LOWER WILMINGTON DEPOCENTER/PROTO-HATTERAS OUTER RIDGE. 8 j_ ...... Berggren, W.A., Kent, D.V., Flynn, J.J. and Van Couvering,. North Atlantic ...
Marine Geology, 103 (1992) 137-164
137
Elsevier Science Publishers B.V., Amsterdam
Paleogene-Neogene depositional history of the middle U.S. Atlantic continental rise: mixed turbidite and contourite depositional systems Stanley D. Locker* and Edward P. Laine** Graduate School of Oceanography, University of Rhode Island, Kingston, RI 02881, USA (Received December 19, 1990; revision accepted March 25, 199 l)
ABSTRACT Locker, S.D. and Laine, E.P., 1992. Paleogene-Neogene depositional history of the middle U.S. Atlantic continental rise: mixed turbiditc and contourite depositional systems. Mar. Geol., 103: 137-164. The construction of the midtYe U.S. Atlantic continental rise from late Paleogene to late Pliocene time included downslope sediment gravity flows and alongslope contourite deposition. Some previous studies have suggested that contour-following bottom currents were the dominant control on depositional patterns. Depositional relief from two large contourite drifts, largely built in late Miocene to !ate Pliocene time, continue to dominate present-day lower rise morphology. Detailed seismic sequence mapping presented here suggests that significant turbidite (fan) systems also were active and are preserved in the stratigraphic section. A single thick depositional sequence is mapped for Oligocene to middle Miocene sediments deposited when inferred fan systems off the Norfolk-Washington canyons were reworked by bottom currents along the lower rise to form the protoHatteras Outer Ridge. Detailed mapping of three late Miocene to late Pliocene depositional sequences indicates that fanchannel systems were active at the same time large contourite drifts were being constructed. Sediment delivered to the lower rise by these elongate turbidite systems undoubtedly was an important local source for construction of the Hatteras Outer Ridge and, through entrainment and resuspension, was an important source for regional contourite deposition controlled by the deep Western Boundary Undercurrent. We suggest that submarine fans and current-controlled sediment drifts may develop simultaneously as companion systems. Types of fan/drift interaction include current-only modification of fans, transitional fan-to-drift, and adjacent or overlapping submarine fan and sediment drift deposition.
Introduction This paper reports on the depositional history o f the middle U.S. A t l a n t i c continental rise d u r i n g the late Palcogene to Neogen¢ based on seismic stratigraphic analysis (Fig.l). The section studied is b o u n d e d below by the H o r i z o n A u u n c o n f o r m i t y (Tucholke, 1979; T u c h o l k e and M o u n t a i n , 1979), a n d above by a late Pliocene u n c o n f o r m i t y (Locker, 1989). Sortie previous investigations o f *Present address: Department of Marine Science, University of South Florida, St. Petersburg, FL 33701, USA **Present address: Geology Department and Environmental Studies Program, Bowdoin College, Brunswick, ME 04011, USA 0025-3227/92/$05.00
this stratigraphic interval have indicated t h a t alongslope, current-controlled sedimentary processes d o m i n a t e d rise c o n s t r u c t i o n ( M o u n t a i n a n d Tucholke, 1985). O t h e r a u t h o r s have highlighted the i m p o r t a n c e of s u b m a r i n e fan systems in the c o n s t r u c t i o n o f the continental rise in this area ( T u c h o l k e a n d Laine, 1982; M c M a s t e r et al., 1989). S e d i m e n t a r y facies and m o r p h o l o g y often reflect the c o m b i n e d influence o f these depositional processes which have varied spatially and temporally. M o r e o v e r , most studies have n o t been detailed enough to resolve specific turbidite systems. T h e
longer term depositional patterns tend to reflect the shaping effects of alongslope bottom currents (McCave and Tucholke, 1986), especially during
