Calcareous graywacke and conglomerate

Unit symbol: Kcgc
Age range Lower Cretaceous, Albian to Aptian (125 to 100.5 Ma)
Lithology: Sedimentary
Group name: Calcareous graywacke and conglomerate
Hard, fine-grained to conglomeratic, locally tuffaceous graywacke and dark-gray finely laminated mudstone, typically exposed in western Alaska from Kuskokwim Bay to the southern Brooks Range. Graywacke contains matrix-supported clasts of intermediate and mafic extrusive and intrusive rocks and chert; quartz clasts and metamorphic and granitic rock clasts are present in subordinate amounts. Some graywacke beds are characterized by a distinctly mottled appearance owing to the presence of laumontite, which occurs most commonly in fine-grained, tuffaceous-rich layers (Patton and others, 2009). Metamorphic detritus is increasingly abundant towards top of unit. Graywacke beds commonly display Bouma-sequence grading from massive at the base to laminated in the middle to cross-laminated at the top. Mudstone rip-up clasts are common at the base of the graywacke beds. Unit has a high graywacke-to-mudstone ratio and is interpreted to represent middle and outer submarine fan deposits (Patton and others, 2009). It appears to correspond with similar rocks that extend in a broad belt from the southeast corner of the Saint Michael quadrangle and the northwest corner of the Holy Cross quadrangle to the south-central part of the Kwiguk quadrangle (map unit Kygc of Patton and others, 2009, in press). Marine mollusks of late Early Cretaceous (Albian) age were identified in this unit in the Bettles, Hughes, and Kateel River quadrangles (Patton and others, 2009). Although no fossils have been found in these rocks in the Saint Michael and Holy Cross quadrangles, the similar rock assemblages in the Yukon-Koyukuk Basin are thought coeval (Patton and others, in press [SIM 3015])

Source map information

Source map Patton, W.W., Jr., and Miller, T.P., 1966, Regional geologic map of the Hughes quadrangle, Alaska: U.S. Geological Survey Miscellaneous Geologic Investigations Series Map I-459, 1 sheet, scale 1:250,000.
Symbol Kgm
Unit name Graywacke and mudstone
Description Fine- to medium-grained, locally calcareous, poorly sorted graywacke interbedded with mudstone
Lithology Sedimentary

Correlated geologic units

Label Kvgc
Description Earliest Cretaceous volcanic graywacke and conglomerate (Replaces 2180 in KW and north)
Geologic age Albian
Geologic setting Sedimentary, marine, shallow to deep association, undivided
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Lithic-wacke < Graywacke < Sandstone < Clastic < Sedimentary Bed Major
Mudstone < Clastic < Sedimentary Bed Minor