Geochemical characterization and dating of R tephra, a postglacial marker bed in Mount Rainier National Park, Washington, USA

Abstract

The oldest postglacial lapilli–ash tephra recognized in sedimentary records surrounding Mount Rainier (Washington State, USA) is R tephra, a very early Holocene deposit that acts as an important stratigraphic and geochronologic marker bed. This multidisciplinary study incorporates tephrostratigraphy, radiocarbon dating, petrography, and electron microprobe analysis to characterize R tephra. Tephra samples were collected from Tipsoo Lake and a stream-cut exposure in the Cowlitz Divide area of Mount Rainier National Park. Field evidence from 25 new sites suggests that R tephra locally contains internal bedding and has a wider distribution than previously reported. Herein, we provide the first robust suite of geochemical data that characterize the tephra. Glass compositions are heterogeneous, predominantly ranging from andesite to rhyolite in ash- to lapilli-sized clasts. The mineral assemblage consists of plagioclase, orthopyroxene, clinopyroxene, and magnetite with trace apatite and ilmenite. Subaerial R tephra deposits appear more weathered in hand sample than subaqueous deposits, but weathering indices suggest negligible chemical weathering in both deposits. Statistical analysis of radiocarbon ages provides a median age for R tephra of ∼10 050 cal years BP, and a 2σ error range between 9960 and 10 130 cal years BP.

Defining the potential source region of volcanic ash in northwest Europe during the Mid- to Late Holocene

“…The unidentified shards include some datapoints whose geochemistries suggest the presence of mineral inclusions in the analyses, but a core group of medium-Ca (CaO ~2.5 wt. %) composition lies with the geochemical field of proximal glass from Rainier(Sisson and Vallance, 2009;Samolczyk et al, 2016; Fig. 8).…”

“…Compositional envelopes are based on published data (excluding outliers): Rainier (>65 wt. % SiO2 only; Sisson and Vallance, 2009;Samolczyk et al, 2016); Mazama (Pyne-O'Donnell et al, 2012;Egan, 2016); Mount St Helens MSH-W (Pyne-O'Donnell et al, 2012;Mackay et al, 2016). Mean values for glass of Mount St Helens Set Y (Ye, Yn, Yb: Westgate, 1977) are also shown.…”

Glacier fluctuation chronology since the latest Pleistocene at Mount Rainier, Washington, USA

“…McNeeley Drift consists of moraines overlain by R tephra (Table 1) that at the time Crandell and Miller published their report was thought to be >8.75 ka (Crandell et al, 1962). More recently, Samolczyk et al (2016) assigned an age of about 10 ka to R tephra (Table 1); the age is discussed further and updated in this paper. McNeeley moraines are rare but where present, they typically lie up to a few hundred meters downvalley from Garda moraines.…”

“…Age (ka) Source W 0.471 a Mount St. Helens (Mullineaux, 1974;Yamaguchi, 1983;Fiacco et al, 1993) C 2.2 a Mount Rainier (Mullineaux, 1974;Swanson, 1993) Y 4.40-3.60 a Mount St. Helens (Mullineaux, 1974;Crandell et al, 1981;Sisson and Vallance, 2009) O 7.777-7.477 b Mazama (Zdanowicz et al, 1999;Egan et al, 2015) R 10.13-9.96 a Mount Rainier (Samolczyk et al, 2016) R (new) >11.6 Mount Rainier (this paper)…”

“…11 and 13). The calibrated median age of 10.05 ka for Rainier R tephra reported in Samolczyk et al (2016) was modeled from five bulk sediment or macrofossil ages from three locations on Mount Rainier (Heine, 1997(Heine, , 1998aSamolczyk et al, 2016). Two new bulk sediment radiocarbon ages from our Tipsoo Lake core, one from 88-cm depth with a LOI value of ∼18%, and one from 98 cm depth with a LOI value of ∼28% and sitting just above the transition to clay at 100 cm depth, calibrate to median ages of 10.83 ka and 12.41 ka, respectively (Table 2).…”