A THREE-DIMENSIONAL VIEW OF THE TUALATIN BASIN, NORTHWEST OREGON, REVEALED BY GRAVITY MEASUREMENTS

New gravity measurements from the Tualatin basin in northwestern Oregon define a broad -40 mGal gravity low, indicating the basin to be one of the deepest along the Puget-Willamette trough. The 110 mGal gravity gradient along its western margin is one of the steepest in Oregon and coincides with the Gales Creek fault zone. The fault forms the boundary between the basin and uplifted Eocene basalt basement of the Coast Range to the west. The Portland Hills and Sherwood faults are associated with gravity anomalies along the basin’s northeast and southeast margins, respectively. Assuming Eocene basalt basement is continuous beneath the basin, inversion of gravity data (constrained by four wells that penetrate Eocene basement rocks) shows that the Tualatin basin is deep (~5km), narrow, and asymmetric, with a steeper (~ 40 degree) western margin at the Gales Creek fault zone. The shape of the Tualatin basin and its proximity to large, basin-bounding faults along the southwest and northeast margins suggest that the Tualatin basin originally formed as a transtensional rhombochasm.

Although the 15-16 m.y. old Columbia River Basalt (CRB) is up to one kilometer thick in the basin, the basin is five times deeper, indicating that basin formation preceded emplacement of CRB. Pre-CRB extension in the Tualatin basin and northern Willamette Valley basin is consistent with an episode of late Eocene extension in the Oregon Coast Ranges. Gravity data also show that the Portland and northern Willamette basins, which border the Tualatin basin to the northeast and south, respectively, are relatively shallow, although the lack of basement control, particularly in the Portland basin, precludes precise estimates of basin depth. The Portland and northern Willamette Valley basins also may be transtensional structures that formed early in the history of northwestern Oregon. The present-day geometry of the Tualatin basin and other basins within the Puget-Willamette lowland likely resulted from late Cenozoic folding of the CRB.