Herendeen Formation and similar units

Unit symbol: Khnl
Age range Lower Cretaceous (139.8 to 125 Ma)
Lithology: Sedimentary
Group name: Coquina and calcarenite
Thin calcarenite, limestone coquina, or similar rock units are widely present in Alaska. Units included are the Herendeen Formation on the Alaska Peninsula, the Nelchina Limestone in south-central Alaska, the Berg Creek Formation of eastern Alaska, and similar unnamed units in southwest and west-central Alaska in the Charley River quadrangle and the Brooks Range (“coquinoid limestone”). The Herendeen Formation was originally described as limestone (Atwood, 1911), but rocks are actually unusually uniform, thin-bedded, medium-grained, calcarenaceous sandstone (Detterman and others, 1996). Inoceramus fragments form major component of formation, although complete specimens have only been found in the Mount Katmai area. Presence of Buchia crassicollis indicates a Valanginian age for the Herendeen in its type area, and ammonite fossils and other collections indicate a Hauterivian and Barremian age at its northern extent (J.W. Miller, written commun., 1983–85; Detterman and others, 1996). The Nelchina Limestone (Martin, 1926) of south-central Alaska and the similar Berg Creek Formation (MacKevett and others, 1978) are lithologically similar to the Herendeen, which consists of “* * * massive dark-colored unaltered fine-grained limestone separated by thin laminae of gray shale. Some beds are highly siliceous and probably ought to be called calcareous sandstone (Martin, 1926).” Buchia crassicollis has also been reported from the Nelchina Limestone. The coquinoid limestone of the northern Alaska is a “[d]istinctive thin marker unit of gray to dark-gray limestone coquina composed of the pelecypod Buchia sublaevis, in beds up to 2 m thick, * * * interbedded with reddish-brown to black clay shale * * *” (Mull and others, 1994); its thickness is less than 10 m. Stratigraphically, unit has been associated with the Okpikruak Formation. Kelley (1990a) states that the unit occurs in both structural and stratigraphic settings; in depositional contact with the Otuk Formation and the undifferentiated Otuk and Shublik Formations; and as tectonic blocks in mélange in his Arctic Foothills assemblage (which is included here in unit JPzs). Tectonic blocks of coquinoid limestone in the Arctic Foothills assemblage, as well as those associated with Okpikruak Formation, may be olistostromal in nature. Other calcareous clastic units of similar age are known from southwestern Alaska (Hoare and Coonrad, 1978), western Alaska (Patton, 1966; Hoare and Condon, 1971; Patton and others, 2009) and in the Charley River area of eastern Alaska (Dover and Miyaoka, 1988). Also includes Kennicott and Kuskulana Pass Formations of eastern Alaska, composed dominantly of thin-bedded, fine-grained feldspathic graywacke and arkosic wacke and siltstone. Also includes shale and some conglomerate whose clasts are predominantly Nikolai Greenstone (unit ^n) at the base (MacKevett and others, 1978). Kennicott Formation is generally dark-greenish-gray, weathers brown, and has crude graded bedding, cross-bedding, sole markings, and spherical limy concretions; the older Kuskulana Pass Formation is generally similar. Both represent fairly rapid shallow marine deposition in a transgressive sea. An Albian age is assigned to the Kennicott on the basis of abundant molluscan fauna, particularly the occurrence of Inoceramus altifluminis McLearn (Patton, 1966) and pelecypods of genus Aucellina, whereas the age of the Kuskulana Pass Formation is defined as Hauterivian and Barremian based on a meager ammonite and pelecypod fauna (MacKevett and others, 1978)

Source map information

Source map Plafker, George, unpublished data
Symbol Kl
Unit name Clastic sedimentary rocks
Description Dark greenish-gray to very dark gray shale, locally concretionary, with subordinate siltstone, arkosic sandstone, and arkosic granule- to pebble conglomerate. Chocolate-brown to reddish-brown weathering colors and hackly to slaty cleavage are conspicuous in the shale, completely obscuring bedding in most places. Locally contain thin coquina and carbonaceous detritus, particularly in sandstone and pebble conglomerate beds near base of unit. Thickness unknown, but exceeds 200 meters. Presumed to correlate with unnamed Neocomian rocks that occur in scattered exposures in the adjacent McCarthy quadrangle (MacKevett, 1978) and extend westward as far as the mouth of the Chitina River (Grantz and others, 1966) and eastward into adjacent parts of Canada (Campbell and Dodds, 19 ) and the St. Elias Mountains (Sharp and Rigsby, 1956).
Lithology Sedimentary

Correlated geologic units

Label Kbc
Description Kennicott and Kuskulana Pass Formations
Geologic age Hauterivian to Barremian
Geologic setting Sedimentary, shallow-marine-siliciclastic
Lithology Form Importance
Conglomerate < Clastic < Sedimentary Bed Major
Calcarenite < Arenite < Sandstone < Clastic < Sedimentary Bed Indeterminate, major
Siltstone < Clastic < Sedimentary Bed Indeterminate, major