Terra Cotta Mountains Sandstone

Unit symbol: Stc
Age range Silurian (443.4 to 419.2 Ma)
Lithology: Sedimentary
Group name: Farewell basinal facies clastic rocks
Mainly composed of sandstone and mudstone and divided into three prominent limestone members. Best described by Churkin and Carter (1996), the Terra Cotta Mountains Sandstone is distinguished from other formations in the area by abundant rhythmically interbedded sandstone and mudstone; more than three-quarters of its total thickness consists of calcareous sandstone interbedded with mudstone. In addition, the formation contains three mappable limestone members (although they are not distinguished here). Stratigraphically lowest part of unit is approximately 90 to 120 m of thick-bedded and massive sandstone that has subordinate argillite or mudstone interbeds. Overlying this part of the unit is a medium-dark-gray, thick-bedded, and relatively pure limestone about 45 to 60 m thick; locally has very pale brown mottling. Most of the limestone is massive or blocky, but its basal part is laminated and platy. No megafossils have been found in it. A second sandstone interval, 150 m thick, is interbedded with cross-laminated mudstone and argillic shale and is overlain by the middle limestone member, a dark-gray, generally thin-bedded, platy, laminated limestone usually about 35 m thick, but as much 60 m thick. The laminations in the limestone are accentuated by silt, mica, and argillaceous impurities. In contrast to other limestone of the Terra Cotta Mountains Sandstone, the middle limestone member contains shelly fossils at several localities. These fossils include pelecypods and straight-shelled cephalopods that are so poorly preserved that they have not been identified by genus or age (Churkin and Carter, 1996). Churkin and Carter (1996) indicate that because of structural complications, the middle limestone member may be a repetition of the lower limestone member. Overlying the middle limestone is another interval of sandstone, siltstone and argillite as thick as 90 m that contains, near its base, a distinctive knobby limestone section 7.5 to 15 m thick (Churkin and Carter, 1996). The third, or upper, limestone member is medium-dark-gray, weathers medium-gray, is generally thick-bedded and massive, and is as thick as 120 m. Locally, where thinner bedded and evenly laminated, the limestone is slabby weathering. This upper limestone member contains dome-shaped structures that may represent stromatolites. Otherwise no fossils were noted (Churkin and Carter, 1996). Stratigraphically highest is a 90- to 150-m-thick calcareous siltstone and sandstone interval that appears to be a finer grained equivalent of the basal, more sandstone-rich lowest interval. Shale beds with phyllitic partings are common, as are slabby weathering argillaceous and silty limestone beds. Sedimentary structures such as graded bedding, flute casts, and cross-lamination indicate that the Terra Cotta Mountains Sandstone was deposited by turbidity currents in a deep marine setting (Churkin and Carter, 1996; Wilson and others, 1998)

Source map information

Source map Bundtzen, T.K., Harris, E.E., and Gilbert, W.G., 1997, Geologic map of the eastern half of the McGrath quadrangle, Alaska: Alaska Division of Geological and Geophysical Surveys Report of Investigations 97-14a, 34 p., 1 sheet, scale 1:125,000.
Symbol mSs
Unit name Terra Cotta Mountains Sandstone; feldspathic-lithic sandstone, limey siltstone, and argillite
Description Medium olive gray to terra cotta, medium-to coarse-grained, thin-bedded to massive, calcareous lithic sandstone, and siltstone containing local gray shale intervals and minor pebble conglomerate beds. Sandstones have well developed oscillation ripple marks, flute clasts, and planer crossbedding. Limey beds of lower part of the unit are thought to be equivalent of Lower Limestone member of Churkin and Carter (1996). Dillinger Subterrane
Lithology Sedimentary

Correlated geologic units

Label Stc
Description Terra Cotta Mountains sandstone, sandstone and graptolitic shale
Geologic age Silurian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major
Limestone < Carbonate < Sedimentary Bed Minor