The prime drilling objectives of Leg 210 were to sample basement and facies corresponding to the overlying U reflection in the Newfoundland Basin, both of which were deep targets. Additional objectives included investigation of the Cretaceous paleoceanography of the NewfoundlandIberia rift and the record of how abyssal circulation developed in Paleogene time through this gateway to the sub-Arctic and Arctic seas.
At our prime site, 1276, basement was estimated to be at 2080 mbsf and depth of U was estimated at 1866 mbsf. Drilling and coring to these depths presented considerable engineering and operational challenges, both expected and unexpected. We were able to follow our operational plan to case the hole at Site 1276 with both 20- and 16-in casing as planned (Fig. F12), but very tight hole conditions prevented us from installing liners to greater depths. Ultimately, these hole conditions also prevented us from logging the hole.
Despite these difficulties, drilling at Site 1276 was an outstanding success. We cored from 800 to 1736.9 mbsf with a remarkable average core recovery of 85%, and we obtained a detailed record of sedimentation from the time that the NewfoundlandIberia rift was a very narrow ocean basin (latest Aptian[?] to earliest Albian) up to the early Oligocene. This record captures an extensive series of major oceanographic events that affected the expanding North Atlantic Ocean during this time period, and it allowed us to accomplish virtually all of our major paleoceanographic objectives.
Site 1276 bottomed in diabase sills that appear to have been intruded into lower Albian sediments at very shallow subbottom depths. These sills are estimated to lie only 100200 m above basement, and it appears completely feasible that future drilling at this site can core to, and into, basement. The sills were intruded at the level of U, and this basin-wide horizon is interpreted to represent a combination of extensive gravity flow deposits and intrusive sills. The occurrence of the sills raises intriguing new questions about the magmatic history of the Newfoundland side of the rift and about its relation to the nonmagmatic exhumation of mantle on the conjugate Iberia margin.
With a few days remaining on Leg 210, we were able to drill at Site 1277, a shallow-penetration basement site ~40 km southeast of Site 1276 on presumed ocean crust. Here we recovered a unique assemblage of ~35 m of basalt flows interleaved with allochthonous slivers of gabbro, serpentinized peridotite, and fine- to coarse-grained sediments at the top of basement. These overlie at least ~45 m of serpentinized peridotite that appears to represent intact basement. The Site 1277 basement rocks record complex processes of magmatism, deep-crust and mantle exhumation, and mass wasting at the eastern margin of the Newfoundland transition zone, in ocean crust that probably accreted at very slow spreading rates.
The hole at Site 1276 was cased to 750 mbsf, and the engineering plan was to extend casing liners to a depth as great as 2060 mbsf, just above projected basement depth (Fig. F12). Hole conditions below the casing, however, prevented liners from being installed. Tight spots at numerous intervals (see "Operations,") indicated that the hole was persistently closing on the drill string, and it was clear that liner could not be forced into this section, even with predrilled, oversize hole. Engineering assessment is that, if drill-in casing had been available for this section, it most likely could have been successfully emplaced and latched into the reentry cone.
Tight spots in the (ultimately) 987 m of open hole below the casing slowed operations throughout the drilling, requiring frequent wiper trips to condition the hole as well as significant redrilling each time we tripped the pipe and reentered. This persistent hole closure also prevented lowering of logging tools, and we ultimately were unable to obtain any logs. Fortunately, the excellent core recovery allowed us to obtain an extensive set of laboratory physical property data that will be used for hole-seismic correlation and synthetic seismogram modeling.
Reentry was another significant operational challenge. Beginning with the first reentry, the reentry cone was found to be completely covered by sediment and its location could only be determined from an overlying craterlike depression in the seafloor. On a number of reentries, muddy water streaming from the crater largely obscured it, and reentry was accomplished only by careful tracking of the crater in both sonar images and in the very limited visual images. Currents presented an additional problem. Bottom currents at times appeared to approach 50 cm/s (estimated from the streaming sediment noted above), and this displaced the drill bit up to 75 m laterally from the location of the moonpool. The conventional assumption on reentry is that the drill bit is within a few meters to tens of meters directly below the ship, so this unexpected offset at one point caused a long search for the cone. We ultimately found the cone by working "upstream" along the trail of muddy water noted above. Subsequently, a transponder was placed on the vibration-isolated television (VIT) frame; this allowed us to track the location of the drill bit with respect to the cone, and it greatly facilitated further reentries.
