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 OZl
Unit name Limestone and dolomitic limestone, York terrane
Description Light-gray- to grayish-orange-weathering, medium-light-gray to medium-dark-gray limestone and dolomitic limestone that occurs widely in the western, central, and northern parts of the Teller quadrangle. The unit forms low outcrops and extensive areas of rubble and is locally intruded by gabbro (|gb of this map; Sainsbury, 1972). Most exposures of OZl are fault-bounded, but in the western Teller quadrangle, it may depositionally overlie unit O<t (Sainsbury, 1969b, 1972). 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 in O<l; quartz silt to very fine sand makes up as much as 20 percent of some samples. Minor to trace amounts of chert, zircon, pyroxene, and antigorite(?) also occur in silty lime mudstone, suggesting that mafic rocks contributed detritus to this unit (Sainsbury, 1969b). Grainstone, composed mainly of intraclasts, peloids and rare bioclasts, is a notable subordinate lithology. O<l contains rocks of more than one age. Sainsbury (1972) considered much of this unit to be Precambrian, but subsequent paleontological studies indicated that at least some strata are Ordovician. OZl has produced dated fossil assemblages (mostly conodonts) at >10 localities (Table A-1; T. Carr and T. Hudson, written commun., 1982; Vandervoort, 1985; Till and Dumoulin, 1994; Toro and others, 2006). A few collections consist only of poorly preserved conodonts, chitinozoans, or acrotretid brachiopods that merely indicate broad, mainly Paleozoic age ranges, but several conodont assemblages are tightly dated as early-middle Early Ordovician (Table A-1) and correlate well with faunas from units Oal and Ol of this map. Three other collections are early Middle Ordovician (Table A-1), slightly younger than any definitively dated faunas in Oal and Ol but coeval with parts of unit Ols in the York terrane and unit Od in the Nome Complex. The gabbro that intruded OZl, however, is earliest Cambrian (Pzgb of this map; Amato and others, 2009), which constrains the age of at least part of the unit to Early Cambrian or older. Metamorphic grade and degree of deformation vary within OZl. Some samples appear unmetamorphosed and undeformed in thin section, whereas others contain layers rich in metamorphic white mica, are recrystallized, and have a phyllitic or schistose fabric. CAI values of conodont collections from this unit are mostly 4-4.5, but are 5 and 6 at several localities (Table A-1); these values indicate that host rocks reached temperatures of 190 to >360¦C (Table A-2). A relationship between variations in CAI, metamorphic textures, and structural level has not been documented. Lithofacies and biofacies of Ordovician strata in OZl match particularly well with those of unit Oal. OZl also correlates with parts of Ol, the Baird Group (Tailleur and others, 1967; Dumoulin and Harris, 1994) in the western Brooks Range (map unit "DOb" of Till and others, 2008b), and the Novi Mountain Formation, lower Telsitna Formation, and related rocks in the Farewell terrane (Dumoulin and Harris, 1994; Dumoulin and others, 2002). Equivalent to "pOt" and "pOl" of Sainsbury (1969a), "pOal", "pOa", "pOl", and "pOlu" of Sainsbury (1969b), and "OpCl", "pCt", "pCld", and part of "pCl" of Sainsbury (1972)
Lithology Sedimentary

Correlated geologic units

Label OZl
Description Limestone and dolomitic limestone
Geologic age Neoproterozoic to Dapingian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major