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SAMPLE INFO : Jensen_2008_Chester_Bluff_Preido_Hill_tephra
Sample ID:Jensen_2008_Chester_Bluff_Preido_Hill_tephra
Station ID:Jensen_2008_Chester_Bluff
Latitude:65.38
Longitude:-142.66997
Datum:NAD83
Sample Type 1:Tephra Fall
Text Description:
Present at all three sites of Chester Bluff, Site A, Site B, and Site C. The Preido Hill (PrH) tephra is reworked with MacGregor Cabin (MC) tephra at Site A1. PrH/MC and SR are heavily faulted, reworked and dipping steeply at both Site A and A1. Loess associated with the reworked PrH/MC is strongly mottled, Fe-stained, andincludes small organic horizons that are reworked and overturned. Solifluction may have caused the mottling and overturning of the tephra beds or, alternatively, a faulted block of silt may have obscured the true stratigraphic position of the tephra beds. At Site B, Tom King (TK), MC, PrH, Ben Creek (BC) and Yukon Tanana (YT) are found within 2m of one another. SR, TK, MC and PrH are deposited within greyish brown loess 0.5 to 1m below a major organic unit that contains BC and YT. At the downstream end of the 6m lateral section the tephra beds are reworked and faulted downward. At the upstream end, the major organic unit above SR, TK, MC and PrH is deformed along its base, causing the tephra beds to be overturned and partially reworked into the base of the unit. A 6m wide trench was excavated to fully delineate the tephrostratigraphic relations of the six tephra beds. PrH is a continuous white-yellow bed up to 4 cm thick with sharp contacts. PrH and MC are very similar geochemically and are distinguished largely by stratigraphy in this relatively undisturbed exposure where they both form distinct, laterally continuous beds consistently separated by 2 cm of silt. At Site C, the reworked PrH/MC and TK are found as an isolated pod. PrH/MC are strongly deformed and down-faulted vertically over 3m toward the downstream side of this exposure. Low percentage of phenocrysts (<20%) and contain a population of distinctively thick-walled pumice and glass shards. The composition reported was calculated from analyses of one sample: UA1217.

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.< 780000 stratigraphy; 10291; Age based on magnetic polarity of the sediments.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 all three sites of Chester Bluff, Site A, Site B, and Site C. The Preido Hill (PrH) tephra is reworked with MacGregor Cabin (MC) tephra at Site A1. PrH/MC and SR are heavily faulted, reworked and dipping steeply at both Site A and A1. Loess associated with the reworked PrH/MC is strongly mottled, Fe-stained, andincludes small organic horizons that are reworked and overturned. Solifluction may have caused the mottling and overturning of the tephra beds or, alternatively, a faulted block of silt may have obscured the true stratigraphic position of the tephra beds. At Site B, Tom King (TK), MC, PrH, Ben Creek (BC) and Yukon Tanana (YT) are found within 2m of one another. SR, TK, MC and PrH are deposited within greyish brown loess 0.5 to 1m below a major organic unit that contains BC and YT. At the downstream end of the 6m lateral section the tephra beds are reworked and faulted downward. At the upstream end, the major organic unit above SR, TK, MC and PrH is deformed along its base, causing the tephra beds to be overturned and partially reworked into the base of the unit. A 6m wide trench was excavated to fully delineate the tephrostratigraphic relations of the six tephra beds. PrH is a continuous white-yellow bed up to 4 cm thick with sharp contacts. PrH and MC are very similar geochemically and are distinguished largely by stratigraphy in this relatively undisturbed exposure where they both form distinct, laterally continuous beds consistently separated by 2 cm of silt. At Site C, the reworked PrH/MC and TK are found as an isolated pod. PrH/MC are strongly deformed and down-faulted vertically over 3m toward the downstream side of this exposure. Low percentage of phenocrysts (<20%) and contain a population of distinctively thick-walled pumice and glass shards. The composition reported was calculated from analyses of one sample: UA1217.Jensen_2008_Chester_Bluff_Preido_Hill_tephraTephra FallCumulateGlass 75.31 0.1 14.85 0.88 0.08 0.27 1.37 4.35 2.75 6162EMP0.88Cl=0.03; H2O=5.94EMP

SAMPLE LOCATION

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