We drilled two sills, one at 16131623 mbsf and a second from 1719 mbsf to the bottom of the hole at 1737 mbsf (the sill base was not reached or recovered). These are alkaline diabases, tentatively identified as "basanites." They were intruded at very shallow levels beneath the seafloor, in lowermost Albian to uppermost Aptian(?) sediments at or near the level of U. They exhibit chilled margins and they developed striking hydrothermal metamorphic effects in the enclosing sediments. It appears that the upper sill may have formed a seal over the underlying sediments, preventing them from becoming normally compacted and isolating fluids (including methane) in the section. Seismic signal reflection from the strong impedance contrasts created by these interbedded igneous and sedimentary rocks appears to explain the poor seismic signal penetration through U, and thus the typically poor reflection definition of underlying basement.
Careful examination of SCREECH site survey seismic reflection data indicates that U may be a double wavelet at Site 1276, and we infer that this character may be related to the presence of the sills. With further study, we identified similar double wavelets at or near U in seismic profiles from a number of locations throughout the Newfoundland Basin. If, indeed, this is a signature characteristic of sills, then such sills may be a common feature in the basin. The double wavelets typically extend for a few kilometers or less, and many coincide with subtle disruptions of the underlying sub-U reflections or with basement highs. This subtle disruption appears to be present at Site 1276. Shore-based synthetic-seismogram studies will examine more closely how variations in sill thickness and spacing may affect the seismic signature in reflection profiles.
The source of magmatism that created the sills is presently unknown, and such magmatism certainly is unexpected considering the nonvolcanic nature of the conjugate transition zone crust off Iberia. A source (hotspot?) associated with development of the Newfoundland Seamounts ~180 km to the south seems to be unlikely, considering the basin-wide scattering of double wavelets (?sills) noted above. However, some kind of regional, postrift magmatic or thermal effect could help to explain the marked asymmetry of basement depth and roughness between the conjugate Newfoundland and Iberia margins (Fig. F4). Shore-based analysis of the Newfoundland Basin reflection data, together with synthetic seismogram modeling based on the Site 1276 physical property data, age dating, and geochemical analyses, are expected to provide significant insights into this issue.
Preliminary shipboard synthetic seismogram modeling and velocity-depth analysis indicate that U is at or near the top of the upper sill noted above. In the lithologic column this level is close to the top of extensive lower Albian sedimentary gravity flows. This is much like the Iberia margin where the orange reflection, coincident with uppermost Aptian fan deposits, lies at the top of a similar, although weaker, reflection sequence. Off Newfoundland, it is somewhat puzzling that there is not a stronger lithologic or physical property contrast at the top of the gravity flow deposits, in light of the fact that U is a very strong reflection throughout the basin. It seems very unlikely that the reflection is widely accentuated by intrusion of sills because the reflection covers a very large area (~600 km x ~150 km) and because its seismic signature is very regular, except for the scattered double wavelets noted above. New insights provided by future analysis of physical property results, however, may help to resolve this question.
A greatly expanded sedimentary sequence of Cretaceous black shales, equivalent to the Hatteras Formation in the western North Atlantic, was cored at Site 1276. The sequence extends from the lowermost Albian, or possibly the uppermost Aptian, upward to the Cenomanian/Turonian boundary. It reflects deposition under relatively low oxygen conditions in the deep basin, probably punctuated by intervals of total anoxia. Various black, organic carbonrich layers contain either terrestrial or marine carbon, or both, indicating that both sources intermittently created reducing conditions at and below the seafloor. There is an overall trend from strong input of terrestrial carbon in the lower part of the section to reduced input in the upper part. This correlates generally with relative abundance of gravity flow deposits derived from the shallow margin, which also decreases upsection. The trend probably reflects, at least in part, increasing sequestration of terrestrial debris on the continental shelf as sea level rose in the mid-Cretaceous.
Site 1276 recovered sediments that may include five major OAEs. These are the latest Cenomanianearliest Turonian OAE 2 ("Bonarelli" event), the mid-Cenomanian event, and OAE 1b ("Paquier" event), OAE 1c, and OAE 1d in the Albian. In addition, one other, possibly new, Albian event is recognized from its characteristic black color coupled with high total organic carbon and related geochemical indicators (see "Geochemistry" and "Biostratigraphy," both in "Site 1276 Synthesis"). Analysis of these sediments will provide a rich data set to examine the paleoceanography of the Cretaceous North Atlantic Ocean as it expanded northward through the NewfoundlandIberia rift.
Interestingly, similar dark sediments with locally black layers were recovered in parts of the uppermost Cretaceous and Paleocene section. This kind of occurrence has also been observed on the southern Bermuda Rise at Site 387 (Tucholke and Vogt, 1979), suggesting that low-oxygen conditions intermittently affected the North Atlantic at times well after the main episodes of anoxia that are documented in the OAEs.
