STRATIGRAPHIC AND GEOCHEMICAL EVIDENCE FOR AT LEAST TWO VOLCANIC CENTERS IN THE CRESCENT FORMATION OF THE SOUTHERN OLYMPIC PENINSULA, WASHINGTON

The Eocene Crescent Formation of the Olympic Peninsula is one of the thickest accumulations of basaltic lava in the world, perhaps exceeding 16 km in thickness. Previous efforts to unravel the stratigraphy and structure of this formation were hampered by faulting, deformation, and difficult terrain. However, new mapping in the southern Olympics suggests that the basalts there were erupted from two separate volcanic centers. The younger lavas were erupted from a newly recognized dike complex onto a submarine fan that caps an older basalt sequence overlying its own feeder dikes. The older submarine basalts, dikes, and sills have less Zr (<125 ppm) than the younger intrusives and volcanics (Zr >125 ppm), implying a more enriched source for the later basalts. Capping the entire sequence are several flows that may indicate yet a third volcanic episode. These flows are coarser and more massive than any others in the region, and their Zr content is once again below 125 ppm.

One or several turbidite sequences up to 160 m thick occur at several locations between the older and younger volcanic sequences, indicating a significant hiatus between eruption cycles. The mineralogy of these turbidites shows that they are of mostly basaltic origin but have some continental input.

Within the younger volcanics a submarine-to-subaerial sequence can be traced for several kilometers across the Olympics. Basalts here are transitional from pillowed to columnar, and shelly basaltic sandstone and conglomerate are abundant.