Stratigraphic anatomy and depositional history of a mass-transport complex, Isaac Formation, Windermere Supergroup, British Columbia, Canada

Abstract

Sandwiched between Isaac Channels 2 and 3 is a 130-m (426-ft)-thick mass-transport complex (MTC) composed mostly of very coarse debris-flow and slump/slide deposits. The succession can be traced laterally from Castle Creek South to Castle Creek North, a distance of at least 2.2 km (1.4 mi). The anomalous abundance of debris-flow and slump/slide deposits compared to super- and subjacent strata suggests a period of slope instability that interrupted otherwise gravitationally stable slope conditions. Additionally, the abrupt increase in the size of sediment particles, composed mostly of quartz, with lesser carbonate and carbonate-cemented clasts, suggests an important change in sediment provenance. In Castle Creek North, the MTC is a tabular unit consisting of a 55-m (180-ft)-thick slide/slump overlain by a 16-m (53-ft)-thick, carbonate-clast-rich, debris-flow deposit. In the slide/slump unit, ductile and brittle deformation structures are common, and clasts consist of a diverse assemblage of low-total-organic-content mudstone and arkosic-sandstone blocks, some ranging up to a few tens of meters (>100 ft) long and several meters (few tens of ft) thick. The debris-flow deposit consists of dispersed quartz pebbles, carbonate-cemented mudstone and common shallow-water stromatolite and oolite fragments. In Castle Creek South, the MTC is a significantly more complicated melange comprising a number of erosively based units. Near the glacier in Castle Creek South, the succession is about 60 m (200 ft) thick and comprises a basal, coarse quartz-pebble-conglomerate that is overlain by a 20-m- (65-ft)-thick slide deposit. The slide deposit is capped by a 30-m (100-ft)-thick mudstone-rich, debris-flow deposit, commonly with dispersed, shallow-water-carbonate clasts. About 500 m (1640 ft) along strike to the southeast, the succession incises deeply into the top of IC2 (Figure 1B) and is approximately 130 m (423 ft) thick. The succession consists mostly of mudstone- and sandstone-rich debris-flow deposits with common quartz-pebble and carbonate clasts. At the base, carbonate clasts consist of carbonate-cemented, siliciclastic sandstone and mudstone, but upward, especially in the uppermost part of the MTC, shallow-water carbonate clasts become common. Interspersed in the debris-flow deposits is a single ≤15-m (≤50-ft)-thick channel-fill unit. Strata consist of thick to very thick-bedded, graded granule conglomerate, or very coarse- to coarse-grained sandstone that infill negative topography along the surface of the underlying debris-flow deposit. Slump/slide deposits tend to be thickest where debris-flow deposits are thinnest, suggesting that they formed seafloor obstructions that influenced the locus of later debris-flow deposition. In addition, slump/slide units consist of strata with mineralogical and total organic contents similar to surrounding strata, which suggests a local (slope) sediment provenance. Debris flow deposits, on the other hand, typically consist of anomalously coarse-grained quartz clasts and have comparatively low feldspar content. In addition, carbonate clasts are common. They show an upward change from sandstone/mudstone clasts that are lithified with early-diagenetic, seafloor-carbonate cements to clasts eroded from a shallow-water, carbonate platform (stromatolites and oolites). Together, these stratal changes suggest an evolution of sediment provenance, which is most likely related to changes of relative sea level. This is similar to the recently reported tectonically active Eocene Ainsa Basin (Pickering and Corregidor, 2005), notwithstanding the fact that MTCs represent a significantly larger proportion of the Ainsa stratigraphy. During the early stages of falling sea level, much of the length of the slope became gravitationally unstable and spawned widespread mass-wasting of slope-derived sediment. With continued fall, the uppermost part of the slope and/or outer part of the shelf became activated. Coarse quartz grains, representing palimpsest and relict sediment related to an earlier rise of relative sea level, were eroded and transported downslope, along with carbonate-cemented, siliciclastic sandstone and mudstone. With more time (late falling stage systems tract), erosion extended into the shallow-water part of the carbonate platform and eroded shallow-water stromatolites and oolites. The eventual onset of lowstand deposition is indicated by the resumption of voluminous siliciclastic sediment input, the termination of carbonate input (IC 3, see Navarro et al., Chapter 25, this volume), and the cessation of mass movement activity. Gravitational stability along the slope was re-established, and the preferential development of channel-levee complexes occurred. In the Castle Creek area, MTCs are thick, well developed, and are vertically separated by hundreds of meters (>1000 ft) of non-MTC deposits. Therefore, the size and resolution are consistent with interpretations based on seismic data from significantly younger passive-margin settings (e.g., Skene and Piper, 2006).