Tucker, D., Scott, K., Foit, Jr., F. and Mierendorf, R., 2007, Age, distribution and composition of Holocene tephras from Mount Baker, Cascade arc, Washington, USA: Geological Society of America Abstracts with Programs, v. 39, n.4, p. 66.
Age, distribution and composition of Holocene tephras from Mount Baker, Cascade arc, Washington, USA
The oldest known Mount Baker tephra is layer SP, 10720 ?? 50-10870 ?? 80 BP, known only from the south flank. SP occurs as a lens of gray sandy 2-px andesite ash ≤ 1 cm thick 7 km S of Sherman Crater. Vitric and lithic pyroclasts predominate; clear glass shards (68 wt% SiO2) are the bulk of the fine fraction. The Schreibers Meadow eruptive period produced two basaltic (51-52% SiO2) units of tephra set SC from the Schreibers Meadow cinder cone, distributed to the NE. Olivine-bearing SC1 scoria lapilli to 3 cm are 1 m thick 4 km north of the vent; fine ash fell 18 km NE of the cone. Charcoal in SC1 gave 14C ages of 8750 ?? 50, 8830 ?? 50, and 8850 ?? 50 BP. SC2 mantles SC1, is entirely ash-sized and is 6 cm thick 3 km NNW of the cinder cone. It resembles distal deposits of SC1. There are two Mazama Park eruptive period tephras. Hydrovolcanic OP (5800 ?? 50 BP), mainly hydrothermally altered lithic material, is ≤ 2 cm thick only 8 km NE of Sherman Crater. OP forms a couplet with overlying magmatic and 2-px andesite BA tephra (glass 69% SiO2). Charcoal in BA gave 5740 ?? 50, 5730 ?? 50, 5670 ?? 60 BP. This most voluminous Baker ash is reported from Copper Ridge, 31 km NE, and Cascade Pass, 66 km SE. Volume is ü0.1 km3. Tephra set YP dates to the 19th C Sherman Crater eruptive period. Stratigraphy strongly suggests that this is the historically reported 1843 Mount Baker ash; at one site, it is directly overlain by an 1845-47 lahar. The root-zone ash is <1 cm thick 31 km SE, 22 km S, and 19 km NE of Sherman Crater. Ash is poorly sorted, and thickens towards Baker. Beds 3 cm thick on the flanks of Mount Baker include both fine glassy ash (<4Φ), and altered lithic andesite lapilli. Microprobe analysis of the glass suggests it is a mixture of MSH Wn, Mazama O, Glacier Peak Dusty Creek ashes (spanning ü5000 years) and more abundant BA-like ash. The mixing process is unknown; no Glacier Peak ash is known on Mount Baker, and YP is found in places where BA is not present. Vesicular lapilli to 4 mm are commonly found in a bed 16 km SE of the crater; some are very well rounded. This may reflect milling in the vent by repeated explosive blasts. These lapilli are not necessarily juvenile- they may have been entrained in eruption columns. Credible reports of 19th C 'glowing sparks' and 'jets of flame' exist. The absence of larger vitric pyroclasts nearer the vent suggests the YP tephra does not have a juvenile magmatic component.