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SAMPLE INFO : Jensen_2008_Chester_Bluff_Chester_Bluff_tephra-2P1
Sample ID:Jensen_2008_Chester_Bluff_Chester_Bluff_tephra-2P1
Station ID:Jensen_2008_Chester_Bluff
Latitude:65.38
Longitude:-142.66997
Datum:NAD83
Sample Type 1:Tephra Fall
Text Description:
Present at Site A and Site C of Chester Bluff. The CB tephra, the upper-most bed at Site A, includes sections of exceptionally preserved primary bedding. CB comprises four individual beds of ash divided by thin laminae of silt, likely representing several eruptions from the same source that occurred over a period of days to weeks to possibly years. The lowermost layer of tephra is pink; less than or equal to 1 cm thick and separated from the overlying beds by less than or equal to 2 cm of silt, and is referred to here as CB2. The next two beds are locally Fe-stained but have a distinct salt and pepper appearance and are up to 2.5 cm thick. The upper-most bed is the thickest, up to 5 cm, also has a ‘‘salt and pepper’’ appearance, but displays winnowing, with heavier phenocrysts concentrated near the base of the bed. These three beds are collectively termed CB1. At Site C, CB is up to 5 cm thick and generally appears as a single bed, although the lower pink bed is locally visible. At Site C, there are two tephra beds directly above CB, the Coal Creek (CC) and Charley Village (CV) tephra. All three beds are faulted steeply toward the downstream end of the bluff. Generally CB2 is distinguishable from CB1 by a higher percentage of thick-walled pumice that is rich in microcrysts and a lower percentage of phenocrysts. CB2 is richer in ortho-pyroxenes and has a distinct population of red amphibole. The composition reported was calculated from analyses of two samples: UA1084 and UA1050.

References:
An extensive middle to late Pleistocene tephrochronologic record from east-central Alaska
Tephrochronology of middle to late Pleistocene loess in east-central Alaska

GEOCHEM DATA
StationIDLatitudeLongitudeGeologistDateVisitedAge InfoVolcanoEruptionLocation DescriptionText DescriptionSample IDSample Type 1Sample Type 2Final UnitMaterialCoeffSiO2TiO2Al2O3FeOTMnOMgOCaONa2OK2OP2O5Total-majorsREF majorsMETH majorsFe2O3/Fe203T origFeO/FeOT origVolatiles csvMETH volatilesCsRbBaSrLaCePrNdSmEuGdTbDyHoErTmYbLuYZrNbHfTaPbThUScVCrFeCoNiCuZnGaMoAsNaKRef trace1METH trace1RbBaSrLaCeNdSmEuGdDyErYbLuYZrNbPbThUScTiVCrNiCuZnGaRef trace2METH trace2Light csvHalogen csvother major csvother lile csvother ree csvother hfse csvother hpe csvother tm csvother misc csv
Jensen_2008_Chester_Bluff65.38-142.66997Jensen, B. J. L. Chester Bluff is located in Yukon Charley Rivers National Preserve (YCNP) in east-central Alaska, on the northwest bank of the Yukon River directly upstream from its confluence with the Charley River. It is a terrace comprising a series of individual bluffs dissected by gullies that extend laterally for about 3 km. The south-facing bluffs are dry and substantial excavation (>2m) is needed to reach frozen sediments. Chester Bluff can be broadly differentiated into four units. The base of the bluff is a bedrock terrace exposed up to ~10 m above river level, composed of argillite, part of the Cretaceous Biederman Formation. Deposited on the terrace are 8-10 m of paleo-Yukon River gravel, which are overlain by 5-10 m of sand and silt rhythmites with minor gravel and planar-bedded sand. Up to 40 m of silt, interpreted as loess and interbedded with multiple organic horizons and tephra beds, cap the sequence. Sites A through C were visited in the area and were found to host 19 distinct tephra beds. A near vertical slope at Site A prevented trench completion. Two trenches were excavated on either side of Site A to examine the lateral continuity of tephra beds. Site B was successfully logged from the top of the bluff to the upper-most sand unit associated with the flood deposits. Thick colluvium at Site C prevented completion of the trench at this site. No coordinates provided; location imprecisely georeferenced from Figure 1 in Jensen et al. (2008).Present at Site A and Site C of Chester Bluff. The CB tephra, the upper-most bed at Site A, includes sections of exceptionally preserved primary bedding. CB comprises four individual beds of ash divided by thin laminae of silt, likely representing several eruptions from the same source that occurred over a period of days to weeks to possibly years. The lowermost layer of tephra is pink; less than or equal to 1 cm thick and separated from the overlying beds by less than or equal to 2 cm of silt, and is referred to here as CB2. The next two beds are locally Fe-stained but have a distinct salt and pepper appearance and are up to 2.5 cm thick. The upper-most bed is the thickest, up to 5 cm, also has a ‘‘salt and pepper’’ appearance, but displays winnowing, with heavier phenocrysts concentrated near the base of the bed. These three beds are collectively termed CB1. At Site C, CB is up to 5 cm thick and generally appears as a single bed, although the lower pink bed is locally visible. At Site C, there are two tephra beds directly above CB, the Coal Creek (CC) and Charley Village (CV) tephra. All three beds are faulted steeply toward the downstream end of the bluff. Generally CB2 is distinguishable from CB1 by a higher percentage of thick-walled pumice that is rich in microcrysts and a lower percentage of phenocrysts. CB2 is richer in ortho-pyroxenes and has a distinct population of red amphibole. The composition reported was calculated from analyses of two samples: UA1084 and UA1050.Jensen_2008_Chester_Bluff_Chester_Bluff_tephra-2P1Tephra FallCumulateGlass 74.19 0.23 14.94 1.42 0.05 0.43 1.87 3.98 2.85 6162EMP1.42Cl=0.05; H2O=4.71EMP

SAMPLE LOCATION

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