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 OZt
Unit name Sandstone, siltstone, and limestone, York terrane
Description Gray- to orange-weathering, gray to brown, locally calcareous sandstone to siltstone, interbedded with gray to black mudstone and local limestone, exposed in the western Teller quadrangle. Beds are typically ?5 cm thick (20 cm max). Outcrop features, which include climbing ripples, cross beds, parallel and convolute laminae, and graded beds, suggest a turbidite origin for these rocks. Sandstone is very fine to coarse grained; some beds contain black shale fragments 1 to 2 mm long. Sand and silt clasts are angular to subrounded and consist of quartz, feldspar, sedimentary and metamorphic lithic grains, mica, chlorite, and minor tourmaline and zircon. Limestone is fine grained, ferroan, and contains as much as 10 percent silt- to medium-sand-size grains of quartz and feldspar. In thin section, samples of OZt appear recrystallized but only mildly deformed. Sainsbury (1969b, 1972) suggested that O<t depositionally underlies unit OZl; its western contact, with Ml, is a fault. OZt likely includes rocks of more than one age. The unit was intruded by gabbro (|gb of this map; Sainsbury, 1972) that at one locality in the northeastern part of the outcrop belt of O<t has been dated as earliest Cambrian (Amato and others, 2009). However, southern exposures of OZt produced a single conodont of middle Early-Late Ordovician age at one site (Table A-1; Till and Dumoulin, 1994) and conodont fragments of indeterminate Cambrian-Triassic age at two other sites (Table A-1; T. Carr and T. Hudson, written commun., 1982). A detrital zircon sample of sandstone from eastern exposures of O<t has a major peak in cumulative probability distribution at 550 Ma and several other notable peaks between 640 and 720 Ma (J. Toro, written commun., 2006, 2007). The 550 Ma peak may include zircons derived from the gabbros (|gb) or their volcanic equivalents. OZt, therefore, is in part Proterozoic (pre-intrusion of gabbros), and in part Ordovician or younger (based on fossil data). OZt could correlate, at least in part, with Middle Ordovician-Lower Silurian graptolitic argillite and turbidites (Iviagik Group of Martin, 1970) exposed south of Cape Lisburne (Grantz and others, 1983; Moore and others, 1994; Harris and others, 1995). Metasandstone in the Iviagik has a range of compositions (Dumoulin, 2001) that are broadly similar to those observed in OZt but it differs in containing more chert, including grains of radiolarian chert not seen in OZt. Equivalent to parts of "pOs" of Sainsbury (1969b) and "pCs" of Sainsbury (1972)
Lithology Sedimentary

Correlated geologic units

Label OZt
Description Sandstone, siltstone, and limestone, York terrane (OZt, TE004; SEWPEN)
Geologic age Neoproterozoic to Ordovician
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Limestone < Carbonate < Sedimentary Bed Incidental