Heterogeneity and lithotype distribution in ancient deep-sea canyons: Point Lobos deep-sea canyon as a reservoir analogue

An evolution and history of filling is proposed for an exceptionally exposed ancient deep-sea canyon on a Paleocene oblique-slip tectonic margin which, on a number of scales, reveals, successive phases of canyon activity. The quantitative methods adopted for this study make it of direct use to modellers as an example of reservoir heterogeneity in an ancient canyon fill, where facies distribution from boreholes can be scaled up to reconstruct the reservoir, using the methods outlined in this paper. The Point Lobos submarine canyon, near Carmel in California, provides a complete cross-section of an ancient canyon, with a fill which displays a whole range of channel morphologies, and laterally extensive coverage of the internal architecture of associated conglomerate packages and related debris flows. This paper presents quantitative documentation of the canyon-fill sediments and canyon-wide fill packages, on scales which vary from bed-to-bed analysis, reflecting processes in operation during individual events, to canyon-wide analysis, reflecting the overall evolution of the canyon. The northern and southern canyon margins are both exposed, and the Paleocene fill onlaps the subvertical canyon wall. The canyon was incised into Cretaceous granodiorite. The fill comprises five thick sequences which correspond to five successive phases of sediment deposition within the canyon. Each sequence typically consists of resedimented conglomerates that are stacked and channelised, with a vertical architecture which resembles that of subaerial braided stream deposits. These are overlain by channelised turbidite sandstones, interbedded with intraformational conglomerates and mud-chip breccias. These in turn are overlain by mudstones and shales, which are commonly slumped and disturbed. Published classification schemes that show the range of deep-water facies were found insufficient to describe the Point Lobos canyon fill. Methods were developed for recording the lithologic proportions of the fill. Twenty-three lithotypes were defined, which show the variation in the sedimentary units, and a range of processes are proposed to explain each lithotype. These methods produced a quantitative database of lithologic proportions for the canyon fill. Over 80% of the fill comprises sandstone and conglomerate. Conglomerate is more common near the canyon margins. Mud and silt comprise the other 20% of the canyon fill; predominantly as laterally extensive, slumped packages which cap the