Chapter 14 The Atlantic Margin Basins of North America

CHAPTER 14

The Atlantic Margin Basins of North America Andrew D. Miall, Hugh R. Balkwill and Jock McCracken

Contents 1. Introduction 2. The Sedimentary Basins 2.1. Introduction 2.2. Rift basins 2.3. Basins of the southern segment: Bahamas to Newfoundland fracture zone 2.4. Basins of the Grand Banks of Newfoundland 2.5. Basins of the northern segment: Labrador to the Arctic Islands 3. Petroleum Resources 4. Discussion Acknowledgments References

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Abstract The Atlantic margin of North America represents the classic ‘‘Atlantic-type’’ continental margin, notably the margin off the east coast of the United States, which was the site of five deep offshore stratigraphic test holes wells (the Continental Offshore Stratigraphic Test, or COST series) drilled in 1976–1979 on the Georges Bank, the Baltimore Canyon Trough, and the Southeast Georgia Embayment. Data from these holes were used in the development of what have become standard backstripping methods and subsidence models for extensional continental margins. Development of the margin began with the initial rifting of Pangea in the Triassic. Sea-floor spreading began in the central Atlantic Ocean during the early Middle Jurassic, and extended northward past Newfoundland beginning in the Late Jurassic. Active sea-floor spreading generated the Labrador Sea and Baffin Bay between the Cenomanian and the end of the Oligocene. Structural styles vary along the Atlantic margin. Off the southern U.S. margin, the edge of the continental margin was affected by magmatic underplating and extensive volcanism during the Jurassic. The Newfoundland continental margin developed by the processes of crustal thinning and crustal detachment. The Grand Banks area was affected by two distinct phases of rifting and flexural subsidence as extension occurred in the central Atlantic, to the south, from Late Triassic to Early Jurassic, and in the North Atlantic, to the northeast of the bank, from Late Jurassic to Mid-Cretaceous. The thickness of Jurassic-Recent sedimentary deposits on the continental margin locally reaches 25 km. Transects across the margin show a series of largely non-marine rift basins, capped by a breakup unconformity, above which is a seaward-thickening wedge of prograding shallow- to deep-water marine deposits. Evaporites are widespread at the base of this section from the Grand Banks to the Bahamas. Carbonates dominate the remaining deposits in the south, notably in the Bahamas area, but as the North American continent drifted northwestward through the Mesozoic, carbonate sedimentation gradually became less important in more northerly parts of the continental margin. On Georges Bank, carbonate sedimentation ended in the Mid-Cretaceous, whereas on the Grand Banks it had essentially come to an end by the close of the Jurassic. Shallow-marine and deltaic clastics comprise much the remaining succession throughout the length of the Atlantic margin. The discovery of major petroleum resources beneath the Grand Banks in 1979 led to extensive seismic and offshore exploration work there, and additional oil and gas resources have been discovered and developed. Gas reserves have been developed off Nova Scotia, and undeveloped gas reserves are located on the Labrador shelf, but no commercial discoveries have been made in the U.S. portion of the Atlantic margin.

Sedimentary Basins of the World, Volume 5 ISSN 1874-5997, DOI 10.1016/S1874-5997(08)00014-2

r 2008 Elsevier B.V. All rights reserved.

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1. Introduction The modern Atlantic margin of North America extends from northern Baffin Island to the tip of Florida and the southeast corner of the Bahamas, a distance of 7,500 km. This is the largest continuous extensional (‘‘passive’’) margin on Earth (Figure 1), comparable in scale to the extensional margin that encircles the western, southern and eastern margins of the continent of Africa. It includes an important area of petroleum production, the Grand Banks of Newfoundland (Figure 2).

Figure 1 Location of the major basins and tectonic elements of the Atlantic margins of North America. CCF, Cobequid--Chedabucto Fault; CGFZ, Charlie Gibbs Fracture Zone.

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Figure 2

Location map, Grand Banks of Newfoundland (Grant and McAlpine, 1990).

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Figure 3 Plate-tectonic evolution of the Atlantic Ocean, showing the paleogeography at 170 Ma, Bajocian; 160 Ma, Oxfordian; 140 Ma,Valanginian; 120 Ma, Aptian; 100 Ma, Albian; 80 Ma, Campanian; 60 Ma, Middle Paleocene; 40 Ma, Middle Eocene; 20 Ma, Early Miocene. Latitude and longitude grids are shown at 201 intervals. Maps were constructed using the interactive mapping program at http://www.odsn.de/odsn/services/paleomap/paleomap.html

The eastern margin of North America was formed during the breakup of Pangea (Figure 3). The gradual evolution of this breakup, from the Triassic to the present, can be broken down into three broad stages, which suggest a three-fold subdivision of the margin: (1) a southern segment, extending from Florida and the Bahamas to the Newfoundland fracture zone, (2) the Grand Banks area off Newfoundland and (3) the Labrador–Baffin Island margin to the north. Breakup commenced with the development of a rift system in the Triassic that affected the entire North American Atlantic margin and adjacent areas of the flanking continents, from Florida to Nova Scotia, parts of northwest Africa, and most of northwest Europe, from Spain to Denmark (Ziegler, 1988),

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but not Labrador and areas to the north. Sea-floor spreading began in the central Atlantic Ocean, between the Carolina Trough and the Scotian basins (the southern of the three segments referred to above), in the early Middle Jurassic (Figure 3). By 170 Ma (Bajocian), oceanic crust was being generated along the full length of the Central Atlantic spreading system, from the Newfoundland fracture zone to the Bahamas and into the Gulf of Mexico (Klitgord et al., 1988). In the late Middle Jurassic (Callovian), the spreading center shifted outboard, leaving a strip of transitional crust marking what is now the continental margin (Sheridan et al., 1988, and see discussion of the Bahama platform, below). Newfoundland began to separate from Europe (the Iberian Peninsula) during the Late Jurassic (Kimmeridgian, about 153 Ma), but separation was probably not complete until the MidCretaceous (Albian, about 110 Ma). Separation of Eurasia from North America was complete by the end of the Albian, at about 97 Ma (Gradstein et al., 1990; Srivastava and Verhoef, 1992). This Late Jurassic to Albian tectonism affecting the Grand Banks defined the final structure of the central of the three segments of the Atlantic margin. Rifting began in the northern segment, Labrador Sea and Baffin Bay, in the Early Cretaceous (Neocomian). Active sea-floor spreading commenced in the Cenomanian and continued until the end of the Oligocene (95–25 Ma). Magnetic anomaly patterns indicate the presence of a triple-point junction off the southern tip of Greenland between anomalies 24 and 20 (Early to Middle Eocene, 52–43 Ma), after which time Greenland moved with North America (Gradstein et al., 1990; Srivastava and Verhoef, 1992). Spreading ended in Labrador Sea in the Oligocene. A large sediment flux was available to fill the rift basins and extend out onto the continental margin as breakup proceeded. As Ettensohn (Chapter 4, this volume) noted, uplift and erosion of the Appalachian orogen reached a peak as the orogeny came to an end in the Permian, shedding sediment on to the incipient Atlantic margin from Newfoundland to Florida from Triassic time on. It has long been thought that the sediment piles off Labrador and Baffin Island were derived in part by erosion of the Cordilleran mountains of western Canada, with large rivers transporting the detritus eastward across the continental interior during the Cenozoic (McMillan, 1973; Duk-Rodkin and Hughes, 1994). The southern portion of the Atlantic margin, from Florida and the Bahamas to the Newfoundland fracture zone represents a fully developed, and still active ‘‘passive’’ margin with a sedimentary record spanning the MidJurassic to modern, resting on Triassic–Early Jurassic rift basins (Sheridan and Grow, 1988). On the Labrador margin, rifting commenced in the Barremian and extended into the Coniacian (about 130–86 Ma), with post-rift deposition continuing to the present day (Balkwill et al., 1990). Geophysical data indicate a thick sedimentary succession extending along the margin of Baffin Island and into the inter-island channels, including Lancaster Sound (Balkwill et al., 1990). Baffin Bay and the inter-island channels extending westward from it were formed during the approximately 70 Myr period of sea-floor spreading that separated Canada from Greenland. During that interval, Greenland underwent rotation and modest lateral (left-lateral) displacement relative to North America, causing extensional deformation in the southeastern Arctic Island and contractional deformation in the northeast, a phase of tectonism termed the Eurekan orogeny (Trettin, 1991). From Newfoundland to Florida the Atlantic margin includes a broad continental shelf, incorporating four areas of particularly wide shelf underlain by extended continental crust: the Grand Banks off Newfoundland, the Georges Bank, Blake Plateau and the Bahamas Platform (Figure 1). A coastal plain underlain by Jurassic– Cenozoic extensional-margin sediments extends from New York to Florida. Northward from New York the sediment wedge is entirely below sea level, except for a few small areas, including outliers in Atlantic Canada and Labrador, and the fill of Eclipse Trough, on Bylot Island, at the northern end of Baffin Island. Triassic–Early Jurassic rift basins within the Appalachian orogen are exposed along a belt from North Carolina to Nova Scotia. They are commonly called ‘‘Newark basins’’ after the Newark Supergroup, the name applied to the synrift sedimentary succession in the eastern United States (Manspeizer and Cousminer, 1988). The underlying basement consists of the Appalachian orogen from Florida to Newfoundland, and the Precambrian Shield from Labrador to Baffin Island. Remnants of the cratonic Paleozoic carbonate succession are present beneath the younger sedimentary cover off Labrador and southern Baffin Island. The North American Atlantic margin has played an important role in the development of concepts about extensional tectonics and basin development. Seismic-refraction studies were carried out by the Lamont–Doherty Geological Observatory in the 1950s, which led to the first realization of the presence of a thick sediment wedge on the continental margin (Keen and Piper, 1990). Petroleum exploration began in the Scotian basin in 1965. Six JOIDES core holes were drilled in the continental margin off Florida in 1965 (Bunce et al., 1965) and offshore exploration began in the U.S. sector in 1967. Five deep offshore stratigraphic test holes wells (the Continental Offshore Stratigraphic Test, or COST series) were drilled in 1976–1979 on the Georges Bank, the Baltimore Canyon Trough, and the Southeast Georgia Embayment, and provided essential data on the sedimentary environments and burial history of this classic ‘‘Atlantic-type’’ margin. These data were used by Steckler and Watts (1978) and Watts (1981) in their development of the backstripping methodology for the quantitative

