Redwood and Poul Creek Formations

Unit symbol: Top
Age range Tertiary, Miocene to Eocene? (38 to 3.6 Ma)
Lithology: Sedimentary
Group name: Redwood and Poul Creek Formations
These units, exposed in northeast Prince William Sound, are a generally coarsening-upward sequence of marine deposits. The stratigraphically higher Redwood Formation has two members: the upper Puffy Member that contains as much as 50 percent conglomerate, conglomeratic mudstone, and sandstone in addition to siltstone, mudstone, and claystone, and the lower unnamed member that consists of thick-bedded sandstone and silty sandstone and siltstone (Winkler and Plafker, 1981). The underlying Poul Creek Formation consists of about 1,600 m of concretionary, glauconitic, dark-gray to greenish-gray siltstone, claystone, and sandstone and subordinate dark-brown, laminated shale rich in organic material, silty shale, and gray calcareous sandstone; it locally includes thin interbeds of basaltic tuff (Miller, 1975; Winkler and Plafker, 1993). Siltstone, mudstone, and claystone of the Redwood Formation are similar in appearance to parts of the underlying Poul Creek Formation but contain few or no concretions, no glauconitic or volcanic beds, and are sandier and more resistant to erosion (Winkler and Plafker, 1993). Fossil control from mollusks suggests an age range of late Eocene through the Oligocene for both units (Addicott and others, 1978), whereas foraminifera indicate younger ages and that the Redwood Formation may extend into the late Miocene (Rau and others, 1977 cited in Winkler and Plafker, 1981; Rau and others, 1983 cited in Winkler and Plafker, 1993). Deposition was in cold to cool waters, below wave base at neritic or bathyal depths (Plafker and others, 1994). Intercalated basaltic fragmental rocks and less common pillow basalt indicate episodic submarine mafic volcanism in the basin during deposition of the Poul Creek Formation (Plafker, 1974)

Source map information

Source map Winkler, G.R., and Plafker, George, 1993, Geologic map of the Cordova and Middleton Island quadrangles, southern Alaska: U.S. Geological Survey Miscellaneous Investigations Map I-1984, 1 sheet, scale 1:250,000.
Symbol Tr
Unit name Redwood Formation
Description Lower sandstone member: about two-thirds thick-bedded sandstone and one-third silty sandstone and siltstone. Upper member: about one-half siltstone, mudstone and claystone, the rest is conglomerate and sandstone.
Lithology Sedimentary

Correlated geologic units

Label Trf
Description Redwood Formation
Geologic age Late-Oligocene to Early-Pliocene
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Claystone < Mudstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Minor