171B Scientific Prospectus


Marine Geology
Blake Nose, or Blake Spur, is a salient on the eastern margin of the Blake Plateau, due east of northern Florida (Figs. 1, 2). It is mostly less than 1000 m deep but drops sharply to water depths >4000 m at the Blake Escarpment-a nearly vertical wall formed by erosion of the continental slope. In contrast, the Blake Nose is a gentle ramp that reaches a maximum depth of about 2700 m at the Blake Escarpment. The Blake Plateau and Blake Nose are composed of an 8-12 km thick sequence of Jurassic and lower Cretaceous limestones that are capped by less than 1 km of Upper Cretaceous and Cenozoic deposits (Benson, Sheridan, et al., 1978).

Seismic records show the presence of buried reef buildups at the landward end of the Blake Nose (Figs. 2, 3). Fore-reef deposits and pelagic oozes, built seaward of the reef front, rest on relatively flat-lying Neocomian-Barremian shallow-water carbonates and serve largely to define the present bathymetric gradient along the Blake Nose. A hiatus probably exists between the Aptian-Albian and the Campanian. Upper Cretaceous to middle Eocene carbonate oozes that rest on the Barremian reef-front deposits are typically <400-600 m thick.

Shallow-water limestone of Barremian age with a foraminiferal fauna that suggests depths of deposition of <50 mbsl was recovered at Deep Sea Drilling Project (DSDP) Site 390 (Benson, Sheridan, et al., 1978). Water depths increased substantially by the late Albian. Aptian-Albian pelagic oozes at the eastern end of the Blake Nose were probably deposited at >500 m water depth based on ratios of planktonic and benthic foraminifers (Benson, Sheridan, et al., 1978). If we assume that the reef was at sea level during the Albian-Aptian and that the depth gradient in the fore-reef deposits has not changed since that time, then the present difference in depth between the reef top and the seaward edge of the Blake Nose (~1500 m) probably reflects the greatest water depth at which the oozes were deposited on Blake Nose during the Aptian-Albian interval. Upper Cretaceous and younger sediments were probably deposited near their present range of water depths (1100-2700 m; Benson, Sheridan, et al., 1978).

Seismic Stratigraphy
Six distinctive reflectors penetrated at Site 390 (Fig. 3) are hypothesized to be of the following ages and lithologies:

1. Oligocene/Eocene nannofossil ooze and phosphorite (Reflector Orange);

2. Paleocene/lower Eocene nannofossil ooze and chert (Reflector Red);

3. Lower/upper Paleocene nannofossil ooze (Reflector Green);

4. Campanian nannofossil ooze (Reflector Purple);

5. Aptian-Albian variegated nannofossil claystone (Reflector Blue); and

6. Aptian-Albian/ Barremian periplatform limestone (Reflector Yellow).

Tracing these reflectors updip along the Glomar Challenger single-channel seismic (SCS) line to its intersection with multichannel seismic (MCS) Line TD-5 shows that the Eocene and lower Paleocene-Campanian sediments appear to have nearly uniform thicknesses across most of the Blake Nose (Fig. 4). The middle Eocene is typically about 75 m thick, as it is at DSDP Site 390, whereas the lower Paleocene-Campanian sequence is about 45 m thick and consists of about 20 m of Paleocene and 20 m of Maastrichtian sediments. The Eocene and Paleocene intervals thin, but do not pinch out, at the extreme western end of the Blake Nose on MCS Line TD-5UW because sediments of these ages are found in USGS borehole ASP-3.

In contrast, the middle to upper Paleocene is highly variable in thickness, ranging from about 20 m at Site 390 to about 200 m in several areas updip of this site. On Gloria Farnella 1987 SCS Line 18, the middle Paleocene varies in thickness and appears to fill a channel in the lower Paleocene/Upper Cretaceous sequence. Hence, there may be an unconformity in the middle Paleocene.