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Figure 4 An early numerical model for £exural subsidence on Atlantic-type margins, in which an actual cross-section through the margin of South Carolina is compared to a numerical simulation (Watts, 1981).

investigation of extensional continental margins, and the development of the first numerical flexural subsidence model (Figure 4). Early ideas about application of the ‘‘simple-shear’’ model of crustal extension were applied to the Grand Banks by Tankard and Welsink (1987) and were subsequently developed in detail with the aid of industry and Lithoprobe seismic data. Oil was discovered at Hibernia in 1979, and subsequently this and several other oil and gas fields have been developed on the Grand Banks and near Sable Island, Nova Scotia. Commercial quantities of hydrocarbons have not been discovered in the U.S. sector. The area has now been extensively mapped by the U.S. Geological Survey and the Geological Survey of Canada. There have also been several Deep Sea Drilling Project (DSDP) and offshore drilling project (ODP) wells drilled along this margin. A significant amount of industry seismic and well data has been obtained, particularly within the Canadian portion of the margin, where several commercial petroleum systems have been proven in the Scotian Shelf and Jeanne d’Arc Basin areas.

2. The Sedimentary Basins 2.1. Introduction The basins of the Atlantic margin display the two-phase architecture characteristic of extensional (‘‘passive’’) continental margins, the ‘‘Texas Longhorn’’ or ‘‘Steer’s head’’ configuration of some authors (Dewey, 1982; White and McKenzie, 1988). Rift basins represent the head of the steer, formed by brittle failure during the initial phase of crustal stretching. They are followed by the broad seaward-thickening sediment wedge deposited during the phase of flexural subsidence that accompanies regional cooling and subsidence as the rifted margins

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move away from the sea-floor spreading center (the horns of the steer). These sediment piles characteristically onlap the continental margin over periods of tens of millions of years as the thinned continental crust gradually subsides (Figure 4; Watts, 1981). Rifting was accompanied by the extensive volcanic activity of the Central Atlantic Magmatic Province (McHone, 2000; Hames et al., 2003). Basaltic dikes, sills and flows formed during a 25 Myr period spanning the Triassic–Jurassic boundary over a vast area, including much of west Africa, northern South America, and the Appalachian orogen of North America. The volume of volcanic rocks generated during this period rivals that of other great basaltic accumulations, such as the Deccan Traps of India (Sheridan et al., 1993). These volcanic rocks are an important component of some of the rift-basin fills (e.g., the Palisades Sills of New York), and also occur in the subsurface, such as at the continental margin in the Baltimore Canyon and Carolina Troughs, as noted later. Rift basins indicate the commencement of crustal extension. Formation of the Newark rift basins of the eastern United States began in the Late Triassic and continued into the Early Jurassic. Rifting began later on the North Atlantic margin. The Grand Banks area was affected by two phases of rifting, as a result of the diachronous opening of the Atlantic Ocean, a Triassic phase extending from the south, and a Late Jurassic to Mid-Cretaceous phase extending from the northeast (Tankard and Welsink, 1987). On the Labrador margin, rifting commenced in the Barremian and extended into the Coniacian (Balkwill et al., 1990). In every case, the end of the rifting phase is marked by a breakup unconformity, followed by a seaward-thickening sedimentary embankment, commonly cut by listric growth faults, which developed by detachment within the cover rocks. As is common in many extensional-margin basins that commence development adjacent to a small ocean in tropical settings ( Jurassic Gulf of Mexico, Cretaceous South Atlantic, Cenozoic Red Sea), the first influx of marine waters leads to the deposition of extensive evaporites. The Late Triassic–Early Jurassic Argo and Eurydice evaporite formations, which began to form during the late rifting stage, are widespread in the Grand Banks, Scotian Shelf and Georges Bank areas. Seismic evidence indicates that diapiric evaporites are present beneath all of the U.S. basins, the Baltimore Canyon and Carolina troughs, the Blake plateau basin and the Bahamas basin (Sheridan and Grow, 1988). Evaporites are not present beneath the Labrador Shelf for two reasons: In the MidCretaceous, when oceanic crust began to form off Labrador, this part of the continental margin was at a latitude of about 401N, within a temperate climatic zone not normally associated with evaporite formation. In addition, by the time the Labrador continental margin developed it faced an already large Atlantic Ocean with normal oceanic circulation. North America underwent a rotational northwestward drift (relative to the hotspot frame) during the opening of the Atlantic Ocean (Figure 3; Lawver et al., 2002). In the Late Triassic, when rifting commenced, the Carolina Trough lay at the latitude of the equator. By the beginning of the Cenozoic this basin was at a latitude of about 401 N, and southern Labrador was at a latitude of about 501 N (it is at 521 N at the present day). This steady northward drift can be seen in the change in sedimentary facies, as documented by Jansa (1981). Carbonate sedimentation began in the Mid-Jurassic on the Bahamas platform, and continues there to the present day. On Georges Bank, carbonate sedimentation ended in the Mid-Cretaceous, whereas on the Grand Banks it had essentially come to an end by the close of the Jurassic (Figure 5). The stratigraphy and the structure in each of the North Atlantic Basins exhibit styles of typical passive-margin basins, with many similarities because of their common histories and tectonic genesis (Figures 6 and 7). Synrift parts of the basins are dominated by coast-parallel, linear, largely listric normal faults, linked by sub-orthogonal faults that commonly are expressed seaward as ocean-floor transform faults (Figure 6). In profile, the rift domains are generally asymmetrical, having a major listric normal fault as their border fault. The deep-seated older terranes affect the younger accumulation; for example where the synrift faults of the Scotian Margin and Grand Banks are aligned with and commonly inherited from underlying Paleozoic Appalachian tectonic fabrics. Triassic to Jurassic evaporites, where present, constitute important synrift lithologies and are the generators of structure along the Scotian Shelf and interior parts of the Grand Banks. The continental margin off the Baltimore Canyon and Carolina Troughs is marked by a thick wedge of Jurassic volcanic rocks (Figure 7), which is discussed below in the section dealing with these basins. Throughout the continental margin, synrift structures are overlapped by oceanward-tilted strata of the continental-terrace wedge, consisting dominantly or exclusively of seawardprograding terrigenous clastic sequences. Parts of the Atlantic-margin continental slope along the system have been broken and detached by down-to-basin gravity glide structures. Northern basins have been strongly affected by glacial activity. In the Canadian sector, virtually the entire continental shelf was under ice periodically, with shelf-crossing glaciations commencing in the Late Pliocene at Hudson Strait and in the Mid-Pleistocene on the Scotian margin (Piper, 2005). These events had important consequences for the topmost basin fills, resulting in till tongues, iceberg scour, plume fallout and turbidite deposition, large sediment drifts, catastrophic meltwater releases and widespread sediment failures linked in some cases to glacio-isostasy (Piper, 2005).

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Figure 5 Generalized stratigraphic columns for selected basins along the Atlantic margin, highlighting the diachronous development of carbonate sediments ( Jansa, 1981).

Figure 6 Sketches showing map and cross-sectional views of the classical two-stage model for evolution of passive continental margin of central eastern North America. A, rifting during the Middle Triassic to Early Jurassic; B, Drifting, beginning in the Middle Jurassic, and continuing today (Withjack et al., 1998).

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Figure 7 Generalized cross-sections through the Atlantic margin, based on synthesis of deep seismic-re£ection data (Withjack and Schlische, 2005).

