Several factors contributed to our decision to renew drilling efforts on the southeast Greenland shelf following Legs 152 and 163:
In particular, the SIGMA study showed that a significant change in the thickness of the oldest oceanic crust occurs between 66° and 63°N. Crust to the north-northeast is ~30 km thick, whereas crust to the south-southwest is slightly <20 km thick. The results from the H/S Dana cruise showed that between 66° and 65°N there is evidence for a change in dip of the dipping reflectors from generally perpendicular to the margin to a more southerly dip oblique to the margin. In addition, the Dana data imaged two prerift basins immediately landward of the limit of the SDRS; one basin at ~64°N and another near the coast at ~65°N. However, little sediment cover was indicated by the Dana data in many areas within the inner continental shelf. Moreover, the smooth, highly reflective seabed in these areas, previously interpreted to consist of Precambrian basement, turned out to show subbasement acoustic layering consistent with a glacially eroded featheredge of the seaward-dipping reflectors, which is known to occur further offshore. As demonstrated during Leg 163X, some areas were sediment-free and teeming with life (see Fig. F7). Based on these new seismic data sets and the wealth of information obtained during Legs 152 and 163, we proposed the following goals for further drilling on the southeast Greenland margin:
ODP sanctioned this leg as a continuation of Leg 163. The "X" designation indicates that funding for the program was provided by external sources (Danish National Science Foundation and the U.S. National Science Foundation) without direct oversight from ODP. Leg 163X extended over 2 yr with cruises in August 1998 and August–September 1999. Drilling during Leg 163X took place in four transect areas north-northeast of 63°N, where drilling during Legs 152 and 163 had provided a good basement coverage.
Drilling of Transect EG68 was proposed to extend the volcanic stratigraphy of the flood basalt succession into the oceanic domain and to link this succession more directly to the Paleogene province of western Iceland. A number of crucial seismic profiles were acquired during the 1997 Dana cruise oriented parallel and perpendicular to the Blosseville Kyst (Hopper et al., 1997). Based on these data, we targeted the region immediately southeast of Nansen Fjord for drilling (Fig. F6). Seismic Line DLC9724 shows relatively even seabed with little sediment cover and seaward-dipping reflectors extending to within 20 km of the coast. The flood basalts along the coast are also seaward dipping and most likely continue into the offshore dipping reflector sequence. Drill sites were selected along this seismic line in two areas. The first was in the smooth ground near the coast and the second was in rougher ground just landward of the detected edge of the postvolcanic sediment cover.
Drilling of Transect EG66 was planned to recover lavas from the newly discovered featheredge of the reflector sequences immediately west of Site 988 likely to comprise the earliest rift volcanic flows and the breakup unconformity. The results from Site 988 had showed the presence of a relatively young igneous flow, possibly a lava flow of an off-axis origin. The Danish Lithosphere Center (DLC) seismic survey with Dana in 1997 specifically targeted the planned drill site area along Transect EG66 with Site 988 located at the intersection of Lines DLC9711 and DLC9722. The seismic lines suggest that seafloor, perhaps representing the eroded edge of seaward-dipping basalt flows, exists landward of the sediment wedge. Transect EG66 drilling was expected to sample the volcanic breakup record close to the Greenland-Iceland rise in order to better constrain crustal age and assess along-margin changes in mantle source composition and melting conditions within a position proximal to the Iceland hotspot track.
Drilling of Transect EG65 was motivated by the results of the Dana seismic survey in 1997 (Hopper et al., 1997) showing the presence of a previously unknown prebreakup basin between the Precambrian basement along the coast and the new igneous (SDRS) crust below the mid- to outer shelf. The transect is located along seismic Line DLC9709. The prime objective of this transect was to sample the continental to oceanic transition of the seaward-dipping volcanic flows that were particularly well imaged by this seismic line. An additional objective was to obtain samples from the prebreakup basin below and landward of the volcanic cover in order to constrain the age and depositional environment of the basin.
A major change in the margin structure occurs between 63° and 66°N. The shelf is very narrow south of 63°N but broadens significantly northeast between 63° and 65°N. This change in morphology is associated with an increase in crustal thickness between 63° and 66°N (i.e., seismic SIGMA-II and SIGMA-III lines). A significant part of this widening of the shelf is caused by recent shelf propagation during glacial times (Larsen, 1990; Clausen, 1998), but an increase in igneous productivity closer to the plume track may also play a role. The broad inner shelf region between 63° and 65°N is also associated with a low-gravity anomaly. The Dana seismic results over this region show the presence of a number of seaward-rotated fault blocks within a newly discovered prerift basin (Hopper et al., 1997) overlain seaward by the volcanic rocks associated with the SDRS. Drilling of Transect EG64 was proposed to sample the earliest volcanic rocks and underlying sediments. We targeted an area in the large embayment south of Ammassalik along the Dana seismic Line DLC9708 extending from Precambrian basement in the west across the prebreakup basin and into the featheredge of the seaward dipping reflectors.