J. Thomas Dutro

Foraminiferal Zonation and Carbonate Facies of Carboniferous (Mississippian and Pennsylvanian) Lisburne Group, Central and Eastern Brooks Range, Arctic Alaska

The Lisburne Group carbonate rocks of the central and eastern Brooks Range contain foraminiferal assemblages assigned to zones of late Tournaisian (Osage) to early Moscovian (Atoka) age. Representatives of both Eurasiatic and American cratonic microfaunas permit correlation with the original Carboniferous type sections in western Europe as well as with the standard Mississippian and Pennsylvanian sequences in the Mid-Continent region of North America. Correlation anomalies in the lower part of the sequence are discussed.

New Mississippian Formations and Faunal Zones in Chesterfield Range, Portneuf Quadrangle, Southeast Idaho

Mississippian rocks in the Chesterfield Range, southeast Idaho, consist of the Lodgepole Limestone overlain by a 1,900-foot limestone-sandstone sequence here designated the Chesterfield Range Group. The lower terrigenous part, about 965 feet thick, is the Little Flat Formation; it has a basal siltstone member, a middle sandstone member, and an upper sandy limestone member. The upper carbonate part of the Chesterfield Range Group, here named the Monroe Canyon Limestone, is about 925 feet thick and contains a lower massive member, a medial medium-bedded member, and an upper cherty member. The Chesterfield Range Group is of Late Mississippian age. Three coral zones and four brachiopod zones are recognized. The coral sequence is composed of a lower Eksvasophyllum Zone succeeded by the Faberophyllum Zone and, at the top, the Caninia Zone. The lowest brachiopod zone of Quadratia hirsutiformis is succeeded by the Echinoconchus cf. E. alternatus Zone, the Striatifera brazeriana Zone, and the Spirifer brazerianus Zone. Correlations with Mississippian sequences elsewhere in the northern Rocky Mountains emphasize relationships with the Dry Lake and Old Laketown Canyon sections in northern Utah and with the Deep Creek Mountains section in southeastern Idaho. Relationships with the faunal zones recently established for the Mississippian in western Canada are indicated.

Mississippian Rocks of Sun River Canyon Area, Sawtooth Range, Montana

The Madison Group (Mississippian) of the Sun River Canyon area, northwestern Montana, is subdivided into two newly named and defined formations--the Allan Mountain Limestone and Castle Reef Dolomite. The Allan Mountain Limestone is composed of three unnamed members: the lower member, middle member, and upper member. The Castle Reef Dolomite consists of an unnamed lower member and the redefined Sun River Member. The recognition of four faunal zones in the Madison permits tentative correlation of these rocks with rocks of Mississippian age in central and southwestern Montana and in Alberta, Canada. Formational boundaries and lithologic types recognized in southern Montana can not be consistently followed in the Sawtooth Range. The faunas indicate that the Allan Mountain Limestone is approximately the same age as the Lodgepole Limestone of southwestern Montana and that the Castle Reef is approximately the same age as the Mission Canyon Limestone. The top of the middle member of the Allan Mountain Limestone is approximately equivalent to the boundary between the Banff and Rundle Formations of Alberta. The Ca/Mg molar ratio substantiates field determinations of the lithologic composition and boundary between the two formations. Local correlation of certain units are indicated by the molar ratio.

Revised Megafossil Biostratigraphic Zonation for Carboniferous of Northern Alaska: ABSTRACT

Carboniferous megafossils are widely distributed in the Kayak Shale and Lisburne Group throughout the northern Brooks Range. Diverse assemblages of brachiopods, corals, and mollusks, with subordinate echinoderms and bryozoans, were collected from 40 measured sections. The combined stratigraphic ranges and abundances of more than 300 species were assessed to construct a biostratigraphic zonation that can be applied regionally for correlation. Preliminary zonations, used for more than a quarter century, were revised to account for the rapidly accumulating data. The 18 assemblage zones, from youngest to oldest, are: Umboanctus? sp., Corwenia jagoensis, Choristites? sp., Delepinoceras sp., Siphonodendron ignekensis, Gigantoproductus striatosulcatus-Stelechophyllum? aff. S.? m lareni, Goniatites americanus-Siphonodendron lisburnensis, Sciophyllum lambarti, Eumetria costata, Stelechophyllum? mclareni, Naticopsis suturicompta-Lithostrotion reiseri, Skelidorygma subcardiiformis, Spirifer tenuicostatus-Siphonodendron dutroi, Sychnoelasma konincki s.1.-Actinocrinites sp., Brachythyris choteauensis, Cryptoblastus-Pentremites, Leptagonia analoga, and Scalarituba-Lepidodendropsis. In the central and western Brooks Range, the deeper water Kuna formation contains a low-diversity fauna of mollusks and brachiopods. Goniatites are found at several levels but never more than two zones in any partial section. Regionally, these goniatite zones, from youngest to oldest, are: Delepinoceras (late Chesterian), Goniatites americanus (late Meramecian-early Chesterian), Beyrichoceras (early to middle Meramecian), Ammonellipsites (Osagean) and Muensteroceras-Protocanites (late Kinderhookian?). Correlations of megafossil zones with the foraminiferal zones of Mamet are discussed. The 15 Mississippian zones have an average age-resolution of about 2 m.y. By themselves, the goniatite zones give an age-resolution of about 6 m.y. zone. End_of_Article - Last_Page 663------------