Fossil Creek Volcanics and similar rocks

Unit symbol: OCv
Age range Ordovician and Cambrian (541 to 443.4 Ma)
Lithology: Igneous - Volcanic
Group name: Fossil Creek Volcanics and similar rocks
Unit includes the Fossil Creek Volcanics of east-central Alaska, which is divided into two members: one largely volcanic, one largely sedimentary. The volcanic member consists of alkali basalt, agglomerate, and volcaniclastic conglomerate. Agglomerate and conglomerate contain well-rounded clasts of basalt, granite, quartzite, limestone, chert, and phyllite. Sedimentary member consists of shale, chert, and limestone and is intruded by gabbro. Late Ordovician brachiopods, trilobites, gastropods, and conodonts have been recovered from sedimentary rocks in the volcanic member (Blodgett and others, 1987); Early Ordovician conodonts and trilobites have been recovered from the sedimentary member (Weber and others, 1992). Unit occurs in the White Mountains stratigraphic belt of the Livengood, Circle, and Tanana quadrangles (Dover, 1994) and is laterally equivalent to the Livengood Dome Chert. In northeast Alaska, unit includes dark-grayish-green to greenish-black and brown mafic vesicular flows, basaltic tuffs, agglomerate(?), pebble- to cobble-sized volcanic conglomerate, calcareous volcanic wacke, tuffaceous limestone, and tuffaceous dolostone of Cambrian age (Reiser and others, 1980). Weathers dark-olive-brown and dark-gray, except the dolomitic facies, which weathers yellowish brown. Minor gray, dark-gray-weathering chert, chert conglomerate, and silicified chert breccia. Locally includes basaltic tuff and mafic intrusive rocks. Probably correlates with the upper part of the Marmot Formation and Dempster Volcanics of Yukon (Gordey and Makepeace, 2003). May be age and lithologically correlative with unit OCdv, which is found in the Doonerak Window of the Brooks Range

Source map information

Source map Weber, F.R., 1998, Written communication.
Symbol SOs
Unit name Siltstone, dolomite, chert, and mafic igneous rocks
Description Gray, black, or olive-gray siltstone and argillite and mudstone. Tan dolomite, gray to dark-gray fine-grained limestone, gray or dark-gray chert. Mafic igneous rocks are dikes, sills, and small lenses-like bodies of dark-greenish-black gabbro and basalt. Two areas of exposure, northern contains chert, southern mafic igneous rocks. This NSAclass assignment is for the southern of the two areas.
Lithology Igneous

Correlated geologic units

Label Ofc
Description Fossil Creek Volcanics, undivided -- Alkali basalt, agglomerate, and volcaniclastic conglomerate
Geologic age Ordovician
Geologic setting Undivided
Lithology Form Importance
Basalt < Mafic-volcanic < Volcanic < Igneous Flow Major
Basalt < Mafic-volcanic < Volcanic < Igneous Volcaniclastic Minor
Limestone < Carbonate < Sedimentary Bed Minor
Chert < Chemical < Sedimentary Bed Minor
Shale < Mudstone < Clastic < Sedimentary Bed Minor