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 Till, A.B., Dumoulin, J.A., Harris, A.G., Moore, T.E., Bleick, Heather, and Siwiec, Benjamin, 2008, Bedrock geologic map of the southern Brooks Range, Alaska, and accompanying conodont data: U.S. Geological Survey Open-File Report 2008-1149, 88 p., 2 sheets, scale 1:500,000.
Symbol OpCc
Unit name Older carbonate rocks of the Central belt
Description Dolostone, metalimestone, marble and subordinate quartzose metasedimentary rocks, carbonate conglomerate, and metabasite exposed in the northeastern Baird Mountains quadrangle, central Survey Pass quadrangle, and along the Dalton Highway in the Wiseman and Chandalar quadrangles. This unit is part of the Central belt.
Lithology Metamorphic

Correlated geologic units

Label OCc
Description Older carbonate rocks of the Nanielik antiform,
Geologic age Neoproterozoic to Middle-Ordovician
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Dolostone < Carbonate < Sedimentary Bed Major
Dolostone < Carbonate < Sedimentary Calcareous Major
Marble < Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major
Metasandstone < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major