Baird Group and similar rocks

Unit symbol: DCbg
Age range Middle Devonian to upper Cambrian (513 to 358.9 Ma)
Lithology: Sedimentary
Group name: Baird Group and similar rocks
Originally defined as three distinct formations (the Kugururok Formation, Eli Limestone, and Skajit Limestone), the Baird Group consists of “beige- to orange-weathering, laminated, partly argillaceous to silty metalimestone and light- to dark-gray, flaggy-bedded to massive metalimestone, marble, and dolostone. The Baird Group was established by Tailleur and others (1967), but was restricted by Dumoulin and Harris (1994) to the carbonate succession of the west-central Baird Mountains quadrangle; broadly coeval carbonate strata of the eastern Baird Mountains quadrangle and Middle and Late Devonian carbonate rocks of the Maiyumerak Mountains were excluded” (Till and others, 2008a). As shown here, the Baird Group is restricted to the western part of Alaska north of Kotzebue Sound, primarily in the Baird Mountains quadrangle and a small area in the Chandalar quadrangle to the east. The Eli Limestone and Kugururok Formation are mapped together here as unit Dke, and the Skajit Limestone is not used here. Rocks commonly assigned to the Skajit Limestone are excluded from the Baird Group and instead included in unit |m here. As shown, parts of two units of Till and others (2008a), DOb (the Baird Group, proper) and DOc (Younger carbonate rocks of the Nanielik antiform) are included in this map unit. As used by Till and others (2008a), the oldest strata of the Baird Group “* * * are Early and Middle Ordovician, locally contain reworked Late Cambrian conodonts, and comprise two roughly coeval lithofacies: dolostone with fenestral fabric and evaporite molds that formed in locally restricted, shallow to very shallow water, and bioturbated to laminated, argillaceous to silty metacarbonate rocks with local cross-bedding, ripples, and flame structures that accumulated in somewhat deeper platform settings. * * * Younger strata include Upper Ordovician and middle Silurian (Wenlock) metalimestone deposited in middle to outer platform settings and upper Silurian and Lower and Middle Devonian (Emsian, Eifelian, and possibly Givetian?) shallow-water dolostone and metalimestone with corals and stromatoporoids; the Emsian section includes an interval of calcareous, chloritic metasandstone. DOb [of Till and others, 2008a] includes local occurrences of metabasalt of unknown age that are especially abundant in western exposures. These rocks contain fine-grained blue amphibole (crossite) at multiple localities, indicating that the Baird Group has experienced blueschist facies metamorphism. The unit has similarities in lithofacies and biofacies to * * * coeval strata of the York Mountains on Seward Peninsula. Conodont assemblages have both Laurentian and Siberian affinities (Dumoulin and Harris, 1994; Dumoulin and others, 2002)” (Till and others, 2008a). The ‘Younger carbonate rocks of the Nanielik antiform’ (unit DOc of Till and others, 2008a) are “very light- to dark-gray (locally orange-) weathering, gray to black, commonly massive dolostone and lesser metalimestone and marble exposed in the Nanielik antiform (northeastern Baird Mountains and northwestern Ambler River quadrangles) and in the northwestern Chandalar quadrangle. DOc was first described in the Nanielik antiform (Sheep Creek section of Dumoulin and Harris, 1987) and later recognized to the east (Dumoulin and Harris, 1994). Meter- to decimeter-scale color banding is characteristic of this unit and reflects shallowing-upward peritidal cycles of darker colored, bioturbated, bioclastic and (or) peloidal carbonate strata overlain by light-colored cryptalgal laminite. Recognizable fossils include corals, stromatoporoids, brachiopods, gastropods, bryozoans, pelmatozoan debris, ostracodes, and dasycladacean algae. Corals and conodonts indicate an age of Upper Ordovician through Silurian for most of the unit. Definitively Devonian rocks have not been found in the eastern Baird Mountains quadrangle, but stromatoporoid wacke-packstone of late Early to early Middle Devonian (Emsian-Eifelian) age occurs locally in the Chandalar quadrangle. The unit is generally fault-bounded but a gradational and apparently conformable contact above OPxc [Older carbonate rocks of the Nanielik antiform of Till and others (2008a), shown here as unit OCls] is locally preserved. Sedimentary structures and fossils indicate warm, shallow-water depositional settings that were shallowest and most restricted during Late Ordovician (Richmondian or Katian) time. DOc has similarities in lithofacies and biofacies to the younger part of DOb, as well as coeval strata of the York Mountains on Seward Peninsula. Ordovician biotas include some megafossils and microfossils with Siberian affinities and others with Laurentian affinities; Silurian and younger fossil assemblages are chiefly cosmopolitan (Dumoulin and Harris, 1994; Dumoulin and others, 2002)” (Till and others, 2008a). Locally subdivided into unit Dke below

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 DOc
Unit name Younger carbonate rocks of the Nanielik Antiform, Central belt and part of Northern Thrust Assemblages
Description Very light- to dark-gray- (locally orange-) weathering, gray to black, commonly massive dolostone and lesser metalimestone and marble exposed in the northeastern Baird Mountains quadrangle in the Nanielik antiform and in the northwestern Chandalar quadrangle. Unit has similarities in lithofacies and biofacies to younger part of DOb, as well as coeval strata of the York Mountains on Seward Peninsula. Ordovician biotas include some megafossils and microfossils with Siberian affinities and others with Laurentian affinities; Silurian and younger fossil assemblages are chiefly cosmopolitan. This unit is part of the Central belt.
Lithology Metamorphic

Correlated geologic units

Label DSb
Description Baird Group; Massive light gray reefoid limestone and dolomite with local thin interbeds of calcareous phyllite
Geologic age Middle-Cambrian to Middle-Devonian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Dolostone < Carbonate < Sedimentary Bed Major
Limestone < Carbonate < Sedimentary Bed Major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Incidental
Shale < Mudstone < Clastic < Sedimentary Bed Incidental