With only 3 days of Leg 210 remaining, we moved 40 km southeast of Site 1276 and drilled into the crest of a prominent basement ridge that we call the Mauzy Ridge ("mauzy" is a Newfoundland word for foggy or misty) at Site 1277 (Fig. F8B in the "Leg 210 Summary" chapter; Figs. F20, F21). The crust here is just on the young side of a magnetic anomaly identified as M1, and it is characterized by a series of high-amplitude, margin-parallel basement ridges (see Shillington et al., this volume). These features suggest that it is oceanic crust formed by early seafloor spreading in the Newfoundland Basin. We drilled Site 1277 to test this inference and to determine the nature of the basement.
Because of the limited time available, we drilled without coring through the thin sediments above the basement ridge. Sediment thickness estimated from multichannel seismic (MCS) stacking velocities prior to drilling suggested a sediment thickness of 132 m, so we planned to drill without coring to 100 meters below seafloor (mbsf). While drilling to this depth we encountered hard layers at 85–89 mbsf and 97.5–100 mbsf, and the recovered "wash core" (210-1277A-1W) contained basaltic and gabbroic debris. Thus, the "basement" surface observed in the seismic reflection record (Figs. F20, F21) may be as shallow as 85 mbsf.
We cored from 103.9 mbsf (wash core) to 180.3 mbsf and recovered a spectacular section of heterogeneous igneous and sedimentary rocks. These rocks include magmatically differentiated gabbros of variable grain size and extent of hydrothermal alteration, together with fine- to coarse-grained sediments that are cemented with calcite spar. Most or all of these rocks reached their present location as mass flows. In addition, the deepest four cores (below ~142.1 mbsf) are dominantly serpentinized peridotite, with minor gabbro and a cataclastic damage zone at the top of this unit. There are also minor intrusive rocks and abundant, mainly calcite, veining similar to Alpine ophicalcites.
The presence of serpentinized peridotite basement indicates that mantle rocks were tectonically exhumed to the seafloor, and they were then covered by thin basalt flows and clastic sediments. These features are characteristic of very slow spreading ocean crust with limited magma supply and extreme tectonic extension. Our results suggest that at least this section of ocean crust in the Newfoundland Basin was formed at a very slow spreading rate.
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Ms 210IR-104