Copper Lake Formation

Unit symbol: Tcl
Age range Tertiary, Eocene to Paleocene (66 to 47.8 Ma)
Lithology: Sedimentary
Group name: Copper Lake Formation
Clastic nonmarine sedimentary rocks 1,025 m thick in a measured section that consists of upper and lower conglomerate members bounding a middle sandstone and siltstone member on the northern Alaska Peninsula (Detterman and Reed, 1980; Detterman and others, 1996). Upper conglomerate unit consists of red-weathering pebble-cobble conglomerate that consists mainly of volcanic rock clasts and contains minor tuff (Detterman and Reed, 1980). This upper member may be an agglomerate rather than conglomerate; clasts are 50 to 100 percent fresh-appearing volcanic rock. Clasts of quartzite, schist, greenstone, rose quartz, limestone, and granitic rocks are present in lower parts of this upper conglomerate member. Sandstone and siltstone intervals vary from thin-bedded to massive and are typically dark- to medium-gray; they are fine- to medium-grained lower in section and become medium- to coarse-grained toward top. Middle member is chiefly medium-gray to greenish-gray, lithic graywacke sandstone and siltstone. More fine-grained clastic parts of formation contain considerable carbonaceous debris and minor coaly material. Grains in the sandstone include abundant quartz, schist, volcanic, and granitic rock fragments. Interbedded siltstone is similar in color and composition to the sandy facies, whereas claystone interbeds are mainly micaceous clay and contain a small amount of montmorillonite. Lower conglomerate member is red-weathering pebble-cobble conglomerate that consists mainly of volcanic rock clasts and contains minor tuff. Volcanic clasts constitute about 25 percent of member, and these appear to be derived from the Talkeetna Formation rather than the Tertiary volcanic units, which have a fresher appearance. Copper Lake Formation was derived from erosion of a Mesozoic source area and is terrestrial. In the southern part of its exposure area, the source area is underlain by the Alaska-Aleutian Range batholith (Reed and Lanphere, 1973) and associated Mesozoic sedimentary and metamorphic rocks. Towards the north, Copper Lake Formation undergoes transition from rocks of Mesozoic provenance to fresh volcanic clasts of probable Tertiary age (Detterman and Reed, 1980, p. B47). Age of the Copper Lake Formation is poorly constrained; sparse megaflora in type section and abundant megaflora on the Alaska Peninsula are restricted to sandstone and siltstone intervals in middle part of unit (Detterman and others, 1996). Detterman and others (1996) correlated the Copper Lake Formation with the Tolstoi Formation of the southwest Alaska Peninsula, where an early Eocene megaflora was collected from a section of sandstone and siltstone. Beneath the sandstone and siltstone, a basal conglomerate contains a late Paleocene flora; beds overlying the sandstone and siltstone contain a middle Eocene flora. By analogy, a Paleocene(?) to early Eocene age was assigned to the Copper Lake Formation. Parrish and others (2010) proposed a new formation they named the Ketavik [sic] Formation whose rocks, we believe, more properly belong to the Copper Lake Formation; the lithology and age assignment they gave to the Ketavik is consistent with the age of the Copper Lake Formation. Upper and lower contacts of the Copper Lake Formation are disconformities with Hemlock Conglomerate and Kaguyak Formation, respectively

Source map information

Source map Detterman, R.L., and Reed, B.L., 1980, Stratigraphy, structure, and economic geology of the Iliamna quadrangle, Alaska: U.S. Geological Survey Bulletin 1368-B, 86 p., 1 sheet, scale 1:250,000.
Symbol Tclu
Unit name Copper Lake Formation -- Upper conglomerate member
Description Red-weathering pebble-cobble conglomerate consisting mainly of volcanic rock clasts and containing minor tuff. Unit may be agglomerate rather than conglomerate; ranges from 50 to 100 percent fresh-appearing volcanic rock. Quartzite, schist, greenstone, rose quartz, limestone, and granitic clasts are present in lower parts of member. Nonmarine unit
Lithology Sedimentary

Correlated geologic units

Label Tcl
Description Copper Lake Formation;
Geologic age Paleocene to Early-Eocene
Geologic setting Sedimentary, continental
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Carbonaceous Indeterminate, major