Lithostratigraphy
The sedimentary record at Blake Nose consists of Eocene carbonate ooze and chalk that overlie Paleocene claystone as well as Maastrichtian and possibly upper Campanian chalk (Figs. 3, 4 ). In turn, Campanian strata rest unconformably upon Albian to Cenomanian claystone and clayey chalk that appear to form a conformable sequence of clinoforms. A short condensed section of Coniacian to Turonian nannofossil chalks, hardgrounds, and debris beds are found between Campanian and Cenomanian rocks on the deeper part of Blake Nose. The contact between the upper Albian and the lower Albian appears to be disconformable, whereas the lower Albian/Aptian boundary may be complete. Albian claystones are interbedded with Barremian periplatform debris, which shows that the periplatform material is reworked from older rocks. The entire middle Cretaceous and younger sequence rests on a Lower Cretaceous, and probably Jurassic, carbonate platform that is more than 5 km thick in the region of Blake Nose.
The middle to upper Eocene is exposed on the seafloor across much of Blake Nose (Fig. 3). The unconsolidated oozes are protected from erosion by a layer of manganese sand and nodules that are up to about 3 m thick in places. The manganiferous sand is composed largely of Pleistocene to Holocene planktonic foraminifers, but in the shallower parts of the Blake Nose mixed assemblages of Oligocene to middle Miocene foraminifers are present. Bivalves and gooseneck barnacle plates are mixed in the sand.
The Eocene consists largely of green siliceous nannofossil ooze and chalk. The upper part of the sequence is typically light yellow, but below this there is a sharp boundary where the color changes to green. This color change is diachronous and probably relates to a diagenetic front (produced by flushing the sediment with seawater?), but does not seem to have altered the microfossil preservation or sediment composition. Planktonic foraminifers, radiolarians, and calcareous nannofossils are well preserved through most of the middle Eocene, but calcareous fossils are more overgrown in the lower middle Eocene and lower Eocene. A distinct feature of the Eocene sequence is the presence of a high number of vitric ashbeds that were found at all sites. The ashbeds serve as excellent beds for marker correlation between the sites and will be very useful for control points when developing the cyclostratigraphic record.
The cyclostratigraphic record based on the color and MST track data is well developed at the shallowest site, Site 1052. The late-middle to late Eocene record displays clear cyclical color changes that were used to splice the records of the individual holes into a composite (Fig. 5). The Milankovitch-controlled cycles will be used to recalibrate the late-middle Eocene and late Eocene time scale. Radiometric dates on the ashes and dating by astronomical tuning will produce an integrated time scale to recalibrate magnetostratigraphy and biostratigraphy. The Eocene cyclostratigraphy is less well developed at the sites downdip of Site 1052, although a splice for most of the record at each site has been produced. Color cycles are also well developed in the Paleocene and early Eocene record and will help calibrate the magnetostratigraphy and biostratigraphy of those intervals as well as aid in intersite correlation (Figs. 6, 7). The color changes were also used to construct a Cretaceous spliced record from the three holes at Site 1049.
The Paleocene and lower Eocene are relatively clay-rich by comparison with the middle and upper Eocene. The upper Paleocene contains chert or hard chalk, and preservation of most fossil groups is moderate to poor. The lower Paleocene is typically an olive-green clay-rich nannofossil chalk or ooze. Calcareous microfossils are typically very well preserved, whereas siliceous components are nearly absent. The sequence is biostratigraphically complete except for a possible unconformity in the upper Paleocene where nannofossil Zone CP5 is missing at some sites. The Danian part of the sequence thickens upslope from ~20 m thick at Site 1049 (2656 mbsl) to ~80 m at Site 1052 (1300 mbsl). Clay-rich nannofossil chalk and ooze continues into the uppermost Maastrichtian. The K/T boundary consists of a 10-17 cm graded bed of green spherules capped by fine-grained, rusty-brown grains that are overlain by dark gray clay of the earliest Danian at Site 1049. This succession is interpreted as fallout from the Chicxulub impact structure on the Yucatan Peninsula and the succeeding deposition of lowermost Danian sediment following the K/T extinction event. Notably, neither of the two K/T boundary sections drilled updip of Site 1049 have well developed ejecta beds between lowermost Danian (foraminifer Zone P0) and uppermostx Maastrichtian (M. prinzi) deposits. The spherules at these sites were either slumped into deeper water very shortly after deposition, or turbidites carrying the ejecta debris bypassed the upper slope and deposited at least part of their load near the tip of Blake Nose.
