Alapah Limestone

Unit symbol: Mlga
Age range Upper Mississippian (330.9 to 323.2 Ma)
Lithology: Sedimentary
Group name: Wahoo, Alapah, and Wachsmuth Limestones (Lisburne Group)
As described by Reiser and others (1971), unit is gray to dark-gray-weathering bioclastic limestone and dolostone and minor black chert. Robinson and others (1989) subdivided unit into two informal parts in the Sadlerochit and Shublik Mountains. The upper member is light- to medium-gray, thin- to medium-bedded limestone and lime mudstone that weathers into small, buff-colored, shard-like irregular pieces. Formation is poorly exposed in Sadlerochit Mountains where it forms distinctive talus aprons below the Wahoo Limestone. In the Shublik Mountains, the upper Alapah is thicker and consists of dark-gray to medium-gray interbedded lime mudstone. At the top of the Alapah is a massive bed of yellow-brown-weathering limestone. Upper unit ranges between 105 and 190 m thick. Lower unit is medium-light-gray to gray and tan, thin- to massive-bedded limestone that consists predominantly of pelletoidal packstone and grainstone that is a distinctive cliff-forming unit below the upper Alapah Limestone. Bedding ranges from less than 1 m to more than 10 m. Locally, massive-bedded pelletoidal grainstone contains large-scale foreset crossbeds that are capped by ferruginous interbeds of hematite-stained sand and shale. Dark-gray to green and red shale is also present locally. Lower unit ranges from 35 to 155 m thick (Robinson and others, 1989) and is mapped only in Mount Michelson quadrangle

Source map information

Source map Reiser, H.N., Brosge, W.P., Dutro, J.T., Jr., and Detterman, R.L., 1971, Preliminary geologic map, Mt. Michelson quadrangle, Alaska: U.S. Geological Survey Open-File Report 71-237, 2 sheets, scale 1:200,000.
Symbol Ma
Unit name Alapah Limestone
Description Bioclastic limestone; dolomite; black chert. Weathers gray to dark-gray. To south lower part is shaly and basal contact is obscure.
Lithology Sedimentary

Correlated geologic units

Label IPMla
Description Lisburne Group, Alapah Limestone
Geologic age Late-Mississippian
Geologic setting Sedimentary, carbonate
Lithology Form Importance
Grainstone < Limestone < Carbonate < Sedimentary Bed Major
Lime-mudstone < Limestone < Carbonate < Sedimentary Bed Major
Chert < Chemical < Sedimentary Bed Minor