Lime mudstone

Unit symbol: Ols
Age range Ordovician (485.4 to 443.4 Ma)
Lithology: Sedimentary
Group name: Farewell platform facies
Limestone and lime mudstone exposed in the northeast Taylor Mountains and southeast Sleetmute quadrangles. Unit consists of distinctive limestone lithologies, boundstone and packstone to wackestone, interspersed with the mudstone: the stratigraphically lowest part of the unit is thin- to medium-bedded, dark-gray, yellow-gray-weathering, burrow-mottled lime mudstone and lesser peloidal mudstone. Age control based on poorly preserved low-spired gastropods and conodonts including Drepanoistodus? sp., Fryxellodontis? n. sp. and other conodonts. This is apparently overlain by medium- to thick-bedded, dark-gray to brown algal-thrombolitic boundstone interbedded with light-gray-weathering, thin- to medium-bedded lime mudstone (Blodgett and Wilson, 2001). Trilobites and conodonts (identified by N.M. Savage, written commun., 1985), including fossils of the genus Hystricurus, indicate an Early Ordovician age for this boundstone Overlying this is an extremely poorly exposed brown and dark gray 'chippy' shale, silty shale and minor silicified limestone exposed in very small areas in the Taylor Mountains quadrangle, where exposures consist only of frost-boil chips of shale and rubbly outcrop; a better exposed area of rubble contains numerous diplograptid graptolites. The uppermost part of unit Ols consists of brown, medium- to thick-bedded skeletal lime packstone to wackestone (Blodgett and Wilson, 2001), which contains abundant pentameroid brachiopods (Tcherskidium, Proconchidium [or Eoconchidium] and a new smooth genus aff. Tcherskidium). Karl and others (2011) provides a comprehensive database of fossil data for the Taylor Mountains quadrangle and adjacent areas to the southwest

Source map information

Source map Blodgett, R.B., Wilson, F.H., and Shew, Nora, 2012, Unpublished data.
Symbol Oab
Unit name Ordovician algal boundstone, Nixon Fork Subterrane Facies Rocks
Description This unit consists of medium- to thick-bedded, dark-gray to brown, algal thrombolites (boundstone) interbedded with light-gray weathering thin- to medium-bedded lime mudstone. The boundstone is comprised of spongiostromate algal buildups that are typically darker in color, thinner bedded, and having a differing accessory algal and faunal component than associated with of the overlying Devonian algal boundstone (Dab) unit. The abundance of algal thrombolite mound buildups and presence of open-marine trilobite fauna suggest an outer platform margin depositional environment for this unit. No age definitive megafossils were recovered from this unit in the Sleetmute A-2 quadrangle, but several localities yielded trilobites (large, smooth trilobite carapaces, probably bumastids) just to the immediate west in the Sleetmute A-3 quadrangle. Conodonts from uppermost platy limestone beds of the COls unit immediately below the Oab unit in the Sleetmute A-3 quadrangle were identified by N. M. Savage as indicating a Middle Ordovician age. On this basis, the Oab unit in the Sleetmute A-2 quadrangle is considered to be of Middle and/or Late Ordovician age. Only one fossil locality (locality 23) is known from this unit in the Sleetmute A-2 quadrangle, which consists only undetermined trilobite exuvae. Minimum thickness of the unit is probably at least 50-100 m; however, the unit appears to be markedly thicker in its more southerly exposures situated immediately to the south the major anticlinorium axis. The unit appears to be lithologically equivalent to the Oab unit recognized to the southwest in the Taylor Mountains D-1 quadrangle (Blodgett and Wilson, 2001). However, the unit there is of Early Ordovician age, thus, it would appear that the reef-rimmed carbonate margin represented by the unit was time-transgressive in a northerly direction. This unit rests conformably above the COls unit and the nature of the contact appears to be transitional, with reef debris appearing commonly in uppermost beds of the underlying unit, rapidly grading upward into the algal boundstone unit proper
Lithology Sedimentary

Correlated geologic units

Label Oab
Description Algal boundstone and lime mudstone
Geologic age Ordovician to Early-Ordovician
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Limestone < Carbonate < Sedimentary Bed Major
Mudstone < Clastic < Sedimentary Calcareous Minor