Reflector Purple forms a strong and laterally continuous reflector from the Blake Escarpment to the Blake Plateau. Immediately below Reflector Purple there is a series of strong reflectors that pinch out against this surface. Apparently, Reflector Purple is a major erosional surface of early Campanian age or older.

SCS lines made during the Glomar Challenger initial survey and more recent SCS and MCS lines show that as much as 800 m of strata is between the Campanian and Aptian reflectors updip of Site 390. These mid-Cretaceous to Barremian deposits form a series of major clinoforms that build out from, and ultimately overlap, a reef complex that underlies Reflector Yellow (Barremian). Reflector Purple (?pre-Campanian) truncates these clinoforms. There may be two cycles of erosion within the clinoform stack because those that overlap the reef complex downlap against underlying clinoform deposits.

The lithology of the mid-Cretaceous(?) sediments that make up the clinoform stack is unknown. However, deposits immediately below Reflector Purple at Site 390 are nannofossil oozes and calcareous clays of Aptian-Albian age. The Aptian ooze at Site 390 can be traced updip in MCS Line TD-5 into the upper sequence of clinoforms below Reflector Purple. These clinoforms are successively cut into by the pre-Campanian erosion surface, which suggests that the remains of the top of the clinoform sequence have a progressively older age updip from Site 390. Hence, it is unlikely that any deposits younger than the Albian are preserved within the clinoform sequence, contrary to the interpretation of the DSDP Leg 44 scientific party. Site 390 is about 60 km northeast of the pre-Barremian(?) reef front so it is probable that the lower sequence of clinoforms may be redeposited periplatform deposits or reef-front debris.

This proposed seismic interpretation is partly corroborated by tracing the Paleogene reflectors updip into the ASP-3 borehole at the extreme western end of MCS Line TD-5UW (Figs. 2, 3). The upper ~80 m of this hole is middle Miocene sediment followed by ~30 m of middle Eocene and >15 m of Paleocene (foraminifer Biozone P2). This hole confirms that Eocene and Paleocene sediments are present at shallow burial depths in a drape across the whole of the Blake Nose.

The interpretation presented here differs from that presented by C.W. Poag in Hutchinson et al. (in press) largely because of differences in how we trace reflectors away from the condensed section at DSDP Site 390 (Fig. 5). Poag's interpretation traces the Maastrichtian and older reflectors to a consistently deeper level than in this interpretation and suggests that Santonian-Coniacian or Turonian-Cenomanian strata may be present on the Blake Nose between shotpoints 1130 and 1700 on MCS Line TD-5 (Figs. 3, 4). These Lower Cretaceous and mid-Cretaceous deposits correspond to a series of reflectors truncated by Reflector Purple and we have interpreted them as Albian-Aptian in age.

Should Poag's interpretation be correct, none of the proposed sites would penetrate deeper than the Maastrichtian. Therefore, the transect would recover extremely thick sequences of Paleocene and Eocene strata. The expanded nature of the section would improve the possibility of recovering an expanded Cretaceous/Paleogene boundary section along most or all of the transect because earliest Danian age fossils were recovered from the comparatively condensed record at Site 390. However, the transect would not have the opportunity to collect Lower Cretaceous deposits.

Regional Sedimentation Rates
Sedimentation rates estimated from the results of Site 390 are uniformly low (0.2-1.5 cm/k.y.). The highest rates are 1.5 cm/k.y. for the middle Eocene (Benson, Sheridan, et al., 1978). These low values almost certainly underestimate sedimentation rates on the shallower portions of the Blake Nose, where thicknesses of Aptian-Albian, Paleocene, and Eocene strata are much greater than at Site 390. For example, if the interval between Reflectors Green and Red is entirely Paleocene, sedimentation rates may have reached >4 cm/k.y. because the interval is >200 m thick. Likewise, sedimentation rates were probably above 1 cm/k.y. in the Aptian-Albian interval below Reflector Purple over much of Blake Nose.

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