Quartz-pebble conglomerate, west-central Alaska

Unit symbol: Kqc
Age range lower Upper Cretaceous to upper Lower Cretaceous (107 to 83.6 Ma)
Lithology: Sedimentary
Group name: Nonmarine to shelf sedimentary rocks
Composed chiefly of well-sorted and well-rounded clasts of white quartz and (or) metagraywacke in a quartzose and micaceous matrix; schist, chert, greenstone, and limestone clasts occur in subordinate amounts. Conglomerate is interbedded with quartzose, cross-bedded sandstone and carbonaceous and micaceous mudstone. Contains rare interbeds of ashy tuff. Plant fossils and thin bituminous coal seams are locally abundant. Unit composed chiefly of debris eroded from the Arctic Alaska and Ruby terranes. Grades downward into unit Kipc, reflecting the progressive unroofing of Arctic Alaska and Ruby terranes beneath Angayucham-Tozitna terrane (Patton and others, 2009). Unit regionally metamorphosed, resulting in stretched-pebble conglomerate, semischist, and phyllite in northeastern part of the Shungnak quadrangle and in adjoining parts of the Ambler River and Hughes quadrangles. Sparse plant fossil collections from this unit range in age from late Early Cretaceous to Late Cretaceous. A K/Ar biotite cooling age of 86 Ma (corrected age is 85.4±2.2 Ma) was reported by Patton and others (2009) and Till and others (2008a) from interbedded ash-fall tuff in the Baird Mountains quadrangle

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., Labay, K.A., and Shew, Nora, 2009, Geologic map of the Yukon-Koyukuk basin, Alaska: U.S. Geological Survey Scientific Investigations Map SIM-2909, 26 p., 2 sheets, scale 1:500,000.
Symbol Kqc
Unit name Quartz- and metagraywacke-clast conglomerate, sandstone, and shale
Description Composed chiefly of well-sorted and well-rounded clasts of white quartz and/or metagraywacke in a quartzose and micaceous matrix; schist, chert, greenstone, and limestone clasts occur in subordinate amounts. Conglomerate is interbedded with quartzose, cross-bedded sandstone and carbonaceous and micaceous mudstone. Contains rare interbeds of ashy tuff. Plant fossils and thin bituminous coal seams are locally abundant. Unit composed chiefly of debris eroded from the Arctic Alaska and Ruby terranes. Grades downward into unit (Kmc) reflecting the progressive unroofing of the Arctic Alaska and Ruby terranes beneath the Angayucham-Tozitna terrane. Unit is regionally metamorphosed to stretched-pebble conglomerate, semischist, and phyllite in northeastern part of the Shungak quadrangle and in adjoining parts of the Ambler River and Hughes quadrangles. Unit exposed along the northern and southeastern margin of the Yukon-Koyukuk Basin from Baird Mountains and Selawik quadrangles eastward to Wiseman and Bettles quadrangles and southwestward to Melozitna quadrangle
Lithology Sedimentary

Correlated geologic units

Label Kqc
Description Quartz-pebble conglomerate, early Late Cretaceous
Geologic age Barremian to Santonian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major