Modern submarine sediment slides and their implications for melange and the Dunnage Formation in north-central Newfoundland

Modern submarine sediment slides produce two features: a slide scar that delineates a zone of removal and a deposit of slide material. The upturned edges of the slide scar form prominent scarps with relief of generally less than 100 m. The zone of deposition includes "hummocky," "blocky," and "debris flow" high-resolution (3.5 - 12 kHz) seismic fades. These echo types probably represent olistoliths, piles and/or folds of deformed sediment, and debris flow deposits, respectively. Cores and bottom photographs exhibit deformed, chaotic material typical of debris flow deposits. Sediment slides are common, not only on active margins, but also on passive margins. Slide complexes are not restricted to base-of-slope sites; rather, a single slide on a passive margin can stretch over 700 km from the continental shelf break to the abyssal plain. Giant submarine sediment slides have implications for studies of melange. Sediment slides provide an extremely important mechanism for generating the internal chaos characteristic of melange. Because sediment slides are not restricted to convergence zones, the presence of a chaotic unit in the rock record does not imply, of itself, a paleosubduction zone. Other characteristics of the melange must be observed and studied before proposing a paleotectonic site of formation. These characteristics include clast lithology and structural fabric history. The Cambro-Ordovician Dunnage Formation is a melange that crops out in northcentral Newfoundland in the Dunnage tectono-stratigraphic zone. The Dunnage melange exhibits soft-sediment deformation features similar to those observed in cores raised from submarine sediment slides. These features are consistent with the interpretation that the chaotic nature of the Dunnage Formation formed initially by sediment sliding. The deformational features include pebbly mudstones with no cleavage and isoclinal folds that are overprinted at high angles by non-axial-planar cleavage. The original site of deposition of the Dunnage Formation is equivocal, but angular clasts in the Dunnage were derived from both an island arc and oceanic crust. This observation suggests that the heads of the slides were very close to subaerial exposures of a combined island arc and ophiolite terrane (perhaps similar to present-day Luzon). By Silurian times, the Dunnage Formation was involved in overthrusting which, considering regional relationships, was probably forearc thrusting.