Drum Sheep Creek Tephra-CC
Start: 190000 yBP [1]
Stop: 39360 yBP [2]
Event Type: Explosive
Description: From Westgate and others (2007): "‘SCt’ refers to a family of tephra beds that closely resemble one another in their petrographic and compositional characteristics. Its members are named as follows: SCt-F (Fairbanks, Alaska, e.g. UT734), SCt-CC (Canyon Creek, Alaska, e.g. UT250), SCt-C (western Yukon, e.g. UT2096), SCt-K (western Yukon, e.g. UT1052), and SCt-A (western Yukon, e.g. UT1051)."
"Sheep Creek-CC tephra is known from one site, Canyon Creek (Fig. 1, locality 1 [in original text])."
"SCt [Sheep Creek Tephra] is now known to be made up of several stratigraphic units: SCt-F (Fairbanks, Alaska; 200 ka), SCt-CC (Canyon Creek, Alaska), and in the western Yukon, SCt-C, SCt-K (80 ka), and SCt-A...Compositional and temporal controls, including Sr isotopes, suggest Mount Drum in the Wrangell volcanic field as the source of the SCt beds."
"Unit 5 [at the Canyon Creek exposure] is the key stratigraphic unit-because it contains angular fragments of tephra believed to have been ‘‘eroded, transported, and redeposited in a frozen state’’ (Weber and others, 1981). This tephra is SCt, here named SCt-CC (Fig. 5 [in original text]). In their 1981 paper, Weber and others stated that this tephra was identified by Westgate as the Dome Ash Bed, but further work showed this identification to be incorrect. Close scrutiny of Fig. 5 [in original text] suggests another interpretation of the relationship of SCt-CC to the gray sands-namely, that the tephra and sands were once a coherent sedimentary sequence, which later was remobilized, at which time both the tephra and sands were deformed. The tephra pods show plastic deformation in places, and elsewhere are preserved as thin beds up to 10 cm long that are parallel to bedding. The gray sands show very variable bedding attitudes. The deformational mechanism is likely downslope movement (solifluction) during the periglacial conditions that prevailed following emplacement of the sands and tephra. This interpretation would mean that the tephra and sands are penecontemporaneous-a very different interpretation to the ‘‘re- worked tephra’’ idea. Indeed, if the tephra was eroded from the upstream river bank and incorporated into the gray fluvial sands as frozen pieces, presumably some of those pieces would contain remnants of the original host sediment. None is seen in the field (see photograph in Fig. 5 [in original text]). Hamilton and Bischoff (1984) noted this point and concluded that the tephra clasts were probably eroded from a recent surface accumulation, in which case, the tephra would be approximately contemporaneous with the alluvium of unit 5. Hopefully, future field studies will be able to resolve this problem of the temporal relationship between the tephra and its host alluvium."
"The mineral assemblage of SCt-CC is similar to SCt in the Fairbanks area. A strongly pleochroic hypersthene with fluid and opaque inclusions is abundant as are hornblende and plagioclase. Ilmenite and magnetite are conspicuous and trace amounts of basaltic hornblende, augite, and quartz are present. Many of the minerals do not have attached glass and are slightly rounded, but otherwise look the same as those with attached glass. A white frothy pumice is the dominant phase, and occurs together with a small amount of brown glass that is not observed in SCt in the Fairbanks area. The pumiceous glass has a rhyolitic composition but is very different from the glass in the SCt occurrences at Fairbanks; SiO2 and K2O are higher and TiO2, Al2O3, FeOt, CaO, and Na2O are lower (Table 2 [in original text]). The broadly similar petrographic features and a co-linear relationship with SCt on the K2O-SiO2 plot (Fig. 6 [in original text]) led to the view, albeit unpublished, that SCt-CC was co-magmatic with SCt, but, being more evolved, was likely slightly older-the more silicic cap of the magma body being tapped first."
"Bones beneath SCt-CC and near the base of unit 5 have a 14C age of 39,360+/-1740 B.P. (Weber and others, 1981) and a U-series age of about 80 ka (Hamilton and Bischoff, 1984). These discrepant age estimates both severely underestimate the 190 ka age suggested by the tephra correlation noted above. Bison bones are present in this fossil assemblage and point to a Rancholabrean land mammal age-in other words, SCt-CC is probably of MIS 6 age or younger."
"Sheep Creek-CC tephra is known from one site, Canyon Creek (Fig. 1, locality 1 [in original text])."
"SCt [Sheep Creek Tephra] is now known to be made up of several stratigraphic units: SCt-F (Fairbanks, Alaska; 200 ka), SCt-CC (Canyon Creek, Alaska), and in the western Yukon, SCt-C, SCt-K (80 ka), and SCt-A...Compositional and temporal controls, including Sr isotopes, suggest Mount Drum in the Wrangell volcanic field as the source of the SCt beds."
