Kiska Harbor and Milky River Formations

Unit symbol: QTvs
Age range Quaternary? and late Tertiary (5.333 to 0.781 Ma)
Lithology: Igneous - Volcanic
Group name: Kiska Harbor and Milky River Formations
Kiska Harbor Formation, exposed in the Kiska quadrangle, is composed of subaerial lava flows, autoclastic breccia, pyroclastic rocks, water-laid pumiceous sand, and conglomerate (Coats and others, 1961). Flows are predominant in northern exposures and sedimentary (water-laid) beds to the south. The flows and sedimentary rocks interfinger; the flows thin southward and grade into autoclastic breccia, whereas sedimentary layers thin northward (Coats and others, 1961). The sedimentary rocks are composed entirely of volcanic debris, and generally are crossbedded. Depending on location, crossbeds are inclined to the east and southeast or south and southwest (Coats and others, 1961). On Little Kiska Island, the Kiska Harbor Formation unconformably overlies steeply dipping beds of the Vega Bay Formation (unit Tvu here). Coats and others (1961, p. 573) noted that the Kiska Harbor Formation resembles the rocks of dissected composite volcanoes on the north end of Tanaga and Kanaga Islands that contain fossils of Pliocene age. Panuska (1981) reported K/Ar whole rock ages between 3.30±0.10 and 5.50±0.7 Ma. The Milky River Formation of the Alaska Peninsula is of variable thickness (but about 600 m at thickest) and consists of volcanogenic, nonmarine sedimentary rocks and interlayered flows and sills; volcanic rocks are thicker and more abundant stratigraphically upward in unit. Lower part of unit consists nearly entirely of coarse, highly crossbedded and channeled, fluvial volcaniclastic sandstone and cobble-boulder conglomerate. Rocks are poorly indurated, dark-brown to gray, and have clasts composed almost entirely of volcanic lithologies. Upper part of unit contains numerous porphyritic andesite flows, lahar deposits, and tuff beds interlayered with sedimentary rocks. Volcanic rock clasts in lahar deposits are as large as 2 m. A flow unit near top of the stratigraphic section in type locality of the Milky River Formation was dated at 3.53±0.27 Ma (K/Ar, whole rock, Wilson and others, 1981). Another flow unit in lower part of the section exposed in the Port Moller D-1 1:63,360-scale quadrangle has been dated at 3.87±0.06 Ma (K/Ar, plagioclase, Wilson, 1989). Unit unconformably overlies the Bear Lake Formation (Tms) and conformably underlies Pliocene(?) and Quaternary volcanic flows (QTv). Included in unit in area northwest of Pavlof group of volcanoes are rocks mapped as agglomerate of Cathedral Valley by Kennedy and Waldron (1955), which they describe as a thick sequence of agglomerate beds and subordinate tuff beds and lava flows that are well exposed. These volcanic rocks crop out northwest of Mount Dutton and Pavlof volcanoes. According to Kennedy and Waldron (1955), the unit in that area is predominantly basalt and basaltic andesite and dips north toward the Bering Sea. Kennedy and Waldron (1955) suggested that the agglomerate of Cathedral Valley flows are probably comparable in age to the Belkofski Tuff, later renamed the Belkofski Formation by Burk (1965), which is included in unit Tuu here; we suggest here that a better lithologic and stratigraphic correlation is with Milky River Formation and, thus, map these rocks as such

Source map information

Source map Wilson, F.H., Weber, F.R., Dochat, T.M., Miller, T.P., and Detterman, R.L., 1997, Revised geologic map of the Cold Bay and False Pass quadrangles, Alaska Peninsula: U.S. Geological Survey Open-File Report 97-866, 34 p., 1 sheet, scale 1:250,000.
Symbol Tvs
Unit name Volcanic sedimentary rocks and agglomerate
Description Unit is of variable thickness and consists of volcanogenic, nonmarine sedimentary rocks and interlayered agglomerate, flows and sills. Included in this unit in the area northwest of the Emmons Lake eruptive center are rocks mapped as the Agglomerate of Cathedral Valley by Kennedy and Waldron (1955), a thick sequence of agglomerate beds and subordinate tuff beds and lava flows that is well exposed in Cathedral Valley
Lithology Igneous

Correlated geologic units

Label Tmr
Description Milky River Formation
Geologic age Pliocene
Geologic setting Sedimentary, continental
Lithology Form Importance
Sandstone < Clastic < Sedimentary Bed Major
Volcanic < Igneous Volcaniclastic, lahar Indeterminate, major
Volcanic < Igneous Pyroclastic Indeterminate, major
Andesite < Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major