25. HEAVY MINERALOGY OF UNCONSOLIDATED SANDS IN NORTHEASTERN PACIFIC SEDIMENTS: LEG 18, DEEP SEA DRILLING PROJECT

Immature, chemically unstable, pyroxene-amphibole heavy mineral assemblages characterize the unconsolidated sands from Leg 18. Sites 174A, 175, and 176 off the Oregon coast have received sediment from the upper portion of the Columbia River drainage system during glacial times. The heavy mineral composi-tion of present-day Columbia River sands, dominated by sands from lower Columbia River sources, is not typical of what it has been in the past. Evidence for a change of sediment source was found in the lower part of Site 174A and suggests that this site may have received sediments from more northerly or southerly sources two million years ago. Sands found at Site 177A have amphibole-rich assemblages except near basement where an epidote-rich assemblage is found. This drastic change in mineralogy at Site 177A, and the observed increased degree of etching of pyroxenes with depth at Sites 174A and 175, indicate that diagenesis is capable of modifying the character of heavy mineral assemblages. Heavy mineral analysis of sediments from the Gulf of Alaska (Sites 178-182) suggest that there is no evidence of a sediment source beyond the coast ranges of Alaska from middle Miocene to the present