Beaucoup Formation, undivided

Unit symbol: Dbf
Age range Devonian (419.2 to 358.9 Ma)
Lithology: Sedimentary
Group name: Beaucoup Formation, undivided
Heterogeneous unit that consists of carbonaceous, siliceous, and calcareous sedimentary rocks and felsic volcanic rocks broadly distributed across the Brooks Range and especially in Central Belt of Till and others (2008a). The name Beaucoup Formation has come to include many rocks of the Brooks Range stratigraphically below the Hunt Fork Shale (unit Degh) and above the loosely defined Skajit Limestone (here mapped as parts of unit Pzm and OCls). Multiple map units of Till and others (2008a) have been assigned to the Beaucoup Formation here. Till and others (2008a) divided their unit Pzw into northern and southern parts whose outcrops straddle the northern part of the Wiseman-Chandalar quadrangle boundary. “The northern belt is composed of metasandstone and argillite; in its eastern part, metasandstone contains abundant detrital white mica that yielded a Late Ordovician 40Ar/39Ar cooling age (Moore and others, 1997a). The northern belt is equivalent to the Trembley Creek phyllite of Moore and others (1997b), and Rocks of Whiteface Mountain of Dillon and others (1986). The southern belt * * * is composed of phyllite, metasandstone with volcanic clasts, argillite, sandstone, pebble conglomerate and rare marble” (Till and others, 2008a). This division into two parts or belts cannot be extended throughout the Brooks Range. “In the northeast Baird Mountains quadrangle, laminated to massive porphyritic rhyolite plugs, flows, and pyroclastic rocks are closely associated with siliceous and calcareous sedimentary rocks (Karl and others, 1989a). Along the northern part of the boundary between the Wiseman and Chandalar quadrangles, felsic to intermediate porphyries, metavolcaniclastic rocks, and rare massive hypabyssal rocks are associated with purple and green phyllite, lithic, quartz, feldspar metasandstone, and meta-argillite pebble conglomerate (Moore and others, 1997b). Elsewhere, volcanic-clast sandstone and conglomerate, feldspathic volcanic wacke or graywacke, and tuffaceous metalimestone occur with other sedimentary rocks of the unit” (Till and others, 2008a). Unit also includes “phyllite, carbonate, and clastic rocks of the Nakolik River, undivided” of Karl and others (1989a). Megafossils and conodonts collected from calcareous black phyllite and metalimestone interlayered with purple and green phyllite in the Chandalar quadrangle are Middle and early Late Devonian in age (Dumoulin and Harris, 1994); Middle and Late Devonian conodonts were recovered from the unit in the northwest Wiseman quadrangle (Till and others, 2008a, table A-1). A foliated felsic metavolcaniclastic rock collected in the northwestern Chandalar quadrangle yielded a U/Pb zircon crystallization age of 393±2 Ma (Aleinikoff and others, 1993, cited in Till and others, 2008a). This unit records the transition from early Paleozoic platform carbonate sedimentation to voluminous, widespread clastic sedimentation represented by rocks of the Endicott Group (Till and others, 2008a). Some rocks that should be assigned to this map unit are likely included in the metamorphosed part of the Hunt Fork Shale in the Chandalar quadrangle and elsewhere. Unit subdivided into the following two units, Dbfl and Dbfw

Source map information

Source map Brosge', W.P., Reiser, H.N., Dutro, J.T., Jr., and Detterman, R.L., 1979, Bedrock geologic map of the Phillip Smith Mountains quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-879-B, 2 sheets, scale 1:250,000.
Symbol Dbs
Unit name Beaucoup Formation, Brown shale member
Description Mostly light brown and orange-weathering partly calcareous gray and greenish-gray shale, slate and shaley micaceous siltstone, and dark-gray and rusty-weathering noncalcareous black shale and slate; ironstone and siltstone concretions common. Interbedded brown weathering, partly calcareous, thin bedded, fine grained, micaceous sandstone and brown and gray, thin to medium bedded micaceous, fine grained quartzite, including both quartz-chert arenite and wacke. Contains lenses or beds conglomerate of chert, quartz and shale pebbles. Few thin beds and lenses of fossiliferous limestone.
Lithology Sedimentary

Correlated geologic units

Label Dbc
Description Beaucoup Formation, carbonaceous, siliceous, and calcareous sedimentary rocks and felsic volcanic rocks that grade downward and to the southwest into Skajit Limestone. Laterally equivalent to Whiteface Mountain and Ambler volcanic and sedimentary rocks
Geologic age Middle-Devonian to Late-Devonian
Geologic setting Undivided
Lithology Form Importance
Siltstone < Clastic < Sedimentary Calcareous Major
Felsic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Felsic-volcanic < Volcanic < Igneous Pyroclastic Indeterminate, major
Marble < Metacarbonate < Metasedimentary < Metamorphic Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Minor
Quartzose-sandstone < Sandstone < Clastic < Sedimentary Calcareous Minor