itransparent) sediments is usually sharp. The gentle undulations of these surficial deposits and the nature of the internal bedding suggest mudwave bedforms and, therefore, that deposition is current controlled. Embley et al. (1980) mapped similar sediments and bedforms approximately 170 n mile southwest of the D T A G S survey area. The scale of the mudwaves (20 30 m high, 4 6 km apart) and the appearance of the layering strongly resembles the profile shown in Fig.7. Typically, the seafloor in the survey area returns
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strong, coherent echoes. Figure 8, however, shows an area of local hyperbolic echoes and reduced surface reflectivity. The roughness elements responsible for these acoustic effects are too small to be resolved by either the surface echosounder or by DTAGS. These effects may, however, be caused by elongate, parallel furrows cut into the sediment• Examples of furrowed bottoms shown by Embley
et al. (1980) are strikingly similar to Fig.8. In addition, hyperbolic eciaoes are widely distributed across the Blake Bahama Outer Ridge (Bryan and Markl, 1966) where they are also caused by long, linear furrows superimposed on mud waves (Hollister et al., 1974; Tucholke, 1979: Flood and Hollister, 1980). The depositional asymmetry exhibited by the
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post-Horizon A v sediments, in general, indicates that deposition of the entire unit is bottom-current controlled (Bowles, 1980); Laine and Hollister, 1981). In the absence of actual sediment samples, one can presume from the higher impedance of the surficial deposits that they may be texturally coarser than the underlying acoustically nonlaminated deposits. Laine and Hollister (1981) indicate that the acoustically laminated deposits on the eastern and northern Bermuda Rise are Pleistocene in age and reflect the deposition of coarser grained (glacial) sediments during the Pleistocene than during the Neogene and Paleogene. Their reasoning seems applicable, as well, for the acoustically laminated sediments south-southwest of Bermuda. The Pleistocene, however, is noted as having periods of intensified bottomwater circulation. As an alternative possibility, the increased impedance and layering may reflect winnowing of fine-grained sediment due to higher bottom-current velocities during glacial intervals compared with interglacial intervals. Sediment thickness
Acoustic penetration by DTAGS (Fig.4) appears to be limited to about 350 m. Although this is probably near the limit of penetration, it is evident in Fig.3 that the AV-A 3 complex of reflectors rests on predominantly acoustically transparent sedi-
ments. Therefore, it is not possible to determine from the data just how far into the bottom DTAGS was able to penetrate. Whatever the depth of acoustic penetration may be, basement along the survey track is generally too deeply buried to be resolved by DTAGS. Some profiles show areas of diffuse scattering and hyperbolae indicating what are probably shallow buried basement peaks (Fig.9). Scattering of acoustic energy by the slopes and/or rough surfaces of these peaks prevents their surfaces from clearly being resolved. Although total sediment thickness cannot be determined continuously along the survey track, observations can be made concerning interlayer thicknesses. Most of the thickness variation within the upper 300 m of sediment, for example, takes place in the post-A ~ sediments. A general thickening of this interval toward the east-northeast reflects the influence of Bermuda as a source of sediment. Superimposed upon this trend are smaller, more random variations which reflect the current-controlled deposition of this sedimentary unit. Below A' the intervals between reflectors show little tendency to change in thickness. The overall thickness of the entire At-A 3 sequence, however, is slightly greater along the western DTAGS track lines. A westward thickening of the turbidite sequence is consistent with the concept of an influx of turbidite sediment from the Atlantic shelf of
GEOLOGY OF THE BERMUDA RISE FROM DEEP-TOWSEISMICDATA
287
Fig.9. DTAGS profile. Areas of diffuse scattering and hyperbolae (arrows) indicate presence of shallow buried basement structures.