2.2. Rift basins The COST-2 well on Georges Banks is the deepest and most important of the offshore wells drilled in the U.S. portion of the Atlantic margin. It was drilled to a depth of 6,667 m, and penetrated a thick section (nearly 2,500 m) of Upper Triassic dolomite, limestone and anhydrite, bottoming in Upper Triassic salt (Manspeizer and Cousminer, 1988). This is the only offshore penetration of a series of some 20 onshore and offshore rift basins that have been mapped south of the Newfoundland fracture zone. Other offshore basins have been delineated on the basis of seismic data. The orientation and basement relationships of these basins indicates that many developed by reactivation of the Late Paleozoic basins formed during the last stages of Appalachian suture. Dextral strike-slip faulting that characterized the last stages of the Acadian orogeny (Gibling, Chapter 6, this volume) was reactivated as leftlateral strike-slip during the Triassic. The Cobequid–Chedabucto Fault is an excellent example. This fault marks the terrane boundary between mainland Laurentia and the Meguma terrane. It became a bounding fault for Late Paleozoic basin development (Gibling, Chapter 6, Figure 16) and evolved into a master fault controlling oblique extension during the Triassic and Jurassic (Withjack et al., 1998). It is now one of the bounding faults of Orpheus Graben. In the U.S. Appalachians, Appalachian thrust faults were reactivated as listric normal faults (Withjack et al., 1998; Figure 8). The transition from rifting to drifting was not synchronous along the length of the southern segment of the continental margin. In the central part of the Florida–Newfoundland segment it commenced in the Middle Jurassic (Withjack et al., 1998). To the south, in the Blake Plateau region, the transition from rifting to drifting appears to have occurred slightly later in the Middle Jurassic (e.g., Klitgord et al., 1988). To the north in the Grand Banks region, the transition occurred much later, in the Early Cretaceous (Srivastava and Tapscott, 1986). Most of the rift basins are asymmetric half grabens, bounded on one side by a system of listric normal faults, and on the other side by sedimentary onlap of the basement (Figure 8). None of the onshore basins exhibit a twosided graben structure. Obliquely crossing normal faults with displacements of up to 3 km, and strike-slip

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Figure 8 Generalized northeast--southwest cross-section through the Newark basin, along the Delaware River. JB, Early Jurassic Boonton Formation; JD, Early Jurassic diabase dikes; JE, Early Jurassic tholeiitic extrusives, with sedimentary interbeds; JPS, Early Jurassic Palisades Sill and related intrusives; BC, border conglomerate; TRP, Passaic Formation (Triassic); TRL, Lockatong formation; TRS, Stockton formation; PC-O, Precambrian and Cambro--Ordvician (Taconic) rocks. Small arrows indicate direction of Taconic and Alleghenian thrust faulting, reactivated as extensional faulting (large arrows) during the Mesozoic (Manspeizer and Cousminer, 1988).

displacements of up to 20 km, cut the basins into rhombic-shaped faults blocks, and attest to the oblique stretching of the Atlantic margin that occurred as Pangea began to break up. The sedimentary succession in these basins may reach 9 km, and consists of a variegated clastic succession of conglomerate, felsic and lithic arenite, siltstone, shale and mudstone, with interbedded basaltic lava flows. Evaporites, eolian sands, coal and kerogen-rich beds are locally important. Siliceous tufas formed locally from hydrothermal systems associated with the lava fields (Birney De Wet and Hubert, 1989). Fossil remains include fish, algae, zooplankton, spores and pollen, the organic remains occurring in sufficient abundance in some cases to qualify the fine-grained deposits as oil shales. Varved and cyclic deposits attest to continually changing climatic conditions, with some workers indicating an orbital control for the cyclicity (Van Houten, 1969; Olsen, 1986, 1990). The North American margin straddled the equator at this time. Tropical temperatures and extreme aridity characterized the environments of northern basins, such as the Fundy Basin, comparable to the modern Death Valley of California. However, progressively wetter facies are present in fills of the Hartford Basin and other basins southwestward along the Atlantic margin, including coals and thicker lake deposits. This regional facies trend may represent a climatic gradient, perhaps coupled with orographic effects and changes in altitude (Olsen, 1990).

2.3. Basins of the southern segment: Bahamas to Newfoundland fracture zone 2.3.1. Introduction to the southern segment The first exploration well was the COST B-2 well drilled in 1975 in the Baltimore Canyon Trough. Up to the end of 1991, 51 additional wells had been drilled. The Baltimore Canyon Trough had 32 exploration wells, the Georges Bank Embayment and the Southeast Georgia Embayment were the location for the remaining 19 wells drilled. Several ODP legs have also explored the regional geology of this area. Hydrocarbon shows have been encountered in several wells, but no commercial quantities have been found. The most significant shows came from the siliciclastic rocks of the Upper Jurassic and Lower Cretaceous interval in the Baltimore Canyon Trough. These prospective zones were drilled by 28 of these wells to test the petroleum system of this area. 2.3.2. South Florida–Bahamas Basin ‘‘The broad, shallow banks and intervening deepwater (800–4,000 m) channels of the Bahama Platform are unlike any other topography on the larger Atlantic margin’’ (Sheridan et al., 1988, p. 329; Figure 9). There has been considerable controversy about the origins of this platform area, since Bullard et al. (1965) pointed out that the best-fit closure of the Atlantic Ocean creates an overlap of the facing continental margins if the Bahama Platform is treated as a continental fragment of North America. A few deep drill holes have been located within the Bahamas. Petroleum exploration reached a depth of 5,700 m in the early 1970s, bottoming in Upper Jurassic carbonates and evaporites, and a few DSDP and ODP holes have penetrated to depths greater than 3 km. The Great Isaac well reached volcaniclastic sediments beneath the Jurassic carbonates. Gravity, magnetic, and deep seismic-reflection data were interpreted by Sheridan et al. (1988) to suggest that the larger, western platform areas (Andros Island, Grand Bahama and other areas approximately to the west of the 781 N meridian) are underlain by transitional continental-oceanic crust formed

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Remote sensed image of the Bahama Platform. Locations of seismic sections are shown.

during an aborted rifting phase that defined the initial continental margin of North America during the MidJurassic. Later seismic work has showed that this transitional crust is in fact a wedge of Icelandic-type subaerial basalts formed during the first phase of sea-floor spreading (Figure 7; Sheridan et al., 1993). The spreading center moved outboard during the Callovian (about 155 Ma) leaving a strip of thickened early oceanic crust located offshore and parallel to the present continental margin, extending from the Georges Banks area to the tip of Florida, and then offset by right-lateral transform faults into the Gulf of Mexico. The transitional crust, thickened by volcanic activity (discussed below), provided a foundation for carbonate platform development, as this crust subsided and underwent transgression during the Late Jurassic. A broad platform or ‘‘megabank’’ developed from Late Jurassic time on, including the present area of the west Florida shelf, the Florida platform, the Bahama Platform and the Blake Plateau. The present topography of the Bahamas, consisting of flat-topped carbonate platforms cut by deep oceanic channels, is thought to have originated in the Late Cretaceous. The channels have developed over graben, although carbonate aggradation and progradation has obscured, in fact, dramatically altered, the deep structure of the platform (Figures 10 and 11). The presence of shallow-water carbonates as young as Early Cretaceous in the downfaulted blocks beneath the inter-island channels indicates that the channels were not present at the time of deposition. They were formed by faulting during the Late Cretaceous. Sheridan et al. (1988) suggested that this was caused by a phase of oblique, left-lateral northeast-directed contractional tectonism generated by interaction between the Caribbean plate and the North Atlantic plate during the Late Cretaceous. Plate-tectonic reconstructions for the Caribbean region (Pindell and Barrett, 1990) indicate that the continental fragment that constitutes the Greater Antilles (Cuba, Hispaniola) collided with and sutured to the Atlantic plate along the southern margin of the Bahama Platform between the Late Cretaceous and Eocene. Masaferro et al. (2002) described the southwestern Bahama Platform as a foreland basin in front of the northeastverging fold-thrust belt that defines the northern margin of Cuba. Tectonized Bahaman reef carbonate deposits of the Bahama foreland occur adjacent to the Greater Antilles suture zone along the northern coastal belt of Cuba (Ball et al., 1985). The tectonism uplifted and warped the megabank. Uplifted blocks provided the foundations for the present large island and bank areas. Downfaulted or folded areas became the deep, inter-island channels. Seismic data from the southwest corner of Great Bahama Bank demonstrate the development of syntectonic carbonate sedimentation formed during contractional deformation of the edge of the bank during the Miocene (Masaferro et al., 2002).

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Figure 10 A northwest--southeast cross section across the northern part of the Great Bahama Bank. Location is shown in Figure 9 (Eberli and Ginsburg, 1989).

Figure 11 Seismic section across the western edge of the Bimini Bank. Location shown in Figure 9 (Eberli and Ginsburg, 1989).

