Floodplain Facies in a Paleo-coastal Wetland: Cretaceous Ferron-Notom Delta, Utah
Famubode, Oyebode A.; Bhattacharya, Janok
The floodplain is an area of land adjacent to the river channel that grows by vertical accretion during seasonal storm-driven floods. Floodplain deposits have been poorly recognized and are rarely described in detail. Because of the importance of mudstones as unconventional hydrocarbon reservoirs, improved facies recognition criteria are required. Outcrop analysis of the Cretaceous Ferron-Notom delta, in central Utah, allows eight floodplain facies to be defined: floodplain-lake, crevasse splay and crevasse channel, levee, abandoned-channel fill, paleosols and associated peat-swamp carbonaceous shales and coals.
Floodplain-lake deposits are characterized by laminated mudstones and siltstones and substantial to high amounts of organic material. Laminations may not be visible due to bioturbation or high degree of compaction. Floodplain-lake deposits grade upward to fine-grained, ripple cross-laminated crevasse-splay sandstones, which show a typical upward coarsening profile. Medium to fine-grained crevasse-channel sandstones usually truncate underlying crevasse splays. Crevasse-channel sandstones are usually cross bedded and ripple cross laminated, and show a typical fining-upward profile. Levee deposits are very fine- to fine-grained, mainly ripple cross-laminated and they prograde onto the adjacent floodplain with individual fining-upward packages stacked to produce an overall coarsening-upward profile.
Abandonment may be a result of avulsion or channel cutoff. Abandoned-channel fills are characterized by a mix of siltstones and fine-grained sandstones and contain a high amount of organic material with a lack of definite sedimentary structures. Paleosols are mostly gleysols, with rooted horizons capped by carbonaceous shales from peat swamps, and coals (or histosols) that are stacked in a single pedostratigraphic unit.
The frequency of deposition or abundance of these facies is observed to be approximately proportional to the rate of increase in aggradation and accommodation creation in the Ferron-Notom Delta. Floodplain cyclicity also matches that seen in related shoreline facies, and suggests allogenic controls on pedostratigraphic cycles.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013