Pinnacle Peak Phyllite

Unit symbol: JTrpp
Age range Jurassic or Triassic? (247.2 to 163.5 Ma)
Lithology: Tectonite
Group name: Kelp Bay Group, undivided
Pinnacle Peak Phyllite and the similar phyllite of Rodman Bay of Karl and others (2015) consist of dark-gray to black carbonaceous phyllite alternating on a meter to decameter scale with subordinate light green phyllite, metagraywacke, metachert, and thin layers of black marble. Protolith is inferred to be carbonaceous mudstone, graywacke, chert, and fine-grained mafic volcaniclastic rocks (Karl and others, 2015). Unit includes a significant amount of chloritic and graphitic schist, as well as graywacke semischist, phyllite, and nonfoliated metagraywacke turbidite and includes the highest grade metamorphic rocks in the Kelp Bay Group (Johnson and Karl, 1985). Graphitic phyllite is locally calcareous and contrast with siliceous argillites of the Khaz Complex mélange. The unit is bounded by faults; stratigraphic relations to other components of the Kelp Bay Group are not clear (Karl and others, 2015). Inclusion of this unit within the Kelp Bay Group is considered provisional here. Age control is not definitive for this unit; Karl and others (2015) report a white mica 40Ar/39Ar age of 155 Ma from a quartz boudin in phyllite near Rodman Bay, which suggests that the phyllite is no younger than Late Jurassic. Loney and others (1975) inferred a Triassic age for the unit based on an apparent stratigraphic relation with the Whitestripe Marble (unit ^lm). Johnson and Karl, (1985) showed that the Pinnacle Peak does not have a stratigraphic tie to the Whitestripe Marble. Hence, assignment of a Triassic age to this unit is not justified on the basis of direct evidence. The Pinnacle Peak Phyllite is considered part of the Kelp Bay Group because of the inferred presence of retrograde high-pressure metamorphic minerals, which then are used to correlate this unit with blueschist-bearing units associated with the Border Ranges Fault System in southcentral and southwest Alaska (unit Jsch). These blueschist units, however, are not generally considered part of the Chugach accretionary complex (units Kaf, Kafv, Ksg, Kumc, and K^m). The character of Pinnacle Peak Phyllite is unique to the Kelp Bay Group part of the accretionary complex; S.M. Karl (unpub. data) suggests that the unit is a phyllonite. While this may hold for the Pinnacle Peak Phyllite proper, which is exposed in a long linear belt; all coauthors do not agree on whether this is likely for the phyllite of Rodman Bay part of the map unit, which does not have a similar outcrop pattern

Source map information

Source map Karl, S.M., 2013, Unpublished data.
Symbol Trp
Unit name Pinnacle Peak Phyllite of the Kelp Bay Group
Description Dark gray to black carbonaceous phyllite alternates on a meter to decameter scale with subordinate light green phyllite, metagraywacke, metachert, and thin layers of black marble. Minerals in the dark gray phyllite include quartz, graphite, chlorite, calcite, white mica, and pyrite. Green phyllite and semischist contain quartz, albite, sericite, clinozoisite, chlorite, fuchsite, and actinolite. Locally the phyllite contains chlorite pseudomorphs of garnet porphyroblasts. The metamorphic mineral replacement textures indicate greenschist facies rocks underwent retrograde metamorphism during a high strain deformational event. Protoliths are inferred to be carbonaceous mudstone, graywacke, chert, and fine-grained mafic volcaniclastic rocks. Graphitic phyllites are locally calcareous and contrast with siliceous argillites of the Khaz Complex. The unit is bounded by faults and stratigraphic relations to other components of the Kelp Bay Group are not clear
Lithology Metamorphic

Correlated geologic units

Label Trpp
Description Pinnacle Peak phyllite and phyllite of Rodman Bay and related phyllite units of southeast Alaska
Geologic age Triassic
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Phyllite < Metaclastic < Metasedimentary < Metamorphic Greenschist Indeterminate, major