Inter- and Intrapillow Limestones of the Olympic Peninsula, Washington

Abstract

Limestones between and within basalt pillows on the Olympic Peninsula originated in at least three ways. Some were generated when subaqueous pillowed flows and pillow breccias were extruded onto layers of pelagic carbonate sediment, incorporating it as clasts or forcing it into pockets between pillows. Others formed when pillowed basalt intruded carbonate ooze; the resulting rock frequently is a loosely packed pillow lava in which individual pillows float in thermally altered limestone. Another kind of limestone, however, was produced through gradual filling of cavities between and within pillows by later sediments; in most of these, precipitation of void-filling drusy calcite preceded and alternated with clastic sedimentation into the cavities. Crucial to the interpretation of these different limestones is whether they are younger or older than associated pillows. Determination of the age relations is nearly always possible by detailed examination of field and petrographic characteristics.