© 1992 - - Elsevier Science Publishers B.V. All rights reserved
138
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the late Neogene. A key objective of this study was to gain a better understanding of the role submarine fans, or turbidite systems in general, play in construction of the continental rise, and how fan development is affected by bottom currents, We present evidence for the co-occurrence of submarine fan systems with previously identified current-controlled sediment drifts (i.e., Chesapeake Drift and Hatteras Outer Ridge) seaward of the Norfolk and Washington canyons. A fan/drift rondel is proposed where submarine fans and abyssal sediment drifts may develop simultaneously as companion systems, the growth patterns of each system partly influenced by the morphol-
ogy and sediment supply associated with the other system. For example, fan channel-levee systems may develop relatively free of bottom current reworking on the low-gradient backslope side of asymmetric sediment drifts. In turn, the locations of fan depocenters are important as a source for sediment entrained or reworked by abyssal boundary currents and thus play a role in the location and accumulation of some drift deposits. Background Previous seismic interpretations focused on resolving the major aspects of Mesozoic and Cenozoic stratigraphy in the western North Atlantic
! 39
PALEOGENE-NEOGENE DEPOSITIONAL HISTORY OF THE MIDDLE U.S. ATLANTIC CONTINENTAL RISE
(Grow a n d M a r k l , 1977; Shipleyetal., 1978;Grow et al., 1979; Tucholke and Mountain, 1979; Klitgord and Grow, 1980; Sheridan et al., 1981; Mountain and Tucholke, 1985; Mountain, 1987; Poag, 197)7; Poag and Scvon, 1989; Poag et al., 1990). Initial identification of Cenozoic seismic horizons was limited to identification of the Horizon-A complex, including the widely recognized late Paleogene unconformity Horizon A u (Tucholke, 1979; Tucholke and Mountain, 1979; Vail et al., 1980), and Horizon X, a problematic middle Miocene horizon identified as the top of a hummocky zone of chaotic and hyperbolic reflectors (Marki
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et al., 1970; Benson et al., 1978; Shipley and Watkins, 1978). Horizon A u is a major erosional unconformity along the North American margin presumably formed near the Eocene/Oligocene boundary in response to intensified abyssal circulation in the western North Atlantic (Miller et al., 1985). It is during post-A u deposition that the major features of the present-day continental rise developed in response to gravity and currentcontrolled sedimentation (Mountain and Tucholke, 1985). Horizon A u is the deepest stratigraphic datum correlated in this study (Fig.2). Two other major regional unconformities in the
Au
A M _ _ A M_
To BE \,A'EOCENE AM I.,..AU.d O1
Fig.2. Correlation diagram comparing seismic sequences mapped in this study with other work. Time scale after Berggren et ai.
(1985). Correlation with DSDP Sites 106 (Hollister et al., 1972) and 603 (Van Hinte et al., 1987) provides primary age control. For Site 106, "x'" marks talculated age range based on core control. Proposed sequence boundary ages shown versus Haq et al. (1988): (l) second order supercycles of Exxon model, (2) third order eustatic cycles, and (3) apparent correlation of sequence boundaries and their ages in this study. Approximate stratigraphic correlations for six other studies shown on right side of figure. Abbreviations: T + L ' 8 2 = T u c h o l k e and Laine (1982); M+ T '85=Mountain and Tucholke (1985); M c M et al. '89=McMaster et al. (1989); P '85 = Poag (1985); P '87= Poag (1987); S + H "87 = Schlee and Hinz (1987). For example, Blue (Mountain and Tucholke, 1985) may correlate with or be slightly above basal T3 boundary, and G (McMaster et al., 1989) may correlate with or be below basal T2 boundary. G, P, M l, M2, X, A", Blue, Merlin, and G identify seismic reflectors or boundaries. Other numbering schemes identify stratigraphic units.