"Unit 5 [at the Canyon Creek exposure] is the key stratigraphic unit-because it contains angular fragments of tephra believed to have been ‘‘eroded, transported, and redeposited in a frozen state’’ (Weber and others, 1981). This tephra is SCt, here named SCt-CC (Fig. 5 [in original text]). In their 1981 paper, Weber and others stated that this tephra was identified by Westgate as the Dome Ash Bed, but further work showed this identification to be incorrect. Close scrutiny of Fig. 5 [in original text] suggests another interpretation of the relationship of SCt-CC to the gray sands-namely, that the tephra and sands were once a coherent sedimentary sequence, which later was remobilized, at which time both the tephra and sands were deformed. The tephra pods show plastic deformation in places, and elsewhere are preserved as thin beds up to 10 cm long that are parallel to bedding. The gray sands show very variable bedding attitudes. The deformational mechanism is likely downslope movement (solifluction) during the periglacial conditions that prevailed following emplacement of the sands and tephra. This interpretation would mean that the tephra and sands are penecontemporaneous-a very different interpretation to the ‘‘re- worked tephra’’ idea. Indeed, if the tephra was eroded from the upstream river bank and incorporated into the gray fluvial sands as frozen pieces, presumably some of those pieces would contain remnants of the original host sediment. None is seen in the field (see photograph in Fig. 5 [in original text]). Hamilton and Bischoff (1984) noted this point and concluded that the tephra clasts were probably eroded from a recent surface accumulation, in which case, the tephra would be approximately contemporaneous with the alluvium of unit 5. Hopefully, future field studies will be able to resolve this problem of the temporal relationship between the tephra and its host alluvium."
"The mineral assemblage of SCt-CC is similar to SCt in the Fairbanks area. A strongly pleochroic hypersthene with fluid and opaque inclusions is abundant as are hornblende and plagioclase. Ilmenite and magnetite are conspicuous and trace amounts of basaltic hornblende, augite, and quartz are present. Many of the minerals do not have attached glass and are slightly rounded, but otherwise look the same as those with attached glass. A white frothy pumice is the dominant phase, and occurs together with a small amount of brown glass that is not observed in SCt in the Fairbanks area. The pumiceous glass has a rhyolitic composition but is very different from the glass in the SCt occurrences at Fairbanks; SiO2 and K2O are higher and TiO2, Al2O3, FeOt, CaO, and Na2O are lower (Table 2 [in original text]). The broadly similar petrographic features and a co-linear relationship with SCt on the K2O-SiO2 plot (Fig. 6 [in original text]) led to the view, albeit unpublished, that SCt-CC was co-magmatic with SCt, but, being more evolved, was likely slightly older-the more silicic cap of the magma body being tapped first."
"Bones beneath SCt-CC and near the base of unit 5 have a 14C age of 39,360+/-1740 B.P. (Weber and others, 1981) and a U-series age of about 80 ka (Hamilton and Bischoff, 1984). These discrepant age estimates both severely underestimate the 190 ka age suggested by the tephra correlation noted above. Bison bones are present in this fossil assemblage and point to a Rancholabrean land mammal age-in other words, SCt-CC is probably of MIS 6 age or younger."
References Cited
[1] Changing ideas on the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America, 2008
Westgate, J.A., Preece, S.J., Froese, D.G., Pearce, N.J.G., Roberts, R.G., Demuro, M., Hart, W.K., and Perkins, W., 2008, Changing ideas on the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America: Quaternary International, v. 178, n. 1, p. 183-209, doi: 10.1016/j.quaint.2007.03.009.[2] Canyon Creek: a late Pleistocene vertebrate locality in interior Alaska, 1981
Weber, F.R., Hamilton, T.D., Hopkins, D.M., Repenning, C.A., and Haas, Herbert, 1981, Canyon Creek: a late Pleistocene vertebrate locality in interior Alaska: Quaternary Research, v. 16, n. 2, p. 167-180.[3] Dating Early and Middle (Reid) Pleistocene glaciations in central Yukon by tephrochronology, 2001
Westgate, J.A., Preece, S.J., Froese, D.G., Walter, R.C., Sandhu, A.S., and Schweger, C.E., 2001, Dating Early and Middle (Reid) Pleistocene glaciations in central Yukon by tephrochronology: Quaternary Research, v. 56, n. 3, p. 335-348, doi: 10.1006/qres.2001. 2274 .Complete Eruption References
Dating Early and Middle (Reid) Pleistocene glaciations in central Yukon by tephrochronology, 2001
Westgate, J.A., Preece, S.J., Froese, D.G., Walter, R.C., Sandhu, A.S., and Schweger, C.E., 2001, Dating Early and Middle (Reid) Pleistocene glaciations in central Yukon by tephrochronology: Quaternary Research, v. 56, n. 3, p. 335-348, doi: 10.1006/qres.2001. 2274 .
Characterization, identity, distribution, and source of late Cenozoic tephra beds in the Klondike District of the Yukon, Canada, 2000
Preece, S. J., Westgate, J. A., Alloway, B. V., and Milner, M. W., 2000, Characterization, identity, distribution, and source of late Cenozoic tephra beds in the Klondike District of the Yukon, Canada: Canadian Journal of Earth Sciences, v. 37, n. 7, p. 983-996.
