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 Jln
Unit name Lower Jurassic? dismembered volcaniclastic turbidites
Description Pervasively deformed and dismembered tuffaceous sandstone-shale sequence, predominately argillaceous. Sedimentary structures variably preserved and indicate deposition from turbidity currents and other mass flow processes. Original thickness uncertain. Sandstones composed of volcanic detritus with very minor chert component. Partially altered to prehnite-pumpellyite mineral assemblages. Unit divisible into two structural facies types, which occur as alternating, fault-bounded blocks. Structural facies type one (SF1) characterized by generally well bedded sections that show varying amounts of pre lithification disruption (i.e. flowage of sandstone beds, thick convoluted sections, numerous bedding-plane decollements). SF2 distinguished by its pervasive cleavage and the occurrence of sandstone boudins in an argillaceous matrix. Cleavage varies from an anastomosing "scaly" fabric of slickensided surfaces to nearly planar slaty cleavage. Fold asymmetry in both SF1 and SF2 and block rotation in SF2 suggest deformation by simple shear mechanisms with a northwest-over-southeast sense. Poorly preserved radiolaria from SF1 have been identified as post-Triassic Mesozoic forms. Clasts of slate similar to SF2 occur stratigraphically below Bajocian fossil occurrence in adjacent Middle Jurassic turbidites (Jmm). Deposition and deformation of this complex therefore Early Jurassic in age.
Lithology Sedimentary

Correlated geologic units

Label Jvt
Description Volcaniclastic turbidites
Geologic age Early-Jurassic
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major