Stratigraphic development, mineral sources and preservation of marine placers from Pleistocene terraces in Southern Oregon, U.S.A.

Abstract

Marine placers containing economic minerals (chromite, ilmenite, magnetite and zircon) have been extensively studied in Pleistocene coastal deposits of southwest Oregon. The two largest placers occur on adjacent, uplifted terraces south of a prominent headland, Cape Arago, and include the older Seven Devils placer, covering 11 km 2 and containing 1.1 × 10 7 m 3 placer sand (heavy minerals > 50% weight abundance) and the younger Pioneer placer, covering about 0.9 km 2 and containing 9 × 10 5 m 3 placer sand. The most highly enriched beds of placer ore, containing at least 6% chromite by weight, are generally concentrated in the bottom 1–4 m of terrace stratigraphic sections. Upsection sequences of nearshore to inner-shelf facies in the Seven Devils deposit, and beach to eolian dune facies in the Pioneer deposit, respectively, indicate placer formation during (1) marine transgression, and (2) sea-level high stand with beachface progradation. Microprobe analyses of opaque minerals establish local river sources of placer minerals for both Seven Devils and Pioneer deposits. In contrast, younger terrace and modern beach deposits are supplied by distant river sources, 100–150 km south, as well as local rivers, indicating increasing net northward littoral drift during successive marine transgressions. Placer thickness in the Seven Devils and Pioneer deposits are controlled in part by platform gradient and paleo-tidal range, respectively. Placer development and preservation in this high wave-energy active-margin setting are strongly controlled by paleo-shoreline orientation and littoral cell bounding headlands.