Notably, the Maastrichtian sections at both Sites 1049 and 1052 are disturbed by slumping. Most of the section consists of gray nannofossil ooze or chalk. Laminations are well preserved but burrowing is not evident, presumably because the sediment was partially liquified during slumping. However, it seems that relatively little if any of the Maastrichtian section has been lost, because the sequence is biostratigraphically complete and in the correct stratigraphic order. It is possible that the slumping was associated with the large magnitude earthquake produced by the Chicxulub impact. At Site 1052, the middle Maastrichtian contains a large slump that lies between light gray, burrowed chalk above and more weakly burrowed, greenish-gray chalk below.
The Maastrichtian apparently unconformably overlies white ooze that contains nannofossil and planktonic foraminifers characteristic of the upper Campanian at Site 1049. A somewhat thicker sequence of upper Campanian strata is present at Site 1050 and overlies a highly condensed section of Coniacian to Turonian hardgrounds that is only ~9 m thick (Fig. 8). Updip at Site 1052, Campanian nannofossils are mixed into the lower Maastrichtian chalk, which rests directly upon the Cenomanian. Evidently, upper Campanian to Turonian sediments were deposited on Blake Nose but were largely eroded prior to deposition of the Maastrichtian sediments.
Cenomanian chalk and claystone are present only in a thin wedge on the upper part of Blake Nose but the wedge thickens considerably down the slope. Seismic data and Leg 171B drilling results suggest that the Cenomanian sequence expands considerably (~70 m) near the center of the Blake Nose and then thins again at the toe of the Blake Nose. Most of the Cenomanian deposits at Site 1050 are slumped black shales and gray claystones. Cenomanian strata are completely absent from the section at Site 1049, which is drilled on a small paleo-high near the northeast tip of Blake Nose.
Multichannel seismic data (MCS Line TD-5) show that the Cenomanian-Albian sequence consists of two sets of clinoforms built over and to the northeast of a buried reef complex. The lower Cenomanian lies on top of a thick package of late Albian aged clinoforms, and the two appear to be conformable. Within the clinoform stack, the Cenomanian includes dominant dark olive gray calcareous silty claystone to clay-rich siltstone and contains very well preserved calcareous microfossil assemblages. The sediment color varies from an olive gray to black, and darker intervals are rich in clay and fine silt.
The upper clinoform set was partly drilled at Site 1052 and proved to consist of alternating green claystone, silty claystone, green laminated claystone, and thin beds of cross-bedded mixed siliceous and calcareous grainstone. The laminated green claystones are rich in humic organic matter and occur in 0.5- to 1.5-m-thick sequences interbedded with thin limestone beds that are poor in organic matter. Age-equivalent strata in Europe are known as 'Oceanic Anoxic Event (OAE) 1d' and consist of a series of laminated beds deposited within oxygen-depleted waters. The upper Albian section appears to overlap the top of the Aptian reef and pinches out entirely near the toe of Blake Nose. Near the bottom of Hole 1052E, the Albian sequence becomes dominated by slight to moderately bioturbated dark olive gray sandy siltstones probably deposited in middle or outer shelf environments. These sandstones were probably deposited near storm wavebase, as suggested by the occurrence of well-sorted grainstone and sedimentary structures associated with sand waves. Apparently, the entire upper Albian clinoform stack represents a deepening upward package.
This clinoform sequence overlaps a second clinoform sequence that probably is represented by lower Albian and upper Aptian variegated claystones recovered at Site 1049 and Deep Sea Drilling Project (DSDP) Site 390. The lowermost Albian contains a 46-cm-thick black shale containing up to 11.5% marine organic matter. This black shale section is correlative with 'Oceanic Anoxic Event 1b'. The upper Aptian section is interbedded with periplatform carbonates that were either eroded from the reef complex located to the west or were deposited at the same time as the reef system.
Interstitial Waters
Interstitial waters were analyzed at every site across the Blake Nose depth transect. The results are consistent with the biogenic and volcaniclastic nature of generally organic carbon-poor sediments and extreme depth (>5 km) to basement.