140
upper middle Miocene and upper Pliocene are recognized by several investigators (Fig.2). These seismic boundaries, termed Merlin (u. middle Miocene) and Blue (u. Pliocene) by Mountain and Tucholke (1985), are closely related to global sealevel lowstands and associated downslope canyon cutting or intensification of abyssal bottom currents (e.g., Miller et al., 1987). Although the various investigators in general agree on the presence of these two major boundaries, minor differences in their stratigraphic position are found due to inherent differences in interpretation among different investigators, to stratigraphic complexities, and to the sparsity of well control, Mountain and Tucholke (1985) correlated Merlin with Horizon X on the upper rise, but placed it above Horizon X on the lower rise in the study area. McMaster et al. (1989) mapped a seismic horizon (G) which corresponds to Merlin and X on the upper rise, but is just below X at DSDP Site 603 (Van Hinte et ai., 1987). Inferred correlations between these seismic markers and the results in this study are shown in Fig.2. We mapped a major sequence boundary at the base of the upper Pliocene-Pleistocene section (sequence T3; Figs.2 and 3), which appears to match Blue on the lower rise, but lies below Blue as identified by Mountain and Tuchoike (1985) on the upper rise. This unconformity forms the upper limit of the section we studied in this paper and bounds an abrupt change in depositional style along the continental margin, which we believe is related to the onset of Northern Hemisphere glaciation, and an increased flux of terrigenous material to the continental rise at about 2.5 to 3.0 Ma (Shackleton and Kennett, 1975; Laine, 1 9 8 0 ; Tucholke and Laine, ! 982; Shackleton et ai., 1984; Mountain and Tucholke, 1985; Poag and Sevon, 1989).
Depositional models The primary depositional processes that have been inferred for the modern continental rise may
S.D. LOCKER AND E.P. LAINE
be grouped into two basic categories: (1) downslope gravity flow processes - - slumping, debris flows, turbidity currents, etc. (Embley and Jacobi, 1986; Pilkey and Cleary, 1986), and (2) alongslope processes controlled by ocean current systems (McCave and Tucholke, 1986). In the latter case, the Wvstern Boundary Undercurrent and Gulf Stream may act independently or interactively to erode, rework, and shape the morphology of the continental rise. Two end-member depositional settings can be interpreted from DSDP drilling in thinned Tertiary sections on the upper and lower continental rise along a transect seaward of New Jersey (Fig.I). The uppermost rise, strongly influenced by gravitycontrolled deposition adjacent to the shallow slope environment, reflects a high terrigenous input and abundant redeposited coarse-grained shell" and slope material (Hoilister et al., 1972; Poag et al., 1987; Van Hinte et al., 1987). The lower rise (Hatteras Outer Ridge) reflects a deep sea environment, strongly influenced by hemipelagic sedimentation and reworking of fine grained clay and silt by bottom currents (Hollister et al., 1972; Benson et al. 1978; Van Hinte et al. 1987). Across most of the intervening expanse of the continental rise, the subsurface depositional history has been inferred from the study of seismic reflection data. Most authors have emphasized tl~,e role of geostrophic bottom currents in shaping the continental rise (Heezen et al., 1966; Ewing and Hollister, 1972; Tucholke and Laine, 1982; Markl and Bryan, 1983; Mountain and Tucholke, 1985; McCave and Tucholke, 1986). Yet widespread mass wasting, predominantly on the upper rise and slope, and turbidity current dispersal systems crossing the rise, reflect the prevailing mechanism for transport of material to the deep sea (Stanley et al., 1971; Embley and Jacobi, 1977, 1986; McGregor and Bennett, 1979; Embley, 1980; Malahoff et al., 1980). The role of submarine fan systems in rise construction has been more difficult to recognize due to coalescence of numerous adjacent fan systerns (Pilkey and Cleary. 1986) and reworking by
Fig.3. Depth sectionsconstructed from digitized seismic interpretations showingstratigraphic framework and depositional geometry of post-Horizon Au sequences and location of depocenters discussed in text. Parts of seismic records used to construct sections indicated along top of each diagram. End points shown in Fig.1. Shaded lines indicate depocenters.
