ALASKAN  SCRAPER PAGE

ALASKAN CHERT:

Is considered to be a hard, dense rock composed of microcrystalline or cryptocrystalline silica--Si02--or silicon dioxide--that breaks with a conchoidal fracture ( A breakage of rock in concentric circles or in a clam shell-like scar pattern. Referring to the characteristic fractures resulting from pressure and percussion flaking of flint and chert) and has a vitreous or glassy luster. This material, when not highly fractured, may yield good raw material for tool making. The term "chert" is also sometimes broadly and loosely used by geologist to describe other dense siliceous rocks that fracture with as dull and somewhat granular appearance, but these rocks are more properly considered Silicified mudstone or limestone and do not yield as high a quality of material for tool making.

Chert is found dominantly associated and inter-bedded with fine grained sedimentary rocks that were deposited slowly on the ocean floor and occasionally is found inter-bedded with fine grained igneous rock such as basalt that was extruded in the oceans of long ago. The chert beds in the picture are geological up-heaves. The beds go for miles underground but are occasionally accessible along a fault that spans the Brooks Mountain Range.

Picture by Del Roerick
 

  Types of Chert:

Banded gray to BLACK Chert (Lisburne group)

Jet Black Chert (Akmalik Chert)
Gray and Maroon to Green Chert (Sisikput Formation and Imnaitchiak chert)
Tan to GRAY to Black Banded Chert (Otuk Formation)
Bright Red, Maroon and Green Chert: (Missippian and Akmalik Formation)

Black Chert:  Kuna Formation of Mississippian age

Green and Gray Chert, some Black:  Etivluk Group of Pennsylvanian through Jurassic age

Chert-Greenish Gray to Very Dark Gray Chert: Imnaitchiak  

Other Types of Lithic Material used for tool making

Chocolate, Banded Gray to Black Chert , Light Tan to Black Mottled Chert

Red , Brown, Translucent,  Maroon and Turquoise Green Chert, Speckled, Vitreous Light Gray

"Kobuck" region

Gallagher, Denbigh Flint, Chalcedony, Graywake, Quartzite, Siliceous Argillite, Basalt, Jasper

Black Chert Group

Raw Black Nodule

Raw Black Chert Slab

Dark Green Chert

Raw  Green Nodule

Raw  Green Chert Matrix

Light Gray Chert Group

Raw Elongate Lense

Insitu Lense

Chocolate Chert Group

Chocolate Chert debitage

Dark Gray Chert Group

Raw Dark Gray Nodule

Chalcedony Group

No Raw Picture at this time

The dominant cherts found in the outcrop in the Brooks Range and its foothills are Black,  Dark Gray,  Light to Medium Gray,  Greenish Gray, or Bluish Gray. Light to Dark Gray to Black banded chert is common in some areas; a distinctive dark gray to black chert with a dull tan-colored outer rind is common in other areas. Maroon, Red, Chocolate , and bright Turquoise blue cherts are found occasionally in outcrop, but are uncommon. These various colors of chert are probably the result of minor impurities that were deposited with the rain of siliceous sediment on the ocean floor or were present when migrating silica moved into and replaced other marine mud's

Banded light to dark Gray and Black chert (Lisburne Group) Gray to Black banded chert is probably largely from scattered nodules and lenses in the massive light-gray cliff forming Lisburne limestone, which is the most widespread of the chert-bearing units in the Brooks Range. The Lisburne limestone, of Mississippian and early Pennsylvanian age (about 313 to 350 million years old) is up to 2500 feet thick in some areas. It extends as a nearly unbroken outcrop belt along the northern mountain front westward

Black chert in the Otuk Formation closely resembles the Black chert from the Mississippian Akmalik Formation. In absence of the wispy mottling or black and tan banding that is characteristic of the Otuk cherts, or absence of fine pyrite flecks that are characteristic of the Akmalik chert, the origin of small black tools and flakes may be difficult to determine Maroon and Turquoise Green Chert: Distinctive brightly colored Red, Maroon, and Turquoise Green Chert is uncommon in the Brooks Range foothills, but is conspicuous where present. This chert seems to be closely associated with basalt that is locally present in scattered small isolated exposures in the Endicott Mountains foothills from the Anaktuvuk River west to the Kuna River

Picture by Del Roerick

Picture by Joel Castanza

These are chert ball with rinds.
The balls range from the size of basket balls to marble size. All have blue
and green chert inside.

