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)."
"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."
"Sheep Creek-F tephra erupted from a vent in the WVF about 190 ka with fallout directed to the northwest across the Fairbanks region of central Alaska. Mount Drum is the most likely source because of strong compositional similarities with Mount Drum pumice, close Sr isotope values, and the fact that Mount Drum experienced a catastrophic eruption sometime after 240 k."
"SCt takes its name from an occurrence in the east wall of the Sheep Creek Cut mining exposure, about 15km north-west of Fairbanks, Alaska (Fig. 2, locality 8 [in original text]; Table 1 [in original text], [sample] UT734). It was initially sampled by T.L. Pewe in 1956 and sent to Westgate in 1971, when it was determined to be chemically and petrographically distinct from other tephra beds then known in the Fairbanks region. At this site, SCt occurs as a 2- to 3-cm-thick white layer 1.5 m below the contact of Gold Hill Loess (GHL) and the overlying retransported loess of the late Pleistocene Goldstream Formation (Preece and others, 1999). The GHL is 20m thick here and overlies the Cripple Gravel of late Tertiary age. The Upper Eva Creek site (Fig. 2, locality 6 [in original text]; [sample] UT746) is important because of the association of SCt with Old Crow tephra ([sample] UT744), and the Dome Ash Bed ([sample] UT745). At this site, SCt has a maximum thickness of 6 cm and has been deformed into a solifluction fold within the GHL. Old Crow tephra and the Dome Ash Bed are 3 and 4.5 m above SCt, respectively. Additional SCt sample sites in the Fairbanks area are shown in Fig. 2 [in original text] and listed in Table 1 [in original text]."
"SCt is a white bed with abundant mafic minerals. It has a maximum thickness of 6 cm and a characteristic tripartite stratigraphy in the Fairbanks area: namely, a lowermost sandy layer with abundant mafic crystals, a finer-grained vitric unit, and a capping layer of crystal-rich reworked tephra. The major mineral species are hornblende, plagioclase showing disequilibrium features, such as embayed cores that were later overgrown, and lesser amounts of ilmenite, magnetite, a strongly pleochroic hypersthene, and trace quantities of basaltic hornblende, quartz, apatite, and augite. The glass consists of frothy, highly vesicular pumice fragments with numerous microphenocrysts. It has a calc-alkaline rhyolitic composition (Table 2 [in original text]), although a few shards are dacitic. Equivalence of all SCt samples in the Fairbanks area is supported by the coherent, tight glass compositional trends seen in Fig. 3 [in original text]."
"Stratigraphic evidence at the Upper Eva site demonstrates that SCt is older than Old Crow tephra, which has a glass-ITPFT age of 140 710 ka (Preece and others, 1999)-an age supported by loess thermoluminescence (TL) ages (Berger and others, 1996). The minimum and maximum probable TL ages for SCt are 178+/-725 and 205+/-731 ka, giving a probable age for SCt in the Fairbanks area of 190+/-720 ka (Berger and others, 1996)."