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 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 Dmu_541
Unit name Metasedimentary and lesser metaigneous rocks, volcanic-rock bearing subunit
Description Part of heterogeneous expanses of calcareous, siliceous, and volcaniclastic metasedimentary rocks, with lesser metaigneous rocks, exposed along the length of the Central belt, thought to be correlative with the Beaucoup Formation. This subunit contains felsic to intermediate volcanic rocks or sedimentary rocks with abundant volcanic clasts. This unit is part of the Central belt.
Lithology Metamorphic

Correlated geologic units

Label Dbv
Description Beaucoup Formation, including volcanic rocks or volcanic rock clasts
Geologic age Middle-Devonian to Late-Devonian
Geologic setting Sedimentary, undivided
Lithology Form Importance
Metasandstone < Metaclastic < Metasedimentary < Metamorphic Greenschist Major
Mica-schist < Schist < Metamorphic Greenschist Major
Metasiltstone < Metaclastic < Metasedimentary < Metamorphic Greenschist Indeterminate, major
Phyllite < Metaclastic < Metasedimentary < Metamorphic Greenschist Indeterminate, major
Metaandesite < Metavolcanic < Metaigneous < Metamorphic Greenschist Minor
Metabasalt < Metavolcanic < Metaigneous < Metamorphic Greenschist Minor