Quantifying Longitudinal Changes in Deep-Water Slope Channel Fill Over a 40 km-long Segment of a Submarine Slope System, Cretaceous Tres Pasos Formation, Chile

Abstract

Outcrops provide the unique opportunity to deduce formative sedimentary processes of submarine channels at the bed through architectural element scale. However, most outcrops feature strike-oriented (across-channel) views of channel systems, providing limited information about downslope changes in deposit character and architecture. Using a 40 km-long dip-oriented segment of an ancient slope channel system from the Tres Pasos Formation, Chile, we document changes in grain size, sandstone thickness, and stratigraphic architecture for discrete channel elements (up to 30 m thick, 300 m wide) over 20-40 km distances. We use this information to investigate evidence for downslope changes in sediment bypass and deposition processes, as well as constrain channel element stacking patterns along dip. Data for this study is derived from 98 distinct channel elements captured within a database of 220 measured stratigraphic sections (6644 m), from which grain size and bed thickness data was extracted. We evaluate these data in the context of a high-resolution stratigraphic framework constrained via differential GPS mapping of channelform surfaces along the outcrop belt.

Mean grain size in channel fill units decreases distally on average from upper coarse sand in updip zones to lower medium sand downdip. Maximum grain size does not vary markedly along dip, ranging from very coarse sand to pebble in most areas. At the channel element scale, sandstone thickness trends are characterized by ratios of: (1) net intra-channel fill sandstone thickness and gross channel fill thickness (NTG); and (2) the thickest amalgamated intra-channel fill sandstone package and gross channel fill thickness (TSTG). NTG values increase distally on average from 67% to 88%; TSTG values also increase downdip on average from 33% to 56%. Analysis of sandstone thickness trends over stratigraphic intervals spanning multiple channel elements reveals that thick (>40 m) amalgamated sandstone packages are more common downslope, which we link to increasingly confined channel fill stacking downdip. Overall, these data suggest that coarse-grained sediment bypass was more significant in updip zones, and that coarse-grained sediment deposition was more prevalent downdip. These results offer a predictive template for deducing sedimentary processes and products along dip in deep-water channels, and provide insight into the position and geometry of updip stratigraphic traps in analogous petroliferous systems.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018