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 ~0.1 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 ~5000 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.