To test the above hypotheses, critical data will be provided by drilling in the Newfoundland transition zone. These data include (1) the structure and composition of basement, (2) the origin and composition of the interval between basement and the U reflection, (3) the origin of the U reflection, and (4) the subsidence history of crust recorded in the basement–U and post-U sedimentary sequence. In addition, coring the post-U sequence will determine the evolution of paleoceanographic conditions in this developing ocean basin, which is a critical gateway between the North Atlantic and the sub-Arctic and Arctic Ocean Basins.
Leg 210 drill sites are arrayed along Ewing line 2MCS (Figs. F2, F3, F5, F6, F7, F8), which is conjugate to the drilling transect of ODP Legs 149 and 173 on the Iberia margin. The prime drill site for ODP Leg 210 is proposed Site NNB-01A. This is a deep hole (~2080 m to basement) which is designed to penetrate a complete stratigraphic sequence below ~800 meters below seafloor (mbsf) (including the U reflection and the U–basement interval) and to penetrate as deeply as possible into basement. Alternate Sites NNB-01B and NNB-01C have the same objectives but they require deeper penetration; it is unlikely that either of these sites will be drilled. Other sites along the transect are designed to sample across the interval from transition zone crust at Site NNB-01A seaward to probable "normal" ocean crust (proposed Sites NNB0-5A and NNB-06B). These sites are outside the limits where the U reflection is observed, and they consequently have lower priority than Site NNB-01A. The order in which these alternate sites might be drilled will depend on the time that each requires balanced against scientific priorities and time remaining during the leg. Site-specific objectives, expected lithologies, and drilling strategies are described below.
This is the primary site for Leg 210 operations (Figs. F2, F3, F5, F7; Table T1). Major objectives are (1) to sample reflection U, the U–basement interval, and basement (penetration of at least 100 m) in order to test hypotheses about crustal origin and evolution; and (2) to obtain as much of the record of the overlying deep sedimentary section as possible in order to examine subsidence history and paleoceanographic evolution in the Newfoundland Basin. On line 2MCS, the site is located slightly east of the crest of a crustal block and basement depth is ~2080 mbsf. On the crossing line 303, basement rises ~500 m within 10 km to the south (see Fig. F7). Reflection U is ~200 m above basement at the drill site and dips southeast on line 2MCS. On the crossing line 303, reflection U forms a gentle swale, rising slightly to the south where it laps onto the shoaling basement. To the north, reflection U is initially nearly flat and then slowly rises.
The nature of the section between reflection U and basement is unknown, and our three working hypotheses predict different lithologies. If the basement is thinned continental crust, the basement–U section could be prerift to synrift sediments (middle Cretaceous and older), possibly including Kimmeridgian beds like the Egret formation source rock in the Jeanne d'Arc Basin ~300 km to the west on the Grand Banks. However, analysis of OBH/S data currently in progress suggests that it is unlikely that the basement is continental. The crust is thin (a few kilometers) and it has velocities and velocity gradients similar to oceanic Layer 3.
The hypothesis that basement is exhumed serpentinized mantle would indicate that the basement–U section must have been deposited in deep water because of isostatic considerations. The section could be basalts, or it could be a sedimentary sequence peculiar to the Newfoundland margin. If the basement is ultra-slow-spreading ocean crust, the basement–U section could also be basaltic or sedimentary, although the depth of the seafloor probably would have been shallower than if it had been over exhumed mantle. OBH/S analyses suggest that the crust has velocity and velocity gradients similar to oceanic Layer 3. If the crust is oceanic, the U–basement section would probably have been deposited in deep water.