|41
PALEOGENE-NEOGENE DEPOSITIONAL HISTORY OF THE MIDDLE U.S. ATLANTIC CONTINENTAL RISE
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lower rise depocenters and contributing to growth of the Hatteras Outer Ridge. These cut-and-fill structures are found in the Type D facies. Much of the middle rise area off the Norfolk-Washington canyons is characterized by irregular reflection patterns interpreted to be a middle-fan depositional environment (Fig,~.6A, liB, and 12). A dominant feature of the T2 sequence is the widespread occurrence of low-amplitude parallel to wavy reflections indicative of current-controlled deposition, Up-building ol the Chesapeake Drift is characterized by relatively continuous parallel reflections (Types B and E; Figs.6A, 11, and 12). On the lower rise, sediment waves associated with the growing Hatteras Outer Ridge and along the
base of the Chesapeake Drift are best developed in subsequences N2 and N3 (Fig.i I B). Figure 14 sdggests a model for the relationship between accumulation patterns, submarine fan deposition (siliciclastic sediment supply), and current-influenced deposition. Three types of sediment wave morphologies are mapped. (!) Migrating sediment waves (Type El) characterize the Hatteras Outer Ridge near DSDP Site 603 and extend to the seafloor as the Lower Continental Rise Hills (Fox et al., 1968; Asquith, 1979; Mountain and Tucholke, 1983) (Fig.4). These are large anti-dune features, which migrate up-slope toward the ridge crest, have an east-west orientation, wavelength of ~ 5 km, height of
156
S.D. LOCKERA N D E.P. LAINE
100km /1~
reflects different sedimentary controls as suggested also by their different orientation and morphology. In addition, these waves occur at a lower position along the drift-flank profile, downslope from planar beds of the Chesapeake Drift (Fig.I IB). In contrast, the migrating sediment waves near Site 603 on the Hatteras Outer Ridge extend throughout the entire drift deposit (Fig.4).
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~ 100 m, and exhibit current scour on the upcurrent sides of waves (Fig. 14). (2) Farther south and overlying the proto-Hatteras Outer Ridge, more subdued wavy/parallel reflections (Type E2) indicate more uniform deposition across the Hatteras Outer Ridge (Fig.8). (3) A thick depocenter of climbing sediment waves (Type E3), the Chesapeake wave field (Fig.6B), was deposited along the flank of the underlying Wilmington depocenter on the lower rise (Figs.3D, 10 and 15). These sediment waves eventually were buried by Quaternary turbidite deposits (high-amplitude flat-lying seismic facies of sequence T3 in Fig.6B). Based on available seismic control, wave crests appear to be oriented from NW-SE to N-S, with wavelengths of ~ 3.75 km and wave heights ~60 m. The Chesapeake wave field appears to be separated from sediment wavesoftheHatterasOuterRidge/Lower Continental Rise Hills (Fig.14). This separation
out crop among them. This suggests that a seaward protrusion of the Hatteras Outer Ridge may have existed at this time, but was later eroded by bottom currents to shape the uniform alongslope trend of the m o d e r n H a t t e r a s O u t e r R i d g e ( F i g . 8 ) .
