Older rocks of York terrane and Grantley Harbor Fault zone

Unit symbol: OPxpt
Age range Middle Ordovician to Proterozoic (635 to 458.4 Ma)
Lithology: Sedimentary
Group name: Older rocks of York terrane and Grantley Harbor Fault zone
Limestone and dolomitic limestone, sandstone, siltstone, and phyllite and low-grade metasiltstone and metalimestone that were collectively called “units of uncertain affinities” by Till and others (2011). The older rocks of the Grantley Harbor Fault zone of Till and others (2011) are also included here. Limestone, which forms the majority of the exposed area of the unit, is light-gray- to grayish-orange-weathering, medium-light-gray to medium-dark-gray limestone and dolomitic limestone widely exposed in the Teller quadrangle. “Beds are even to irregular and mostly 2 to 30 cm thick. Much of the unit is parallel laminated, but crossbedding occurs locally and some intervals are bioturbated. Other sedimentary features include fenestral fabric and intraclast conglomerate with clasts as much as 5 cm long. Lime mudstone, in part dolomitic and (or) argillaceous, is the main lithology” (Till and others, 2011). Silt to very fine sand-sized quartz grains are a minor component locally. Metamorphic grade and degree of deformation vary from apparently nonmetamorphosed and undeformed to recrystallized areas that are rich in metamorphic white mica, and are phyllitic or schistose. Till and others (2011) also report CAI values for conodonts that are mostly 4–4.5, but are locally 5 and 6, indicating that temperatures of 190 °C to more than 360 °C were reached. Most exposures of the carbonate rocks are fault-bounded, but Sainsbury (1972) reported that the carbonate rocks transitionally overlie sandstone, siltstone, and phyllite. The underlying clastic unit consists of gray- to orange-weathering, gray to brown, very fine- to coarse-grained, locally calcareous sandstone to siltstone, interbedded with gray to black mudstone and, locally, limestone. Beds are typically less than about 5 cm thick but may be as thick as 20 cm. Climbing ripples, cross beds, parallel and convolute laminae, and graded bedding suggest a turbidite origin (Till and others, 2011). Gray-weathering, sericite-rich pelitic phyllite and brown-weathering calcareous phyllite is nonfossiliferous (Till and others, 2011) and is intruded by gabbro (<gb) that crystallized at about 540 Ma (Amato and others, 2009). Rocks of the Grantley Harbor Fault zone consist of metasiltstone, metasandstone, and phyllite that may be a more deformed and metamorphosed equivalent of the clastic part of this unit (Till and others, 2011). Additionally, thinly layered to laminated, orange- and gray-weathering, color-banded, white to dark-gray metalimestone may be a more metamorphosed equivalent of the carbonate rocks part of this unit. Fossil collections, mostly conodonts, indicate that multiple ages are present (Till and others, 2011). The best controlled conodont assemblages are tightly dated as early-middle Early Ordovician and other collections are early Middle Ordovician (Till and others, 2011). The intruding gabbro (<gb) is earliest Cambrian, which constrains the age of at least part of the unit to Proterozoic. The clastic rocks, also intruded by gabbro in northeastern exposures, have also produced a single conodont of middle Early to Late Ordovician age at one site and conodont fragments of indeterminate Cambrian-Triassic age at two other sites at southern exposures (Till and others, 2011). A detrital zircon sample of sandstone has a major peak in its cumulative probability distribution that is similar to the age of the gabbro at 550 Ma; there are also a large number of zircons of varying Proterozoic age and a few Archean grains (Amato and others, 2009)

Source map information

Source map Till, A.B., Dumoulin, J.A., Werdon, M.B., and Bleick, H.A., 2011, Bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Scientific Investigations Map 3131, 2 sheets, scale 1:500,000, 1 pamphlet, 75 p., and database, available at http://pubs.usgs.gov/sim/3131/.
Symbol OZp
Unit name Phyllite, York terrane
Description Fine-grained, mainly pelitic and (or) calcareous phyllite exposed in the west-central part of the Teller quadrangle. Outcrops occur on creeks, weather gray to grayish-brown, and are layered and locally finely laminated. The phyllitic foliation surface is crenulated. Gray-weathering material is sericite-rich; brown-weathering rocks are calcareous. In thin section, grain size, composition, and oriented lenses of sericite and opaques define foliation. Thin sericitic layers alternate with layers that contain interlocking crystals of calcite that are matrix to disseminated rounded to subrounded plagioclase and quartz grains. Locally, carbonate grains are stained with iron oxide. No fossils have been found in this unit, but it was intruded by gabbro (|gb of this map) that crystallized in the earliest Cambrian (Amato and others, 2009). PzZt may be a more deformed and metamorphosed correlative of OZt. Equivalent to parts of "pOs" of Sainsbury (1969a, b) and "pCs" of Sainsbury (1972)
Lithology Metamorphic

Correlated geologic units

Label OZp
Description Slate of the York region, slightly to moderately metamorphosed graphitic siltite, slate, graywacke, and calcareous siltite
Geologic age Neoproterozoic to Ordovician
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Phyllite < Metaclastic < Metasedimentary < Metamorphic Greenschist Major