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doi:10.2204/iodp.pr.339.2012

Expedition synthesis

Expedition 339 drilled five sites in the Gulf of Cádiz and two sites off the west Iberian margin from November 2011 to January 2012 (Fig. F23). In total, we recovered nearly 5.5 km of core, with an average recovery of 86.4%, from a region never before drilled for scientific purposes. The Gulf of Cádiz represents a key location for the investigation of MOW through the Strait of Gibraltar gateway and its influence on global circulation and climate and is a prime area for understanding the effects of tectonic activity on evolution of the Strait of Gibraltar gateway and on margin sedimentation. The Gulf of Cádiz has also become known as the world’s premier contourite laboratory in which to thoroughly investigate and challenge existing models of contourite sedimentation.

Extensive previous work, both onshore and offshore and including seismic surveys for oil company exploration, has allowed us to develop a good regional understanding. Most importantly, we have been able to establish a firm seismic stratigraphic framework into which we could fit the ages of key seismic reflectors as determined by the drilling results from this expedition.

The principal results of Expedition 339 can be summarized as follows.

Mediterranean Outflow Water onset and evolution

We penetrated the Miocene at two different sites and established the strong signal of MOW in the sedimentary record of the Gulf of Cádiz following opening of the Strait of Gibraltar gateway at 5.3 Ma. Our present documentation shows clear contourites in the record from ~4.2 to 4.5 Ma. However, MOW was not well developed at this stage and the signal is relatively weak. Additional seismic evidence for sheeted drift development exists in the early Pliocene. The contourite signal is also mixed with considerable downslope resedimentation and hiatuses in the record. Further work will establish how far back in time we can extend the onset of contourite sedimentation.

The Pliocene succession was penetrated at four sites, all of which show relatively low bottom-current activity linked with a generally weak MOW, with some evidence for a slow increase in activity through the later Pliocene. Significant unconformities are apparent at ~3.0–3.2 and 2.1–2.4 Ma to a variable extent at different sites. We interpret these as indicative of enhanced bottom currents related to intensified MOW, the principal phase occurring from ~2.4 Ma.

In general, the Quaternary succession shows a much more pronounced phase of contourite deposition and drift development throughout the region. Although there is some variation between sites, we recognize two periods of current intensification, noted by increased sandy and silty contourites in the sedimentary record. The first is from ~2.0 to 0.9 Ma and culminates in a regional hiatus of variable duration (~0.7–0.9 m.y.). The second is from 0.9 Ma to the present; this also includes a more locally developed hiatus at ~0.4 Ma.

In part, there is an important climate control on this long-period cyclicity in the development of MOW and bottom-current activity. Another part is tectonic (see below).

Tectonic pulse at a plate boundary

Regionally, there appears to be very strong tectonic control on margin development, downslope sediment transport, and contourite drift evolution. From the occurrence, nature, and disposition of the sedimentary record, as well as from the known timing of closure and opening of the Atlantic-Mediterranean gateways, we recognize a clear signal of this tectonic activity.

We have established a clear signal of tectonic pulsing over the past 6 m.y. in this region that has controlled

1. Closure of Atlantic-Mediterranean connections in Spain and Morocco,

2. Initial opening of the Strait of Gibraltar gateway and probable subsequent deepening,

3. Continental margin instability and episodes of active downslope resedimentation,

4. Basin subsidence in the Gulf of Cádiz,

5. Local uplift and diapiric intrusion within the basin, and

6. Constriction of MOW and development of narrow core bottom currents instead of a broad tabular.

According to the timing of these different events, we propose an ~1 m.y. duration of tectonic pulsing with an overprint of larger 2.5 m.y. cycles. We further relate this tectonic pulsing to regional asthenosphere activity that is especially apparent at the plate boundary between the African and European tectonic plates.

Testing the contourite paradigm

Of the 5.5 km of core recovered, at least 4.5 km is from the Cádiz CDS in the world’s premier contourite laboratory. This was the ultimate testing ground for the contourite paradigm. In general, we have found the models for contourite deposition to be in very good order, contrary to recent doubts expressed in the literature. Sedimentation rates ranged from moderate (~20 cm/k.y.) to extremely high (>100 cm/k.y.).

The contourites recovered are remarkably uniform in composition and textural attributes. They have a noted absence of primary sedimentary structures and an intense continuous bioturbation throughout. They are particularly characterized by bi-gradational sequences from inverse to normal grading with a range of partial sequence types, as predicted by the models.

However, very interesting modifications are required, for example, to the detail of the sand-silt contributions and the role of sediment supply. These are very significant for future use of contourite systems in paleoceanographic studies and in hydrocarbon exploration. We have documented very interesting interactions between contourite and turbidite processes that are completely new and different from the current models.

Paradigm shift for oil exploration

We have verified an enormous quantity and extensive distribution of contourite sands (and bottom-current-modified turbidite sands), and have begun to establish their detailed characteristics. Drilling at the proximal site (U1388) managed to penetrate only the uppermost 220 m of what we had interpreted as a very thick sandy contourite drift. Hole instability and collapse of these unconsolidated sands prevented further penetration. At other proximal sites (U1389 and U1390), we also encountered thick contourite sands (as thick as 10 m) within the muddy contourite drifts.

These are completely different deepwater sands than the turbidite sands that are currently dominant as deepwater oil and gas plays and are formed in different depositional settings, have different depositional architectures, and are clean and well sorted. These characteristics would provide good quality potential reservoirs when buried deeply.

In addition, the associated contourite muds are very thick, rapidly deposited, and moderately rich in organic carbon (up to 2 wt%). These could provide potential source rocks in the subsurface, as well as suitable seals in stratigraphic traps.

These new findings could herald a paradigm shift in exploration targets in deepwater settings.

Cracking the climate code

At the Shackleton site (U1385), we succeeded in recovering multiple sets of a pristine Quaternary record over the past 1.4 m.y. These sets will form the basis of a detailed shore-based scientific collaboration to establish a marine reference section of Quaternary climate change for comparison with ice-core and terrestrial records. There are many years of research ahead to decode the full climatic signal from these cores.

However, we have also shown that exactly the same climate signal is evident in all the contourite drift sites at which the sedimentation rate is 3–10 times as high. This will allow even more detailed sampling, a possibly better marine archive, and a hugely important comparison with the Shackleton site for documenting the nature and variation of MOW. The comparison between the influence and effects of MOW compared with NADW (above the Shackleton site) will also be of great interest. We can be confident of identifying hiatuses where these occur in the contourite records and therefore can avoid this problem in their interpretation. At the most distal site on the Portuguese margin, for example, hiatuses were minimal, although sedimentation rates for the last 1.5 m.y. were about three times as high as those of the Shackleton site.

Preliminary work has shown a remarkable record of orbital-scale variation in bulk sediment properties at several of the drift sites, a good correlation between sites, and, significantly, a very close correlation with the same records at the Shackleton site. The climate control on contourite sedimentation is clearly significant at this scale. Further work will determine the nature of controls at the millennial scale.

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