Quartz-carbonate sandstone and pebbly mudstone

Unit symbol: Kqcs
Age range Lower Cretaceous, Aptian to Valanginian (139.8 to 113 Ma)
Lithology: Sedimentary
Group name: Coquina and calcarenite
Lithic sandstone, conglomerate, and shale about 5 km thick (Hoare and Coonrad, 1983), informally called the graywacke of Buchia Ridge by Hoare and Coonrad (1978). Lower part of unit is about 2,400 m thick, composed mainly of well-bedded, fine- to medium-grained sandstone with siltstone interbeds and pebble-cobble conglomerate. In lowest part of section, beds range from 0.1 to 3 or 4 m thick; the upper part of this lower section consists of 1,400 m of increasingly massive conglomerate (Hoare and Coonrad, 1983). Well-rounded clasts, as large as 10 cm, consist of “graywacke and siltstone, cherty tuff, and less abundant porphyritic volcanic rocks” (Hoare and Coonrad, 1983); graywacke clasts resemble graywacke of Kulukak Bay (unit Jk here). Lithic conglomerate, near the top of Buchia Ridge, contains coquinas of Buchia crassicollis shells (Hoare and Coonrad, 1983). Upper part of unit, about 2,500 m thick, is “mostly shale and thin-bedded sandstone” (Hoare and Coonrad, 1983) that is poorly exposed. It is “dominantly fine-grained * * * calcareous black shale, thin-bedded calcareous siltstone, sandstone, calcarenite, and minor gritstone” (Hoare and Coonrad, 1983). Box (1985) interpreted lower part of the unit to be of shallow marine origin and the upper part of the unit as probable deeper marine origin. According to Hoare and Coonrad (1983), the graywacke of Buchia Ridge “is the thickest, least deformed section of Lower Cretaceous sedimentary rocks known in southwestern Alaska.” Outcrop area covers approximately 350 km2 in a thrust plate (Hoare and Coonrad, 1983). Lower part of section contains abundant Valanginian fauna, primarily Buchia crassicollis; upper part of section yields sparse Inoceramus, belemnites, and a single Hauterivian ammonite, according to Hoare and Coonrad (1983)

Source map information

Source map Box, S.E., 1985, Mesozoic tectonic evolution of the northern Bristol Bay region, southwestern Alaska: Santa Cruz, University of California, Ph.D. dissertation, 163 p., 7 tables, 21 figures, 2 plates.
Symbol Klb
Unit name Lower Cretaceous graywacke of Buchia Ridge
Description Lithic sandstone, conglomerate, and shale at least 2-5 km thick (Hoare and Coonrad, 1983). Clastic rocks composed mostly of slaty volcaniclastic rock fragments, similar to Jurassic rocks to the southeast (Jln, Jmm). No paleocurrent data available. Divisible into lower, coarse clastic member (Klb1) of shallow marine origin, and an upper calcareous black shale member (Klb2) of probable offshore marine origin. Klb1 preserves abundant Valanginian fauna; Klb2 yields rare Hauterivian ammonites. Probable basal angular unconformity obscured by later faulting, except north of map area (Hoare and Coonrad, 1983)
Lithology Sedimentary

Correlated geologic units

Label Ksm
Description Quartz-carbonate sandstone and pebbly mudstone, Early Cretaceous, Valanginian-Aptian
Geologic age Valanginian to Aptian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Minor
Mudstone < Clastic < Sedimentary Bed Minor