ABSTRACT

Nearshore sands (<25-30 meters) on the northern Washington shelf (Grays Harbor-Cape Flattery) have high percentages of clinopyroxene, garnet, and amphibole that indicate local sediment supply along the coast. Orthopyroxene, which characterizes the Columbia River sediment, does not form a significant proportion of these sands, as it does in the nearshore sands to the south, between Grays Harbor and the Columbia River. In contrast to these differences, a heavy mineral-rich zone at 44 to 51 m has relatively abundant orthopyroxene all along the shelf, especially in the southern part. This feature indicates that orthopyroxene-rich sediments were more abundantly supplied to the southern part of the shelf in the past and/or the northward sediment drift was stronger than at present, possib y due to the lack of headlands. Although the Columbia River is the most obvious source for the sediment, the Chehalis River could have delivered considerable quantities of sediment during the Pleistocene. The mineral composition of the sediments from these two sources may have been somewhat different. Glacial outwash exiting through Grays Harbor during lowered sea level supplied the gravel found on the adjacent inner, middle and outer shelf. Except near the Columbia River where a blanket of modern silt covers the shelf, anomalously coarse deposits or zones rich in heavy minerals are found discontinuously at depths of 18 to 33 m, 44 to 51 m, 73 to 82m and 165 to 180 m. The latter deposit is at the shelf break but the other three depths may represent stillstands of lowered sea level. Corre ations of these depths with other shelves should not be attempted because of complex, local Quaternary tectonism.