Conglomerate, sandstone, and lignite

Unit symbol: TKs
Age range lower Tertiary to Upper Cretaceous (83.6 to 33.9 Ma)
Lithology: Sedimentary
Group name: Conglomerate, sandstone, and lignite
Sandstone, mudstone, thin coal seems, and conglomerate exposed in the Charley River and Rampart areas. In the Charley River area, the unit consists of poorly consolidated sandstone, grit, pebble-to cobble-conglomerate, and carbonaceous mudstone with coal seams and occurs just east of the Kandik basin and along the Tintina Fault Zone. Late Cretaceous, Paleocene, and Eocene pollen have been recovered from exposures within and immediately south of the map area (Miyaoka, 1990) and from shallow core holes (unpublished oil industry data, 2002, cited in Till and others, 2006a). In the western Rampart area, along the Victoria Creek Fault Zone, quartz- and chert-rich fluvial conglomerate, sandstone, and mudstone are typical, and palynoflora of probable Maastrichtian age (Farmer and others, 2003) as well as of early Tertiary age (Chapman and others, 1982) have been collected. Younger part of the unit likely correlative to rocks of unit Tsu, sedimentary rocks, undivided. Rocks of this unit also occur in a 25-m-thick section along the Sethkokna River in the northeast Medfra quadrangle (Patton and others, 1980). There, the conglomerate contains clasts of quartz, chert, felsic volcanic rocks, and talc schist, which Patton and others (1980) provisionally assigned a latest Cretaceous age (Campanian-Maastrichtian) on the basis of pollen in the lignite beds. Unit is overlain by rhyolite and dacite flows. On the Seward Peninsula, Till and others (2011) mapped two separate sedimentary sequences as their unit TKs. Here, only the more northern Late Cretaceous to Tertiary rocks are included; the older middle-Cretaceous rocks are assigned to map unit Kcc of this map. The rocks included here from the unit TKs of Till and others (2011) consist of gray and brown siltstone, mudstone, sandstone, coal, and minor conglomerate poorly exposed in narrow slices along the Kugruk Fault Zone in the northeastern and southeastern Bendeleben quadrangle; it has been explored for coal and uranium (Retherford and others, 1986; Dickinson and others, 1987). Pollen assemblages of Late Cretaceous and Tertiary (Eocene to early Miocene?) ages have been found in finer grained parts of the sequence (Till and others, 1986; Haga, in Retherford and others, 1986)

