Early Cretaceous Wedgetop-Clinothem Deposition along the Southern Margin of the Colville Foreland Basin, Arctic Alaska
The Early Cretaceous southern margin of the Colville foreland basin has proven difficult to reconstruct due to structural disruption and stratigraphic complexity. A key to understanding this part of the basin lies in the stratigraphic architecture of the Aptian-early Albian Fortress Mountain Formation (Kfm), which unconformably overlies a northward tapering wedge of allochthonous rocks. Field work along the Brooks Range front enables the recognition of a composite Kfm stratigraphy composed of basal olistostrome, basin-floor fan system, marine slope, and shallow marine to nonmarine deposits.
Local olistostromes comprise chaotic deposits with pebble- to house-sized olistoliths of chert, sandstone, shale, and limestone, all in a matrix of silty mudstone to scaly argillite. Olistoliths are inferred to have been “recycled” from older mélange, and field relations suggest the olistostrome is a mass-transport complex that onlaps the emplaced tectonic wedge. Kfm basin-floor fan facies of the “Cobblestone sandstone” appear to onlap and interfinger with the basal olistostrome. Facies range from amalgamated sandstone and conglomerate interpreted as channelized and axial deposits to heterolithic sandstone and mudstone successions interpreted as off-axis lobe deposits. Kfm slope facies comprise a kilometer or more of silty mudstone, siltstone, and local sandstone. Various slope facies are distinguished based on the occurrence of gravity-mass-failure features, as well as sandstone and conglomerate units inferred to represent incised slope channels and sediment-gravity-flow by-pass deposits. Kfm topset deposits fine abruptly northward and include repeating parasequences of mudstone, sandstone, and conglomerate exhibiting features consistent with fluvial, wave, and mass-flow processes typical of fan-delta environments.
An upward trend towards greater textural and mineralogical maturity of clasts in the Kfm suggests a gradation from local to more distant provenance, likely reflecting progressive burial of the orogenic wedge tip and an increase in sediment yield from the growing subaerial orogen. This observation and the vertical succession of facies suggest that the Kfm is a clinothem characterized by southward onlap onto the tectonic front and northward offlap into the Colville foredeep. As such, the clinothem is a “healing wedge” that buried the upper part of a complex Early Cretaceous orogenic wedge and defined the southern margin of the Colville basin.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California