PALEOECOLOGIC INTERPRETATIONS OF THE MIDDLE EOCENE COALEDO FORMATION OF OREGON

Tectonic activity in the Pacific Northwest during the Eocene included the accretion, subduction, subsidence, uplift, and basin filling resulted in paleoecological changes that are recorded by the microfossils. Based on sediments and macrofauna, the Middle Eocene Lower Coaledo Formation near Coos Bay, Oregon, is interpreted as a shoaling upward sequence within a wave dominated delta; however, this unit contains abundant lower bathyal foraminifers suggesting lower bathyal water depths (2000-4000 m). Sedimentologists argue that these deep-water foraminifers have either been reworked, migrated to shallower water depths, or are displaced due to an upward movement of bottom water indicative of an upwelling environment. A new study of over 300 samples documents the paleoecology of the Lower Coaledo Formation (primarily Cycle 2) based on the foraminiferal assemblages, and suggests that deposition occurred at lower bathyal depths, near the base of the continental slope as part of a submarine fan complex. This interpretation includes analysis of the preservation, biofacies, dominant benthic foraminiferal species, and planktic organisms. Preservation of shelf indicates downslope transport, as these specimens are few and poorly preserved, in contrast with the moderately well preserved lower bathyal species that argue for in situ deposition. Biofacies analysis indicates that species with upper depth limits in the lower bathyal biofacies dominate (~50 %) the microfauna. Key indicator species suggest suboxic depositional conditions with abundant organic material. Agglutinated foraminifers with dissolution resistant tests suggest proximity to the calcium compensation depth which was at 4-4.5 km bsl during the early Middle Eocene. Common planktic organisms (planktic foraminifers, diatoms, and radiolarians) at the base of each cycle, is consistent with deposition below the shelf edge (>200 m). Deposition of the Lower Coaledo Formation on a submarine fan at lower bathyal depths is consistent with faunal and lithologic associations seen in modern submarine fans. Taken together, the multiple lines of evidence presented here support the interpretation that the deposition of the Lower Coaledo Formation is consistent with lower bathyal depths.