The present configuration of the continental slopes around the platform and inter-island channels is partly the result of carbonate progradation and partly the result of increased submarine erosion, probably reflecting the gradual acceleration of thermohaline oceanic circulation as the climate became cooler with increasing latitudinal temperature gradients, during the Cenozoic. Eberli and Ginsburg (1989) recognized multiple sequence boundaries in the Cretaceous–Cenozoic carbonate cover (Figure 10), reflecting the interplay between vertical motions of the platform and eustatic sea-level changes. Coral reefs and beaches were formed at 5 m above sea level at 125 ka BP on many of the islands, and constitute the bedrock throughout much of the land areas of the Bahamas and Florida (Neumann and Moore, 1975). Pleistocene and Holocene carbonate eolianites are also important. Two cored holes located on and near the center of the seismic line shown in Figure 11 (within the topsets and foresets of the deposits), drilled into the Bahama platform down to a Middle Miocene level at a maximum depth of 678 m, and revealed much about the stratigraphy, sedimentology, fluids and other features of the platform rocks

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(Eberli et al., 1997). Analyses indicate the importance of ‘‘highstand shedding’’ as the primary control on the depositional evolution of the Neogene western Great Bahama Bank: ‘‘The principal source of sediment to the slope is the extensive offbank transport of suspended, fine-grained bank-top ‘background’ sediment during periods of sea-level highstands when the entire platform was submerged providing the bulk, more than 80%, of the slope sediment. During sea-level falls, the supply of fine-grained sediment to the slope environment is reduced or completely stopped. These deposits of reworked margin-derived material form thin intercalations in the ‘background‘ sediment. Factors controlling the thickness, composition and diagenesis of the deposits, and the formation of discontinuity surfaces are (1) the morphology of the platform, hardgrounds may develop at the base (ramp morphology) or at the top of the lowstand deposits (flat-topped platform), (2) the frequency and amplitude of sea-level changes and (3) the water depth and distance to the margin’’ (Eberli et al., 1997, p. 35). A suite of ODP cores drilled mainly on the lower foresets and bottomsets along the seismic line of Figure 10 provided additional details. They showed that much of the lower foreset deposits consists of carbonate turbidites, while the bottomset deposits include contourites (Betzler et al., 1999). Depositional slopes on the clinoforms reach a maximum of 471, but this is exceptional. Most slopes are less than 41 (Betzler et al., 1999), a point that is obscured by the large vertical exaggerations characteristic of seismic displays. These cores reveal three scales of sequence cyclicity, large scale cycles 60–170 m thick, a medium scale of cyclicity tens of meters thick, and a small-scale cyclicity on a meter scale (Eberli et al., 1997). The highestfrequency sequences represent sea-level cyclicity on a 20-ka time scale, according to Betzler et al. (1999). Three major progradational episodes, of Late Miocene, late Early Pliocene and latest Pliocene age, are considered to indicate sea-level lowstands (Eberli et al., 1997). The major lowstand unconformity dated as Late Pliocene–basal Pleistocene has been interpreted as correlating to the global lowstand that records the build-up of continental ice cover in the northern hemisphere. The modern sedimentary environments and deposits of the Bahama Platform have long been regarded as the type area of carbonate sedimentology. This goes back to the work of such pioneer sedimentologists as Illing (1954) and Beales (1956, 1958). Based on his work in the Devonian Palliser Formation of southwest Alberta, Beales (1958), suggested the term bahamite ‘‘for the granular limestones that closely resemble the present deposits of the interior of the Bahama Banks’’. Systematic study of modern Bahaman sediments by Imbrie and Purdy (1962), Ginsburg et al. (1963), Purdy (1963) and others led to the erection of the classic facies classification of carbonate sediments illustrated in Figure 12.

Figure 12

Modern sediments of the northern Great Bahama Bank. Based on Purdy (1963).

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The modern bank margin is a zone up to 10 km wide covered by skeletals sands and oolite shoals. Detritus composed of fragments of corals, molluscs and foraminifers comprises the commonest particle type. Coral reefs and eolian sands are locally present, particularly on the windward (northeastern) margins of the bank areas. Carbonate eolian dunes may reach heights of 100 m above sea level. Back reef areas, covered by extremely shallow but fully marine waters, are areas of quiet water sedimentation, characterised by peloidal and grapestone muds. Deep-water deposits have been documented by Mullins and Neumann (1979) and Cook and Mullins (1983). Carbonate debris flows and turbidites are common. Deep-water currents along parts of the continental slope, especially on the windward margin, have led to hardground development. Contourite drifts of carbonate sand are common on the adjacent basin floor areas. 2.3.3. Blake Plateau Basin and Carolina Trough The Blake Plateau is named for an area of unusually wide continental shelf located off the east coast of Florida. At the latitude of Cape Canaveral the shelf reaches 460 km in width. The total thickness of the sedimentary cover above the post-rift unconformity reaches more than 13 km (Dillon and Popenoe, 1988). Etheridge et al. (1989) noted an observation made by a number of authors regarding the asymmetry of the continental margin structures bordering the central Atlantic Ocean between the United States and African margins. Large basins on one side face small basins on the other side. Etheridge et al. (1989) suggested that this basic surface evidence can be explained by the development of conjugate extensional margins by a detachment or simple-shear model (Wernicke, 1985). The Blake Plateau was interpreted by Etheridge et al. (1989) as a marginal plateau underlain by continental crust thinned by mid-level crustal detachment. The Carolina and Baltimore Canyon troughs to the north were interpreted as lower-plate margins, with the dip of the master detachment reversing direction (from easterly beneath the Blake Plateau to westerly beneath the Carolina and Baltimore Canyon troughs) at a transfer fault corresponding to the Blake Spur fracture zone (this compares to the interpretation of the Paleozoic continental margin of western Canada based on the detachment model, as summarized in Chapter 5 of this volume; see Figure 7). Deep reflection-seismic data have since become available to test this interpretation (Sheridan et al., 1993), and a different interpretation is now available for the Baltimore Canyon and Carolina Troughs (Figure 7 Sections C and D). Seismic and magnetic data indicate the presence of a seaward-dipping wedge up to 15 km thick at the transition between continental and oceanic crust, interpreted to consist of basalts and volcaniclastic rocks. These rocks are part of the Central Atlantic Magmatic Province, and are thought to have originated as a series of volcanic islands along the length of the U.S. Atlantic margin during the early rift phase of margin development. Beneath this wedge is a zone of interpreted magmatic underplating. Magnetic and gravity data are consistent with this zone consisting of mafic and ultramafic intrusions, probably the source for the overlying volcanic wedge. Seismic data and COST samples indicate that Jurassic rocks at the base of the sedimentary section consist of a mixed terrigenous-carbonate suite. Seismic reflections indicate reef structure at the shelf margin (Figure 13), and salt diapirs in the deep offshore, beneath the foot of the continental slope of the Carolina Trough. A broad carbonate platform covered the area during the Early Cretaceous. Rising sea-levels caused a retrogression of the carbonate platform during the Aptian–Albian, with deeper-water sedimentation replacing a carbonate environment on the outer part of the Plateau (Dillon and Popenoe, 1988). A radical change in depositional style from carbonate-platform to deep-water marls occurred during the Cenomanian over the Blake Plateau Basin. This may have been a result of climatic cooling caused by the northward drift of the continent ( Jansa, 1981) slowing carbonate production to the point that it was unable to keep pace with rising sea levels. By contrast, to the south, over the Bahama Platform, carbonate production continued to the present day. The physiographic boundary between the Bahama Platform and the Blake Plateau is abrupt, marked by an increase in water depth to the north of some 800 m. Dillon and Popenoe (1988) suggested that this does not necessarily indicate a control by tectonism, such as an active fault, but could simply indicate that the climatic gradient across the boundary enabled Bahaman carbonate reefs to ‘‘keep up’’ with increasing accommodation due to sea-level change, whereas those over the Blake Plateau were unable to do so. Landward, the deep-water marls of the Upper Cretaceous succession pass into shallow-water, feldspathic sandstones and claystones. Marine transgression carried shelf sedimentary conditions inland as far as the present fall-line at the foot of the Appalachian Mountains. During the latest Cretaceous (Campanian–Maastrichtian) eustatic sea-levels had risen to flood the entire modern coastal-plain area and all of Florida. A marine channel, the Suwannee Strait, opened a connection between the Gulf of Mexico and the Atlantic Ocean across southern Georgia, dividing the shelf into an area of open-marine carbonate banks and reefs to the south, from an area of sandy, calcareous shales to the north. The channel itself is an area of non-deposition and periodic erosion, as indicated by seismic stratigraphic interpretations (Dillon and Popenoe, 1988).

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Figure 13 Two closely-spaced west--east cross sections through the Blake Plateau Basin, showing the Upper Cretaceous progradation wedge, the thick build-up of Campanian--Maastrichtian strata at the mouth of the Suwanee Channel (left end of Line 19), and the erosional unconformity on the top of the Paleocene. Location shown in Figure 1 (from Pinet and Popenoe, 1985).

