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GRAVELS IN THE ATLANTIS II FRACTURE ZONE 1

The sedimentology of gravel intervals was studied in three cores recovered at Site 734 at a water depth of —3700 m on the eastern wall of the Atlantis II Fracture Zone. At two holes, coring recovered single beds <l m thick that grade upward from moderately well-sorted igneous gravel to foraminiferal ooze. The origin of the stratigraphy in the cores is problematic, but sedimentological arguments are used to infer that the recovered core is intact. The sediments are tentatively interpreted as turbidity current deposits that originated from a nearby cliff face. Lithology, particle size, and particle shape suggest that mass-wasting of the cliff face was influenced as much by hydrothermal alteration and brecciation at depth as jointing and fracturing of the cliff face caused by stress release. INTRODUCTION Fracture zone ridges are inferred to be oceanic crust uplifted during a complex tectonic history (Karson and Dick, 1983; OTTER, 1985). The nonbiogenic portion of the sediments on the walls of active transform faults should provide a representative sample of local rock types and a record of recent history of tectonism and surface erosion. Episodic downslope transport from the unstable, steep slopes of the transform wall will produce gaps in the sediment record at any one location. If gathered over a wide enough area of the ridge and fracture zone floor, however, the sediments should provide a picture of the development of fracture zone ridges. One of the key contributions from studying these sediments is the interbedded pelagic microfossils that can be used to date mass-wasting and transport events. This history can be used to test models of fracture zone ridge development. To use sediment stratigraphy to solve tectonic questions, one must know where the sediments came from and how they were deposited. To date, little sedimentological and stratigraphic data from fracture zone walls have been published. The objective of this study is to report sedimentological data from two cores obtained during Leg 118 and to present interpretations regarding source and transport mechanisms to help guide future studies of fracture zone sediments. Knowledge of sediments on fracture zone walls is based on observations by towed camera and submersible and, to a lesser extent, on sampling by dredging, coring, and submersible. Seafloor features indicative of a variety of downslope sedimentary processes have been described. At the Kane Fracture Zone, Karson and Dick (1983) observed both angularand tabular-shaped blocks in talus ramps at the base of faulted basement walls. These ramps appeared to feed debris slides that flowed downslope along curvilinear tracks controlled by topography. Debris slides contained variably consolidated chalks, massive-to-foliated plutonic rocks, and breccias. Where faults offset debris slides, the thicknesses of beds attributed to single depositional events measured up to tens of meters. At the Oceanographer Fracture Zone, OTTER (1984, 1985) found much more evidence of faulting, mass-wasting, and downslope transport, coincident with rapid vertical uplift 1 Von Herzen, R., Robinson, P. T., et al., 1991. Proc. ODP, Sci. Results, 118: College Station, TX U.S.A. (Ocean Drilling Program). 2 Woods Hole Oceanographic Institution, Woods Hole, MA 02543. near the ridge-transform intersection than along the relatively tectonically stable walls of the active transform. Near the ridge-transform intersection, OTTER (1984) observed masswasting features similar to those at the Kane Fracture Zone. They also noted broad gullies several meters deep eroded in terrace deposits, presumably by gravity-driven sedimentary transport. Thick, lobate talus wedges having seafloor slopes up to 30° occur at the base of fault escarpments and coalesce along strike. Scarp erosion produces block formation along joints and fractures due to stress release after exposure at the seafloor. Pelagic carbonates drape over undisturbed surfaces and fill gaps in the talus deposits. Exposures along fault scarps reveal that the carbonates recrystallize quickly and weld the talus blocks. Along the walls of the active transform, OTTER (1985) observed no evidence of recent faulting. Thick, carbonate sediment draped over most areas indicated slow talus formation rates. Relatively old, originally planar fault scarps having heights of a few to several meters were cut by narrow, steeply dipping erosional gullies. In these gullies, jointing of ultramafic rock produced rounded and distinctly oblate talus fragments, whereas jointing of plutonic rock (gabbro and diorite) produced angular fragments. Fracture zone rocks have been identified on land in conjunction with ophiolite sequences in Newfoundland (Karson and Dewey, 1978) and Cyprus (Simonian and Gass, 1978). In Cyprus, sedimentary rock units range up to 0.5 km thick and are interbedded with and abut eruptive pillow basalts. No carbonate sediments are present, indicating deposition below the calcite compensation depth. Grain sizes range from cobbles to clay. Simonian and Gass (1978) observed a wide range of downslope, gravity-driven sedimentary deposits, including proximal rockfall talus and poorly sorted mass flow deposits grading laterally to distal, graded turbidite deposits. Sediments in the Skinner Cove Sequence of the Coastal Complex, Newfoundland (Karson and Dewey, 1978), are more highly deformed and metamorphosed than those exposed in Cyprus. Relationships among sedimentary rocks and both seafloor morphology and igneous source rocks have not been preserved. Karson and Dewey (1978) observed carbonate cemented breccias, shales, limestones, cross-bedded arenites, and coarse calcarenites. From the observations summarized above, fracture zone sediments appear to have been transported and deposited by gravity-driven mechanisms similar to those described elsewhere in marine environments. Sedimentation on the walls of fracture zones, however, differs in a number of ways from