Deep-water facies of the Lisburne Group, west-central Brooks Range, Alaska

Abstract

Deep-water lithofacies of the Lisburne Group (chiefly Carboniferous) occur in thurst sheets in the western part of the foreland fold-and-thrust belt of the Brooks Range and represent at least three discrete units. The Kuna Formation (Brooks Range allochthon) consists mostly of spiculitic mudstone and lesser shale; subordinate carbonate layers are chiefly diagenetic dolomite. The Akmalik Chert (Picnic Creek allochthon) is mostly radiolarian-spiculitic chert; rare limy beds are calcitized radiolarite. The Rim Butte unit (Ipnavik river allochthon) consists chiefly of calcareous turbidites, derived from shallow- and deep-water sources, interbedded with spiculitic mudstone. Much of the material in the turbidites came from a contemporaneous carbonate platform and margin, but some fossils and lithic clasts were eroded from older, already lithified carbonate-platform rocks. All three units appear to be roughly coeval in the Howard Pass area and are chiefly late Tournaisian and early Viséan (late Early Mississippian) in age. Shallow-water lithofacies of the Lisburne Group exposed in the Howard Pass area (Brooks Range allochthon) are mostly of Viséan and younger (Late Mississippian) age. Thus, these carbonate-platform rocks were not the source of the calcareous turbidites in the Rim Butte unit. Rim Butte turbidites could have been derived from older carbonate-platform rocks such as the Utukok Formation of Tournaisian age (Kelly River allochthon) exposed mainly to the west of the Howard Pass quadrangle.