The initiation of a long-term drop in global sea levels in the Cenozoic led, in the Late Paleocene or Early Eocene, to a diminishing in importance of the Suwanee Channel and the development of the modern Gulf Stream current, flowing out of the Gulf and northward through the Straits of Florida. Eocene and Early Oligocene deposits, consisting of argillaceous limestones and carbonate muds, built the shelf out to the point where they encountered the erosive effects of the Gulf Stream. Subsequently, during the Late Oligocene and Neogene, generally lower sea levels and cooler climates led to fine-grained clastic aggradation and progradation of the shelf, with many erosional breaks caused by sea-level changes. During periods of low sea level, erosion by the Gulf Stream shaped the continental slope into the Blake Escarpment. The current flowed northward across the shelf, eroding sediment that spilled over the edge of the escarpment, generating turbidity currents that, in turn, eroded gullies and canyons as they moved down the slope. Meanwhile, deep Atlantic circulation at the foot of the slope, flowing southward, built up sediment drifts on the floor of the deep Atlantic Ocean. Dives by submersibles have recorded deep ocean currents of more than 4 km/hr moving along the escarpment at the present.

2.3.4. Baltimore Canyon Trough The Baltimore Canyon Trough lies immediately north of the Carolina Trough, north of Cape Hatteras, between the Norfolk Fracture Zone and the Long Island Platform and Atlantic Fracture Zone (Figure 14). The Baltimore Canyon Trough ranges from 50 to 150 km wide. The trough is separated from the Georges Bank Basin to the north by the broad, low-relief Long Island Platform and separated from the Carolina Trough to the south by the Carolina Platform. The first well, COST B-2 drilled on the United States Continental Margin was drilled here in 1975. There have been 32 additional wells drilled (up to 1991). The geology of this basin is summarized by Grow et al. (1988), and additional analysis of the well and seismic data is provided by Prather (1991). The seaward edge of the trough is bounded by the Mesozoic carbonate and siliciclastic shelf-margin complexes. The Salisbury Embayment to the west of a hinge line defined by the approximate updip edge of the Middle Jurassic strata, contains the younger Upper Cretaceous to Cenozoic (Recent) coastal plain deposits. During the Jurassic the shelf margin prograded oceanward by as much as 40 km. Initial subsidence of the Baltimore Canyon Trough is thought to have been the result of Triassic and Early Jurassic continental rifting and crustal thinning as the North American craton separated from Africa. Rapid but

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Location map, Baltimore Canyon Trough area. Adapted from Grow et al. (1988) and Prather (1991).

variable subsidence along the North American continental margin during the Early and Middle Jurassic was controlled in part by transverse fracture zones, which segmented the margin into sedimentary basins and intervening platforms. The stratigraphy in this basin is similar to the Georges Bank and Scotian Shelf. Dip-oriented seismic lines in the Baltimore Canyon Trough show a wedge of sediment up to 15 km thick overlying a prominent angular unconformity that separates an onlapping sequence of Lower Jurassic and younger sedimentary rocks from underlying Triassic and Lower Jurassic synrift basin-fill and eroded continental basement (Figure 15). Analysis of seismic data and the COST-2 well section indicates that the synrift deposits include sandstone, dolomite, volcanic debris and salt. The basal post-rift deposits comprise limestone, dolomite and anhydrite (Grow et al., 1988). A Jurassic–Cretaceous carbonate platform extends through this area for 650 km, from the Carolina Platform to the Long Island Platform, and constituted the major objective of the drilling program in the 1970s and 1980s (Prather, 1991). The platform complex, varying between 4 and 8 km wide, was situated at the edge of a shelf margin that prograded some 20–60 km during the Middle and Upper Jurassic. This succession was followed, in the Kimmeridgian, by a coastal deltaic and shelf clastic complex. During the Berriasian a raised carbonate rim developed at the edge of the shelf, backed by a clastic belt. An oolite bank developed at the edge of the shelf during the Aptian, and represents the last phase of bank-edge carbonate development. No hydrocarbon accumulations have been located in this basin. Core and sample data indicate a lack of potential source rocks, and there is some indication that hydrocarbons may have dissipated through fractures (Prather, 1991).

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Figure 15 Cross-section through the Baltimore Canyon Trough. Location shown in Figure 14. The section and interpretation are from Grow et al. (1988). Subsequent seismic exploration) has shown that the post-rift unconformity corresponds to the surface labeled ‘‘basement hinge zone’’. The wedge of seaward-dipping re£ectors above this is the initial oceanic crust identi¢ed by Sheridan et al. (1993) (see Figure 7).

2.3.5. Atlantic coastal plain The coastal plain extends from Long Island to Florida (Figure 1), and is underlain by a succession of Middle or Upper Jurassic to Late Cenozoic sediments forming a seaward-thickening wedge. The updip edge of the wedge, the zero isopach, occurs at the Fall Line, along the foot of the Appalachian Mountains. Seaward, the succession thickens to 1,500 m at the New Jersey Coast, to 1,500 m at the Georgia–Florida state line, and to more than 5 km beneath southern Florida. The succession underlying Florida consists mainly of carbonate platform deposits, with some anhydrite and terrigenous deposits at the base of the section (Gohn, 1988). Elsewhere along the coastal plain, the succession is lithologically much more varied, and contains the evidence, in the form of disconformities and unconformities, of the repeated sea-level changes that affected coastal regions during the Cenozoic. Chronostratigraphic study of the successions in outcrop and well records has contributed significantly to the debate regarding the record of global eustatic sea-level cycles (e.g., Miller et al., 2004).

2.3.6. Georges Bank Basin The Georges Bank lies between the southern tip of Nova Scotia and Cape Cod, eastward of the Gulf of Maine Platform (Schlee and Klitgord, 1988; Figure 16). Physiographically, the bank, an important fishing ground, lies at depths of less than 60 m. The main Georges Bank Basin lies beneath the Georges Bank. A deep-water channel separates the bank from the Scotian Shelf. Geologically, this channel lies over the Yarmouth Arch, a prominent Paleozoic basement high, which separates the Georges Bank Basin from the Scotian Basin (Figure 17). Two COST wells were drilled here, and eight exploration wells were drilled in the U.S. portion between 1976 and 1982 (Poppe and Poag, 1993). Oceanic crust offshore is interpreted to be either late Early Jurassic (Toarcian) or possibly Mid-Jurassic (Bajocian/Bathonian), generated during initial separation of Africa and North America. This is the oldest known oceanic crust bordering Canada. Initial spreading is represented stratigraphically by a breakup unconformity in rocks of the continental-terrace wedge. Above that level, Mesozoic–Cenozoic sequences prograded and offlapped progressively seaward as landward- and seaward-tapering wedges, attaining a thickness of 15 km along parts of the outer continental margin. Pre-spreading rifting is interpreted to be Late Triassic–Early Jurassic, based on onshore outcrops of synrift red beds and volcanics in Fundy Basin, and rocks drilled in some of the wells. Basement under the synrift beds consists of the Meguma Group (Lower Paleozoic metasediments of the Meguma Terrane) and Devonian plutons. The bottom 12 m of the COST G-2 well penetrated evaporites thought to represent the Argo Salt (Early Jurassic). The Argo Salt regionally underlies the Iroquois Formation throughout much of the Scotian Basin, but is areally restricted in the Georges Bank Basin. These layers of bedded salt and anhydrite accumulated in isolated elongate depressions and are part of the transition from synrift (continental rifting) to post-rift (drifting) deposition (Poppe and Poag, 1993). Middle and Upper Jurassic strata consist of alternating carbonates (including the Iroquois Formation of Toarcian–Aalenian age) and intervening deltaic clastics (including the Mohican Formation of Toarcian age), with a well-developed shelf-edge carbonate succession straddling the flank of Yarmouth Arch and continuing northeastward from there along a large part of Scotian Shelf (Wade and MacLean, 1990; Poppe and Poag, 1993;

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Figure 16 Location map, Georges Bank and Scotian Shelf basins.

Figure 17 1990).

Cross section from the Georges Bank to the Scotian Shelf. Location is shown in Figure 16 (Wade and MacLean,

Kidston et al., 2002; Figure 16). Interfingering tongues of limestone and shale spanning the Bathonian to Berriasian are referred to several different lithostratigraphic units, the Abenaki, Mic Mac or Mohawk formations. The Iroquois Formation represents a lower part of the broad carbonate platform comprising the Bahama– Grand Banks megaplatform. Siliciclastics dominated in the shoreward part of the basin, where the carbonate platform was bifurcated by a clastic wedge (fed by source terrains in the New England Appalachian highlands) that extended across the central part of the shelf and spread onto the incipient continental slope and rise. The muddy, evaporitic and dolomitic carbonate lithologies and seismic facies of the Iroquois Formation indicate that a barrier reef existed along the seaward edge of the platform during Iroquoian deposition. Behind this reef, the platform is characterized by a series of mounds and oolitic shoals. During Abenaki/Mic Mac time the elevated barrier reef system, which continued to exist along the paleoshelf edge (Figure 16), sheltered the adjacent back reef shelf permitting the accumulation of muddy lithologies. Two broad gaps in the carbonate platform along the southern edge of the basin, however, allowed the siliciclastic deltaic sediments to pass onto the continental slope. Cretaceous strata are seaward-prograding clastic wedges with a regionally developed, seismically discernible thin limestone (‘‘0 Marker’’) at the top of the Barremian. The Cretaceous succession was tilted seaward and the landward part eroded prior to basal Tertiary deposition. Tertiary strata thicken to about 1 km at the present-day shelf edge.

