The Gold Hill Loess is a geologic formation in Alaska that consists of green, brown or tan massive loess. It contains several well dated volcanic ash beds. The maximum thickness of the Gold Hill Loess is up to thick. On bases of volcanic ash beds and two major internal unconformities, Gold Hill Loess is informally divided into upper, middle, and lower subdivisions. It unconformably overlies either the Cripple and Fox gravels, the Tanana Formation, or pre-Quaternary bedrock. It also unconformably underlies the Eva and Goldstream formations. It is the lateral, downslope equivalent the lower part of the Fairbanks Loess. In Alaska, the Gold Hill Loess Formation represents a discontinuous, but important, record of high-latitude, pre-Sangamonian fossil faunas, paleoenvironments, paleosols, and distal volcanism.Péwé, T.L., Westgate, J.A. , Preece, S.J., Brown, P.M., and Leavitt, S.W., 2009. Late Pliocene Dawson cut forest bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska. Geological Society of America Bulletin, 121(1-2), pp. 294-320.Westgate, J.A., Stemper, B.A. and Péwé, T.L., 1990. A 3 my record of Pliocene-Pleistocene loess in interior Alaska. Geology, 18(9), pp. 858-861.
Nomenclature During the late 1800s and early 1900s, field geologistsDall, W.H., 1868. Explorations in Russian America. (from a letter addressed to J. D. Daya, dated Russian Extension, St. Michaels, R. Am., Aug. 13, 1867). American Journal of Science , 2(133), pp.96-99.Dawson, G.M., 1894. Notes on the Occurrence of Mammoth remains in the Yukon District of Canada and in Alaska. Quarterly Journal of the Geological Society, 50(1-4), pp.1-9.Spurr, J.E., 1898. Geology of the Yukon Gold District, Alaska: By Josian Edward Spurr; with an introductory chapter on the history and condition of the district to 1897 by Harold Beach Goordich. U.S. Geological Survey Annual Report 18-III, 328 pp. discovered that a blanket of silty, often clayey, organic-rich, massive to weakly bedded sediments that flank both sides of the Yukon River and its tributaries. These fine-grained sediments are as much as , or more, thick and overlie bedrock and gold-bearing stream gravels. Where cut into by streams and rivers and gold mining operations, they form steep bluffs. These sediments were found to contain Pleistocene and, possibly Pliocene, terrestrial vertebrate and invertebrate fossils. In 1892,Dall, W.H. and Harris, G.D., 1892. Correlation papers: Neocene. Bulletin of the United States Geological Survey, 84, 349 pp., the name Kowak Clays was proposed for these sediments. In 1898, the name Yukon Silt was also applied to the same set of fine-grained sediments. Both terms were used to designate to the entire thickness of silty sediments overlying gold-bearing gravels on either side of the Yukon River and its tributaries until being abandoned after 1937.Frick, C., 1937. Horned ruminants of North America. Bulletin of the American Museum of Natural History, 69, 699 pp.
After 1937, the silty sediments that were named either Yukon Silt or Kowak Clays became known informally as either muck or Alaska muck in the published literature.Hibben, F.C., 1941. Archaeological Aspects of the Alaska Muck Deposits. New Mexico Anthropologist, 5(4), pp. 151-157.Rainey, F., 1940. Archaeological investigation in central Alaska. American Antiquity, 5(4), pp. 299-308.Taber, S., 1943. Perennially frozen ground in Alaska: its origin and history. Bulletin of the Geological Society of America, 54(10), pp.1433-1548. Muck is an informal term used by miners and geologists in the Arctic to refer to predominately dark gray to black silt, which contains a quantity of plant remains, vertebrate fossils, lenses of ice, permafrost, and thin beds of peat and volcanic ash that directly overlie gold-bearing fluvial gravels. Instead of muck, still other researchers used the informal term Alaska silts for the same silty sediments. The term upland silt designated loess deposits blanketing the upland terrain that grade laterally downslope into muck and accumulated contemporaneously with it.
Starting in 1936, Giddings recognized and studied buried forest beds found at various levels within the muck (Yukon Silt or Kowak Clays) deposits. This research started while he was employed for the Fairbanks Exploration Company's placer gold mining operations along Engineer Creek, north of Fairbanks, Alaska.Nash, S.E., 2000. James Louis Giddings' archaeological tree-ring dating in the American Arctic: A forgotten legacy. Arctic Anthropology, 7(1) pp.60-78. Within the muck deposits, GibbonsGiddings, J.L., 1938. Buried Wood from Fairbanks, Alaska. Tree-Ring Bulletin, 4(4) pp. 3-5. mapped four, laterally persistent beds composed of buried spruce forests at different depths below its surface. Within the muck, the oldest forest grew on a buried surface just above the top of the gold-bearing gravels and the youngest buried forest grew on a buried surface below the pre-mining, modern surface. Using tree rings, he was able to construct separate floating, or undated, dendochronologies for each level of buried spruce forest found within the local muck.