Source map information

Source map Till, A.B., Dumoulin, J.A., Werdon, M.B., and Bleick, H.A., 2010, Preliminary bedrock geologic map of the Seward Peninsula, Alaska, and accompanying conodont data: U.S. Geological Survey Open-File Report 2009-1254, 1 pamphlet, 57 p., 2 plates, scale 1:500,000, and database.
Symbol TKs
Unit name Carbonate-rich conglomerate and sandstone; mudstone, siltstone and coal, Kugruk Fault zone
Description Two separate sedimentary sequences, of broadly different age: tan to light-gray siltstone, sandstone, and pebbly sandstone, and light-gray-weathering conglomerate composed mostly of marble, metalimestone, and dolostone clasts, probably of mid-Cretaceous age; and a separate sequence of gray and brown siltstone, mudstone, sandstone, coal, and minor conglomerate of Late Cretaceous and Tertiary age. Both sequences are exposed in narrow slices along the Kugruk fault zone. The mid-Cretaceous carbonate-rich sandstone and siltstone occur in the southeastern and east-central parts of the Bendeleben quadrangle; conglomerate is found in these areas as well but also forms scattered outcrops in the southeastern Solomon and southwestern Norton Bay quadrangles, where it is the dominant lithology. The younger sequence is poorly exposed in the northeastern and southeastern Bendeleben quadrangle; it has been explored for coal and uranium (Retherford and others, 1986; Dickinson and others, 1987). The older, carbonate-rich sedimentary rocks are the better exposed. Carbonate-rich sandstone and siltstone typically occur as rubble-covered hills but are best exposed in river-cliff outcrops in the Bendeleben C-2 quadrangle. Rocks are friable to well indurated, calcite-cemented, and form beds 5 to 60 cm thick; sedimentary structures include graded bedding, channels, small scale ripples, and cross-beds. Some fine-grained layers are rich in carbonaceous plant debris and coal seams are locally well developed. Most samples consist of poorly to moderately well sorted, angular to rounded grains. Clasts appear to have been derived primarily from adjacent metamorphic rocks; clast lithologies include marble, dolostone, plagioclase, monocrystalline and polycrystalline quartz, volcanic rocks (with felsitic and lathwork textures), blueschist-facies metabasite, radiolarian chert, phyllite, quartz-mica schist, and amphibolite. Carbonate clasts predominate at most localities. Conglomerate, associated with minor sandstone and pebbly sandstone, forms rounded knobs up to 25 m high and more extensive areas of rubble crop. Bedding is rarely evident, but sandy interbeds and crude grading are locally present. Rocks are very poorly sorted and have a matrix of calcite cement and carbonate sand. Cobbles are sub-rounded to rounded; maximum clast diameter at outcrops studied ranges from 52 to 72 cm. Pebble counts indicate that carbonate clast content varies from a low of 81 percent in the Bendeleben C-2 quadrangle to a high of 98 percent in the Bendeleben A-1 quadrangle. Marble and metalimestone versus dolostone ratios range from 1:2 in the Bendeleben A-1 quadrangle to 3:1 in the Bendeleben C-2 quadrangle. Non-carbonate clasts include chert, monocrystalline and polycrystalline quartz, quartz-mica schist, chlorite schist, and various greenstone lithologies that were likely derived from the adjacent unit (}|m of this map). Two small outcrops of mafic clast conglomerate underlie carbonate conglomerate in the Bendeleben C-2 quadrangle. Sorting in these rocks is poor; clasts are rounded to angular and as much as 30 cm in diameter. Clasts are mainly metavolcanic rocks, some of which contain blue amphibole; subordinate clast lithologies include radiolarian chert, marble, quartz, and quartz-mica schist. The calcareous sediments are locally intruded by sills of probable Tertiary or Cretaceous age too small to show on this map; the sediments are unmetamorphosed but deformed and vertical beds occur locally. No fossils constrain the depositional age of the conglomerates, but ages of some clasts have been determined. Carbonate clasts from six localities yielded 21 conodont collections, 19 of which have CAI values of 5-6 (Table A-1). Relatively well-constrained clast ages are mainly Silurian and Devonian; the tightest ages are Middle to Late Ordovician, early to middle Silurian, middle to late Silurian (Wenlock-Ludlow), late Early Devonian (late Emsian), and Middle Devonian. Conodont biofacies of several of the Silurian clasts indicate a high-energy, shallow-water depositional setting (Till and others, 1986). The lithofacies, thermal level, age, and biofacies of the carbonate clasts correlate well with those of units Od, Sd, and Ddm; carbonate cobbles in TKs thus most likely were derived largely from these units. Carbonate clasts of Silurian age and shallow-water biofacies appear to be volumetrically over-represented relative to the present areal distribution of such rocks. The carbonate-rich clastic rocks of TKs may have been deposited in a series of small alluvial fans. Mid-Cretaceous carbonate-rich sedimentary sequences are exposed east of Seward Peninsula in the Yukon-Koyukuk basin (Nilsen, 1989; Patton and others, 2005), and are thought to be correlative with the carbonate-rich sedimentary rocks of unit TKs. The younger sequence in unit TKs is exposed in creek bottoms in the Bendeleben D-1 quadrangle and in a small area south of Death Valley, in the Bendeleben A-1 quadrangle. In the northern locality, lignite, up to 30 m thick, is interbedded with siltstone, mudstone, and sandstone along a strike length of almost three-quarters of a mile, based on drilling results; bedding is steeply-dipping (Retherford and others, 1986). Pollen assemblages of Late Cretaceous and Tertiary (Eocene to early Miocene?) ages have been found in finer grained parts of the sequence (Till and others, 1986; Haga, in Retherford and others, 1986). Drilling results show that schists of the Nome Complex sit structurally above parts of the sequence (Retherford and others, 1986). In the southeastern Bendeleben quadrangle, the sequence is exposed in a small area south of Death Valley. Early Eocene and younger sedimentary and basaltic rocks accumulated in a small graben and are interleaved with basaltic rocks (Dickinson and others, 1987). Sandstone, mudstone, conglomerate and coal beds up to 55 meters thick were documented in drill core. Sandstones in the core host an epigenetic and supergene uranium deposit (Dickinson and others, 1987). A similar sequence of sedimentary and volcanic rocks probably underlies Death Valley. The unit is equivalent to "Kc" of Miller and others (1972), "Klcg" of Sainsbury (1974), "TKs" and "TKc" of Till and others (1986), and "Kcc" of Patton and others (2005); also partly equivalent to "Kss", "Kls", and "TKs" of Sainsbury (1974)
Lithology Sedimentary

Correlated geologic units

Label TKcs
Description Conglomerate and sandstone
Geologic age Late-Cretaceous to Eocene
Geologic setting Sedimentary, continental
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Coal < Sedimentary Bed Incidental