Elevated silica concentrations in the pore waters are consistent with significant alteration of biogenic and volcaniclastic siliceous sediments, particularly in the lower Eocene and Paleocene sequence. Excellent preservation of radiolarians around ash layers, especially at Site 1051, may indicate that the volcaniclastics are the more important of these two silica sources in the Blake Nose area.
Volcaniclastics are dispersed throughout the section and also seem to be the dominant control on the pore-water calcium and magnesium concentration depth gradients (formation of authigenic dolomite is an additional control, especially at Site 1052). Calcium and magnesium gradients are weak in the Blake Nose in comparison to many deep-sea sequences where gradients are often controlled by seawater/basalt interaction in the underlying upper oceanic crust. The weakness of these gradients at Blake Nose is consistent with the extreme depth (>5 km) of basement in this area. General increases with depth in Sr concentrations and calculated Sr/Ca values at all Blake Nose sites are consistent with the recrystallization of biogenic carbonate in the sediment column and alteration of volcaniclastics.
Perhaps the most remarkable results from the interstitial pore waters at Blake Nose are the extreme concentrations of lithium (up to 20 times seawater concentration; Fig. 9). Extreme distance from basement and the shape of the pore-fluid lithium profiles suggest that the source for Li to the pore waters is likely within the sedimentary column and that high concentrations are most likely caused by alteration of the volcaniclastics.
Anoxic Events
Finely laminated sediments occur periodically through the middle Cretaceous section drilled during Leg 171B. One of our goals was to recover laminated deposits along a depth transect to study the vertical extent of low-oxygen waters in the Cretaceous Atlantic. Our results suggest that the laminated sediments recovered at the shallow Blake Nose transect sites have very different characteristics than those drilled on the deep end of the transect.
The upper Albian sections within drill sites located on the upper end of the Blake Nose transect consist partly of dark-green, laminated claystones that alternate with lighter colored limestones. We observed 15 cycles of light-colored, strongly bioturbated, coarse-grained clayey limestones with moderately bioturbated silty claystones and dark, laminated claystones. The organic carbon content of the laminated beds is less than 1%, and the kerogen is of terrestrial origin. These laminated deposits are part of a deepening upward cycle from strata deposited near storm wavebase to sediments deposited near the shelf-slope break. The pervasive lamination of the deeper water part of the sequence suggests that the top of the low-oxygen zone impinged on the margin near the depth of storm wavebase. The interval where the laminated dark-gray sediments occur is time equivalent to similar sediments deposited during 'Oceanic Anoxic Event 1d' found in Tethyan deposits (Fig. 10). The widespread occurrence of the cyclic alternation of dark laminated claystones and limestones indicates that environmental conditions were similar throughout the Tethys and the young Atlantic basin.
Although not the deep-water equivalent of the laminated claystones (OAE 1d) found updip on Blake Nose, the early Albian black shale (OAE 1b) recovered at Site 1049 probably reflects the relative clastic starvation of the deep-water seaward edge of the Blake Escarpment compared to the shallower areas that represent the ancient shelf and shelf-slope break (Figs. 2, 3, 10). The abundance of marine organic matter suggests that the black shale is the distal equivalent of more terrigenous sediments that are presumably present in a clinoform stack that overlaps the updip reef complex on the Blake Plateau (Fig. 3). The black shale may represent a relatively brief period during which low oxygen conditions extended well down into intermediate water masses, because most upper Aptian and lower Albian sediments at the seaward end of Blake Nose are not organic-rich.
Mid-Maastrichtian Deep Water Reversal
The Maastrichtian was a time of global cooling marking the end of the Cretaceous greenhouse climate. Widespread geochemical and biological shifts, including extinction among rudistid and inoceramid bivalves, seem to be concentrated in the middle of the Maastrichtian. It has been proposed that rudists thrived in a hypersaline 'Supertethyan' province and inoceramids at bathyal depths where the bottom water was warm and saline (e.g., MacLeod and Huber, 1996). Thus, extinction among both groups could be explained by reorganization of Maastrichtian oceans with temperature differences replacing salinity differences as the dominant force driving circulation. A reversal in deep ocean circulation patterns would affect many paleoclimatological and paleoecological variables such as latitudinal heat transport, benthic carbon budgets, and deep ocean ventilation. Whereas some data support each of these propositions (e.g., MacLeod and Huber, 1996), there are more predictions than actual evidence of this ocean circulation hypothesis.
