Graywacke of Kulukak Bay of Hoare and Coonrad (1978)

Unit symbol: Jk
Age range Upper to Middle Jurassic (174.1 to 152.1 Ma)
Lithology: Sedimentary
Group name: Graywacke of Kulukak Bay of Hoare and Coonrad (1978)
Thick marine sedimentary unit that consists of very hard dark-green or gray, massive graywacke and siltstone and contains local coarse pebble conglomerate horizons. Compositionally, “varies from quartz- and plagioclase-rich wackes to quartz-poor volcanic wackes. Generally contains black argillite or tuff chips” (Hoare and Coonrad, 1978). Unit is a thick marine sedimentary unit earlier referred to as the “Weary graywacke” by Hoare and others (1975). Unit is widely exposed in the southern Goodnews Bay and Nushagak Bay quadrangles as well as the adjacent southeastern Dillingham quadrangle (Hoare and Coonrad, 1978). Buchia, Inoceramus, belemnites, and rare ammonite fragments that range in age from Middle to early Late Jurassic age have been found in this unit. Unit may be correlative with the Koksetna River sequence of Wallace and others, 1989, which is mapped as unit KJgn here. Locally subdivided by Box (1985) into a coherently and pervasively deformed volcaniclastic turbidite units

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 Jmm
Unit name Middle and Upper Jurassic turbidites of Metervik Bay
Description Coherently deformed volcaniclastic turbidites southeast of Right Hand Fault. Reconstructed 6 km section, divided into four turbidite facies units (using criteria of Walker and Mutti, 1973): (from bottom to top) a conglomeratic inner fan facies unit; a well-bedded, outer fan lobe facies unit; a sandy, often massive, midfan channel facies unit; and a conglomeratic inner fan facies unit. Sandstone compositions almost entirely derived from plagioclase-rich volcanic sources. Minor slaty clasts near base of lowest unit, and minor component of slate, dioritic plutonic and chert detritus in upper unit. Interbedded slump folds suggest southeast-facing paleoslope. Rare fossils range from Bajocian (base of third unit) to Oxfordian (in upper unit). Tightly folded around gently northeast-plunging axes and subvertical axial planes. Northwestward vergence suggested by longer southeast-dipping limbs. Axial planar slaty cleavage weak except near northwestern fault boundary. Adjacent to Right Hand thrust cleavage is folded and bedding dismembered by slip along cleavage surfaces. Deformation probably latest Jurassic-earliest Cretaceous, based on appearance as clasts in Klb1. Partially replaced by prehnite-pumpellyite mineral assemblage.
Lithology Sedimentary

Correlated geologic units

Label Jkmb
Description Turbidites of Metervik Bay, subdivision of graywacke of Kulukak Bay (Weary Graywacke)
Geologic age Bajocian to Kimmeridgian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Conglomerate-sandstone < Mixed-clastic < Clastic < Sedimentary Bed Indeterminate, major