Sandstone, shale, and conglomerate deltaic deposits

Unit symbol: Ksd
Age range Upper and upper Lower Cretaceous (113 to 66 Ma)
Lithology: Sedimentary
Group name: Nonmarine to shelf sedimentary rocks
Nonmarine and delta-plain deposits that grade downward into marine delta-front deposits in east- and west-central and southwestern Alaska. In west-central Alaska, unit includes polymictic conglomerate of the Yukon-Koyukuk Basin (Patton and others, 2009; in press [Scientific Investigations Report 3015]. Fluvial deposits are characterized by fine-grained, locally cross-bedded quartzose sandstone interbedded with micaceous shale and siltstone. Near the base, the fluvial beds are composed of fine- to coarse-grained, lenticular, crossbedded, friable sandstone and conglomerate that contain pebble- to grit-size clasts of quartz and chert and lesser amounts of mafic intrusive and extrusive rocks, and schist clasts. Contains bituminous coal seams as thick as 90 cm. Sandstone and conglomerate clasts suggest a mixed metamorphic and granitic provenance and, in combination with limited paleocurrent data, indicate that the unit prograded westward from a source area in the Ruby terrane bordering the southeastern margin of the Yukon-Koyukuk Basin. Fluvial deposits contain abundant fresh- and brackish-water mollusks and well-preserved plant remains of Late Cretaceous (Cenomanian to Turonian?) age. Shallow marine deposits contain an abundant molluscan fauna of late Early (Albian) and early Late (Cenomanian) Cretaceous age. Unit poorly exposed along the eastern and western margins of the Yukon-Koyukuk Basin. On the western margin of the basin, exposures of this unit are confined to a few small isolated outcrops on the coastal plain in the northern Kwiguk and the southeastern Saint Michael quadrangles. In east-central Alaska, unit includes the sedimentary rocks of the Cantwell Formation (see unit Tpcv for the volcanic rock unit of the Cantwell Formation) as well as continental sedimentary rocks of Turonian to Albian ages in the McCarthy region. Unit consists of a fluvial, intercalated sequence, containing various proportions of conglomerate that is dominantly polymictic, sandstone (including arkose), siltstone, argillite, shale, and a few thin coal beds (Csejtey and others, 1992). Thin volcanic flows and thin tuff layers are present locally. Conglomerate clast lithology varies greatly within the outcrop area, indicating different geologic source areas and deposition by several river systems. Ridgeway and others’ (1994) recent palynological analyses suggest an early Campanian to late Maastrichtian age and they report a maximum thickness of 4,000 m for the Cantwell. As mapped here, unit Ksd also includes other nonmarine sedimentary rocks that crop out in east-central and southern Alaska. These chiefly consist of medium- to dark-gray phyllitic shale, sandstone, grit, and conglomerate containing minor carbonaceous shale and tuffaceous sandstone. Locally subdivided into unit TKis

Source map information

Source map Csejtey, Bela, Jr., Mullen, M.W., Cox, D.P., and Stricker, G.D., 1992, Geology and geochronology of the Healy quadrangle, south-central Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-1961, 63 p., 2 sheets, scale 1:250,000.
Symbol Tcs
Unit name Cantwell Formation -- sedimentary rocks subunit
Description Fluviatile, intercalated sequence, in various proportions, of dominantly polymictic conglomerate, sandstone (including arkose), siltstone, argillite, shale, and a few thin coal beds. Locally contains thin flows and related tuff layers of mafic to intermediate composition. Conglomerate clast are of locally variable composition, indicating derivation from a number geologically different source areas
Lithology Sedimentary

Correlated geologic units

Label Kcs
Description Cantwell Formation, sedimentary rocks subunit
Geologic age Campanian to Maastrichtian
Geologic setting Sedimentary, continental
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Mafic-volcanic < Volcanic < Igneous Flow Indeterminate, major
Mafic-volcanic < Volcanic < Igneous Pyroclastic, tuff Indeterminate, major
Argillite < Metaclastic < Metasedimentary < Metamorphic Indeterminate, major
Arkose < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major
Shale < Mudstone < Clastic < Sedimentary Carbonaceous Minor
Sandstone < Clastic < Sedimentary Tuffaceous Minor
Coal < Sedimentary Bed Incidental