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No wells have been drilled in the Canadian side of Georges Bank; the region is presently under exploration moratorium. Petroleum exploration has not been successful in this basin. No hydrocarbon occurrences have been reported. Source-rock studies of the U.S. wells indicated that the region has poor (richness) potential; the prospectivity of the Canadian segment is perhaps enhanced by the presence of a thicker succession containing salt. 2.3.7. Scotian shelf The continental margin off Nova Scotia (Figure 16) has long been known as the definitive Atlantic-style passive margin: a pull-apart margin followed by thermal sag and a prograding shelf with a carbonate bank, major river delta system and a mobile salt substrate. The three major analogue passive margins are all Atlantic facing, namely the Gulf of Mexico, offshore Brazil and offshore West Central Africa, all which are petroleum productive. The Mesozoic and Cenozoic section of the Scotian Shelf may be divided into four broad tectonostratigraphic units: pre-continental break-up rocks, post continental break-up rocks, deltaic wedge and Mid-Cretaceous and Tertiary transgressive sediments (Wade and MacLean, 1990; Wade et al., 1995; Kidston et al., 2002; Figure 18). In the Late Jurassic, three major deltas existed along the Scotian margin: Laurentian, Sable and Shelburne Deltas. Carbonate banks, ramps and reefal complexes flourished on stable platforms and interdeltaic regions. By Early Cretaceous, carbonate deposition ceased and the Sable Delta became the dominant depositional system in the region and expanding during a period of relative sea level highstand. It may be more than 5 km thick in the Sable sub-basin. Small outcrops of Early Cretaceous sediments onshore in Nova Scotia and New Brunswick represent the drainage systems — some developed over karst terrain — that fed the delta (Stea and Pullan, 2001; Gobeil et al., 2006; Falcon-Lang et al., 2007). In Nova Scotia, thick (up to 30 m) saprolites developed on deeply weathered granitic rocks as a result of warm latest Paleozoic and Mesozoic climates, and such widespread regoliths may have fed silica sand and kaolinite-rich clays to Cretaceous drainages (O’Beirne-Ryan and Zentilli, 2003; Pe-Piper et al., 2005). By Middle Oligocene, a major lowstand exposed the entire shelf. A series of shelf-margin deltas and upper slope canyon systems developed, providing sources and conduits for coarse-grained clastic sediments to reach deepwater depocenters. The deepwater Scotian Slope is located on the seaward portion of the 25+ km thick Mesozoic and Cenozoic sedimentary prism that was deposited along the rifted continental-oceanic crustal hinge line zone. Early synrift Late Triassic–Early Jurassic sediments and evaporites (salts) were deposited in a heavily faulted and rifted terrane. During the subsequent drift phase that followed the separation of Morocco and Nova Scotia, the shelf prograded seaward, with the slope region the locus for deposition of fine-grained sediments. Shelf advancement was punctuated by periodic sea level falls with resultant gravity slides and turbidite flows carrying coarser-grained sediments into very deepwater, with deposition over and around the seafloor topography created by salt halokinesis. Salt structures of the Slope Diapiric Province are widespread, and include allochthonous salt canopies that were mobilized as a result of loading by prograding slope sediments, migrating up to 100 km basinward and causing the overlying strata to become detached and extended (Shimeld, 2004; Ings and Shimeld, 2006). The slope area has been significantly modified by subaerial and submarine erosion during lowstands, especially in the Tertiary and even quite recently, with major canyons carved into the slope following Pleistocene glaciation. Six erosional unconformities ranging in age from Oligocene to Pliocene have been identified on a seismic section across the Laurentian Channel, indicating that this channel has a long history as a major drainage conduit from the continental interior (Wade and MacLean, 1990). As noted above, the Scotian margin represents a classic ‘‘Atlantic-type’’ continental margin. Cross sections interpreted from seismic-reflection data (Figure 19) reveal the presence of a suite of rift basins at depth. The breakup unconformity appears nearly everywhere to be blanketed by the Argo Salt, which has also been deformed into a series of diapirs. However, the Argo has been dated as pre-breakup in age (Hettangian). The seismic interpretation shown in Figure 19 suggests a smearing out of the salt along the unconformity, a result of salt mobilization during loading. However, structural details in evaporites are difficult to resolve on seismic data and the interpretation shown in this cross-section is probably over simplistic. The Mesozoic–Cenozoic stratigraphic succession which follows the evaporites consists of a series of seaward-thickening clinoform packages, cut by numerous listric faults. The thermal evolution of the upland region bordering the Scotian Basin probably represents in part cooling due to rift-flank uplift and exhumation, along with changes in surface temperature regimes (Grist and Zentilli, 2003). The basin was affected by tectonic events through the Cenozoic, as a result of motion on the linked strikeslip fault systems of the Newfoundland Fracture Zone, southwest Grand Banks Transform and Cobequid– Chedabucto Fault Zone (Pe-Piper and Piper, 2004). Local magmatic rocks formed during Triassic to Early Jurassic rift phases and Early to Mid-Jurassic post-rift phases (Pe-Piper et al., 1992). Close to the Newfoundland Fracture Zone lie the Fogo Seamounts of Cretaceous age, which represent local volcanic activity associated with detachment faulting and the separation of Iberia and the Grand Banks (Pe-Piper et al., 2007). Large failures of the

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Figure 18

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Stratigraphic table for the Scotian Basin (Kidston et al., 2002).

continental margin have become a prominent feature of the margin in the past 40 million years, with the largest failures taking place during non-glacial periods but frequent smaller failures during periods of deglaciation (Campbell et al., 2004). Hydrocarbon source rocks range from Middle Jurassic (Misaine Shale) to Albian (Sable Shale). The Upper Jurassic to Lower Cretaceous Verrill Canyon Shales are the most likely sources for gas, condensate and oil that have been discovered in the Mic Mac, Missisauga, Logan Canyon and Dawson Canyon sandstones. Most of these hydrocarbon discoveries are within shallow-water deltaic-wedge clastics.

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Cross-section across the Scotian Shelf and slope. Location is shown in Figure 16 (Wade and MacLean, 1990).

2.4. Basins of the Grand Banks of Newfoundland The Grand Banks of Newfoundland became an important Canadian petroleum province with the discovery of oil at the Hibernia P-15 well in 1979 (Figure 20). Several other fields have been developed since that time. As a result of this exploration activity, a wealth of drilling and seismic data has been collected from the area, and several specialized collections of research papers have been published containing descriptions of the geology of the Grand Banks, and of other comparable Atlantic-margin areas (e.g., Beaumont and Tankard, 1987; Tankard and Balkwill, 1989). There is, in addition, a major Decade of North American Geology chapter discussing this area (Grant and McAlpine, 1990). The key to understanding the regional development of the Grand Banks is to explore the relationship between the evolution of the oceanic crust off the Grand Banks, and the orientation, sense of movement and timing of the complex pattern of extensional structures that dominate the geology of the area (Tankard and Welsink, 1987; Welsink et al., 1989; Figure 20). Most published discussions deal with the Jeanne d’Arc basin, which is the main focus of this section. There were four broad phases of basin subsidence. (1) The first phase, of Late Triassic–Early Jurassic age, lasted some 20–30 Myr, and resulted in the deposition of a red bed complex capped by evaporites. (2) Movement on major northwest–southeast faults, including the Newfoundland fracture zone and the parallel transfer faults on the Grand Banks initiated the fragmentation of the shelf platform. (3) A phase of slow thermal subsidence during the Early and Middle Jurassic led to the deposition of a monotonous mudstone–carbonate succession (epeiric basin phase in Figure 21). (4) Africa began to separate from Nova Scotia in the Middle Jurassic, at about 175 Ma, and this led to about 40 km of continent–continent displacement along the Newfoundland fracture zone and the transfer faults until Valanginian time. Synrift deposits developed in the Jeanne d’Arc and other basins. This phase of movement came to end with the separation of Iberia at about 115 Ma. (5) During the post-rift phase, which followed, oceanic crust began to develop off the northeast margin of the Grand Banks, and crustal extension led to the development of a suite of crossing faults oriented northeast–southwest (Figure 20). The stratigraphic section of the Jeanne d’Arc basin is illustrated in Figure 21 and the structural geology is shown in two seismic cross sections, Figures 22 and 23. The evolution of the structure was complex. During the Late Jurassic and Early Cretaceous, extension oriented northwest–southeast initiated the major basin-bounding master faults, including the Murre fault, and caused right-lateral strike-slip displacement on a suite of faults perpendicular to the basin-bounding master faults (Tankard et al., 1989). The Murre Faults is interpreted as a master detachment, flattening out at a depth of about 22 km (Figure 22). During the Late Cretaceous, the changing trajectories of sea-floor spreading that led to the opening of the North Atlantic Ocean, imparted extensional stresses on the Grand Banks in a direction more or less perpendicular to the earlier pattern, that is, northeast–southwest. The master faults display evidence of right-lateral strike slip superimposed on the earlier extensional movement, while the northwest–southeast-oriented strike slip faults developed extensional dip slip

