Volcaniclastic and volcanic rocks

Unit symbol: Jvc
Age range Upper and Middle Jurassic (174.1 to 152.1 Ma)
Lithology: Sedimentary
Group name: Volcanic and sedimentary rocks of southwest Alaska
Basaltic and andesitic volcanic and volcaniclastic rocks on Hagemeister Island that have been extensively altered to prehnite-pumpellyite facies (Hoare and Coonrad, 1978; Box, 1985). Volcanic rocks, characterized by plagioclase and clinopyroxene┬▒hornblende porphyritic rocks are more common in lower third of exposed section. Interbedded clastic rocks composed almost entirely of volcanic lithic clasts, except lower conglomerate, which contains a minor proportion of granitic plutonic rock clasts similar to adjacent pluton. Sedimentary facies range from fluvial deltaic to nearshore marine to subwave base turbidites. The contact between this unit and similar older volcanic and sedimentary rocks that are structurally and stratigraphically below it on Hagemeister Island (unit J^vs) were mapped by Box (1985) as a low-angle thrust(?), which he suggested may be a faulted unconformity. Fossils of Middle Jurassic age are locally common (Box, 1985). Conglomerate includes clasts of granitic rocks similar to nearby granodiorite of the Middle Jurassic Hagemeister Island pluton (Box, 1985); mapped herein as unit Jegd. More northern exposures of this unit are tuffaceous marine sandstone, shale, and conglomerate (Box and others, 1993; Patton and others, in press [Scientific Investigations Map 3015]) that contain minor basaltic and andesitic lava flows. Felsic pyroclastic rocks are found locally in the middle part of the unit in the Bethel quadrangle and near base of unit northward in the Russian Mission quadrangle. Unit is as much as 2 km thick; coarser parts are composed primarily of rounded clasts of basalt and andesite and minor plutonic rock fragments of mafic to intermediate composition (Box and others, 1993). Marine pelecypods of Bajocian age (Middle Jurassic) are found near the base of the sequence and marine pelecypods of late Kimmeridgian to early Tithonian age (Late Jurassic) were reported from the Russian Mission quadrangle (Box and others, 1993)

Source map information

Source map Patton, W.W., Jr., Wilson, F.H., and Labay, K.A., in press, Reconnaissance geologic map of the lower Yukon River Region, Alaska: U.S. Geological Survey Scientific Investigations Series Map SIM-3015, scale 1:500,000.
Symbol Jvc
Unit name Volcaniclastic conglomerate, sandstone, and shale
Description Tuffaceous marine sandstone, shale, and conglomerate. Minor basaltic and andesitic lava flows. Felsic pyroclastic rocks locally near base of unit. Unit rests conformably(?) on unit Jab and is conformably(?) overlain by unit KJb
Lithology Sedimentary

Correlated geologic units

Label Juc
Description Unnamed marine sedimentary rocks, sandstone, siltstone, mudstone
Geologic age Middle-Jurassic to Late-Jurassic
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Mafic-volcanic < Volcanic < Igneous Flow Minor