In the summers of 1946, 1947, 1948, 1949, and 1951, Péwé conducted detailed field research concerning the geomorphology and Quaternary geology of the Fairbanks region while an employee of the United States Geological Survey. As a result of this research, he initially subdivided the Alaska muck (Alaska silt) into four informal stratigraphic units. These units include the (1) Dawson muck, (2) Goldstream muck, (3) Gold Hill loess, and (4) Engineer muck. After years of additional detailed field research for the United States Geological Survey, Péwé subdivided the informal Alaska muck (Alaska Silt) into formal geologic formations and formally defined them.Péwé, T.L., 1975. Quaternary Geology of Alaska. U.S. Geological Survey Professional Paper, 835, 245 p.Péwé, T.L., 1975. Quaternary stratigraphic nomenclature in central Alaska. U.S. Geological Survey Professional Paper, 862, 32 pp. From youngest to oldest, these geologic formations include the Ready Bullion Formation (Holocene), Engineer Loess (Holocene), informal Gibbons forest bed (Holocene), Goldstream Formation (Wisconsin), Eva Formation (forest bed)(Sangamon), Gold Hill Loess (PreSangamon to Pliocene), and Dawson Cut Formation (forest bed)(Pliocene). The Fairbanks Loess became the formal name for the upland silts. The gold-bearing gravels were formally divided into the Cripple and Fox gravels.Péwé, T.L., 1989. Quaternary stratigraphy of the Fairbanks area, Alaska. In Carter, D.L., Hamilton, T.D., and Galloway, J.P., eds., pp. 72-77, Late Cenozoic history of the interior basins of Alaska and the Yukon. U.S. Geological Survey Circular, 1026. Reston, Virginia, 114 pp. Péwé, T.L., Berger, G.W., Westgate, J.A., Brown, P.M., and Leavitt, S.W., 1997. Eva Interglaciation Forest Bed, Unglaciated East-Central Alaska: Global Warming 125,000 Years Ago. Special Paper, 319. Boulder, Colorado, Geological Society of America. 54 pp.
Description The Gold Hill Loess is the oldest and thickest loess blanketing the lower slopes and in valley bottoms of the Tanana River basin. This perennially frozen loess is exposed in the vertical cuts and highwalls of placer mines. It does not naturally outcrop on the surface. When freshly exposed, the Gold Hill Loess consists of perennially frozen, either brown or green, massive, well-sorted silt in the form of primary or retransported loess. The Gold Hill Loess directly underlying either the Eva or Goldstream formations is green because of the reduction of the ferric iron to ferrous iron. Upon exposure, the Gold Hill Loess oxidizes, thaws, and turns brown. Stratification and bedding in the Gold Hill Loess is generally indistinct or absent except where mass movement or reworking by water has created either some crude irregular or laminated bedding. Organic-rich paleosols and volcanic ash beds are common; serve as marker horizons; and define stratification in the otherwise massive loess. Immediately beneath the contact of the Gold Hill Loess with either the overlying Eva or Goldstream formations, the volcanic ash beds, paleosols, and enclosing loess are deformed by conspicuous soft sediment faulting and folding.Péwé, T.L., 1952. Geomorphology of the Fairbanks area, Alaska. Palo Alto, California, Stanford University, Ph.D. dissertation, 220 pp.
Nineteen volcanic tephras occur within the Gold Hill Loess. They include the pink tephra and the Old Crow Ash Bed. The pink tephra occurs as a thick, pink, semi-lithified volcanic ash bed that is the oldest laterally persistent volcanic ash bed in the Gold Hill Loess. It is dated at 2.2 million years ago. In one outcrop, it lies about above the base of The Gold Hill Loess. The ubiquitous Old Crow Ash Bed consists of a thick to thin layer of white tephra that outcrops in the upper Gold Hill Loess. It is typically folded or faulted wherever exposed. It provides a date of about 140,000 BP for upper Gold Hill Loess.Preece, S.J., Westgate, J.A., Stemper, B.A. and Péwé, T.L., 1999. Tephrochronology of late Cenozoic loess at Fairbanks, central Alaska. Geological Society of America Bulletin, 111(1), pp.71-90.
The Gold Hill Loess also contains numerous loess kinchen. Where its loess has been washed away water spray of hydraulic jets, they form a gravelly lag composed of abundant tubular calciteconcretions about in diameter and long are left behind. Locally, the Gold Hill Loess contains marcasite concretions at its base. Many of these concretions contain vertebrate bones or pebbles of shattered bedrock as nuclei.
At the Gold Hill mining cut, a thick gravel layer composed of rounded silt fragments, carbon fragments, small sticks and small pebbles was observed near the base of the Gold Hill Loess. This layer is slightly more resistant than the rest of the loess and it formed a ledge when it had been exposed in the wall of the placer excavation. Also, at this location, very thin gravel seams about thick and composed of angular pebbles about one-half to 1 inch in diameter were once exposed. Several, similar gravel layers were observed in exposures along the right descending bank of upper Engineer Creek.
Contacts and Unconformities The most complete known thickness of the Gold Hill Loess exists at it type section in the Gold Hill Mining cut about west of Fairbanks, Alaska. Based on paleomagnetic data, radiometric dating of tephra and silt, and sedimentary structures it has been determined that this exposure of the Gold Hill Loess records for the past 3 million years major periods of loess deposition in glacial tundra or steppes and associated permafrost alternating with major periods of erosion of the loess blanket, permafrost degradation, and thermokarst formation associated with interglacial boreal forests. The result of the alternation of these processes, is numerous layers of loess or retransported loess separated by laterally persistent unconformities. Of the numerous unconformities, two form the upper and lower contacts of the Gold Hill Loess. Two other prominent unconformities subdivide the Gold Hill Loess into three members, upper, middle, and lower Gold Hill Loess. These unconformities also record periods of severe erosion of the accumulated loess, interglacial climates, and regional boreal forests.