Overlying U, and up to reflection Au, we expect the following ascending series: black claystones equivalent to the Hatteras formation (Barremian–Cenomanian) drilled in the western North Atlantic, multicolored claystones (±limestones) equivalent to the Plantagenet formation (Turonian–Paleocene), siliceous claystones (possibly cherty) equivalent to the Bermuda Rise formation (Paleocene–Eocene), and, possibly, claystones equivalent to the Blake Ridge formation (Eocene and younger). Because the black claystones in the western North Atlantic can contain significant amounts of organic matter and because any equivalent in the Newfoundland Basin is buried up to ~1800 m, we must consider the potential in this interval for hydrocarbon generation. Within the Newfoundland–Iberia rift, black claystones of Barremian–Albian age were drilled at Deep Sea Drilling Project Site 398 (water depth = 3900 m) on the conjugate Iberia margin at subbottom depths of ~1180–1670 m. These sediments presumably were deposited under conditions similar to those in the Newfoundland Basin. Although the sediments contain up to 3% organic matter, Kendrick et al. (1979) considered the sequence to have low petroleum-generating potential. Nonetheless, the environment may have been different in the Newfoundland Basin and it will be important to monitor continuously for any hydrocarbons during drilling at this site.
The shallowest part of the hole has a thin turbidite sequence, and there is the possibility of sand in this section that could cause hole instability. Because proposed Site NNB-01A is located at the western feather edge of this sequence, this possibility is minimized.
Previous drilling results on the conjugate Iberian margin (Legs 149 and 173) have been used to predict drilling conditions and penetration rates at Leg 210 sites, including Site NNB-01A. As these previous legs encountered substantial hole stability problems (unconsolidated turbidites and unstable deep sediments and basement), we are prepared to use multiple casing strings to stabilize the entire sedimentary cover and uppermost basement at Site NNB-01A. Operations at Site NNB-01A are expected to occupy the entire leg. To be able to achieve the prime objectives of the leg (sampling the deepest sediment section, the U reflection, and basement) in the time allotted, coring the uppermost sediments (0–800 mbsf) will be deferred until achieving the main goals of the cruise.
Our first operation will be to drill-in a reentry cone with ~80 m of 20-in casing (Table T1). Next, we will drill without coring from 80 to 800 mbsf and install 16-in casing to 775 mbsf. Rotary core barrel (RCB) coring will start at 800 mbsf and continue down to ~1375 mbsf; the hole will be logged and a 133/8-in casing liner set to ~1350 mbsf. We will core from ~1375 mbsf down to basement (~2080 mbsf) and possibly into the upper few tens of meters of basement, hole conditions permitting. This section will be logged before setting a 103/4-in casing liner to ~2080 mbsf. Then basement will be penetrated 100–200 m, depending on hole and drill-bit conditions as well as time available (see Table T1). Logging will be conducted in the same sections that are cored and will consist of three runs (triple combination [triple combo] and Formation MicroScanner [FMS]-sonic tool strings and the Well Seismic Tool [WST]). After the primary deep objectives have been achieved, if time permits, we may core the upper ~800 m of section in a second hole to obtain a complete sedimentary record at this site.
This site is an alternate to drilling at proposed Site NNB-01A (Figs. F2, F3, F5, F7; Table T2). The uppermost turbidite section at Site NNB-01B is slightly thicker than that at Site NNB-01A. Based on recent time migration of MCS data and velocity analyses, basement depth is picked at ~7.90 s (~2177 m subbottom), ~100 m deeper than at Site NNB-01A. The Site NNB-01B basement depth is shallower than indicated by recent analysis of the other alternate (proposed Site NNB-01C), and thus Site NNB-01B is the preferred alternate. The site is located on a small basement rise on line 2MCS. On the crossing line 108, basement is depressed at the site, rising both to the north and the south. The overlying U reflection is flat to gently depressed, with a small rise to the south side of the drill site.
The objectives, the stratigraphic section, and the drilling, logging, and monitoring procedures are similar to those for proposed Site NNB-01A, but they are outlined in Table T2 as a three casing string scenario.
This site is a second alternate to Site NNB-01A (Figs. F2, F3, F5, F7; Table T2). There is no turbidite section at the top of Site NNB-01C. Based on recent time migration of MCS data and velocity analyses, basement depth is picked at ~7.76 s (~2358 m subbottom), requiring ~280 m more penetration than at Site NNB-01A. Proposed Site NNB-01C is located just east of the crest of a basement high on line 2MCS, and on the crossing line 305 it is in a basement swale. The overlying U reflection dips eastward on line 2MCS and forms a local swale in the crossing line 305.
The objectives, the stratigraphic section, and the drilling, logging, and monitoring procedures are similar to those for proposed Site NNB-01B.