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Figure 20 Structural framework of the Grand Banks, showing major faults, magnetic lineaments and the trajectories of the developing oceanic crust (dashed lines). The parallelism between structural trends in the continental and oceanic crust indicates a close genetic linkage (Welsink et al., 1989).

during this phase. This was the major phase of movement that developed the Orphan Basin, on the northeast flank of the Grand Banks, facing the Late Cretaceous oceanic crust of the North Atlantic Ocean. Extension of the crust that created Jeanne d’Arc Basin has been calculated to be about 50%, compared to a regional average of about 20%. The basin is 17 km deep, including a 3-km-thick post-rift succession (Tankard et al., 1989). Each of the phases of subsidence and deformation can be linked to discrete phases of sedimentation in the basin. The earliest phase of subsidence, during the Late Triassic to Early Jurassic (Carnian–Sinemurian) was accompanied by deposition of a suite of non-marine argillaceous red beds and evaporites, the latter deposited primarily in very shallow-water to non-marine sabkha environments. A carbonate-dominated lagoon developed during transgression in the Pliensbachian, and this was followed by the uniform subsidence of the post-rift epeiric basin phase (Toarcian–Callovian; Figure 21) during which calcareous shales and limestones were deposited. Phase two subsidence (Callovian–Aptian) created accommodation for six unconformity-bounded sequences, each spanning 7–10 Myr. It is this synrift succession that is involved in the large roll-over structure in the hanging wall of the Murre Fault (Figure 23), and the structure has been further modified by salt diapirism. Folds, minor internal faults and erosional unconformities yield a detailed history of subsidence during this phase, the details of which are beyond the scope of this chapter. The Hibernia oilfield contains multiple intervals of oil-bearing reservoir rock, including the Ben Nevis and Avalon Formations, the Catalina Member, and the Hibernia and Jeanne d’Arc Formations. Most of the resources, however, are contained in the Ben Nevis–Avalon and Hibernia reservoirs. The petroleum was sourced from the Egret Shale, of Kimmeridgian age, the same age as one of the principle source rocks in the North Sea Basin. A widespread unconformity at the Barremian–Aptian contact marks the end of rift phase two. Subsequently, deposition took place more uniformly across the Grand Banks during the flexural subsidence phase.

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Figure 21 Stratigraphic table for the Jeanne d’Arc basin, showing the unconformity-bounded sequences, tectonic phases and the evolving style of basin development (Tankard et al., 1989).

A continental-terrace wedge was developed, showing a gentle northward tilt. Late Cretaceous to Eocene successions include shelf-slope sand bodies on the western basin margin and small, sand-prone submarine fans on the basin floor that were fed by canyons incised through the western margin (Deptuck et al., 2003). The Orphan Basin on the northeast margin of the Grand Banks exhibits a similar structural and stratigraphic style to the Jeanne d’Arc Basin, with the phase 2 subsidence the dominant basin forming episode. Well control in the Basin is sparse, with seven exploration wells drilled in the basin up to 2003. The fist well was drilled on a basement high, but encountered only thin Mesozoic sediments before entering the Paleozoic. A DSDP well drilled on the Orphan Knoll has also provided some geologic data (Smee et al., 2003). Basins of the Grand Banks and Iberian facing margins originated as intra-continental rifts during the early stages of separation of North America from Europe. In consequence, they share many stratigraphic features in

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Figure 22 A northwest--southeast oriented Lithoprobe seismic line through the southern end of the Jeanne d’Arc Basin, showing the half-graben form of the basin, the marginal roll-over structure, and the basal detachment, which is interpreted to sole out at a depth of about 22 km along a band of subhorizontal re£ectors. Location is shown in Figure 20 (Tankard et al., 1989).

common, although local paleogeographic settings resulted in systematic variations in storm and tidal energy that influenced the nature of their reservoir sand bodies (Hiscott et al., 1990a, 1990b).

2.5. Basins of the northern segment: Labrador to the Arctic Islands Labrador Sea, Davis Strait, Baffin Bay and Nares Strait collectively comprise a 3,500 km-long seaway, initiated by Cretaceous rifting and plate separation, connecting the Atlantic and Arctic oceans. In simplistic terms, this vast tectonic domain consists of elongate, northwest-trending, stretched and deeply subsided, small, rhombic ocean basins, which are linked by narrow, northeast-trending transform zones. Plate motion along this lithospheric network was initiated in Early Cretaceous (?Neocomian) (or possibly Late Jurassic) and ceased in latest Eocene or Early Oligocene (prior to anomaly 13) when ocean spreading aborted and shifted to its present-day location between Greenland and Europe (Balkwill et al., 1990). Twenty-eight exploration wells were drilled on the Labrador Shelf and three holes were drilled off southern Baffin Island during the period 1974–1983. There has been no drilling activity on the Canadian side of Baffin Bay or further north, whereas there has been exploration on the Greenland side of the bay. The Canadian plate margin of this tectonic region has first-order attributes typical of Atlantic-type passive margins: a nearly flat continental shelf margin merges outboard with a more steeply inclined continental slope; the inner part of the shelf is an erosional surface developed on Precambrian crystalline basement (and locally, on Lower Paleozoic platform strata); the outer shelf and slope are constructed by a thick prism of seaward-dipping Cretaceous and Tertiary terrigenous clastics; the more landward part of the shelf prism lies on extended cratonic crust, locally containing large fault-bounded wedges of Lower Cretaceous and lower Upper Cretaceous synrift clastics; the outer part of the prism lies on and is intercalated with thick Upper Cretaceous and Tertiary basalts, representing transitional cratonic/oceanic crust or oceanic crust (Balkwill et al., 1990). 2.5.1. Labrador shelf Initial Labrador Basin rifting is probably recorded by Early Cretaceous Alexis Volcanics (K-Ar age 122 Ma at Bjarni H-81) (Figures 24 and 25). Magnetic anomaly 33, of Santonian age, is the oldest recognized in the Labrador Sea. Anomaly 27 (61 Ma, Early Paleocene) indicates the onset of spreading in the northern Labrador Sea (Harrison et al., 1999). The presence of Late Jurassic mafic dikes along the West Greenland coast indicates the possibility of older synrift rocks in undrilled structures. Bjarni Formation synrift strata are divisible into lower and upper members. The former is dominated by arkosic, coaly, generally coarse-grained fluvial sandstones and the latter by siltstones and sandy shales. The transition from rifting to sea-floor spreading and flexural subsidence of

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Figure 23 Seismic line oriented northwest--southeast through the Hibernia ¢eld. The ¢eld is a roll-over trap adjacent to the Murre Fault, at left. Salt that has risen along the Murre fault is shown by the pattern of squares. Location is shown in Figure 20 (Tankard et al., 1989).

the continental margin lies in the lower part of the thick, marine, shale-dominated Markland Formation. The remainder of the shelf prism deposited during this phase is shale dominated, with some locally developed, inboard sandstone intervals in Upper Cretaceous and Lower Tertiary (Freydis, Gudrid and Leif members). The termination of sea-floor spreading is represented by the regionally mappable Kenamu–Mokami sequence boundary. The upper part of the terrace prism (Mokami and Saglek formations) is increasingly coarser upward, having been deposited as the Labrador (and Baffin) coasts were uplifted and eroded as a result of post-rift lateral heat flow toward the craton, amplified by flexural uplift from sediment loading of the seafloor. Hopedale and Saglek sub-basin depocenters are separated by a broad basement arch (Okak Arch). Synrift faults in Hopedale Basin occupy a relatively wide part of the shelf; they consist of semi-orthogonal patterns of seaward-dipping basement faults, linked by short transfer faults (Figures 24 and 25). Lying discordantly above the

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Hopedale Basin, Labrador Shelf, showing depth to basement and well locations (Balkwill et al., 1990).

basement fault array in the outer part of the shelf, are clusters of seaward-dipping growth faults, detached in Markland shales, and having rotated structural traps at mid-Tertiary levels. In contrast with the foregoing, large synrift faults in Saglek Basin are landward-dipping, leading to the likelihood that Okak Arch forms a lithospheric zone across which there was a polarity reversal in crustal delamination. Onshore in Labrador, a widespread Mesozoic regolith was probably present, the erosion of which generated economic, iron-rich lacustrine deposits of the Late Cretaceous Redmond Formation (Umpleby, 1979). Gas and minor oil shows were discovered at the Bjarni well off southern Labrador, and condensate in the Hekla well in the southeast Baffin shelf. Five large gas fields were subsequently discovered in synrift structures in Hopedale Basin (Bell and Campbell, 1990). Synrift, Lower Cretaceous Bjarni clastics, forming the reservoir level at Bjarni, North Bjarni, Hopedale and Snorri, were probably sourced by interbedded coals and carbonaceous shales. Lower Paleozoic dolomite is the reservoir at Gudrid.