Lower and upper Maastrichtian sediment was recovered at Sites 1049, 1050, and 1052. The shipboard studies of material from Site 1052 are particularly exciting because mid Maastrichtian changes seem well defined and are provocatively ambiguous. The observed distribution of inoceramid shell fragments in paleontological sample residues indicates that the disappearance of inoceramids at Site 1052 falls within an ~40 m interval of good to excellent recovery. There is also a change in the amount of bioturbation across this interval from dominantly laminated or slightly bioturbated to thoroughly bioturbated. This is consistent with increasing ventilation of the bottom waters and, potentially, a reversal of bottom water circulation patterns. On the other hand, the abundance of organic carbon is lower in the less bioturbated intervals raising the possibility that food supply rather than benthic oxygenation might have limited the activity of (pre-extinction) burrowing organisms. Detailed documentation of the relationship among paleontological, geochemical, isotopic, and sedimentological data at this site and the others along the Blake Nose transect will provide better data than has previously been available to document paleoceanographic change during the last 5 m.y. of the Cretaceous.
Cretaceous/Tertiary Boundary
A complete Cretaceous/Tertiary (K/T) boundary interval was recovered in Holes 1049A, 1049B, and 1049C (Fig. 11). At Sites 1050 and 1052, partial K/T boundary sections were recovered. At Site 1049, boundary ejecta intervals are 17, 9, and 10 cm thick in Holes 1049A, B, and C, respectively. With the exception of the apparent compression in Holes 1049B and 1049C, the boundary interval seems to be virtually undisturbed and exhibits the same stratigraphic sequence in each hole (Fig. 11). The lowest bed is a graded, faintly laminated layer consisting almost entirely of green spherules that range in size from 2 to 3 mm at the base to less than 1 mm at the top. This spherulitic layer is capped by a 3-mm-thick orange limonitic layer that contains flat goethite concretions. The limonitic layer is overlain by 3 to 7 cm of dark, burrow-mottled clay that represents the P0 foraminiferal biozone. The final bed in the sequence is a 5- to 15-cm-thick, white foraminiferal-nannofossil ooze that contains a Zone Pa foraminiferal assemblage. The K/T boundary sections at Site 1049 are complete and, thus excellent for studying the response of marine biota to the extraterrestrial event. For instance, the planktonic foraminifers are extremely well preserved, ideal for stable isotopic studies that we hope will reveal the chain of climate events caused by the impact.
Paleocene/Eocene Boundary
Leg 171B recovered an apparently complete, or nearly complete, upper Paleocene carbonate sequence that should help resolve many of the issues concerning the biochronology and geochemistry of this period. Upper Paleocene sections are frequently interrupted by clay beds or a rapid switch from carbonate deposition to siliceous deposition that has been related to a short-lived intensification of low-latitude deep-water production. Added to this oceanographic switch in the deep-water source area is geochemical evidence for potential rapid release of buried gas hydrate, including the potent greenhouse gas methane. Unfortunately, the changes in type of sedimentation and the frequent unconformities typical in the upper Paleocene have previously inhibited study of this 'super greenhouse' event.
The uppermost Paleocene at Site 1051 consists of color-banded, greenish nannofossil chalk that contains moderately to poorly preserved foraminifera and calcareous nannofossils. Nonetheless, species are sufficiently well preserved that it will be possible to construct a detailed biochronology. The material is suitable to produce a very detailed carbon isotope profile from the late Paleocene to early Eocene that can be used to test the gas hydrate hypothesis for upper Paleocene warming as well as aid in correlation of Site 1051 to other localities around the globe. Finally, the sequence displays pronounced cyclic sedimentation of dark green and pale green alternations that may reflect orbital cycles. In conjunction with the magnetostratigraphy for this site, the color cycles should help produce a very detailed chronology of the duration of the thermal maximum.
Geological History of Blake Nose
Blake Nose is composed largely of Jurassic to middle Cretaceous carbonate platform deposits. The platform rests on basement rocks formed by intrusion and volcanism through attenuated continental crust during the rifting stage of the Atlantic. As much as 10 km of carbonates accumulated in this area. By Barremian-Aptian time, the reef stepped back 40-50 km from the lower Cretaceous margin and formed a long tract of coral-rudist reefs such as the one evident in seismic profiles in the shallow subsurface at the head of Blake Nose (Fig. 3).
