A Long-Lived Compound Incised Mega-Valley in the Cretaceous Ferron Sandstone, Utah

Abstract

The Notom delta wedge of the Ferron Sandstone in Utah contains a compound, stratigraphic mega-valley, that is extensively exposed over an area of about 1000 km². The valley records at least 3 episodes of cut and fill producing a regional composite stratigraphic scour surfaces that can be mapped over a distance of 40 km in width and 40-50 km from proximal to distal. This compound incised valley contains both tributary and trunk rivers. Trunk rivers achieved a maximum bankfull flow depth of 6.6 m and maximum channel width of about 70 m. Localized terrace deposits include thin alluvial intraformational conglomerates associated with rooted interfluves. The extensive valley width reflects significant periods when the majority of the valley was not occupied by an active channel, and during which the valley was filled by floodplain mudstones. Abundant tidal facies, such as double mud drapes, suggests that much of the valley lies within the backwater limit, which is estimated to be about 6-7 km from the paleo-shoreline. Tidal facies are particularly well preserved within tributaries. The valley is exceedingly heterogeneous, consisting of coarse-grained, conglomeratic braided fluvial systems, floodplain deposits, and large-scale coarse-grained meander belts with a preserved height of 5.3 m. Time stratigraphic analysis suggests that the entire compound valley system was cut and filled over a 60,000 year period suggesting that and each cut and fill episode may represent 20,000 year Milankovitch scale cyclicity and represent high frequency stratigraphic sequences. The nature and frequency of incision in this valley system with respect to Milankovitch scale cyclicity is similar to Quaternary Gulf Coast incised valleys and indicates that mechanisms responsible for driving valley cut and fill are largely consistent regardless of climatic setting (greenhouse or icehouse conditions). The width of the valley is significantly larger than the scale of associated topographic valley that existed at any one time. The Ferron mega-valley exhibits considerable architectural complexity and may represent an analog for other extensive Cretaceous incised valleys, such as the heavy-oil reservoirs of the McMurray Formation in Western Canada, versus smaller simple valley systems such as the Crystal Field in the Viking Formation.

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