The Extent of the White Chuck Tuff, a High Temperature Pyroclastic Flow Deposit, Glacier Peak, Washington

• Main Author: Ladd, Gerald T.
• Format: Text
• Language: unknown
• Published: Western Washington University 2004
• Subjects: Glacier Peak; Kennedy Creek; North Fork; Geomagnetic Pole
• Online Access: https://dx.doi.org/10.25710/8jt1-qq48; https://cedar.wwu.edu/wwuet/671

The White Chuck Tuff, a massive deposit approximately 15 m thick, caps two terraces in the White Chuck River valley covering an area of approximately 5 km 2 at the base of Glacier Peak, Washington. Three major post-glacial eruption cycles from Glacier Peak reportedly occurred approximately from 12,000 to 11,250 years ago (White Chuck Assemblage), from 5,500 to 5,100 years ago (Kennedy Creek Assemblage), and from 1,800 to 250 years ago (recent eruptions). West of Glacier Peak, pyroclastic and lahar deposits from all three episodes are found in drainages out to Puget Sound 100 km away. The White Chuck Tuff has been assumed to be approximately 11, 500 years old (Beget, 1981) and not found west of Camp Creek in the White Chuck River Valley, approximately 17 km from Glacier Peak. Anisotropy of magnetic susceptibility, paleomagnetic, petrographic, and geochemical procedures were conducted to characterize the tuff deposit. Similar laboratory procedures were conducted on five distal pyroclastic deposits to determine if they were unconsolidated runout of the White Chuck Tuff. Flow direction, paleomagnetic directions and paleomagnetic poles, mineralogy, and chemical composition of the proximal indurated White Chuck Tuff indicate that it was emplaced as a single unit at temperatures exceeding 580° C. The overall flow direction during emplacement was northwesterly down the White Chuck River Valley. The paleomagnetic directions, mineral and chemical compositions are similar amongst the seven sampling sites. Five distal deposits WC-1, SR-1, SR-2, SR-3, and ST-1 were products of cold to warm (from 22°C to 375°C) debris flows that made their way down the White Chuck River Valley into the Sauk and North Fork Stillaguamish River Valleys. The paleomagnetic directions amongst the distal deposits were not well defined. The anisotropy of magnetic susceptibility of these deposits had no preferred orientation of the magnetic fabric indicating that the flow direction and individual clast anisotropy were independent of emplacement mechanisms. A viscous magnetization was measured in many of the pumiceous clasts sampled at all of the cooler distal sites and upward directions were measured in the low unblocking temperature range of 100 to 300° C in many of the samples from these sites. The clasts from these deposits had a previous magnetic history before they came to rest at their present location. The chemistry and mineralogy of all five distal sites is similar and indicate dacitic composition. The virtual geomagnetic pole of distal deposit WC-1 closely corresponds with the 9180 +290/-200 virtual geomagnetic pole of Hagstrum and Champion (2002), which post dates the White Chuck Assemblage. Virtual geomagnetic poles of other distal deposits are not well enough defined to be useful. From field relationships associated with the deposition and location of other deposits (Dravovieh et al, 2003), the Glacier Peak distal deposits were probably produced during the Kennedy Creek Assemblage eruption cycle. The virtual geomagnetic pole of the White Chuck Tuff deposit matches the 12,750 b.p. virtual geomagnetic pole of Hagstrum and Champion (2002). This virtual geomagnetic pole dates the tuff's deposition before the White Chuck Assemblage eruptive cycle. Therefore the previously assumed age of 11,500 b.p. is probably incorrect.