North America (Ewing et al., 1969). The observed thickening is at apparent odds with DSDP results which show a thicker turbidite sequence at site 386 than at site 387. However, it has been noted (McCave, 1979) that the thickness of the turbidites at site 386 is due partly to the fact that the site was a seafloor depression. That the At-A 3 shows little tendency to change in thickness indicates, further, that no major structure (and therefore nearby sediment source) existed prior to the Eocene volcanic events that built the present Bermuda Seamount. Sediment structure
D T A G S data reveal that the upper 300 m of sediment are largely undisturbed, even on a very fine scale. Undulating and upwarping of reflectors are most commonly observed. Although basement is not directly observable, as previously noted, these effects are presumably due to underlying basement structure. In most cases the gentleness of the bowing suggests differential compaction of the sediments rather than basement uplift. Conventional surface-tow seismic systems show that the Horizon A-complex in the Bermuda area has been deformed primarily by differential compaction (Bowles, 1980). The deformation is evident in the quasi-draped appearance of these predominantly turbidite layers. Individual reflectors within the Horizon A-complex, traced horizontally, show only occasional, minor, fault-like discontinuities. Many of these "discontinuities" appear to be acoustic artifacts caused by a change in layer
reflectivity (i.e. fadeout). Some, however, may reflect stratigraphic peculiarities (e.g. overlapping edges of turbidite deposits, pinchout, etc.), In either case, these types of reflector discontinuities are not evident in adjacent reflectors and, therefore, are clearly not faults. There are, however, a few instances where the Horizon A-complex has undergone minor post-depositional faulting (Fig.10) as well as major uplift and faulting (Fig.ll). The absence of Horizon A v in Fig.ll indicates that uplift occurred before the deposition of A v turbidites, but after the deposition of the A t (i.e., early to middle Eocene). Thus, the uplift is probably related to the development of the Bermuda Rise itself. The absence of any significant surface expression of this uplift is indicative of sediment redistribution (smoothing) by bottom currents. Gettrust et al. (1988) suggest that the termination of the A v horizon (Fig.4B) is related to a fault with surface expression. Re-examination of these data suggests that this "fault" is the result of an anomalous instrument depth reading; therefore, the termination of the A v layer shown in Fig.4B is not related to faulting. Control of the deposition of the volcaniclastic turbidites is principally a function of their coarse-grained nature resulting in rapid deposition once the turbidites reach the fiat seafloor at the base of the Bermuda seamount. Figure 12 shows three areas where the continuity of Horizon A v is disturbed, apparently by faulting. Only profile 12C appears to be an example of a true fault as evidenced by small offsets in the layers below. This criterion does not apply to profiles
12A and 12B in which the reflectors beneath A' a.,c actually rising (smoothly) instead of bemg downfaulted. In both profiles it appears, instead, that the upper part of Horizon A' is missing (exposing older material). In profile A. there is also the impression of truncated beds and more downcutting on the left side. These observathms are indicative of erosion and suggest that the features in profiles 12A and 12B represent turbidity current channels or, less likely, slump scars. Both interpretations are consistent with the deposition history of Horizon A'. Bermuda is clearly the predominant volcanic..tectonic feature of the Bermuda Rise. Considering the proximity (60 n mile) of the survey area to the seamount, it is surprising that seismic reflection data through the area reveal little secondary tectonic activity (faulting, piercement, etc.) related to the origin of the seamount. This suggests that most of the basement relief in the area existed prior to the development of the rise. Thus, it appears that the rise formed as a broad, but gentle upwarping that caused little change in the basement structure (with the exception of Bermuda) or deformation of the sediments. The apparent absence of significant disturbance to Horizon A" indicates that Bermuda has been inactive since its development during early/middle Eocene to middle Oligocene time.
P-wave velocity inversion Fig.[0. DTAGS protiles. Arrows poml I~, mintq bedding discontinuities caused by post-depositional faulling
Velocity-depth functions derived from the DTAGS data (Gettrust et al, 1988) using the Dix
0
Fig. ll. DTAGS profile. Arrows point to major post-depositional uplift and faulting of Horizon A-complex.
