A geoarchaeological perspective on late Holocene Puyallup River delta history using lithostratigraphic data from Xaxtl'abish 1 on the Hylebos Creek distributary in Tacoma, WA

Geoarchaeological data from the pre-contact archaeological midden site Xaxtl'abish 1 (45PI974) on the Hylebos distributary of the Puyallup River delta contribute to understanding local site formation processes and the regional pre-contact landscape near what is now Tacoma, Washington. The post-glacial history of deltaic progradation, non-seismic deltaic subsidence, tectonic subsidence, and eustatic sea level rise along the Puyallup River delta front provides temporal limitations for late Holocene occupation of the delta, as well as expectations for past land use of the Hylebos Creek vicinity. The formation history of Xaxtl'abish 1 was developed by examining the stratigraphy and lithology of the natural and cultural deposits hosting, composing, and preserving the site using an allostratigraphic approach and considering post-depositional alteration and preservation. Seven strata, including intertidal delta, delta plain, midden, peat, wetland, floodplain, and fill were identified at Xaxtl'abish 1 based on the distribution of 22 discrete lithofacies. Radiocarbon dates collected from Xaxtl'abish 1, nearby archaeological sites, and other geological contexts across the Puyallup River delta allow estimates of late Holocene aggradation rates and provide limiting dates for human occupation of the delta front. Preliminary results show the lower Puyallup River delta aggraded at an average rate of about 15 cm a year during the middle to late Holocene. Late Holocene archaeological sites may be buried within the upper 11 m of sediment on the delta front and are younger than 3140 cal. BP. The site formation history of Xaxtl'abish 1 argues for the presence of other deeply buried archaeological sites in similar sub-environments on deltas around Puget Sound.