Event Name : Martin Salt and Pepper Ash
|Start: 2660 (± 130 Years) || Years BP C-14 (raw) || |
From Fierstein (2007): "Only a few centimeters beneath the upper pale tan ash is another sharply defined and persistent pale tephra 0.5-cm thick (K-2500K in this [Angle Creek] section). Distinctive in glass composition (?71-72 wt.% SiO2, CaO ?1.5-1.8 wt.%) and yielding microprobe data that plot in a tight group (Fig. 13 [in original text]), this sandy layer (Md=0.35 mm; Table 1 [in original text]) is similarly coarse eastward (Md=0.34 mm) and thickens to 7 cm atop the Martin coulees (K-2661C; Figs. 3, 14 [in original text]) only 3 km SE of section K-2500. It apparently thins eastward from there, however, being only 3 cm thick in the headwaters of Angle Creek and 0.5 to 1 cm thick in the lower VTTS (K-2675; Figs. 3, 14 [in original text]). Both thickness and clast-size data point to Mount Martin as the source for this fallout. Most noteworthy of the maximum clast data in Fig. 14 [of original text] are the 1-to-1.7-cm lithics in the fall layer at the head of Angle Creek. One of the most distinctive aspects of this tephra is its "salt and pepper" look in the field. Not only is it crystal rich with pyroxenes (opx and cpx) and feldspars contributing this dark and light coloring, but it also includes a mixture of white-to-clear glass and distinctive dark brown to black, granular and friable lithic clasts. These lithics, similar in appearance to those in the mica-bearing ash, are loosely coherent crystal sand, with dark-light stratification evident in some of the larger clasts. Likely to be a sort of tuff deposit at the vent disrupted by the ash eruption, these lithic clasts-although not a unique lithology, are distinctive-add a useful identifying criterion for this tephra layer. Only in Angle Creek section K-2500 do the brown-black granular clasts (up to 8 mm in size) make up much of the sample; at the head of Angle Creek, although these tuffaceous clasts are present, the predominant lithics are the denser bedrock (Jurassic Naknek) siltstone. At other more distal locations sampled, white fibrous pumiceous granules are the dominant component, with only minor amounts of the tuffaceous lithics."
"At all three locations with microprobe data, the glass plots in a tight group distinctively less silicic than all the tephras from Mount Mageik (e.g., lower grey and ODLF) and more silicic than those thought to be from Mount Peulik (e.g., upper grey; Fig. 13a [in original text]). FeTi-oxide data from these same three samples are less definitive, however, especially because some of the magnetite may have been derived from the friable, tuffaceous lithics; K-2500K has a bimodal distribution of low and high-MgO magnetite (2.2 wt.% and 4.2 wt.% MgO), K-2675A has magnetite that spans that entire range, and K-2661C has magnetite restricted to ?2.2 wt.% MgO (Fig. 13b [in original text]). Of note is that this range of magnetite compositions is similar to that from other tephra layers suggested here to have been derived from Mount Peulik (upper grey ash, mafic orange fine ash, and mafic crystal ash). Relatively coarse grain size and thickness distribution, however, preclude Mount Peulik as a source for this ash layer. Instead, this tephra may have erupted during growth of the summit-forming ejecta cone (?59-61 wt.% SiO2) that caps the stubby dacite lava flows high on Mount Martin (?64 wt.% SiO2; Figs. 7, 8 in Fierstein and Hildreth 2001), which are in turn perched atop the older major mid-Holocene sprawling dacite coulees in Angle Creek (?60-63 wt.% SiO2). 14C ages for soils loosely bracket the "salt and pepper" ash ("K" in section K-2500 in original text) between 2,310+/-120 and 3,355+/-135, and soil directly beneath the ash atop the Martin coulees (section K-2661 in original text) yields 2,700+/-100 and beneath the ash in upper Angle Creek (K-2069) yields 2,515+/-195. On this basis, and considering that the upper pale tan ash was also deposited during this interval, the favored deposition time for the salt and pepper ash is between 2,700 and 2,800 14C years B.P."