Age and Environment of Allochthonous Peat Clasts from the Bogachiel River Valley, Washington

Abstract

Peat clasts that are 18,800 ± 400 radiocarbon yrs old, redeposited in alluvial sediments of the lower Bogachiel River, date from the Evans Creek Stade of Fraser Glaciation. Pollen analyses of eight samples of the clasts show grass and sedge in amounts between 77 and 86 percent of the pollen sums and indicate treeless or near-treeless vegetation and environment. These analyses agree with analyses of sediments of a comparable age from a sea-cliff section near Kalaloch, about 28 km south of the Bogachiel site. At this locality, grass and sedge reach 81 percent of the pollen sum and portray a tundra across this portion of the western Olympic Peninsula. Farther south at Point Grenville, where lodgepole pine–dominated forest is evident, environmental conditions were less severe. The tree line, therefore, was located somewhere between Kalaloch and Point Grenville at that time. The Bogachiel clasts were redeposited during the early part of the Holocene, about 9,500 yrs ago. Pollen analyses from a measured section exposing the clasts disclose striking values for Sitka alder, amounting to as much as 65 percent of the pollen sum at the time of redeposition. These amounts of alder are representative of the regional, early Holocene pollen sequence reconstructed from bog and lake stratigraphy. A cool but ameliorating climate was manifest when the clasts were emplaced. The glacier that flowed farthest down the Bogachiel River valley, tentatively dated as early Salmon Springs, advanced to a position some 6 km beyond the clast site. Ice of late Salmon Springs age reached only to the Hoh-Bogachiel drainage divide, 6.5 km southeast of the site. It is not known how far the Vashon glacier extended down the Bogachiel during Fraser Glaciation, but available evidence indicates that the glacier terminated in the upper part of the valley. In the Hoh drainage to the south, the Vashon ice front stood about 6 km from the Hoh-Bogachiel divide, and recession began some time prior to 15,600 radiocarbon yrs ago. It is suggested that glacier ice in the Bogachiel valley melted away by 6,500 B.P.