DRILLING STRATEGY

The three proposed sites (Fig. F3) jointly form a logic suite, representative of the Belgica mound province and relevant to a number of key scientific questions related to modern mound settings. This "Challenger" downslope transect addresses the key hypotheses regarding the paleoceanographic and stratigraphic significance of the mounds on the eastern slope of Porcupine Basin. The lower sequence of interest within the Belgica province is characterized by an alternation of parallel reflectors with relatively high amplitude, locally grading into a set of sigmoidal units. A thick package of drift deposits with rhythmic buildup onlaps the mound flanks.

Based on recommendations from EPSP on drilling site order, drilling will start on the off-mound Site PORC-04A, followed by the Challenger mound Site PORC-03A, and finalize with the stratigraphic drill site PORC-02A (Table T1). Drilling will help identify and date the major underlying sequences and unconformities and the moundbase. The proposed program includes using standard logging tools to determine downhole physical properties and hole-core correlation, as well as a zero-offset vertical seismic profile (VSP) at proposed Site PORC-03A.

Proposed Drill Sites

Site PORC-04A: Challenger Off-Mound Site

The pre-IODP coring exercise carried out with the Marion-Dufresne implied a systematic on-mound/off-mound comparison. This has already yielded exciting results, in particular in comparing the mode in which environmental signatures are being recorded, in comparing densities of corals, and in comparing relative rates of sedimentation (Foubert et al., 2003; Van Rooij, 2004). Proposed Site PORC-04A (Fig. F11) should provide valuable insight into the drape history of the mounds and the off-mound transportation of mound-related skeletal and nonskeletal grains. The flanking sediments contain mixed sedimentary signatures of both contouritic and turbiditic origin. Many horizons onlapping the mound flanks display enhanced amplitudes, which again might be of various origin (Figs. F18, F19). Cores acquired from the Marion-Dufresne at both Sites PORC-03A and PORC-04A document the full paleoceanographic record down to marine isotope Stages (MIS) 4–5. The MD01-2450 Calypso core (length = 11 m) taken off-mound close to the Challenger mound, moreover, features intriguing rhythmic deposits of sulfidic nature.

As noticed on the north-northwest–south-southeast seismic section crossing the site, penetrating the whole thickness of the sigmoidal units will allow us to groundtruth two different and overlying sigmoidal sequences. Below Thérèse mound, the 3-D migration patterns of such two different sigmoidal sequences have been elucidated by a pseudo–3-D seismic grid of very high resolution (Galanes-Alvarez, 2001).

Coring strategy for this site includes triple coring using the advanced piston corer (APC) or extended core barrel (XCB) (Table T1).

Logging at this site will include the triple combination (triple-combo) and Formation MicroScanner (FMS)-sonic tool strings to correlate core depth with hole depth and to gain information about physical properties.

Challenger mound can be imaged with very high resolution seismics, including its basal sequences (Figs. F5, F13), which is in contrast with many other mound sites in Porcupine (Hovland mounds) and Rockall basins, where energy is frequently scattered by the mound surface (hardgrounds). Hence, it is a prime target for linking seismic images with coring and logging information. Some polarity inversions may argue for residual patches of shallow gas below the mound, which might help to address the first hypothesis. The quality of the data has supported some level of attribute analysis (polarity, instantaneous frequency, etc.) (Figs. F12, F13). A time-to-depth conversion based on a compilation of velocity determinations in shallow subsurface sediments and on the flattening of a deeper horizon led to the depth model shown in Figure F17. A remarkable observation is the need to introduce a slightly higher velocity in the mound core (2000 m/s), which can argue for a beginning of lithification.

Coring at proposed Site PORC-03A (Table T1) may require the XCB, due to the higher degree of lithification and cementation (velocity pull-up on lower reflectors). Use of microbial contamination tracers will be necessary. Sampling has to be done carefully because this site may provide valuable insight into the microbially mediated early cementation. Samples for microbiological analysis will require the possibility to aseptically acquire whole-round core sections at selected depths for subsequent subsampling. Triple coring will be preferred, considering the risk of lower core recovery within a mound.

We propose triple combo and FMS-sonic tool string runs to correlate core depth with hole depth and to gain information about physical properties, as well as a zero-offset VSP (pending approval).

Proposed Site PORC-02A, situated upslope Challenger mound (Figs. F6, F14), completes the Challenger slope transect (PORC-02A–PORC-03A–PORC-04A). It will primarily shed light on the enigmatic, rather transparent layer that underlies the southern part of the Belgica mound province and will refine dating of the basal unconformity below the mounds, as the sequence below the unconformity encompasses younger strata, including the quasi-transparent layer and some of its cover. The site is situated in water depths of ~650 m; it is proposed for a total penetration of 200 m.

At this site we propose double APC coring (Table T1).

We propose employing the triple combo and FMS-sonic tool strings to correlate core depth with hole depth and to gain information about physical properties.