Marine Geophysical Researches 22: 369–399, 2001. © 2002 Kluwer Academic Publishers. Printed in the Netherlands.
369
Neogene-Quaternary contourite and related deposition on the West Shetland Slope and Faeroe-Shetland Channel revealed by high-resolution seismic studies John E. Damuth∗ & Hilary Clement Olson Department of Geology, P.O. Box 19049, University of Texas at Arlington, Arlington, Texas 76019 USA Institute for Geophysics, University of Texas at Austin, 4412 Spicewood Springs Road, Austin, Texas 78759 USA ∗ Author for correspondence (Tel: +1-817-272-2976; Fax: +1-817-272-2628; E-mail:
[email protected]) Received 8 June 2001; accepted 17 December 2001
Key words: Faeroe-Shetland Channel, contourite, mass-transport deposits, seismic facies
Abstract The Neogene and Quaternary sediments of the Faeroe-Shetland Channel and West Shetland shelf and slope rest upon a major regional unconformity, the Latest Oligocene Unconformity (LOU), and have been deposited through the interaction of downslope and parallel-to-slope depositional processes. The upper to middle continental slope is dominated by mass-transport deposits (debris flows), which progressively diminish downslope, and were largely generated and deposited during glacial cycles when ice sheets supplied large quantities of terrigeneous sediment to the upper slope and icebergs scoured sea-floor sediments on the outer shelf and uppermost slope. Large-scale sediment failures have also occurred on the upper slope and resulted in deposition of thick, regionally extensive mass-transport deposits on portions of the lower slope and channel floor. In contrast, large fields of migrating sediment waves and drift deposits dominate most of the middle to lower slope below 700 m water depth and represent deposition by strong contour currents of the various water masses moving northeastward and southwestward through the channel. These migrating sediment waves indicate strong northeastward current flow at water depths shallower than ∼700 m and strong southwestward current flow at water depths from ∼700 to >1,400 m. These flow directions are consistent with present-day water-mass flow through the Faeroe-Shetland Channel. The Faeroe-Shetland Channel floor is underlain by thin conformable sediments that appear to be predominantly glacial marine and hemipelagic with less common turbidites and debris flows. No evidence is observed in seismic or core data that indicates strong contour-current erosion or redistribution of sediments along the channel floor.
Introduction and background The modern Faeroe-Shetland Channel is a narrow elongate basin that trends NE–SW between the West Shetland Shelf and the Faeroe Shelf and that provides a major passageway for the exchange of cold and warm water masses between the Norwegian Sea and the North Atlantic (Figure 1). The Faeroe Basin, which is a several kilometer thick accumulation of sediments beneath the channel floor and the West Shetland slope and shelf, has become an important hydrocarbon province. Exploration for hydrocarbons by Mobil North Sea Ltd. led us to participate in a detailed seismic-stratigraphic analysis of the Paleogene strata of the Faeroe Basin using conventional multi-fold seismic data (Mitchell et al., 1993). Concurrent with this study, we undertook a preliminary study of the depositional processes of the Neogene/Quaternary section of this area with a partial grid of high-resolution airgun
(50–250 Hz) seismic data collected by the British Geological Survey for the purposes of developing depositional analogs for the Paleogene prospective deposits, and for assessing sea-floor geohazards (Damuth and Olson, 1993). This study revealed that a significant portion of the West Shetland Slope contains extensive fields of migrating sediment waves or drift deposits presumably deposited by contour-current activity. In addition, significant downslope movement of sediments through mass-transport and turbidity-current processes was also observed. The fairly wide spacing of the seismic lines available for this preliminary study left some uncertainties about the depositional processes in some portions of the study area (Damuth and Olson, 1993). The present study is an outgrowth of, and builds on, our original study, and was undertaken when a more robust data set consisting of the entire BGS grid of high-resolution (50–250 Hz) airgun seismic
370
Figure 1. Bathymetric map (redrawn from Roberts et al., 1977) of the Faeroe-Shetland Channel and West Shetland continental shelf and slope showing location of study area (outlined by black box; see Figure 2). Locations of airgun seismic profiles illustrated in figures are identified by heavy lines and letters. Contour interval is 200 m. Small inset shows location of area of this map.
lines and 3.5 kHz echograms (Figure 2) was made available to us for study by Mobil North Sea Ltd. and the British Geological Survey. In addition, Mobil North Sea Ltd. collected 50 piston cores from the continental slope and basin floor in the study area made them available to us (Figure 2). The present study was undertaken to evaluate the character and extent of contour-current and downslope sedimentation processes in the Faeroe-Shetland Channel and on the West Shetland Slope using high-resolution seismic facies analysis and echo-character interpretation and mapping.