Along the middle rise, up to 1 km of sediment deposited to form the Chesapeake Drift (Fig. 15). On the upper rise, a 900-m thick depocenter adjacent to the slope thins to about 100 m landward of the Chesapeake Drift. Evidence for truncation and drift morphology suggests that the thin section is due to either non-deposition or erosion caused by the Western Boundary Undercurrent (Fig.3, see cross-sections A and B about 100 km seaward of slope). Accumulation patterns are more complex for the N I-N3 subsequences where isopach maps reveal downslope trends related to a NorfolkWashington fan system (Figs.16, 17, and 18). was
SubsequenceN1 During the late Miocene, an increasingly complex variety ofdepositional and erosional processes were active across the continental rise (Fig. 16). A localized thick deposit on the upper rise may be evidence that sediments supplied through slope canyons may have been reworked by strong contour-following bottom currents along the upper rise. On the upper to middle rise, the presence of
PALEOGENE-NEOGENEDEPOSITIONALHISTORYOF THE MIDDLE U.S. ATLANTICCONTINENTALRISE
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Fig. 15. lsopach map of depositional sequence f2 (middle Miocene-upper Pliocene). Note dominant alongslopc depositional trends. Thickness greater than 500m is stippled. WV=Wiimington Valley; CD=Chesapeake Drift; NWF=Norfolk-Washington Fan;
HOR=Hatteras Outer Ridge. subparallel and discontinuous seismic reflections suggests some development of downslope turbidite systems. Downslope on the middle rise, a uniformly weak to reflecu~;, free seismic facies (Fig.llA) suggests tila': the Norfolk-Washington fan system was not yet well developed. Across the middle rise, the Chesapeake Drift was beginning to take shape (Fig.12). In the southwest corner of the study area, an accumulation up to 450 m thick is eviaence of sediment eroded from the Blake Plateau by the Gulf Stream (Pinet and Popenc, e, 1982).
Subsequence N2 Alor~gslope accumulation patterns during the early Pliocene are marked by a major growth phase of the Chesapeake Drift and the Hatteras Outer
Ridge (Figs.,~ and 17). Accumulation i~1the Chesapeake wave field also continued at a high rate, as suggested by the steeply climbing nature of sediment waves (Fig.6B). At the same time, thickness patterns and seismic reflection character reveal that a Norfolk-Washington fan-channel system crossed the southern end of the Chesapeake Drift upslope from the Chesapeake wave field (Fig.12). Digitally processed single-channel Fay seismic data show evidence of a leveed channel adjacent to the lower rise in this location (Fig.13). This channel indicates that an active Norfolk-Washington fan system was transporting sediment to the lower rise during this time. A thin sedimentary section on the upper rise suggests that a zone of strong bottom currents there may have prevented the accumulation of fan deposits (Fig.17).
158
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along with b o t t o m - c u r r e n t e r o s i o n o r non-deposition along the upper rise. The NorfolkWashington system remained active, with a shift intheaxisofchannel-leveedepositionon thelower rise, inferred from isopach maps, to the northern end of the Chesapeake wave field (compare Figs. 17 and. !8). tO g r o w ,
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Subsequence N3 During the early to late Pliocene, similar patterns of accumulation continued to form due to both current-controlled drift and submarine fan deposition (Fig.18). The Chesapeake Drift, Hatteras Outer Ridge, and Chesapeake wave field continued
The uppermost sequence (late Pliocene-Recent) identifiedas T3 in this paper, is dominated by downslope turbidite systems (Tucholke and Laine, 1982; Mountain and Tucholke, 1985; Poag and Sevon,1989). Additionally, sequence T3 is often characterized by higher amplitude seismic facies (Figs.4,6,7,8and 12). This part of llhe stratigraphic sectionis the focus of a separate paper and is not discussed in detail here. It is important to note, however, that subsequent evolution of turbidite systems and sediment accumulation pa:terns in the late Pliocene to Recent were strongly controlled by the prominent alongslope topographic trends of the Chesapeake Drift and Hatteras Outer Ridge, which were well-developed drifts by late Pliocene time (Fig.3).
PALEOGENE-NEOGENE DEPOSITIONAL HISTORY OF THE MIDDLE U.S, ATLANTIC CONTINENTAL RISE
Discussion
This study provides new insight regarding the interaction between submarine fan development and bottom currents. Long-term depositional patterns (e.g., sequences Tl and 1"2; Figs.9 and 15) reflect the large-scale control of deposition by bottom currents. However, mapping subsequences N l - N 3 (on the order of third order Exxon cycles), combined with seismic facies analysis, begins to reveal the importance of downslope channel-levee systems of the Norfolk-Washington fan system (Figs.ll, 16, 17 and 18).
