Description: From Fierstein (2007): "From the vent at an elevation of 2,010 m, two of the flows extend 6 km to termini as low as 300-350 m in Martin Creek; we call these the "south lobes". One of the youngest flows bifurcates to the northeast; we call this the "Y-flow". The rim of the ice-filled crater on the East Summit is strewn with coarse scoriae and dense glassy blocks of phreatomagmatic ejecta, deposited upon the surfaces of thin lava flows exposed locally. Compositional data for one such ejecta block is used here to represent the "East Summit" in the following correlations."
"Considerable overlap in the microprobe data between the different East Mageik lavas and tephras (Fig. 23 [in original text]) preclude firm correlations, but we tentatively suggest that the magnetite data correlate the south lobes with ODLF [orange dacite lapilli fall] ash (3,800-4,000 14C years B.P.)..."
"One of the most distinctive tephra layers in Windy Creek and the lower reaches of the VTTS is an orange-stained dacite fallout unit that typically lies 20-60 cm beneath the 1912 deposits and stratigraphically below the two grey ash layers just described. The dacite lapilli themselves are not particularly distinctive, having phenocrysts typical of most other dacitic fallout in the region (plagioclase+FeTi oxides+sub-equal proportions of opx and cpx; Table 1 [in original text]). In uppermost Windy Creek, where both the "lower grey ashfall" and ODLF units are coarse granules and lapilli and are rarely separated by a soil horizon, the two are distinguished in the field principally by abrupt fining at the top of the ODLF unit (Fig. 9 [in original text]). Only there are the two fall units so similar; farther down Windy Creek, the ODLF unit is always coarser than the lower grey ash (Figs. 8, 9 [in original text]). Soil does separate the two fall units in the lower VTTS area, where the ODLF layer is distinctive because it is widely preserved and is coarser-grained than the other pre-1912 ash layers there. Where best preserved in the upper reaches of Windy Creek, this 20-cm-thick fall unit is reversely graded at its base, with the coarsest pumice lapilli (as large as 5.5 cm, commonly to 2-3 cm) in the middle of the layer, but grading normally up to fine grey ash at its top (Fig. 9 [in original text]). Another 11 km northward in lower Windy Creek, the ODLF thins to 5 cm, with the largest pumice lapilli just over 1 cm; and 8 km still farther ENE, it thins to half that (2.5 cm) and the largest clasts fine to only a few millimeters. Only a whiff of this ash is found 1.75 km still further ENE, where locally preserved, poorly defined ash remnants are barely 3-4 mm thick."
"ODLF lapilli and granules are conspicuous on cutbanks because of their orange color, which is pervasive in smaller clasts, but only surficial on larger (fundamentally grey to tan) ones. The nearly ubiquitous orange coloring of almost all lapilli and granules indicates eruption-related oxidation, not in-situ weathering of the dacitic clasts. The thickness and coarseness of this fall unit in Windy Creek suggests a source at one of the nearby Holocene vents, Mount Martin or East Mageik. The layer is also preserved along Angle Creek, where it is markedly thinner and finer than along Windy Creek (Fig. 8 [in original text]). Isopachs and isopleths suggest a narrow northerly distribution of the ODLF fallout."
"Whereas in the lower VTTS and Windy Creek the lower grey fall unit is difficult to distinguish in the field from the upper grey ash, in Angle Creek the difficulty becomes distinguishing it from the ODLF. Both are orange-stained lapilli-to-granule falls in the headwaters of Angle Creek, indistinguishable in the field or by titanomagnetite analyses in the lab (Fig. 10a [in original text]). Discrimination is achieved, however, by their glass compositions. Although the data fields overlap, the ODLF is, overall, less silicic (71-76% SiO2) than the lower grey ash (74.5-78% SiO2; Fig.10b [in original text]). Where both are preserved in a single section farther downstream in Angle Creek they are separated by as much as 30 cm of dense brown peat (section K-2500; Fig. 3 [in original text]). There, the ODLF (K-2500H) is a 1-cm thick fine to medium ash (maximum clasts ∼0.3 cm) whereas the "lower grey ash" is a 5-cm-thick coarser granule layer (maximum clasts to 0.6 cm). Thickness data thus clearly show that the ODLF came from East Mageik, as it thins toward Mount Martin from its thickest exposures in Windy Creek. 14C dates on soils beneath and above the orange dacite lapilli fall bracket its deposition between ∼3,600 and ∼4,300 14C years (Figs. 3, 4; Table 2 [in original text]), although stratigraphy and evaluation of the 14C dates favors 3,800 to 4,000 years."