Marine volcaniclastic and arkosic sandstone

Unit symbol: Jvs
Age range Jurassic (201.3 to 145 Ma)
Lithology: Sedimentary
Group name: Volcanic and sedimentary rocks of southwest Alaska
Consists of two units of Box and others (1993) (1) volcaniclastic sandstone and (2) arkosic sandstone and shale, both turbidite facies. Volcaniclastic strata is generally sandstone, but locally is as coarse as fine-grained pebble conglomerate and consists of predominantly volcanic rock fragments of intermediate composition and detrital feldspar, quartz, clinopyroxene, and hornblende. Minor rock fragments of plutonic, felsic volcanic and hypabyssal, sedimentary, and low-grade metamorphic rocks are consistently present components (Box and others, 1993). Unit also contains arkosic sandstone and shale primarily composed of detrital plagioclase, quartz, and potassium feldspar, as well as minor hornblende, biotite, and clinopyroxene apparently derived from weathering of plutonic rocks (Box and others, 1993). Arkosic sandstone is on strike with volcaniclastic part of unit (Box and others, 1993). Age control is based on stratigraphic position above Early Jurassic and Late Triassic phyllite and chert of map unit JTrp of Box and others (1993) and below Early Cretaceous and (or) Late Jurassic argillite and tuffaceous chert of their map unit KJc)

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 Jmt
Unit name Middle Jurassic volcaniclastic turbidites of Togiak Bay
Description Tightly folded volcaniclastic turbidites at least 1.7 km thick. Sandstones and conglomerates composed entirely of volcanic and hypabyssal igneous clasts. A few tool marks and interbedded slump folds suggest ENE-directed paleocurrents. Middle Jurassic age constrained by occurrence of Bajocian pelecypods northeast of map area in Togiak River valley (Hoare and Coonrad, 1978a). Intruded by Aeolus Mtn. gabbro body with 174 +/- 8 m.y. K-Ar amphibole age. Section in seacoast divisible into buff-colored lower member (1 km thick) and bright green-colored upper member (0.7 km). Lower unit consists of thin-bedded facies D and E fine-grained turbidites (terminology of Walker and Mutti, 1973), with channelized facies A, B, and C medium- to coarse-grained sandstones and minor chaotic facies F interbeds. Upper, more tuffaceous member mostly thin-bedded facies D and E turbidites with coarse conglomerates in upper part showing poorly defined thinning and fining upward cycles. Similar conglomeratic rocks intruded by Aeolus Mtn. gabbro. Entire sequence interpreted as deposited on proximal channelized portion (i.e., upper to midfan) of turbidite basin fill complex. Unit is folded around subhorizontal, northeast-trending axes and steep axial planes, and lacks a penetrative cleavage.
Lithology Sedimentary

Correlated geologic units

Label Jvt
Description Volcaniclastic turbidites of Togiak Bay - Goodnews terrane
Geologic age Middle-Jurassic
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Conglomerate-sandstone < Mixed-clastic < Clastic < Sedimentary Tuffaceous Major