Site 642 on the V°ring Margin penetrated a more than 750-m-thick upper series of tholeiitic basalts and terminated in a more than 100-m-thick lower series of andesites and dacites (Eldholm, Thiede, Taylor, et al., 1987). Identified in the hole were 137 lava flows and 63 thin interbedded sedimentary units. Downhole data reveal characteristic changes in seismic properties within the flow units, with low P-wave velocities in the flow top (2.5-3 km/s) and high P-wave velocities in the lava interior (5-6 km/s) (Planke, 1994).
Seismic reflection data over Site 642 show a well-defined high-amplitude, rough top-basement reflector at about 2.15 s (Fig. 3A). No coherent reflectivity is found in the following 0.35-s interval, but a positive reflector at 2.5 s can be correlated with reflector K, which is the base of the SDR unit. Synthetic seismogram modeling suggests that K is an interference reflector originating from two thick flow units near the base of the upper series, not the upper/lower series transition (Planke and Eldholm, 1994). Seismic facies interpretation of profile BGR74-1 shows that Site 642 is located within the Landward Flows facies unit, a few kilometers east of the termination of the SDR (Fig. 3A). It is not possible to correlate reflectors from the SDR to the Landward Flows, with the exception of the top and base reflector.
DSDP Leg 81 was designed to drill a representative transect across a volcanic rifted margin, and volcanic basement was reached at Sites 552-555 (Fig. 1) (Roberts, Schnitker, et al., 1984). The sites were located on the flank of the Outer High (Site 554) and within the SDR (Site 553) and the Landward Flows (Site 552) (Fig. 3B). Site 552 terminated 31 m into the basement, penetrating only one thick, massive basaltic unit, which was possibly deposited in a marine environment. Farther seaward, a 181.5-m-thick basaltic pile was drilled at Site 553. Thirty-four basalt units were identified, with an average thickness of ~6 m. The basaltic lava flows frequently had red flow tops suggesting subaerial weathering. In addition, a 1-m-thick submarine tuffaceous hyaloclastite unit was recovered from the basement section. Site 554 drilled 82 m into basement on the flanks of the Outer High, with a recovery of only 16%. Pillow-lava fragments, submarine hyaloclastites, and coastal eroded and deposited volcaniclastic conglomeratic and sandy sedimentary rocks were recovered.
None of the Rockall Margin holes penetrate any reflectors below the top-basement event, and thus they cannot provide direct information about the nature of the intrabasement reflectivity. However, Sites 552-553 show that the upper part of the Landward Flows and SDR may consist of subaerial or shallow-marine-deposited basaltic lavas, and that hydroclastic material also may be present. Site 554 reveals that the Outer High has a strikingly different volcanic morphology, consisting of hyaloclastites, fragmented basalts, and coastal eroded volcaniclastic rocks deposited in a near-shore environment.