Provenance of the Hillsboro Formation: Implications for the Structural Evolution and Fluvial Events in the Tualatin Basin, Northwest Oregon
Quantitative heavy mineral and geochemical analyses of Neogene nonmarine clastic deposits are used to compare provenances of two sedimentary sequences lying in adjacent forearc basins in northwestern Oregon and to interpret structural and depositional events affecting both basins. The northwest-trending Tualatin Mountains-Portland Hills (TMPH) faulted anticline presently separates fine-grained sediments of the Hillsboro Formation in the Tualatin Basin from coarse-grained sediments of the Troutdale Formation in the Portland Basin.

Augite-hornblende-hypersthene relative percentages of sands and instrumental neutron activation analysis of silts and clays from the Hillsboro and Troutdale formations indicate a difference in provenance between the two formations. Hillsboro Formation sands are low in hypersthene (< 7%) and are derived from Paleogene basalts and sedimentary rocks of the northern Oregon Coast Range and surrounding highlands. Troutdale Formation sands may contain either relatively high amounts of hypersthene (up to 85%) or little hypersthene, reflecting mixed provenances from the Cascades volcanic (hypersthene source) arc, the Rocky Mountains, and the Oregon Coast Range. Elemental geochemistry plots of silt and clay samples in both formations illustrate overlapping yet distinctive basin groups, supporting the interpretation of separate provenances for the two sedimentary sequences.

The provenance difference between the two basin sequences suggests that a highland, the faulted TMPH anticline, blocked the Columbia River system from depositing sediments in the Tualatin Basin since late Miocene time. The basin-fill sequences in each basin evolved independently, although they are geographically separated by only a few kilometers.

Elevated hypersthene levels in Hillsboro Formation sands from the eastern margin of the Tualatin Basin, along with a correlative diatomaceous, lacustrine mudstone in the central part of the basin, indicate that the Tualatin Valley was inundated during the upper Pliocene or lower Pleistocene; Boring Lava flows dammed and diverted the Willamette River in the Portland Basin into the lower Tualatin River valley.