GEOLOGY OF THE BERMUDA RISE FROM DEEP-TOW SEISMIC DATA
289
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(1955) depth model indicate P-wave velocity inversions within the upper sediments of the survey area. One hundred and fifty-six inversions were observed and are distributed stratigraphically as follows: 102 occur between A c and A 3, 36 between A ' and A c, and 18 above A v. Nearly 90% of the inversions occur below Horizon A v within the siliceous turbidite deposits; of these, 65% are
located beneath Horizon A c which marks the top of interbedded porcelanitic cherts (Tucholke, 1981). The inversions range in magnitude from roughly 50 m/s to 2000 m/s with a median magnitude of about 680 m/s. Nearly one-half of the inversions have magnitudes below 500 m/s. The magnitudes of the inversions are generally far in excess of any error we believe would be introduced
by the calculation technique. As an estimate of error, we note that our calculations of the compressional wave velocity in water are within t% of the known (CTD) velocity. This is the simplest case, of course, and one can assume a larger error for the subbottom. Error will increase with the computation of interval velocity (choice of stacking velocity) and decreasing signal noise ratio with subbottom penetration. Nevertheless, the large magnitude of most of the mversions indicates that they are indeed caused by true subbottom variability. Summarizing the above paragraph, three aspects concerning the inversions are significant: (1) the inversions occur sporadically along the survey track, i.e., an inversion is not always found at each shot point, (2) the inversions occur principally, but randomly distributed, within the siliceous turbidite layers, and (3) the magnitude of the inversions differ greatly. We emphasize that within a particular seismic interval (e.g., A1-A 2) there is considerable horizontal variability in both the magnitude and location of the inversions. Such variability is not easily explained in light of the apparently uniform deposition that is indicated by the D T A G S profiles. Measured velocities on core samples recovered at sites 386 and 387 show that large fluctuations in sound velocity, typically several hundred m/s, occur below horizon A ~ (i.e. within the calcareous turbidites and cherty claystones). The high velocities are associated with layers of chert, limestone, and cemented claystone and very likely give rise to the strong reflecting horizons (A t, A ~, A 1 , etc.) seen in the D T A G S profiles. One could assume that the inversions we measured are associated with these high-velocity layers alternating with low-velocity layers, i.e. the inversions are lithological in nature. The high-velocity layers, however, are generally only a few tens of centimeters in thickness and cannot be resolved by DTAGS. We cannot measure the velocity of these layers, we can only measure the velocity of the intervals between these marker horizons. It is within these intervals, which are on the order of 20-50 m in thickness, that the inversions occur. Moreover, it is important to note that although we measure large inversions between A' and A ~, velocity measurements on core samples
Irom this interval are very unil\wm. In hght ol ~Jic above considerations, we suggest that flae invclsions may be related to excess pore pressures and present the following hypothesis. The distribution of Horizon A ~ (Fig.2t shows that the western portion of the DTA(;S survey track crosses the outer edges of this volcaniclastic turbidite layer. The appearance of A ~ can be seen in the right side of Fig.4B which, including 4A, shows the profile along which Gettrust et al. (1988) made their velocity-depth determinations. Horizon A ~ represents a series of rapid depositional events. The effect of these events was to rapidly load the sediments beneath A ~ causing increased consolidation and higher (excess) pore pressures relative to the adjacent unloaded sediments (i.e. where A ~ is absent). The increased consolidation is reflected by higher internal velocities in the sediments beneath A ~ (Fig.3; Gettrust et al., 1988). The effect of excess pore pressure is to decrease the effective stress and, therefore, the strength (rigidity) of the sediment. The reduced strength is expressed acoustically by lower seismic velocities where the fluids are primarily trapped. Velocity depth functions show that the inversions occur in the sediments beneath A ~, particularly beneath A ~ which marks the top of interbedded porcelanitic cherts, limestones, and cemented claystones (Tucholke, 1981). This suggests that pore fluids are being squeezed between these layers, resulting in excess pore pressures. Variations in the degree of chert development and limestone/claystone lithification, as well as variations ira the lateral continuity of these layers, could account for variations in pore fluid entrapment and, therefore, the patchy distribution of the inversions. Some vertical migration of fluids probably takes place (through cracks, gaps, etc.), however, one could expect a predominantly lateral migration of" the fluids as they move from beneath A' toward regions of lower overburden pressure. Figure 4 shows that the interval A~-A 3 is characterized by fadeouts, i.e. patches of diffuse reflectivity or even no reflectivity. These areas may be locations where the pore fluids are selectively migrating upward through "breaks" in the entrapping layers. It is important to point out that the fadeout phenomenon is common to seismic reflection
GEOLOGY OF THE BERMUDA RISE FROM DEEP-TOW SEISMIC DATA
291