The Aptian reef tract ceased growth during the late Aptian, at which time periplatform debris was no longer delivered to the outer edge of Blake Nose, and the deposition of green and red variegated clays began. These nannofossil clays must have been deposited at a depth of at least 1500 m since the top of the Aptian reef to the west does not appear to have been subaerially eroded and, hence, was probably near or below wavebase. Aptian clinoforms built out in front of the reef and also partially overlap the reef top. The nature of these rocks is not known except for where they have been recovered from the distal edge of the deep-water facies at Site 1049. At least part of the clinoform sequence is probably correlative with the black shale of latest Aptian age found at Site 1049, suggesting that the low-oxygen conditions associated with the organic-rich sediments extended to a water depth of at least 1500 m.
The lower Albian sequence at Site 1049 contains a number of hardgrounds and firmgrounds suggesting that there were periods of nondeposition or erosion. Some of these short hiatuses may correlate to the tops of deepening-upward sequences within the Aptian clinoform stack, similar to deepening-upward sequences within the overlying upper Aptian clinoforms. There are also some truncated clinoforms within the Aptian sequence that are visible on the seismic reflection profile across the Leg 171B drill sites (MCS Line TD-5), and these may correlate with nondeposition or erosion surfaces near the toe of Blake Nose. Unfortunately, the entire middle Cretaceous section at Site 1049 is so condensed that it is difficult to correlate reflectors unambiguously updip into the various clinoform wedges and, in any event, the Aptian/lower Albian section does not produce strong reflectors except within the clinoform stack.
Upper Aptian and lower Albian strata are overlapped unconformably by a second major clinoform wedge of late Albian age. These clinoforms are composed mostly of micaceous claystones that are interbedded with mixed carbonate and terrigenous grainstones near the base of the stack and that pass upward into cyclic limestone-laminated dark green claystones. The lower part of the clinoform sequence was probably deposited near storm wavebase as the sands are well washed and exhibit sedimentary structures indicative of megaripples or sand waves. The upper part of the sequence was apparently deposited in deeper water, perhaps near the shelf-slope break as it contains few, if any, sand layers and no evidence of substantial bottom current activity. Presently, the top of the clinoform sequence is located ~500 m below the top of the adjacent reef. Decompaction of the clay-rich strata in the Albian clinoforms by 60%-80% suggests that these sediments were deposited at about 100-200 m water depth if the top of the reef was at sea level. The presence of Albian marine rocks landward of the Blake Nose suggests that the Blake Plateau was also submerged and implies depths greater than those estimated above for the clinoform sequence.
Clinoforms within the upper Albian sequence pinch out against a surface of erosion or nondeposition, which is overlain by lower Cenomanian strata. There is little time missing across this hiatus at Site 1052, which is close to the center of the upper Albian clinoform stack, and there is no discernable hiatus judging from the planktonic foraminifer or nannofossil biochronologies. The hiatus probably represents a reduction in sediment supply to the slope either because of a sea level rise that trapped sediment inshore or diversion of sediment to some other location on the slope. Cenomanian sediments are absent from most of the Blake Plateau, suggesting that they may not have been deposited there. Alternatively, they may have been removed by later erosion and were only preserved on the upper slope. In any case, the downdip pinchout of Cenomanian strata in the middle of Blake Nose is evidence that the Cenomanian sediments recovered at Site 1052 were a relatively deep-water deposit near the most seaward extent of deposition. Deep-water conditions are supported by the presence of rich assemblages of planktonic foraminifera including the keeled rotaliporids that are rare or absent in epicontinental seas and shallow shelf strata.
There is a widespread, major unconformity above the Cenomanian on Blake Nose from which upper Cenomanian to lower Campanian strata were largely removed. A thin section of Turonian and Coniacian strata is present as a series of manganiferous hardgrounds, beds of ripup debris, and red nannofossil chalks. A very thin sequence of upper Campanian foraminifer ooze is present at Site 1049, but no sediment of corresponding age was recovered from the shallower parts of the slope. However, the Maastrichtian sequence contains numerous slumps, including one located at the Maastrichtian/Cenomanian contact at Site 1052, so it is possible that Campanian sediments were removed from the area of Site 1052 by downslope transport. Despite the slumping, much of the Maastrichtian appears to be present as a drape of nannofossil chalk and ooze. The preserved record has a well-developed color banding that may record orbital cycles. The sequence also appears to preserve an expanded record of the mid Maastrichtian and changes in bioturbation intensity. The abundance of inoceramid prisms suggest that the biological crisis recognized in low-resolution sections from other parts of the globe is probably preserved at Blake Nose, as well.
