Downslope variability in deep-water slope channel fill facies and stacking patterns

Abstract

Lithologic variations in deep-water slope channel systems provide critical insight into sedimentary processes on deep-water slopes, and are a key control on natural resource distribution and connectivity in subsurface reservoirs associated with ancient deep-water systems. While many studies have described these variations at outcrop-scale along depositional strike (across-channel), few have focused on documenting along-depositional-dip changes due to: (1) the dearth of along-slope perspectives afforded by outcrops; or (2) limited resolution and coverage of subsurface data sets. In this study, slope channel elements (≤30 m thick; ≤400 m wide) and composite channelform bodies composed of ≥2 stacked channel elements (channel complexes and channel systems; >30 m thick; >400 m wide) were characterized along a 50-km-long depositional-dip-oriented outcrop belt of the Campanian-Maastrichtian Tres Pasos Formation (Chile) to constrain longitudinal changes in channel fill character and stacking patterns. Results show that channel elements become more sandstone-rich downdip. Outcrop observations are supported by channel element net-to-gross ratios (measures of sandstone proportion), which increase downdip. Sandstone-prone channel elements within channel complexes and channel systems are more prevalent downdip; however, variability in lateral and vertical offset between successive channel elements downdip results in a poor correlation between net-to-gross values and paleoslope position when composite channelform bodies are considered. These findings suggest that the along-slope distribution of coarse-grained sediment in channel systems is tied to the degree of coarse-grained sediment bypass and erosion that occurs along a slope. The results of this study provide a detailed perspective into downdip changes in slope channel fill and stacking patterns, and help clarify aspects of analogous subsurface reservoirs, including sandstone proportion trends for several scales of deep-water slope channel architecture.