Evolution of Depositional Systems Fan deposition during the Oligocene-middle Miocene (sequence T I) is inferred from subtle downslope trends of the Wilmington depocenter, seismic evidence for channel-levee features (McMaster et al., 1989), and a thick section of faint parallel/ discontinuous reflections below the hummocky zone (Fig.6A). We infer that most of the thick section below the hummocky seismic fac;~ is downslope deposition, stratigraphically equi~alcnt to thick Miocene deltaic sequences mapped upslope along the shelf (Schlee, 1981; Poag, 1987; Poag and Sevon, 1989). Later in this depositional phase, the hummocky seismic facies developed as a result of widespread reworking by abyssal boundary currents, as proposed by Mountain and Tucholke (1985). The section containing faint seismic reflections above the hummocky zone on the lower rise (Fig.8) may be an indication that a change to more uniform (less reworked?) deposition began first in deep water during T! time, before becoming more widespread in sequence T2. Except for construction of outer ridge deposits along the lower rise, the effects of bottom-currents during the Oligoc~ne-middle Miocene appear to have been mostly erosional in nature. This early shaping of post-horizon A u deposition on the lower continental rise by bottom currents formed the proto-Hatteras Outer Ridge, and thereby established a primary morphological element of the lower rise that continued to grow and to influence sedimentary processes and sediment dispersal patterns up to the present time.
159
From the middle Miocene to early Pliocene (sequence T2), fan and drift systems coexisted (Fig.12). The overall alongslope geometry of the T2 depositionai sequence (equivalent to second order Exxon sea-level cycles) reflects the dominance of drift deposition in the study area during this time, along with localized current erosion (Fig. ! 5). However a submarine fan system seaward of the Norfolk-Washington canyons continued to build at the same time (Fig.14). This far~ system was a significant local sediment source with which the deep boundary currents built the Hatteras Outer Ridge, and presumable was an important source for fine particles carried farther south to be deposited on the Blake Outer Ridge. Fail channel systems in the upper Miocene to upper P!iocene section (T2) are indicated by downslope oriented depocenters in the N I-N3 subsequences, combined with reflection patterns that suggest channel-levee features (Figs. 12 and ! 3). By late Piiocene time (top of sequence T2) the Hatteras Outer Ridge and Chesapeake Drift had formed significant relief that influenced subsequent downslope turbidite systems (Fig.3A). The Hatteras Outer Ridge had by this time become a continuous ridge along the lower rise, which created a basin behind it that later filled with horizontally strztified turbidites in the late Piiocene and Pleistocene (Tucholke and Laine, 1982; Locker, 1989) (Figs.4 and 6A). The highly oblique path (from north to south)of a major erosional channel, referred to as the paleo-Wilmington Valley (Figs.12,15 and 18), cuts into the top of sequence T2 and appears to have been controlled by the topographic relief of the Chesapeake Drift.
Interaction between fans, drifts aad bottom currents A lack of well-developed morphologies resembling modern submarine fans (e.g., Normark, 1978) makes delineation of fan subenvironments within these ancient deposits difficult, and is one reason the role of fan channel-levee complexes in rise construction have been difficult to assess. We attempt in Fig. 19 to summarize the fan, drift, and bottom-current relationships inferred from this study.
I~
S.D. LOCKER AND E.P. LAINE
segmented, leaving discontinuous and partly pre-
CUM~EN'TEFFECTSONL¥ A . Uniform flow. low relief
served fans (Fig. ! 9B).
B. Localized core flow.
truncated fan
rAN ~
4-'~-
~
~
.,G.
4-,ow ,,G.