Geological mapping in the Brooks Range shows that high quality chert for tool making is abundant in sedimentary rocks in a number of areas along the northern flank and particularly in the foothills of the central and Western Brooks Range. These cherts are dominantly black, light to dark gray, greenish gray, banded gray to black, or tan. Some distinctive chert colors can be correlated with specific rock units and the geographic distribution of these formations has been mapped; this may help narrow the search for sources of specific lithic types. Minor gray, brown to reddish brown, or bright turquoise blue-green chert is associated with basalt in a few areas on the north side of the range. On the south side of the mountains, minor gray to black chert pebbles in conglomerate are probably derived from a linear belt of basalt. Except in a few isolated localities, chert is not present in the main part of the range itself.

Atigun Valley- Picture by Del Roerick

ALASKAN CHERT SOURCES

The original source of the silica in chert is generally thought to be from siliceous sediment derived from micro-organisms such as *radiolaria and some varieties of sponges that use silica as skeletal material. Other cherts appear to have formed by replacement of fine-grained limestone or mudstone by silica that migrated through the sediment as it was being compacted and lithified through geological time.

*Radiolaria are holoplanktonic protozoa widely distributed in the oceans. They occur throughout the water column from near surface to hundreds of meters depth. As with many planktonic organisms, their abundance in a geographical region is related to quality of the water mass, including such variables as temperature, salinity, productivity, and available nutrients.

Chert Ball

Chert is not present in sandstone or conglomerate, except sometimes as pebbles or cobbles recycled from older rocks, because sandstone and conglomerate are deposited rapidly in settings in which the slow rain of silica-bearing organisms is diluted by the abundance of coarse detritus.

Picture by Del Roerick  and Joel Castanza

 The chert in the Brooks Range is commonly found as scattered nodules or elongate lenses in limestone, or as evenly thin bedded and occasionally nodular units inter bedded with less Silicified thin shale, mudstone, or limestone.

Chert-bearing limestone is also present in the headwaters of some of the south-flowing streams that are tributaries of the Noatak River and the Wulik River in the western DeLong Mountains of the western Brooks Range. The alluvium in these drainages may contain chert pebbles and cobbles that could be a source of lithic material. And finally, limestone with chert nodules that could be a source of tool making material is widespread in the Lisburne Hills between Cape Thompson and Cape Lisburne at the extreme western end of the Brooks Range. In addition, a linear belt composed dominantly of basalt that forms the southern flank of the range may contain minor amounts of gray or black chert.

Alaskan Chert Sources cont.

The quarry site in the DeLong Mountains,

near the head of the Kelly River  contains a distinctive high quality gray chert with a very uniform color and texture. Cherts of this age of a quality for tool making are not known in the eastern Brooks Range foothills or mountain front east of the Anaktuvuk River. Siliceous rocks of this age are present on the southern side of the eastern Brooks Range in the Christian River area southeast of Arctic Village, but I do not know if high quality chert is present in these rocks.

Tan Thin bedded, banded tan to black chert with wispy mottling is widespread in the central Brooks Range foothills. It commonly has distinctive light gray to tan or cream-colored upper and lower bedding surfaces that grade to black in the bed centers. When examined closely with a hand lens, the tan to gray upper and lower bands can commonly be seen to contain tiny translucent light gray spherules about the size of a small pin head. These small spherules are re crystallized radiolaria; although radiolaria are present in other cherts in northern Alaska, they are most visible in the Oh& Formation This distinctive chert is confined to the limestone member of the Otuk Formation of middle and late Triassic age (215-240 million years). This unit is usually less than 50 feet thick but contains a number of 2" to 6" thick beds of the banded chert and limestone; it is relatively resistant and forms conspicuous tan-weathering slopes and hill sides covered with chert rubble. Fossil pelecypod shell fragments are common on some bedding surfaces but are not common in the better quality chert. A lower chert member of the Otuk Formation consists of thin bedded black chert and Silicified limestone, but appears to be too intensely fractured to yield good tool making material. The cherty beds in the Otuk Forrnation are present discontinuously in many areas along the mountain front and foothills of the Endicott Mountains from the Anaktuvuk River west to the Kuna River area in the western Howard Pass quadrangle. The Otuk Formation is also present in the DeLong Mountains foothills.