The lower Aptian lithologic assemblages at Sites 897 and 899, which include serpentinized peridotite breccias and igneous and sedimentary rocks, are here conceived and described as an olistostrome (Fig. 2). Our interpretation is based on inferred transport mechanisms and depositional setting criteria, the range of sediment ages, and, to a great extent, on our interpretation of the origin of the serpentinized peridotite breccias.
Since the work of Abate et al. (1970), the term olistostrome (sometimes called "gravitational mélange") has been widely used in the Alpine and Tethyan literature. It usually denotes large, thick, heterogeneous stratiform units that accumulated somewhat chaotically as a result of tectonically induced massive gravitational sliding, that is, from an active fault escarpment, in different tectonic settings (Füchtbauer and Richter, 1983). The transport distance for the sliding can vary from several dozen up to several hundred kilometers (Einsele, 1992). Olistostromes may contain large blocks called olistoliths, from several meters up to several hundreds of meters thick, that preserve their internal coherence and in which the original facies can still be established. In the displacement of the olistostromes, sliding materials can move as a more or less rigid plug through a basal shear zone (Lowe, 1982; Stow, 1986). In slide masses composed of hard rocks and semi-indurated and soft sediments, softer and friable materials make up the basal mobile phase, which may have even been liquefied and then disintegrated progressively during displacement. In contrast, hard rock elements, traveling in a passive phase in the flow, can maintain their original coherence. Because liquefaction may be restricted to the base and periphery of a large volume of slide material, most of the elements in the olistostrome may not be subject to any type of mechanical differentiation.
On the basis of the olistostrome characteristics indicated thus far, we interpret the lower Aptian lithologic complex (Unit IV) sampled at Sites 897 and 899 as follows:
1. The drilled intervals of serpentinized peridotite breccia are believed to be single bodies of former breccia that were involved in gravitational sliding (Fig. 2). Intervals of breccias up to 5 m that preserve their internal coherence and original petrology and fabric (that is, olistoliths) were recovered in Hole 897C from Sections 149-987C-66R-1 to 70R-3, and in Hole 899B from Cores 149-899B-16R to 25R. Intervals of serpentinized peridotite recovered in Hole 897C from Core 149-897C-63R to 65R, in Hole 897D from Core 149-897D-10R, and in Hole 899B from Cores 149-899B-26R to 31R are also considered boulders or clasts included in the olistostrome. From drilling data we can discern a possible major olistolith about 95 m thick (the Upper Breccia named by Shipboard Scientific Party, 1994b). However, incomplete recovery obscures whether this interval corresponds to a single body and prevents us from discussing the maximum size of olistoliths and boulders.
2. The different original sizes of breccia elements influenced the preservation of the original fabrics. Smaller elements, clasts, or fragments included in sediments at Site 899 from Core 149-899B-26R downhole were crushed and extensively altered during the transport process. In general, this alteration resulted in the replacement of the original clast mineralogy by calcite and the intrusion of sediments into the serpentinized peridotite breccia (Pl. 1, Figs. 1, 2).
3. Contorted and fractured heterogeneous sediments, cracked serpentinite breccias, and serpentinite gouges mixed with serpentine boulders and clasts (Hole 897C from Cores 149-897C-63R to 65R, and Hole 897D from Cores 149-897D-7R to 10R) are interpreted as the plastic and mobile phase in the olistostrome, which contains a record of the deformational fabrics generated during transport. The intervals of highly altered and rounded clasts of mafic rock mixed with sediment, recovered at Site 899 (from Core 149-899D-26R downhole), may also be regarded as a mobile and deformed phase in the olistostrome. Pebbles and clasts of mafic igneous rocks are believed to be reworked, altered, and disintegrated residual elements from a major volume of igneous rock originally involved in the sliding mass.
4. Shear zones affecting sediments and peridotite gouges (Cores 149-897C-65R and 897D-10R) and the sheared boundaries of olistoliths (top of Core 149-899B-16R) are interpreted as the result of deformation during the emplacement of the olistostrome. The latest generation of calcite veining, which crosscuts sediment and serpentinite lithologies (Core 149-897C-65R) or bounds bodies of different rheology (thick dipping calcite veins in Hole 899B from Cores 149-899B-17R to 25R; see Sawyer, Whitmarsh, Klaus, et al., 1994, pp. 518-526), can be interpreted as having originated from fractures that occurred in the sliding mass during transport over a long distance across the seafloor (Pl. 2., Figs. 4, 6).
5. The source area for the olistostrome was a fault-controlled escarpment, located above a gentle slope. This escarpment exposed to the seafloor the following different lithologies: serpentinite, former serpentinite cataclasites, breccias, and fault gouges; metabasites; basalts and microgabbros, sheared amphibolites, and older sedimentary rocks. The contemporaneous (Aptian) soft sediments involved in the olistostrome could have been derived from the source area, or could be seafloor sediments that were progressively incorporated into the sliding mass during its transport.
6. The incomplete recovery of Unit IV at both sites makes it impossible to determine whether the olistostrome was formed through successive sliding events or from a single massive event.
Certain characteristics of the Aptian olistostromes at Sites 897 and 899, such as their lithologic heterogeneity, strong internal deformation, and the roundness and alteration of the clasts involved, suggest long-distance transport during the gravitational sliding. We therefore define the olistostrome as distal. This implies that the tectonically active fault escarpment that generated the sliding was located far from both Site 897 and Site 899.
Some sedimentary rock samples from olistostromes at Sites 897 and 899 (Sawyer, Whitmarsh, Klaus et al., 1994) are quite similar to the Hauterivian to late Barremian dark marlstones and microturbidites recovered at Site 638 (Unit IIB and III, in Boillot, Winterer, Meyer, et al, 1987), and at Site 398 (Subunit 4C, DSDP Leg 47, in Shipboard Scientific Party, 1979). Noteworthy affinities can be seen between the mafic rock associations in the olistostrome, basalts, metabasites, and underplated gabbros sampled above the peridotites in the Galicia Margin (Boillot et al., 1988; Kornprobst et al., 1988; Beslier et al., 1990).
These similarities point to the Galicia Margin, that is, to an escarpment north of the Iberia Abyssal Plain, as the probable source of these deposits. Furthermore, the alkaline affinity and non-oceanic character of the basalt fragments within the olistostrome supports this interpretation.