Faeroe-Shetland Basin and Latest Oligocene Unconformity (LOU) The Faeroe-Shetland Channel is up to 200 km wide between respective shelf breaks (∼200 m contour) off the Shetland and Faeroe islands, and deepens northeastwardly along its axis from about 1,000 m at the southwestern end, where it joins the Faeroe Bank Channel near the Wyville-Thomson Ridge, to >1,700 m, where it enters the Norway Basin. The Faeroe-Shetland Basin beneath the channel and the West Shetland Shelf and Slope is a major depocenter, where sediments have been accumulating since the Paleozoic (Duindam and van Hoorn, 1987; Haszeldine
371
Figure 2. Echo character map showing the distribution of 3.5 kHz echo types (seismic facies) I through VII. Echo types are described in detail in Table 1 and text. Locations of airgun and 3.5 kHz seismic lines used in this study are shown by solid lines (BGS 84/05) and dashed lines (BGS 79/14, long dashes, and BGS 77/7, short dashes). Thicker solid lines along ship tracks show locations of illustrated 3.5 kHz profiles; number beside each bar (e.g., 15D) identifies figure number that illustrates profile. Letters along some seismic lines (e.g., A–B, C–D, etc) indicate locations of seismic profiles illustrated in this paper (also shown in Figure 1). Solid black circles represent locations of piston cores. Contours are redrawn from Roberts et al. (1977). Location of area shown in Figure 1.
et al., 1987; Hitchen and Ritchie, 1987; Ziegler, 1988; Stoker, 1990c; Mitchell et al., 1993). A prominent mid-Tertiary unconformity, designated the Latest Oligocene Unconformity (LOU) by Damuth and Olson (1993) forms a major erosional
surface and prominent sequence boundary beneath the West Shetland Shelf and Slope and the FaeroeShetland Channel (identified by dashed horizon labeled LOU on Figures 6–8, 13, 14, and 17 displaying seismic profiles). Damuth and Olson (1993) presented
372
Figure 3. A. 3.5 kHz echogram profile illustrating Type I echoes: Sharp bottom echoes with no sub-bottom reflections; generally no acoustic penetration below the seafloor; smooth to gently undulating sea floor (Table 1). This echo type is returned from the inner continental shelf, and in some areas the outer shelf and uppermost slope (Figure 2). B to E: 3.5 kHz echogram profiles illustrating Type II echoes: Sharp bottom echoes with regular single hyperbolas; no sub-bottom reflections and generally little or no acoustic penetration below seafloor; seafloor is hummocky and rough (Table 1). This echo type is generally recorded from the uppermost continental slope (Figure 2). Location of profiles shown in Figure 2.
373 evidence that suggests the LOU formed during the latest Oligocene or possibly early to middle Miocene. The LOU may be equivalent to a prominent unconformity beneath the adjacent Rockall Trough and Hebrides Slope that appears to be middle Miocene in age (Stoker, 1995, 1997a; Stoker et al., 1998). Stoker (1990a) and Stoker et al. (1991) originally suggested that this unconformity formed in response to the initiation of intense bottom-water flow from the Arctic to the North Atlantic. If the LOU is equivalent to this prominent unconformity reported by Stoker and coworkers, then the LOU may be as young as middle Miocene. Other plate-tectonic and paleoceanography data from the Norwegian-Greenland Sea also suggests that intense bottom-water flow was not initiated until at least the middle Miocene (Eldholm, 1990 and references therein). However, we prefer to remain consistent with our earlier paper (Damuth and Olson, 1993) and refer to the unconformity throughout this paper as the Latest Oligocene Unconformity (LOU). The strata above the LOU are well imaged throughout the study area, especially seaward of the modern continental shelf edge (e.g., Figures 6–8, 13, 14, 17). Neogene to recent sedimentation processes The LOU is overlain by a clastic wedge of Miocene (?) to Holocene sediments, which ranges in thickness from 400 m beneath the West Shetland slope and shelf (Mudge and Rashid, 1987; Stoker, 1990a, b, c, 1995; Stoker et al., 1991, 1998; Mitchell et al., 1993; Damuth and Olson, 1993). Our observations and interpretations that follow in the present study are limited to this Neogene-Quaternary clastic wedge. Recent studies using various high-resolution seismic records, side-scan sonar, shallow cores (
