Wet Avulsion Sequences in the Cretaceous-Paleogene Raton Formation, Colorado

Abstract

Avulsion sequences are well established depositional models in fluvial floodplain successions. Most documented avulsion sequences are found in well drained floodplain deposits, however it is accepted that avulsion sequences should be found in poorly drained floodplains as well. In this study we provide evidence for avulsion sequences in the Cretaceous-Paleogene Raton Formation of Colorado. Measured sections of 8 outcrops and accompanying photopans with facies architecture were completed at varying stratigraphic positions within the Raton Formation analyzing the lithologic and architectural components of these strata. Previous work has identified the Raton Formation as having been deposited by a meandering to braided fluvial system that experienced prolonged periods of high accommodation with poorly drained, swampy floodplain conditions between channel belts. Floodplain preservation was dominant within this system and the Raton Formation can be separated into three facies. Facies A represents poorly drained floodplain conditions consistent with swampy deposition with lithologies comprised of coal and carbonaceous shale rich in woody debris. Facies B is dominated by weakly developed paleosols dominated by mudstone and siltstone that are rooted with well preserved leaf fossils, and beds of ferruginous nodules. These are interpreted to be poorly developed paleosols in splay deposits, lacking distinct gradational changes. Facies C is comprised of very fine to medium, laterally extensive sheet sandstones with ripple and planar laminations indicative of splay sand deposition. Sheet sandstones have abrupt contact with underlying heterolithic floodplain strata or coals, and fine upward into siltstones and mudstones. Facies B and C are lateral to channel-belt sandstone. The presence of channel belts and channel bodies greater than 2m in thickness lateral to splay deposits records avulsion cycles. These are capped by coals recording histosol development between avulsion events. Laterally extensive sandstone sheets deposited by crevasse splays within coal beds are proximal to channel belts and have potential to charge sand beds. Furthermore, these thin beds have implications for reservoir connectivity, as they have to potential to connect otherwise discrete channels, increasing reservoir volume.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016