INTRODUCTION

The geology of the Deer Island quadrangle is dominated by four main packages of deposits separated by regional unconformities: Paleogene bedrock, Miocene flood-basalt flows of the Columbia River Basalt Group, late Miocene and Pliocene alluvial deposits of the ancestral Columbia River, and Quaternary deposits of the modern river and its tributaries. The latter two packages constitute the thick sedimentary fill of the Portland Basin, a Neogene structural depression developed in the older rocks. Late Eocene volcanic and volcaniclastic rocks and rare small intrusions, early products of the Cascade volcanic arc, underlie the dissected, southwest-sloping surface on the Washington side of the river. In Oregon, similar volcanogenic rocks of the Goble Volcanics and unconformably overlying Oligocene marine sedimentary strata are sporadically exposed beneath a mantle of younger deposits. Following mild folding, faulting, and erosion, the Paleogene bedrock units formed a low-relief terrain that was inundated by several of the areally extensive lava flows of the Columbia River Basalt Group. These lavas erupted from fissures in eastern Washington and Oregon, traversed the Cascade Range via an ancestral Columbia River valley, and spread out to cover large areas of the Coast Range province. After the basaltic eruptions ceased, fluvial silt, sand, and gravel of Columbia River provenance were deposited on the subhorizontal surface of the flows within and adjacent to the subsiding Portland Basin. Owing partly to late Neogene regional uplift, the Columbia River has cut through the Miocene and Pliocene deposits into the subjacent bedrock. In addition to fluvial sediments transported by the Columbia River, the fill of the modern river valley includes strata of colossal late Pleistocene glacier-outburst floods and volcanic debris carried down the Lewis River following eruptions of Mount St. Helens.

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