Chert of Interior Alaska

Unit symbol: Oc
Age range Ordovician (485.4 to 443.4 Ma)
Lithology: Sedimentary
Group name: Chert of Interior Alaska
Dark-gray and black banded chert, siliceous slate, argillite, and less abundant greenstone, and fine-grained, thin-bedded, impure limestone and dolostone. Beds separated by carbonaceous partings. Unit interpreted to be a deep-water sequence composed of turbiditic and hemipelagic deposits (Patton and others, 2009). Unit locally contains conodonts and graptolites of Early and Middle Ordovician age in the Medfra quadrangle (Patton and others, 2009) and in adjoining parts of the Mount McKinley quadrangle and has graptolites in the Livengood quadrangle occurrences (Weber and others, 1992). Includes the Livengood Dome Chert in the Livengood, Circle, and Tanana quadrangles (Foster and others, 1983; Weber and others, 1992; J.N. Dover, written commun., 1997), the Nilkoka Group in Fairbanks quadrangle (Péwé and others, 1966), and chert and slate in the Mount McKinley and Kantishna River quadrangles (unit Doc of Bela Csejtey, Jr., written commun., 1993; Chapman and others, 1975). Locally, also included is a distinctive marker bed of lime mudstone within the chert unit, locally characterized by coated ooids in the Tanana and Circle quadrangles (J.N. Dover, written commun., 1997). Thickness varies from 10 to several tens of meters, probably in part due to structural thickening and repetition. Unit exposed in two areas and assigned to three different terranes. The northern exposures of this unit are in the Circle, Livengood, and Tanana quadrangles between thrust splays associated with the Tintina Fault System and are typically considered part of the Livengood terrane (Dover, 1994). The southern exposures are in the Medfra, Ruby, Mount McKinley, and Kantishna quadrangles and are considered part of the Minchumina and Farewell terranes (Patton and others, 2009). Unit may correlate with part of the Road River Formation of eastern Alaska and Yukon. Unit is exposed near the Amy Creek unit of Weber and others (1992), included here in unit DPxkb

Source map information

Source map Reifenstuhl, R.R., Dover, J.H., Newberry, R.J., Clautice, K.H., Liss, S.A., Blodgett, R.B., and Weber, F.R., 1998, Geologic map of the Tanana A-1 and A-2, central Alaska: Alaska Division of Geological and Geophysical Surveys Public Data File 98-37a, 19 p., 1 sheet, scale 1:63,360.
Symbol Pzlo
Unit name Orum limestone
Description Light to medium gray, tan to reddish brown-weathering, extensively recrystallized, typically thin- to medium-bedded lime mudstone. Unit is locally thick-bedded, and locally includes ooid grainstones and cryptalgal(?) lamination. The platy character recognized in thinner-bedded intervals may be due to well developed axial planar fracture cleavage. This unit is a distinctive regional marker unit, which can be traced from the Tanana B-1 Quadrangle (Reifenstuhl and others, 1997a), southwest into the Tanana A-1 and A-2 quadrangles; it is also traceable west of the Stevens Creek fault, where it is exposed in both the Tanana A-2 and A-3 quadrangles. The Orum limestone type area was “the area of limestone exposures in the south wall of Orum Creek valley lying between 4½ and 5½ miles upstream from the confluence of Orum Creek and Stevens Creek” (Hopkins and Taber, unpublished manuscript). The exposures along Orum Creek are poor and inaccessible. Better, and more typical, exposures are in the Tanana B-1 Quadrangle (southern side of Baldry peak) and in the Tanana A-3 Quadrangle (NE¼ of Section 6, T.4N., R.17W.). The age of the unit is not known with certainty; it is here included with the Livengood Dome Chert assemblage, which is situated at least in part above the Orum limestone. Consequently its age is probably pre-Late Ordovician. No megafossils have been identified from the unit and no conodonts were recovered. Based on regional correlations with similar appearing units in Alaska, this informal unit may be as old as Neoproterozoic, or as young as Middle Ordovician
Lithology Sedimentary

Correlated geologic units

Label Oldl
Description Limestone marker bed within Livengood Dome Chert
Geologic age Ordovician
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major