The middle to upper Eocene (Narizian) Coaledo Formation, well exposed along the southwestern Oregon coast, includes seven basic lithofacies types which constitute a complex marine-deltaic sedimentary sequence. The lithofacies are defined by lithology, fossils, bedding characteristics and sedimentary structures.

Petrographic examination of Coaledo sandstones indicates a dominantly andesitic volcanic source with subordinate plutonic, metamorphic, and sedimentary sources. Paleocurrent indicators suggest that these source areas were located to the southeast, from the initial calc-alkaline vents in the southern ancestral Cascades and from the northern Klamath Mountains. Regional paleocurrent analysis also suggests open-ocean conditions to the west, offshore from a wide, swampy coastal plain.

The pre-Coaledo units exposed in the Cape Arago area are interpreted to be delta foreset deposits. Coarsening-upward sequences recognized within the lower Coaledo member are interpreted as delta topset deposits, including subaqueous distributary channel, interdistributary tidal flat, lagoonal, and barrier-bar facies. The lower part of the middle member is characterized by deeper marine facies. The upper part of the middle Coaledo member represents prodelta deposition during a second progradational phase. Coarsening-upward sequences are recognized in the upper Coaledo member, again representing delta topset deposits. Uppermost Coaledo sands and overlying units show a gradual return to deeper marine conditions.

Coaledo sediments unconformably ovelrie gently folded fore-arc sediments of the Tyee basin. Deposition of coal-bearing deltaic sediments far to the west of the axis of the basin suggests regional tectonic uplift along the flank of the basin, or progradational filling of the continental flank of the basin.

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