Description: From Fierstein (2007): "The youngest pre-1912 tephras widely preserved in and near the lower Valley of Ten Thousand Smokes are a pair of thin, fine-grained, grey to grey-tan ash layers sandwiched between the 1912 fall and a distinctive orange-stained dacite lapilli fall (described below [in original text]). Where best preserved in the lower VTTS, the younger of the two is typically 1.5 cm thick (ranging from 0.5 to 3.5 cm, Fig. 6 [in original text]) although it is not uncommonly expressed as discontinuous grey lenses or wisps of grey fine ash (a little gritty to touch) aligned in the enclosing soil. Its dominant crystals are plagioclase and two pyroxenes, with an opx/cpx ratio of 20:1 (Table 1 [in original text]). Because of its wide preservation here (even though it is thin), and because the ash is gritty (rather than silty), the source was originally presumed to be local. However, microprobe data for glass and Fe-Ti oxides (Fig. 7 [in original text]), permit this ash to be correlated with a grey ash layer found as far as 42 km to the southeast (beyond Mount Katmai) and ∼30 km SSE in Martin Creek (Fig. 6 [in original text]). In almost all sections south of the volcanic axis this layer is a slightly gritty ash about 1 cm thick, with no obvious thinning or fining trends to indicate the source vent. An exception is in the well-preserved peat section in Angle Creek (Fig. 3 [in original text]) where the uppermost ash is actually a composite of two grey ashfalls, the lower (2 cm) of which is fine ash and the upper (1.5 cm) is medium ash, each including a 0.5-cm cap of very fine vitric silt. These sublayers appear to represent two pulses of the same eruption, as microprobe data for the young grey ash at 15 other locations indicate correlation, with the single layer being a more distal expression of the more proximal Angle Creek sublayers."
"Glass analyses suggest this eruption was compositionally zoned, with silicic and mafic components represented in the analyzed shards (Fig. 7a [in original text]). It is possible that the two sublayers in Angle Creek represent separate mafic and silicic pulses, but samples are not in hand to test that possibility. Of the 14 upper grey ash samples analyzed, three have only silicic glass (∼64 to 68 wt.% SiO2; 3 to 4wt.% CaO), two have only mafic glass (∼57 to 64 wt.% SiO2; 7 to 5 wt.% CaO), and nine have glass representing that entire range. Similarly, magnetite analyses of eight of these samples, both mafic and silicic (Fig.7b [in original text]), have distinctive MgO-TiO2 trends (MgO ∼2.7 to 4.3 wt.%); two samples have MgO from 1.2 to 2.8 wt.%, and five samples, including the one from Angle Creek, have magnetite that plots in both groups. Although the subordinate population of lower-MgO magnetite is similar to other Katmai group tephras, the higher-MgO magnetite is distinct from all other locally derived fall layers yet analyzed. Considering this, and the 3.5-cm thickness and two-layer stratigraphy preserved in Angle Creek-west of all the Katmai volcanoes studied-suggests the vent for this ashfall was westward. In agreement, the fairly constant ∼1-cm thickness measured over a wide area around the Katmai group of volcanoes suggests this ash was not locally derived. In contrast, the gritty (not silty) grain size suggests the source could not have been too far away. Mount Peulik, the next young stratovolcano, 95 km SW down the volcanic chain from Mount Katmai, seems a likely source. If so, Millers (2004) descriptions of Mount Peulik narrow the possible source of this upper grey ash to the opx-rich lavas of the present steep-sided cone, because older flows and younger domes include hornblende and biotite, which are not present in this tephra."
"Seven of the radiocarbon dates for soils beneath this ash layer are in good agreement and range between ∼400 and 700 14C years (Table 2 [in original text]). Another soil 6.5 to 8.5 cm beneath the ash yields an age of 1,190+/-70 14CyearsB.P., which-considering its stratigraphic position-is consistent with the other seven. Two additional ages, however, are considerably older (1,130+/-60 14C years B.P. on top of the Martin coulees; and 1,980+/-70 14C years B.P. on top of a Mageik lava), and shed some doubt on those young ages. Although rootlet contamination (in spite of careful sample preparation) could contribute toward the youngest soil dates, slow soil development or erosion of the uppermost soil layer prior to tephra deposition might result in soil ages that are much older than the overlying ash layers. The number of (seemingly good) younger dates favors an age for the upper grey ash of ∼700 14C years. However, because the entire soil section on top of the Martin coulees (K-2661) yielded organic-rich samples free of any modern rootlets, one cannot rule out that∼1,100 14C years B.P. is a better record of the age of this tephra."