Most Turonian and younger sediments are facies characterized by reddish, brown, green gray, and other light colors that indicate a well-oxygenated basin, in marked contrast to the underlying black shales. This change is well documented in the main North Atlantic Basin, where the multicolored sediments form the Plantagenet Formation (Jansa et al., 1979). The paleoceanographic change is thought to be associated with development of longitudinal deep circulation between the North and South Atlantic Oceans when these two oceans first became fully connected at abyssal depths near the end of Cenomanian time (Tucholke and Vogt, 1979). Documentation of this oceanographic change off Newfoundland indicates that the widening rift probably was connected to the main North Atlantic over full ocean depth.
We recovered one of the few nearly complete upper Maastrichtian to lower Danian abyssal sedimentary sections across the K/T boundary at Site 1276. Extensive reworking and frequent carbonate-free sediments prevent this section from being suitable for analyzing processes of biotic extinction, but the succession of biotic changes is obvious. High sedimentation rates in the lower Paleocene section will facilitate high-resolution study of biotic recovery following the impact event at the K/T boundary.
The Paleocene/Eocene boundary interval is characterized worldwide by an abrupt warming event referred to as the PETM, which is recorded by clay-rich precursor beds, followed by a sharp negative 13C excursion and a benthic foraminiferal extinction event. We cored this sequence at Site 1276. Although the specific boundary clay layer appears to be missing in our cores, a complete succession of the calcareous nannofossil events occurring immediately above the boundary clay layer was recognized, and it will provide important information on biotic recovery after this event.
Sedimentation patterns in the North and South Atlantic Oceans have been profoundly affected by bottom currents ever since the initiation of strong abyssal circulation in Paleogene time. Determining when this initiation occurred, however, has been problematic because the currents created major unconformities (e.g., Horizon Au in the main North Atlantic Basin) and thus left little lithologic or biostratigraphic record to establish the timing of the event. Current interpretations are that the abyssal circulation developed near the Eocene/Oligocene boundary (Miller and Tucholke, 1983; Davies et al., 2001).
At Site 1276 we cored through a seismic marker that appears to coincide with this circulation event. It matches an unconformity between lithologic Units 1 and 2 and a middle Eocene hiatus identified from preliminary biostratigraphy. The hiatus seems to represent a limited length of geologic time compared to other places where the unconformity has been cored, probably because gravity flows were flooding the Newfoundland deepwater margin with abundant sediment at the time. If our preliminary shipboard conclusions about the age of the hiatus are correct, then the age of this major paleoceanographic event may be 47 m.y. older than previously supposed. Shore-based analyses are planned to investigate this phenomenon in detail.
With only 3 days of Leg 210 remaining, we moved 40 km southeast of Site 1276 and drilled into the crest of a prominent basement ridge at Site 1277 (Figs. F8B, F9). The crust here is just on the young side of a magnetic anomaly identified as M1, and it therefore is presumed to be oceanic. We cored the basement from 103.9 mbsf (wash core) to 180.3 mbsf and recovered a spectacular section of mixed basalts, fine to very coarse grained gabbros, serpentinized peridotites, fine- to coarse-grained sediments, and calcite-cemented conglomerates. Most of these rocks appear to have been displaced to their present location in local slumps, slides, and debris flows. The distance of transport cannot have been large because they were cored from the crest of the ridge, and they must have been displaced only from local basement irregularities. The deepest four cores, below ~135 mbsf, consist dominantly of serpentinized peridotite with minor gabbros and a cataclastic damage zone. The serpentinites are interpreted to may represent in situ basement.
The occurrence of coarse-grained gabbros and serpentinites at the crest of this ridge suggests that the ridge must have been formed by large-scale fault displacement that unroofed lower crust and upper mantle or that magmatic crust at this site is extremely thin, or both. Both of these features are expected in very slow spreading ocean crust that is characterized by limited magma supply and extreme tectonic extension. We conclude that at least this section of ocean crust in the Newfoundland Basin was formed at very slow spreading rates.
We undertook Leg 210 with a full understanding that drilling, casing, coring, and logging a 2+ km hole during only one drilling leg would be a major challenge. Although we were unable fully to meet this objective at Site 1276, in the final analysis we judge that the leg was a clear success. We cored to 1739 mbsf and recovered an exceptionally complete sedimentary record. Engineering analysis indicates that Site 1276 can reasonably be deepened to reach and core basement if casing is drilled into the lower part of the hole. In addition, Site 1277 returned a remarkable record of lower oceanic crust and upper mantle from a very short-penetration hole. As is true of the best scientific problems, results from these two sites raise exciting new questions. Among the most important of these is: How do we reconcile a record of extreme tectonic extension in an apparently slow spreading and magma starved environment (Site 1277) with a record of significant postrift magmatism in nearby crust at Site 1276? We look forward to future drilling in the Newfoundland Basin to answer this kind of question and to determine the nature of the crust in the Newfoundland transition zone.
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