COt~gANIONFAN/DRIR"SYSTEMS C. Transitional F ~ ~ . P/--.,h .~~ _.____.j--~//,f~/, o,~tt " /
IL~'"L_.~ ! ~ ~ ~ l _ ,.~l'-'-'J.,~-~A ~ CUlrrent E. Overlapping
D. Adjacent
-- "~" ~ ~_.. Fan--,~
"~-~-ff~ " ~ - ~ _-. D'[-ii~--~
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---"
\, Fig.19. Model depictir.g proposed effects of bottom currents on fan development. (~,) Uniform current flow might smooth fan reliefand facilitate channel avulsion. (B) High-velocity core flows may erode part of a fan deposit while other areas are preserved. (C) Transitional fan/drift systemwheredistal margin of fan is reworked by geostrophic bottom currents inlo a slopeparallel ridge. (D) Adjacent fan/drift system in which fan channel systems are active alongside a growing contourite drift, (E) Overlapping fan~drift systems.
Although we found localized evidence within the N I-N3 sequences that continuous fan-channel systems crossed the rise, they did not build large leveed fan-valleys with high relief (Figs.rA and 12). Fan-channel systems in this area appear to have been shorter lived, frequently having shifted in location. Several factors, such as the rate and type of sediment supply, may have been involved in this process. Bottom currents might also have played an important role in limiting levee developmerit and making it easier for flows frequently to abandon channels (Fig.19A). If high-velocity bottam-current flow paths were present and operated out-of-phase wi',h fan deposition, the normal progression of fan subenvironments could have been
We suggest three possible configurations for companion fan/drift systems on the middle U.S. Atlantic continental rise (Fig. 19). (I) Transitional fan/drift system (Fig.19C): These would be characterized by downslope transition from a specific fan channel-levee system to a current-swept drift deposit - - the drift area having replaced the lower fan subenvironment. An exampie of this type of relationship is the Wilmington Depocenter/proto-Hatteras Outer Ridge transition (e.g., McMaster et al., 1989)(Fig.9,. At this transition, the location and growth pattern of the protoH a t t e r a s Outer Ridge appears to have been primarily linked to the sediment yield of the NorfolkWashington Fan system, rather than to longdistance sediment transport by the Western Boundary Undercurrent. In time, however, this relationship was blurred as the Hatteras Outer Ridge merged with the Mytilus Outer Ridge and established a continuous outer ridge system (Tucholke and Laine, 1982) (Fig.15). (2) Adjacent fan and drift (Fig.19D): A second type of association occurs where fan-channel systems build out adjacent to drift accumulations (Fig. 12). Our seismic data suggest that discrete fan and drift systems developed at the same time, and that individual depositional sequences encompass both depositional systems. The lateral change from one depositional system to the other is reflected by a change in seismic facies within sequences, while no alternating downlap or onlap relationships are observed. The location of the NorfolkWashington system across the southern end of the Chesapeake Drift suggests that syndepositional drift relief to the north may have controlled the location of channel-levee systems. The GulfStream may also have been a factor, perhaps having impeded the southwerd flow of the Western Boundary Undercurrent in this cross-over area (Fig.l). (3) Overlapping fan/drift (Fig. ! 9E): A third relationship involves overlapping alongslope and downslope deposits, which in a broad sense, is characteristic of the entire continental rise. Yet certain areas, such as the Chesapeake wave field, iilustrat.e a specific depocenter attributable to both
PALEOGENE-NEOGENE DEPOSITIONAL HISTORY OF THE MIDDLE U $, ATLANTIC CONTINENTAL RISE
fan and drift deposition (Fig.14). The important distinction here is that turbidites and contourites would be intercalated and preserved due to high accumulation rates, The Chesapeake wave field appears to reflect the combined influence of alongslope flow at the base of the Chesapeake Drift, and downslope sediment supply from the Norfolk-Washington Fan system. Sediment waves extending from the Chesapeake wave field and mapped further north along the flank of the Chesapeake Drift, were considered to be a facies component of the currentdeposited drift by Mountain and Tucholke (1985). However, the locally thick, non-linear depocenter shape and northward progradation of the Chesapeake