Description: From Coombs and others (2017): "The earliest and most substantial Holocene deposit on the island is a widespread ignimbrite, produced during the latest caldera-forming eruption.
Semisopochnoi caldera covers approximately 43 square km and has walls that vary in prominence.
The pyroclastic-flow deposit, or ignimbrite, that was ejected during the caldera-forming eruption covers large parts of Semisopochnoi Island, and it can be identified by its fairly flat but incised morphology where it has been covered by younger tephra.
The stratigraphy of the ignimbrite of the caldera-forming eruption varies considerably by sector and distance from the caldera. In several places, a several-meter-thick tephra-soil complex mantles the deposit and limits its exposure. Despite stratigraphic variations, the general depositional sequence typically consists of a brick-red, oxidized, and lithic-rich top unit that is as much as several meters thick. Many tephra sections bottom out on this unit, which is indurated in most exposures. The lithic-rich top overlies the main body of the ignimbrite, which is typically massive, poorly sorted, and rich in tan-to-black, variably vesicular pumiceous clasts.
The ignimbrite is variably welded and (or) indurated. Where exposed near the south and east caldera walls, a welded horizon is typically as much as several meters thick, dark gray to black, variably oxidized, and fairly well indurated. Clasts in the welded parts are dense to moderately vesicular and contain striking 2- to 3-mm-diameter plagioclase phenocrysts. This subunit, which forms resistant bands in outcrop, is probably the rock type mapped by Coats (1959) as unit Qbt, which he described as basaltic tuff breccia with minor basaltic flows. Along the southern caldera margin, the welded band is 1-3 m thick, whereas east of Fenner Lake it is >10 m thick.
The exact location of the vent for this violent explosive eruption is unknown but almost certainly lies within the area of the current caldera. The initial strata deposited by this eruption evidently are not exposed, and so it is unknown whether the eruption began with a Plinian fall, although that is a likely supposition on a comparative basis with similar eruptions. The subsequent ignimbrite commonly is partly welded near the vent, suggesting that it may have been emplaced by pyroclastic flows fed by a collapsed eruption column(s) issuing at high mass-eruption rates.
On the basis of our tentative age assignment, the Semisopochnoi ignimbrite should be found below the intermediate tephra and above the main tephra on Adak Island."
From Derkachev and others (2018): "The ~ 12.2 ka SR2 tephra forms a visible layer in the sediments on the Shirshov Ridge and at the northeastern margin of the Aleutian basin (Figs. 1, 2,and 3). Its thickness varies from 2 to 4 cm.
The extent and volume of the SR2 tephra, and consequently the magnitude of the parent eruption, can now bpreliminary constrained from these data. The dispersal axis was directed NNW from the source (Fig.1) [in text]. Based on the known thicknesses of the SR2 tephra in all studied cores, we can here draft a preliminary tephra dispersal area (Fig.1 [in text]; Online Resource 5). The sites on the Shirshov Ridge are located close to each other and have a median tephra thickness of 3 cm. These sites, together with sites Lv63-30, -22, and -23 located at the northeastern margin of the Aleutian basin (Fig. 1) [in text], permit us to delineate a 2-cm isopach as a minimum concave line bounding all of these sites. The area within the 2-cm isopach is 767,400 km2, so that by applying the single-isopach approach (Legros 2000) we obtain a minimum ash-fall volume of 54 km3. This corresponds to a volcanic explosivity index (VEI) six (Newhall and Self 1982). Assuming an ash density of 0.6 g/cm3 (Kutterolf et al. 2008b), we obtain SR2 tephra mass of 3.2 × 104 Mt, and aDRE volume of 12 km3. This corresponds to an eruption magnitude 6.5 (Pyle 1995; Mason et al. 2004). These are minimal estimates as they are based on only a few thickness measurements for SR2 tephra, and our calculations do not include the volume of the caldera fill or of ignimbrite dispersed beyond the caldera.
Based on the estimated size and geochemical similarity to the Semisopochnoi ignimbrite, we suggest that the SR2 tephra was a product of the caldera-forming eruption. Our age estimate of ~ 12.2 ka, as well as the stratigraphic position of the SR2 tephra within the Younger Dryas sediments, do not agree with a preliminary age estimate of 7.7-7.8 ka suggested by Coombs et al. (2018) for this ignimbrite. This discrepancy needs further investigation. At the same time, we do not observe any younger tephra of SR2 (and ignimbrite) composition in any of our cores. As a result, two closely spaced different and large eruptions from Semisopochnoi seem unlikely. The SR2 tephra is widely dispersed in the Aleutian basin, and can serve as a marker for the Younger Dryas climate interval."