Matanuska Formation and correlative rocks

Unit symbol: Km
Age range Upper Cretaceous to upper Lower Cretaceous (113 to 66 Ma)
Lithology: Sedimentary
Group name: Shallow to moderate depth sedimentary rocks
Matanuska Formation is well-indurated, thinly bedded, dark-gray, fossiliferous marine shale deposited at shallow to moderate depths thatcontains conspicuous calcareous concretions, volcanic-lithic siltstone, sandstone, graywacke, and subordinate conglomerate (Winkler, 1992). According to Martin (1926), unit is at least 1,250 m thick. Grantz (1964) reported results of a stratigraphic reconnaissance of the Matanuska Formation and found it to include several mappable lithologic units and suggested that when these units are defined as formations, the Matanuska Formation merits elevation to group rank. Winkler (1992) described unit as diverse shallow to deep marine (in part, turbiditic) deposits derived from a northern source, either from an unidentified mid-Cretaceous magmatic arc or from the Jurassic arc represented in part by the Talkeetna Formation. Upper part of unit is coeval with the flysch of the Valdez Group to the south. Unit overlies and is separated from Early Cretaceous and older strata by a pronounced angular unconformity (Csejtey and others, 1978) and contains locally abundant marine fossils. In Wrangell Mountains, unit consists of Upper Cretaceous sedimentary rocks as defined by MacKevett (1978), which are locally subdivided into 1) Moonshine Creek Formation, siltstone and sandstone, with minor conglomerate; 2) Schulze Formation, porcellanite, with minor sandstone and conglomerate; 3) Chititu Formation, mudstone and shale, subordinate porcellanite, sandstone, and impure limestone; and 4) MacColl Ridge Formation, coarse sandstone and minor granule and pebble conglomerate. These correlate to part of the Matanuska Formation in the Talkeetna Mountains and Matanuska Valley

Source map information

Source map Winkler, G.R., Silberman, M.L., Grantz, Arthur, Miller, R.J., and MacKevett, E.M., Jr., 1981, Geologic map and summary geochronology of the Valdez quadrangle, southern Alaska: U.S. Geological Survey Open-File Report 80-892-A, 2 sheets, scale 1:250,000. and modifications by Winkler, 1997).
Symbol Kmu
Unit name Matanuska Formation -- upper part
Description A very thick sequence of marine siltstone, claystone, and sandstone in interbedded with lenticular and channelized deposits of sandstone and conglomerate. Contact with lower part of unit presumed to an unconformity. Deposition thought to occur on the fringes of a deep-sea fan. Thinly interbedded siltstone, claystone, and sandstone, which predominate, represent deposition in interchannel areas; whereas thick beds and lenses of sandstone and conglomerate represent distributary channels.
Lithology Sedimentary

Correlated geologic units

Label Km
Description Matanuska Formation, fossiliferous marine shale, late Early Cretaceous to Late Cretaceous
Geologic age Albian to Maastrichtian
Geologic setting Sedimentary, marine, shallow to deep association, undivided
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Graywacke < Sandstone < Clastic < Sedimentary Bed Indeterminate, major