Abstract

Carboniferous rocks of arctic Alaska, widely distributed in the outcrop and the subsurface, consist of more than 1,500 metres (5,000 feet) of clastic and carbonate sediments assigned to numerous formations. Detailed analysis in the field and laboratory of more than 40 sections shows that the Carboniferous rocks consist of (1) a transgressive suite, (2) a platform suite, and (3) a deeper water suite.

The transgressive suite, consisting of basal clastic rocks overlain by spiculitic to pelletal limestone, is believed to represent alluvial-deltaic and interdeltaic environments succeeded by shallow marine to brackish marine and finally by open-marine, predominantly subtidal carbonate environments. This suite of rocks ranges in age from Early to Late Mississippian and includes the Kekiktuk Conglomerate, Kayak Shale, Noatak Formation, Utukok Formation, Nasorak Formation, Wachsmuth Limestone and Itkilyariak Formation.

The platform suite, composed of shallow-water cyclic carbonate sediments, represents intertidal, supratidal, and subtidal environments probably in an arid climate. This suite ranges in age from Early Mississippian to Middle Pennsylvanian and includes the Kogruk Formation, the Alapah Limestone and the Wahoo Limestone.

The poorly known deeper water suite consists of radiolarian-and cephalopod-bearing dark chert, shale and limestone. It may represent deeper shelf, slope and basinal environments. Only Mississippian fossils are known from this suite which is represented by the Tupik Formation, unnamed formations in the western Brooks Range, and thin units in other formations.

Because of limited data and the structural complexity of the Brooks Range where thrust fault displacements of as much as 250 kilometres (150 miles) may have occurred, the outline and geometry of the Carboniferous basin of deposition are poorly understood. Paleogeographic restorations and facies analyses indicate deposition on the north side of a generally east trending basin. Transgression during Mississippian time was northward over an irregular surface consisting of several major positive and negative elements. The platform suite of carbonate rocks developed after most of the positive elements were submerged by the transgression. A lack of evidence for abrupt shelf margins, an absence of reefs, and apparent gradual changes in facies suggest that a ramp configuration is more likely than a shelf one for the Carboniferous of arctic Alaska.

New data from the Mount Doonerak area, central Brooks Range, suggest that Carboniferous rocks there are autochthonous and that there may be a minimum of 60 kilometres (36 miles) tectonic displacement for this part of the range. Palin-spastic restoration and facies analysis for this area indicate shallow-water carbonate deposition south of deeper water carbonate deposition. This is the first documented occurrence in this region of shelf carbonate rocks south of basinal carbonate rocks. These shelf carbonate rocks may be part of: (1) a shelf developed on the south side of the Carboniferous basin, (2) an intrabasin carbonate bank, or (3) an irregular-shaped northern shelf margin.