is markedly reduced. This suggests fundamental changes in the physical nature of the reflectors such as density, porewater, etc. Presently, we are examining our data in greater detail as we address the fadeout phenomenon. Horizon A ~ is a coarse-grained, high-velocity layer overlaying finer grained, lower velocity material; thus, it is likely that one would find inversions immediately beneath A ~ even without the buildup of excess pore pressures. However, the roughness at the bottom of A ~ makes it impossible to derive reliable velocity estimates for this layer alone. The A ~ A t interval velocity reported by Gettrust et al. (1988) is an average of the highvelocity A ~ material and low-velocity material directly beneath it (i.e. overlying At). As a result, we are unable to resolve inversions between A ~ and A t"
profiles and that there may be other explanations for the fadeouts that we observe. Figure 13A and 13B shows expanded seismic traces of the fadeout area indicated by the arrow in Fig.4A. Some preliminary observations can be made. Although the signal amplitudes of reflectors A ¢ and the one immediately below diminish sharply in shots 100 102 (Fig.13A), the A G C profiles (Fig.13B) demonstrates the coherency of these reflectors through the fadeout area. This suggests that (1) the actual layers represented by the reflectors are continuous (vis-a-vis, separations) and that (2) the layers have not thinned below some critical threshold thickness of detectability. The strength of reflector A 2 indicates, furthermore, that energy is penetrating the fadeout area. We also observe no diffraction effects associated with the fadeout. Both observations indicated that energy is not being scattered in the vicinity of the fadeout. The above observations indicate that changes in reflector thickness, gaps in the reflectors, or acoustic scatter, do not (in this case) appear to be the cause of the fadeout. It is clear that the impedance of the reflectors within the fadeout area
98 I
~-
~
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100 ~
~
The inversions above A ~ are associated with the weak reflectors occurring approximately 0.06 s (98 m) and 0.08 s (132 m) below the seafloor. These inversions are most unusual because they occur above A ~ and within predominantly transparent sediments. As a result, neither excess pore pressure
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nor lithology, as discussed above, seem appropriate reasons for these inversions. Note, however. that the reflectivity (i.e., impedance contrast) o1' these layers is highly variable, indicating that the physical phenomenon (e.g,, density, porosity, etc.) causing the inversions have considerable spatial variability. Silva and Booth (1986)state that there is evidence showing the sediments over large areas of the Bermuda Rise to be underconsolidated at depths below 8 m. Typically, underconsolidated sediments are soft, watery, and lower m rigidity than normally consolidated sediments. Thus, the presence of inversions within the post-A ~ sediments may be related to underconsolidation (Gettrust et al., 1988).
Conclusions (1) The geometry achieved by towing a relatively high frequency seismic source (250-650 Hz) and multichannel receiving system near the seafloor (500-700 m altitude) produces major improvement in resolving power for both structure and compressional velocities within the sediments over conventional surface-tow seismic methods. (2) The upper 300 m of sediment appear largely undisturbed. Although evidence of faulting associated with basement uplift is found, reflecting horizons can generally be traced horizontally with virtually no offset. Gentle undulating of reflectors is most common and is thought to reflect differential compaction over basement relief. These observations indicate that development of the Bermuda Rise caused little change in the basement structure (with the exception of the Bermuda Seamount) or deformation of the sediments, i.e. the rise formed as a broad but gentle upwarping, (3) The acoustic reflectivity and character of the reflectors is consistent with DSDP results from sites 386 and 387. Post-Horizon A" sediments, for example, are characterized by low impedance contrast which is consistent with their clay/ooze nature. The lateral continuity and even bedded, rhythmic appearance of earlier reflecting horizons is indicative of distal turbidite deposits. A slight tendency of these deposits to thicken toward the west suggests a source area along the North American continental margin.
(4) Velocity-depth functions derived i,~m~ lhcsc data (Gettrust et al., 1988) resolve seismic velocit~ reversions that occur principally below l torizo[~ A' and most occurring within a sequence of strong, closely spaced reflectors associated with Horizop, A ~. The lower velocities are thought to result from a buildup of excess pore pressure between layers oF porcellanitic chert, limestone and cememed clays-tone. The excess pressure is, in turn, thought to result from the rapid loading (hence, consolidation) of the sediment column by the deposition of volcaniclastic turbidites (i.e. Horizon A'). Areas of little or no reflectivity in the reflection profiles are interpreted as possible vertical migration sites lk~r the escaping pore fluids. (5) Velocity inversions were resolved within the predominantly transparent sediments overlying the A ~ horizon. The existence of these inversions is inconsistent with present models depicting increasing velocity gradients within the upper (t00-170 m) unconsolidated sediment column and emphasizes the spatial variability of physical properties within these sediments. (6) Although the post-Horizon A ~ sediments are, in general, acoustically transparent, the topmost 25 m or so of sediment are reflective and acoustically laminated. The shaping of these surficial deposits into low-amplitude mudwaves is indicative of the current-controlled deposition that has characterized post-A ~ deposition in general. The greater reflectivity and laminated nature of the surficial deposits is thought to reflect intensified bottom-current velocities during Pleistocene glacial periods.