The end of the Cretaceous and the earliest events of the Cenozoic are well preserved on Blake Nose. We recovered a thick spherule bed, which presumably represents the ejecta debris from the Chicxulub crater, only at Site 1049. Nonetheless, the K/T boundary sections drilled at all Blake Nose sites preserve the earliest Danian biozones and the nannofossil markers for the latest Maastrichtian (see above). Hence, we succeeded in coring the boundary beds along a depth transect and made it possible, for the first time, to study the boundary beds and events over a range of 1300 m water depths.
Deposition of a nearly uniform drape of pelagic sediment continued into the Paleocene. The Danian is unusually thick on Blake Nose in comparison with most sites in the deep sea, and the claystone preserved the calcareous microfossils very well. Paleocene strata are the first sediments to preserve geochemical and lithologic evidence of abundant volcanic ash on Blake Nose‹a trend that continued throughout the Eocene. The upper part of the Paleocene is increasingly siliceous, and chert stringers are present where the Paleocene beds thin toward the landward and seaward ends of the Blake Nose.
By the latest Paleocene, deposition was concentrated into a major clinoform stack that reached its greatest thickness near the center of the Blake Nose transect. At least two hiatuses are present in this sequence. One is within the uppermost Paleocene and occurs close to the upper Paleocene 'Thermal Maximum' when the deep oceans appear to have abruptly warmed for a few hundred thousand years. However, the hiatus is either absent or very short near the center of the clinoform stack where the Paleocene/Eocene transition is biostratigraphically complete. A second hiatus is present near the lower-middle Eocene transition where almost 2 m.y. of the Eocene are absent. This hiatus is present across the whole of Blake Nose and is represented by foraminiferal packstones, grainstones, chert, and green clay. The hiatus cuts out the top of the Paleocene through the lower-middle Eocene on the upper part of Blake Nose, where foraminiferal packstones and nannofossil claystones contain highly mixed assemblages within a stratigraphic interval that is only about 5 m thick.
Sediment depocenters apparently backstepped up the slope of Blake Nose during the middle and late Eocene. Upper middle Eocene siliceous nannofossil oozes and chalks are thickest at Sites 1051 and 1053, which are probably close to the depocenter of these units. Indeed, analysis of the compaction state of the sedimentary sections at Sites 1050 and 1049 suggest that these areas probably were never buried by more than 100-150 m of sediment. This observation implies that the middle and upper Eocene thinned toward the toe of Blake Nose. The updip continuation of the middle and upper Eocene was removed almost completely by erosion, making it difficult to estimate how much Eocene strata were originally deposited there. However, our ability to piston core more than 150 m of the section at Site 1053 indicates that the sediment was not compacted by burial beneath a thick blanket of younger sediments.
Sea-level changes and current intensification have probably contributed to the fact that the youngest sediments on the Blake Nose are of latest Eocene age. The Oligocene is associated with widespread hiatuses in the North Atlantic. The Gulf Stream assumed its present course, for the most part, in the Oligocene and cut into the surface of the Florida Straits and the Blake Plateau. In addition, a sea-level highstand in the late Oligocene shifted sedimentation from the shelf to the coastal plain, starving the outer shelf and slope landward of the Blake Escarpment. In the Blake Basin, Oligocene cooling at high latitudes intensified the southward flow of deep water along the Blake Escarpment and formed the widespread Au seismic reflector that represents an unconformity distributed over most of the western North Atlantic. Erosion of the base of the Blake Escarpment occurred during the Oligocene as well, and large blocks of debris are present on the northwest and southeast slopes of the Blake Nose. In contrast, the northeastern tip of the Blake Nose has likely experienced relatively little erosion. Not only are there no slump blocks at the base of the escarpment in this area, but all sedimentary sequences younger than the late Aptian appear to thin considerably or pinch out there. It appears that there has not been substantial erosion of the plateau in this area, whereas the northwestern and southeastern sides have experienced considerable erosion. Indeed, the Aptian reef is exposed in the escarpment on both sides of Blake Nose. If the reef tract stepped back a similar distance all along the Blake Plateau during the Aptian, more than 70 km of the escarpment must have been eroded to create the present bathymetry.