wave field may reflect instead localized input from ~he Norfolk-Washington Fan system (Figs.6B and 15). We infer that the upslope climbing wave crests indicate a rapid sediment input, which may reflect additional fan-derived levee-like deposition, as has been identified in other submarine fans (Embley and Langseth, 1977; Damuth, 1979; Normark et al., 1980). Depositional sequences and &,undaries The importance of submarine fans within the sequences we mapped, combined with sequence boundary correlations to DSDP Sites, supports a correlation to Exxon sea-level cycles as has been proposed from stratigraphic analysis of drilling results from the New Jersey transect (Poag et al., 1987; Van Hinte et al., 1987). Subsequences N2 and N3 are the only sequences mapped here which appear to correlate with specific third order eustatic cycles (e.g., Haq et al., 1988). The ability to resolve third order cycles becomes more difficult with increasing age and burial depth. This is due not only to the inherent downward decrease is seismic resolution, but may also indicate depositional processes in the study area were more dominated by deep-sea controls prior to the late Miocene. With acquisition of additional high-quality seismic data, further subdivision of sequence T2 will be achieved. Subdivision of the TI interval will continue to be a difficult task on a regional scale, Overall it appears the sequence boundaries
161
mapped primarily reflect depositionaleventslinked to the transfer of terrigenous material to the deep sea due to lowered sea-levels. However, the vailability in location and/or velocity of bottom currents sweeping the continental rise appears to have controlled, in some areas, the develrpment of seismic sequence boundaries, and has complicated chronostratigraphic calibration of the sequences. Paleoceanographic studies of the western North Atlantic have demonstrated that tectonism and paleoclimatic change have independently controlled abyssal circulation (e.g., Tucholke and Mountain, 1986). Further study, including drilling, is needed to determine more fully the chronostratigraphic significance of sequence boundaries within the continental rise stratigraphic section. Conclusions Construction of the middle U.S. Atlantic continental rise from late Paleogene to late Pliocene time is attributeble to both downslope turbidite deposition and alongslope current-controlled erosion or sediment drift deposition. The ge,mc~ry of depositionai sequences and associated seismic facies reveal the co-occurrence of submarine fans and current drifts. During the Oligocene to middle Miocene, fan deposition occurred mainly on the upper rise, whereas current-controlled ridges developed on the lower rise. From middle Miocene to late Pliocene time, the Chesapeake Drift and Hatteras Outer Ridge were the dominant alongslope features. The Norfolk-Washington fan system continued to extend fan-channel systems to the middle and lower rise, however, and channel systems shifted their locations in successive sequences. Correlation of the seismic stratigraphic framework to DSDP Sites indicates that prominent sequence boundaries on the continental rise formed during lowstands of the Exxon sequence model. Three types of companion fan/drift systems are suggested: (!)transitional fan-to-drift, (2)adjacent, or (3) overlapping submarine fan and sediment drift deposition. In addition, variation in the velocity/depth structure of abyssal circulation may resuit in partial erosion of fan deposits, or prevention of the buildup of large channel-levee systems. The location and growth patterns of current drift or
162
submarine fan depocenters may be partly influenced by the morphology of, and sediments supplied by, the other system, Acknowledgements Support for this work included the Low Level Ocean Waste Disposal Program, Sandia National Laboratories, contract 53-3793; National Science Foundation Grant OCE-84161 !1; and student computer funds from the University of Rhode Island, Graduate School of Oceanography. T. O'Bden and D. Twitchell (U.S. Geological Survey, Woods Hole) made available original R.V. Fay seismic tapes and recording equipment. P. Lemmond, D. Nelson, M. Sundvik, R. Detrick, N. A d a m s a n d B. S a n d p r o v i d e d technical a n d cornp u r e r assistance. H e l p f u l reviews b y R . L . M c M a s -
ter, D.E. Owens, J.F. Sarg and C.W. Poag are
appreciated. References
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