Acknowledgements The authors thank Dr. Peter Fleischer for critically reviewing the manuscript. This work was supported by the Office of Naval Research, Program Element 61153N through the NORDA Defense Research Sciences Program, and the Office of Naval Technology (Program Element 624 35N). NORDA Contribution Ja 362:042:89.
References Bowles, F.A., 1980. Stratigraphy and sedimentation of the archipelagic apron and adjoining area southeast of Bermuda, Mar. Geol., 37: 267-294.
GEOLOGY OF IHE BFRMUDARISEFROMDEEP-TOWSEISMICDAFA Bryan, G.M. and Markl, R.G., 1966. Microtopography of the Blake-Bahama region. Lamont-Doherty Geol. Obs. Tech. Rep. 8 CO-8-66, 44 pp. Dix, C.H., 1955. Seismic velocities from surface measurements. Geophysics, 20:67 86. Embley, R.W., Hoose, P.J., Lonsdale, P., Mayer, L. amd Tucholke, B.E., 1980. Furrowed mud waves on the western Bermuda Rise. Geol. Sco. Am. Bull., 91: 731-740. Ewing, M.. Worzel, J.k., Beall, A.O., Bergren, W.A., Burky, D., Burk, C.A., Fischer. A.G. and Pessagno, E.A., 1969. Initial Reports of the Deep-Sea Drilling Project, 1. U.S. Gov. Print. Off'., Washington, D.C., 672 pp. Flood, R.D. and Hollister, C.D., 1980. Submersible studies of deep-sea furrows and transverse ripples in cohesive sediments. Mar. Geol., 36: MI M9. Gettrust, J.F., Grimm, M., Madosik, S. and Rowe, M., 1988. Results of a deep-tow multichannel survey on the Bermuda Rise. Geophys. Res. Lett., 15 (12): 1413 1416. Hollister, C.D., Flood, R.D., Johnson, D.A., Lonsdale, P.F. and Southard. J.B., 1974. Abyssal Furrows and hyperbolic echo traces on the Bahama Outer Ridge. Geology, 2: 395 4O{). Laine, E.P. and Hollister, C.D., 1981. Geological effects of the Gulf Stream on the northern Bermuda Rise. Mar. Geol., 39: 277 310. McCave, I.N., 1979. Diagnosis ofturbidites at Sites 386 and 387 by particle-counter size analysis of the silt (2-40 gm) fraction.
293 In: B.E. Tucholke, P.R. Vogt et al., Initial Reports of the Deep-Sea Drilling Project, 43. U.S. Gov. Print. Off., Washington D.C., pp.395 402. Reich, V. and Von Rad, U., 1979. Eocene porcellanites and early Cretaceous cherts from the western North Atlantic Basin. In: B.E. Tucholke, P.R. Vogt et al., Initial Reports of the Deep-Sea Drilling Project, 43. U.S. Gov. Print. Off., Washington D.C., pp.437 448. Silva, A.J. and Booth, J.S.. 1986. Seabed geotechnical properties and seafloor utilization. In: P.R. Vogt and B.E. Tucholke (Editors), The Geology of North America. Vol. M, The Western North Atlantic Region. Geol. Soc. Am., Boulder, Colo., pp.491 506. Tucholke, B.E., 1979. Relationships between acoustic stratigraphy and lithostratigraphy in the western North Atlantic Basin. In: B.E. Yucholke, P.R. Vogt et al., Initial Reports of the Deep-Sea Drilling Project, 43. U.S. Gov. Print. Off., Washington, D.C., pp.827 846. Tucholke, B.E., 1981. Geologic significance of seismic reflectors in the deep western North Atlantic Basin. Soc. Econ. Paleontol. Mineral. Spec. Publ., 32:23 37. Tucholke, B.E. and Mountain, G.S., 1979. Seismic stratigraphy, lithostratigraphy, and paleosedimentation patterns in the North American Basin. In: M. Talwani, W. Hey and WB.F. Ryan (Editors), Deep Drilling results in the Atlantic Ocean: Continental Margins and Paleoenvironment. Am. Geophys. Union, Washington, pp.58 86.