2.5.2. Davis Strait transform Reaching northeastward from the northern end of Saglek Basin, Davis Strait Transform is a broad zone of lithospheric, left-hand strain translation linking Labrador Sea and Baffin Bay (Balkwill et al., 1990). The transform zone consists of transtensional and transpressional basement structures (positive and negative flowers),

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Figure 25 Seismic cross-section through the Hopedale Basin, Labrador Shelf. Approximate location shown in Figure 24 (Balkwill et al., 1990).

with draped, rotated and faulted Cretaceous and Tertiary sedimentary and volcanic cover rocks. Truncation/ onlap relationships indicate that volcanism and mafic intrusion ended with cessation of Labrador Sea spreading. Gas and condensate were recovered from Paleocene (Gudrid) sandstones, draped over a volcanic flow at the Hekja structure, in the southwestern part of the transform zone (Bell and Campbell, 1990). Six dry holes have been drilled on the West Greenland shelf, in structures approximately at the northeastern end of Davis Strait Transform.

2.5.3. Baffin Bay No exploration wells have been drilled along the Baffin Island shelf or slope. Seismic control is limited to the northern part of the shelf, near the mouth of Lancaster Sound. The ages and nature of the Mesozoic–Cenozoic stratigraphic succession have been interpreted from outcrops on Bylot Island and vicinity (Eclipse Trough: Miall et al., 1980; Harrison et al., 1999), and it is considered probable that the tectonic history of the shelf can be anticipated from events and stratigraphy known along Labrador Shelf, Davis Strait and the West Greenland shelf. The succession, which is as thick as 14 km at the mouth of Lancaster Sound, is interpreted to consist of Lower Cretaceous (Bjarni-like) synrift, largely non-marine clastics, overlain by Upper Cretaceous–Lower Paleocene marine shales, and Upper Paleocene and younger marine shales and sandstones. The interpretation carries a caveat of risk, because of uncertainty with regard to the age and kinematics of Baffin Bay tectonism. The northern Baffin shelf has (at times) attracted exploration interest because of the presence of large structures, the probability of sandstone reservoirs, the presence of an active oil seep at Scott Inlet (at a midpoint along the Baffin Island coast), and outcrops of organic-rich Upper Cretaceous marine shales (Kanguk Formation) on Bylot Island. Lancaster Sound, a narrow, fault-bounded submarine graben, is an appendage to the Mesozoic–Cenozoic succession in northern Baffin Bay, and is presumed to hold similar stratigraphy. It contains 7 km of Paleozoic and younger sedimentary rocks. As reported by Balkwill et al. (1990, p. 333) it has been suggested that this graben represents a Precambrian aulacogen reactivated in the Mesozoic. The present structure of the basin is probably the result of extensional tectonism associated with the rotation of Greenland away from Baffin Island.

3. Petroleum Resources There are four areas along the Atlantic Margin where commercial and ‘‘stranded’’ (not-commercial-at-thistime) hydrocarbons have been found: Sable Island area on the Scotian Shelf, Jeanne d’Arc Basin on the Grand Banks, the Hopedale Basin on the Labrador Shelf and the Saglek Basin on the Baffin Shelf.

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The first commercial production from the Scotian Shelf Basin was 7.1  106 m3 (44.5 million barrels) of oil from Panuke and Cohasset in the Sable Island area between 1992 and 1999. Gas from the Sable Basin has since been under production since 1999 and has produced 31  109 m3 (1.1 trillion cubic feet) from Thebaud, Venture, North Triumph, Alma and South Venture. It is expected that the total recovery from this project alone could be 85  109 m3 (3 tcf ) of gas. (Canada–Nova Scotia Offshore Petroleum Board, Annual Report 2006–2007). Another project, Deep Panuke is being planned. The Canada–Nova Scotia Offshore Petroleum Board has published that the ultimate recoverable resource in this area could be 159  109 m3 (5.6 tcf ) of gas and 28.8  106 m3 (181 million barrels) of oil/condensate from 26 discoveries (Canada–Nova Scotia Offshore Petroleum Board, 2000). Wade et al. (1989) reported that this Scotian Shelf area including the Shelbourne, Abenaki, Mohican and Sable Sub-basins could have the potential of 510  109 m3 (18 tcf ) of gas and 170  106 m3 (1.07 billion barrels) of oil/condensate. A similar study interpreted that the Laurentian Sub-basin, the easternmost extension of the Scotian Basin, could have the potential of 227–255  109 m3 (8–9 tcf ) of gas and 95–110  106 m3 (600–700 million barrels) of oil (MacLean and Wade, 1992). There also has been further additional assessments of the deep-water gas potential on the extensive Scotian slope (Kidston et al., 2002). The Jeanne d’Arc Basin has been under production since 1997 and has produced 116  106 m3 (733 million barrels) of oil from Hibernia, Terra Nova and White Rose. This prolific basin has the potential of an additional 377  106 m3 (2.37 billion barrels) of oil and natural gas liquids and almost 170  109 m3 (6 tcf ) of gas from the above 3 fields and 15 other discovered accumulations (The Canada–Newfoundland and Labrador Annual Report, 2006–2007) There has been some recently announced expansions on the existing fields as well as the probable production from the Hebron discovery. Bell and Campbell (1990) estimated that the potential of the Jeanne d’Arc Basin and area has the approximate potential of 1335  106 m3 (8.4 billion barrels) of oil and 340  109 m3 (12 tcf ) of gas. Oil exploration has been expanding outside of this prolific area into the Flemish Pass and Orphan Basin in the last few years. On the Labrador Shelf the resources estimated for the five gas discoveries of North Bjarni, Gudrid, Bjarni, Hopedale and Snorri total 119  109 m3 (4.2 tcf ) and 20  106 m3 (123 million barrels) of natural gas liquids. The GSC estimate from Bell and Campbell (1990) for the resource potential of the Labrador to Baffin Shelf could be from 80–134  106 m3 (500–843 million barrels) of condensate and 539–737  109 m3 (19–26 tcf ) of gas.

4. Discussion The outlines of the continents were what first caught Wegener’s attention in the 1920s, and inspired him to draw his famous map showing how the world’s continents might once have fit together (Wegener, 1929). From the outset, his map contained problems, notably the fit of Greenland back against Canada, which opened up a hole between Greenland and the Canadian Arctic Islands. This was the beginning of the notorious (to Danish and Canadian geologists) ‘‘Nares Strait problem’’ (Dawes and Kerr, 1982). There were other overlaps, too, such as that in the vicinity of the Bahama Platform, when Africa was pushed back against the United States Atlantic margin (referred to earlier in this chapter; see Bullard et al., 1965). Modern research in extensional tectonics has provided answers to these problems, and the Atlantic margin of North America is where many key solutions have been developed. From the very beginnings of the modern era of plate tectonics, the Atlantic margin has provided a model for comparison. The western Paleozoic margin of North America was compared with the modern Atlantic margin in the early 1970s (see Chapter 5). Detailed studies of paleomagnetic lineations in the Atlantic Ocean have yielded extremely well-constrained kinematic models of Atlantic spreading (e.g., Srivastava and Tapscott, 1986; Srivastava and Verhoef, 1992), providing, for example, relative rotation paths for Greenland and North America which need to be accommodated to the observable geology along the contact between these plates in the northeastern Arctic Islands. This has now largely been done (see discussion in Harrison et al., 1999; the Nares Strait problem has largely been solved). Detailed studies of crustal extension, aided by the COCORP and Lithoprobe projects (see Chapter 17) have provided explanations for what happens when continental crust is stretched and breaks, with the generation of new oceanic crust along the fracture. Stretching of the lower crust, the pure shear model of McKenzie (1978), could help to explain how continental margins could be extended to positions that overlap if a simple geometric fit of post-rift margins was attempted. However, the simple-shear model developed initially by Wernicke (1985) has become the basis for a much more important and comprehensive explanation of Atlantic margin evolution. The collection of research papers on the North Atlantic margins edited by Tankard and Balkwill (1989) is a landmark in the field of applied plate tectonics, because it represents a rigorous, detailed examination of the simple-shear model as applied to a single, complex, extensional margin. The constituent papers demonstrate, through detailed analysis of many of the basins bordering the North Atlantic Ocean, how

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brittle failure of the upper crust is translated into extension along detachments that may bottom out at depths of more than 20 km, thereby explaining substantial modifications to the configurations of the continental margins. This analysis provides explanations for the complex structures in the resulting basins that are of so much interest to the petroleum geologist. The development of the Atlantic margin of North America reflects a relatively uncomplicated evolution of the Atlantic spreading center, which, while repeatedly undergoing subtle changes in trajectory, did not involve major jumps in the position of the spreading center, unlike the evolution of the European margin (Ziegler, 1988), and unlike the evolution of Tethys by the repeated fragmentation of the northern Gondwana margin (AudleyCharles and Hallam, 1988). For all these reasons, the emergence of the term ‘‘Atlantic-type margin’’ is entirely understandable.

ACKNOWLEDGMENTS Thanks are due to Martha Withjack and R. W Schlische for arranging for us to receive a copy of their recent seismic interpretations of the U.S. portion of the Atlantic margin (Figure 7), and to Martha and to Bob Sheridan for their comments on the structural interpretations. David Piper and Marcos Zentilli provided many useful references relating to recent work, at the request of Martin Gibling, who reviewed the manuscript and provided many important corrections and additional information.

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