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 Miller, M.L., and Bundtzen, T.K., 1994, Generalized geologic map of the Iditarod quadrangle, Alaska, showing potassium-argon, major-oxide, trace-element, fossil, paleocurrent, and archaeological sample localities: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219A, 48 p., 1 sheet, scale 1:250,000.
Symbol Ks
Unit name Sandstone and siltstone
Description Medium-grained to very fine grained sandstone, tuffaceous sandstone, and siltstone only exposed as colluvial chips in western part of Iditarod quadrangle.
Lithology Sedimentary

Correlated geologic units

Label Kme
Description Melozitna group, informal name; Shaktolik (sp) Group (abandoned name), graywacke, shale, grit, and conglomerate
Geologic age Albian to Turonian
Geologic setting Sedimentary, deltaic-and-nearshore
Lithology Form Importance
Shale < Mudstone < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Major
Conglomerate < Clastic < Sedimentary Bed Indeterminate, major
Bituminous < Coal < Sedimentary Bed Incidental