Changing ideas on the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America, 2008
Westgate, J.A., Preece, S.J., Froese, D.G., Pearce, N.J.G., Roberts, R.G., Demuro, M., Hart, W.K., and Perkins, W., 2008, Changing ideas on the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America: Quaternary International, v. 178, n. 1, p. 183-209, doi: 10.1016/j.quaint.2007.03.009.
Late Cenozoic geology, Ancient Pacific Margin NATMAP Project, report 5: paleoecology and proxy climatic change records, south Klondike placer region, Yukon Territory, 2002
Jackson Jr., L.E., Froese, D.G., Telka, A.M., Westgate, J.A., Preece, S.J., Storer, J.E., and Huscroft, C.A., 2002, Late Cenozoic geology, Ancient Pacific Margin NATMAP Project, report 5: paleoecology and proxy climatic change records, south Klondike placer region, Yukon Territory: Geological Survey of Canada, Currrent Research, 2002-A2, 16 p.
Late Cenozoic history of McQuesten map area, Yukon Territory, with applications to placer gold research, 1997
Bond, J.D., 1997, Late Cenozoic history of McQuesten map area,
Yukon Territory, with applications to placer gold research: University of Alberta, Edmonton, unpublished M.S. thesis, 161 p.
Continuous late Quaternary proxy climate records from loess in Beringia, 2001
Beget, J. E., 2001, Continuous late Quaternary proxy climate records from loess in Beringia: Quaternary Science Reviews, v. 20, n. 1-3, p. 499-507.
The Klondike goldfields and Pleistocene environments of Beringia, 2009
Froese, D.G., Zazula, G.D., Westgate, J.A., Preece, S.J., Sanborn, P.T., Reyes, A.V., Pearce, N.J.G., 2009, The Klondike goldfields and Pleistocene environments of Beringia: GSA Today, v. 19, n. 4, p. 4-10.
Paleoecology of two marine oxygen isotope stage 7 sites correlated by the Sheep Creek Tephra, northwestern North America, 2003
Schweger, C. E., 2003, Paleoecology of two marine oxygen isotope stage 7 sites correlated by the Sheep Creek Tephra, northwestern North America: in Westgate, J. A., (ed.), Special issue dedicated to Troy Pewe, Quaternary Research, v. 60, n. 1, p. 44-49.
Ash Bend Section, 1987
Hughes, O.L., Harington, C.R., Schweger, C.E., and Matthews, J.V. Jr., 1987, Ash Bend Section, in Morison, S.R, and Smith, C.A.S., eds., Guidebook to Quaternary research in Yukon: XII INQUA Congress, Ottawa, Canada, National Research Council, Ottawa, Ontario, p. 50-53.
Late Pliocene Dawson Cut forest bed and new tephrochronological findings in the Gold Hill Loess, east-central Alaska, 2009
Pewe, 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, v. 121, n. 1-2, p. 294-320, doi: 10.1130/B26323.1 .
Age of Sheep Creek Tephra (Pleistocene) in central Alaska from thermoluminescence dating of bracketing loess, 1996
Berger, G. W., Pewe, T. L., Westgate, J. A., and Preece, S. J., 1996, Age of Sheep Creek Tephra (Pleistocene) in central Alaska from thermoluminescence dating of bracketing loess: Quaternary Research, v. 45, n. 3, p. 263-270.
Stratigraphy, major oxide geochemistry, and 40Ar/39Ar geochronology of a tephra section near Tok, Alaska, 2002
Schaefer, J. R. G., 2002, Stratigraphy, major oxide geochemistry, and 40Ar/39Ar geochronology of a tephra section near Tok, Alaska: University of Alaska Fairbanks unpublished M.S. thesis, 62 p.
full-text PDF 2.06 MB 
Eva Interglaciation Forest Bed, unglaciated east-central Alaska: Global warming 125,000 years ago, 1997
Pewe, 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: Geological Society of America Special Paper 319, 54 p.
Fullglacial paleosols in perennially frozen loess sequences, Klondike goldfields, Yukon Territory, Canada, 2006
Sanborn, P.T., Smith, C.A.S., Froese, D.G., Zazula, G.D., Westgate, J.A., 2006, Fullglacial paleosols in perennially frozen loess sequences, Klondike goldfields, Yukon Territory, Canada: Quaternary Research, v. 66, p. 147-157.
Late Pleistocene mountain glaciation in Alaska: key chronologies, 2008
Briner, J.P., and Kaufman, D.S., 2008, Late Pleistocene mountain glaciation in Alaska: key chronologies: Journal of Quaternary Science, v. 23, n. 6‐7, p. 659-670.
Canyon Creek: a late Pleistocene vertebrate locality in interior Alaska, 1981
Weber, F.R., Hamilton, T.D., Hopkins, D.M., Repenning, C.A., and Haas, Herbert, 1981, Canyon Creek: a late Pleistocene vertebrate locality in interior Alaska: Quaternary Research, v. 16, n. 2, p. 167-180.
