Limestone, northern Alaska

Unit symbol: OPxls
Age range Ordovician through Proterozoic? (1000 to 443.4 Ma)
Lithology: Sedimentary
Group name: Older carbonate rocks of northern Alaska
Unit largely corresponds to unit O<c, Older carbonate rocks of the Nanielik antiform of Till and others (2008a) and in the central Brooks Range, the lower part of the Skajit Limestone. Description here derived from Till and others (2008a). Unit consists of “dolostone, metalimestone, marble and subordinate quartzose metasedimentary rocks, carbonate conglomerate, and metabasite exposed in the Nanielik antiform (northeastern Baird Mountains and northwestern Ambler River quadrangles), central Survey Pass quadrangle, and along the Dalton Highway in the Wiseman and Chandalar quadrangles.” Lower part of unit is primarily orange- to light-gray-weathering, dark- to light-gray dolostone, metalimestone, and marble and subordinate quartzose metasedimentary rocks, carbonate-cobble conglomerate, and metabasite. Well-preserved tabular to club-shaped stromatolites and coated grains occur locally in dolostone and suggest an intertidal to shallow subtidal depositional setting. Matrix-supported conglomerate may represent debris flows (Till and others, 2008a). Metabasite, known only in the Baird Mountains quadrangle, consists of metamorphosed pillow breccia, pillow lava, and mafic pyroclastic rocks that contain blue amphibole. Three subunits overlie this lower unit: (1) impure metalimestone, marble, dolostone, and subordinate phyllite and calcareous and chloritic schist that display preserved sedimentary structures including grading, parallel and cross-laminae, which suggest deposition by turbidity currents; (2) massive marble that grades upward into thin couplets of bioturbated metalimestone and laminated dolostone, which is interpreted as shallowing-upward peritidal cycles. This subunit contains protoconodonts, chancellorid sclerites, hyolithids, and steinkerns of monoplacophoran mollusks that indicate a maximum age of Early Cambrian for the marble; presence of acrotretid brachiopods and agnostid arthropods demonstrate Middle and Late Cambrian ages for the middle and upper parts of the subunit; (3) condensed Lower and Middle Ordovician section, deposited in a shallowing-upward regime, consists of carbonaceous phyllite with subordinate layers of radiolarian chert and fine-grained metalimestone. Subunit grades upward into platform-margin carbonate turbidites and then into middle- to inner-platform bioclastic support stones; graptolites and conodonts indicate an age of Arenig (Lower Ordovician) to Caradoc (approximately Upper Ordovician) for this subunit. The stratigraphic succession of O<c is most complete and lithologies are most diverse in the Baird Mountains and western Ambler River quadrangles. There, the close spatial relationship of the unit with Proterozoic metamorphic rocks (unit <am of Till and others, 2008a) is thought to reflect an original depositional relationship (Till and others, 2008a)

Source map information

Source map Karl, S.M., Dumoulin, J.A., Ellersieck, Inyo, Harris, A.G., and Schmidt, J.M., 1989, Preliminary geologic map of the Baird Mountains and part of the Selawik quadrangles, Alaska: U.S. Geological Survey Open-File Report 89-551, 65 p., 1 sheet, scale 1:250,000.
Symbol PzPcb
Unit name Carbonate rocks and metabasite
Description Orange- to light-gray-weathering, dark-to light-gray, fine- to medium-grained crystalline dolostone, metalimestone and marble, with subordinate intercalated calcareous quartzite, quartz-mica schist, green and silver phyllite, carbonate cobble metaconglomerate, and blue amphibole-bearing metabasite.
Lithology Metamorphic

Correlated geologic units

Label PzZcb
Description Carbonate rocks and subordinate metabasite
Geologic age Neoproterozoic to Silurian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Marble < Metacarbonate < Metasedimentary < Metamorphic Major
Quartzite < Mica-schist < Schist < Metamorphic Indeterminate, major
Metabasalt < Metavolcanic < Metaigneous < Metamorphic Minor
Quartzite < Metaclastic < Metasedimentary < Metamorphic Minor