Proposed Site NNB-03A is 22 km seaward of Site NNB-01A (Figs. F2, F3, F5, F8; Table T2). It is our next highest priority for drilling if there are problems at Site NNB-01A and enough time remains to drill at the site. Proposed Site NNB-03A lies over a flattened basement block that the U reflection laps onto and partially circumscribes. Thus the block predates the formation of reflection U, and a key objective is to determine the nature of this pre-U crust. The block is near magnetic Anomaly M3, the interpreted age of the oldest ocean crust on the conjugate Iberia margin. However, the block exhibits reflections that are significantly more coherent and lower frequency than large basement ridges farther to the east and closer to Anomaly M0. This might indicate that the Site NNB-03A crust contains a continental component, or, like the basement ridge drilled at Site 1070 near Anomaly M3 off Iberia, that it consists of serpentinized peridotite ± gabbroic intrusions. Other key objectives are to determine the subsidence history of the basement and the paleoceanographic history from the overlying sedimentary section.
On line 2MCS, the site is east of the crest of a basement high, and on the crossing line 208 it is on the flank of the basement high. Reflection U is not present at the site. The sedimentary section over basement, up to reflection Au, probably consists of claystones and siliceous/cherty claystones of Late Cretaceous to Eocene age. There may be a thin interval of black shales (Hatteras formation equivalent) immediately above basement. Turbidites are present from the seafloor to a depth of ~400 mbsf.
The drilling/logging plan consists of a two-casing string strategy with a 103/4-in liner as contingency to ensure we can obtain deep sediments and basement (Table T2). We will drill 20-in casing in to ~80 mbsf and then 16-in casing to ~475 mbsf. RCB coring will extend from 500 mbsf down to, and into, basement. Logging will consist of the two standard logging strings and a checkshot with the WST. After the primary deep objectives have been achieved, if time permits, we will core the upper ~500 m of section in order to obtain a complete sedimentary record at this site.
Seaward of Site NNB-03A out to just beyond Anomaly M0 is a set of three large basement ridges (Figs. F2, F5, F6). These ridges are much higher amplitude than any other basement blocks known either landward or seaward, but they are similar to the large peridotite ridges drilled on the conjugate Iberia margin. Proposed Sites NNB-04A to NNB-06B are located on these large ridges, and a primary drilling objective is to determine whether they are normal ocean crust, serpentinized peridotite as off Iberia, or possibly some other composition. Site NNB-04A is the highest priority for drilling in this group, and it is also a short hole (~6 days).
Basement at Site NNB-04A is picked on line 2MCS at 6.27 s (~136 m subbottom) at the top of a basement high. In the crossing line 206 (Fig. F8), an elongated ridgelike character of the basement high is apparent and basement rises southward nearly to the seafloor. Overlying sediments are Pleistocene turbidites.
Coring/logging at this site will be RCB to basement, plus at least 100-m basement penetration (Table T2). Logging will consist of the two standard logging strings and a checkshot with the WST.
This site is located to the side of the crest of a basement high which is elongated in a northeast-southwest direction (Figs. F2, F6, F8). Basement depth at the drill site is ~355 m. The overlying sediments are turbidites, fan sediments, and contourites, probably of Pliocene (?Miocene) to Pleistocene age.
In the first hole we will use the advanced piston corer/extended core barrel (APC/XCB) to core to basement (Table T2). In the second hole we will drill without coring to ~355 mbsf (just above basement) and then core at least 100 m in basement. Logging will consist of the two standard logging strings and a checkshot with the WST.
This site also is located to the side of the crest of a basement high which is elongated in a northeast-southwest direction (Figs. F2, F6, F8). Basement depth at the drill site is ~760 m. The overlying sediments are turbidites, fan sediments, and contourites, probably of Oligocene–Miocene to Pleistocene age. Horizon Au is interpreted to be at ~650 mbsf, and Eocene sediments are presumably present below this reflection.
Coring/logging at this site includes APC/XCB coring to ~500 mbsf (Table T2). The second hole will be drilled without coring to just above ~500 mbsf and then cored downward to at least 100 m into basement. Logging will consist of the two standard logging strings and a checkshot with the WST.