Calcareous and metabasaltic schist and phyllite

Unit symbol: JPs
Age range Early Jurassic to Permian (298.9 to 174.1 Ma)
Lithology: Metamorphic
Group name: Calcareous and metabasaltic schist and phyllite (Togiak-Tikchik Complex)
Consists of a number of interrelated map units defined by Hoare and Coonrad (1978) and Box (1985) in the Goodnews Bay and Hagemeister Island region of southwest Alaska. Consists of three main subunits; the rocks of the first subunit include recrystallized schistose and phyllitic calcareous sandstone, shale, limestone, limestone conglomerate, greenish tuffaceous rocks, mafic volcanic rocks, and volcanic conglomerate; the second subunit consists of metamorphosed conglomerate, sandstone, and shale; and the third subunit is partially to completely recrystallized glaucophane-hornblende epidote schistose rocks derived from mafic igneous rocks that include pillow basalt, angular volcanic breccia, diabase, and gabbro and associated sedimentary rocks, that include pebbly mudstone, fine-grained tuffaceous sedimentary rocks, and rare chert (Box, 1985). Turbidite characteristics are locally preserved in clastic metasedimentary rocks. Mineral assemblages indicate greenschist to transitional greenschist-blueschist facies metamorphism (Box, 1985; Dusel-Bacon and others, 1996); structurally the unit shows evidence of multiple metamorphic events (Box, 1985). Metamorphic fabric is similar in all three subunits, which suggests that all three units were deformed during the same metamorphic event (Box, 1985). Locally, unit is structurally overlain by Devonian to Ordovician limestone and the Kanektok metamorphic complex (Wilson and others, 2013). West of Goodnews Bay, the unit structurally overlies Permian volcaniclastic rocks and underlies Permian limestone. Intruded by nonfoliated Middle Jurassic gabbroic rocks (Jgb). Schist has yielded K/Ar mica ages of 155±8 Ma and 150±8 Ma (Wilson and others, 2013), possibly related to a Late Jurassic thermal event suggested by Box (1985). The ages are suspect, however, because the K2O content of the mica (1.71 and 2.94 percent, respectively) is much lower than typical for mica (8 to 10 percent), suggesting alteration. A K/Ar cooling age on amphibole of 231.2±6.9 Ma (Wilson and others, 2013) from the metasedimentary rocks suggests Middle Triassic metamorphism; age is constrained to be older than the intrusion of Middle Jurassic gabbroic rocks. Age of protolith may be Permian or older (Hoare and Coonrad, 1978; Box, 1985). Main part of unit is spatially associated with a Triassic ophiolite complex

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 JTrmvss
Unit name Upper Triassic-Lower Jurassic volcaniclastic schists
Description Partially recrystallized schistose rocks with sandstone, shale, and conglomerate protoliths derived from feldspar-rich volcanic source. Rounded conglomerate clasts mostly plagioclase +/- pyroxene +/- hornblende amygdaloidal porphyritic volcanic rocks, but include rare clasts of ultramafic rocks. Greenschist to transitional greenschist-blueschist facies metamorphic mineral assemblages. Strong fabric development (S2) by synmetamorphic flattening and (or) mylonitization. Lineation (L2) defined by cobble elongation and mineral lineation (amphibole, plagioclase). Syn- and post-metamorphic structural history similar to structurally overlying (JTrmb) and underlying (JTrmc) schists. Also records evidence of pre-metamorphic (D1) deformation, characterized by noncoplanar isoclinal folding and bedding dismemberment, possibly the result of pre-consolidation deformation. Apparently grades into similar unfoliated rocks (JPvss) in northwestern map area. Protolith age uncertain (Permian to Middle Jurassic). Intruded by Middle Jurassic plutons (Jmg, Jmu). Metamorphism probably Upper Triassic to Lower Jurassic, based on constraints on metamorphic age of JTrmb. Single K-Ar determination yields a Late Triassic amphibole age.
Lithology Metamorphic

Correlated geologic units

Label TrPvs
Description Volcaniclastic glaucophane-hornblende-epidote schist
Geologic age Permian to Middle-Jurassic
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Metaclastic < Metasedimentary < Metamorphic Indeterminate, major