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.
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.
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.
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.
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.