NODAL BASIN(?) SEDIMENTATION IN AN ANCIENT OCEANIC FRACTURE ZONE, INGALLS OPHIOLITE COMPLEX, WASHINGTON

The Late Jurassic (~161 Ma) Ingalls Ophiolite Complex is interpreted to represent a back-arc basin cut by an oceanic fracture zone. The eastern portion of the ophiolite includes the largest and most continuous sedimentary section, which consists primarily of mudstone with chert, graywacke, and pebble conglomerate beds located near the base. Lenticular deposits of sedimentary breccias, located at the base and throughout the section, are < 1 km in length with thicknesses ranging up to 10s of m. Ophiolite-derived olistoliths, typically lens-shaped and reaching in excess of 100 m in length, are associated with the breccia deposits. The breccias consist mainly of mafic igneous and/or serpentinite clasts. The poorly sorted mafic breccias are clast- to matrix-supported, boulder-to pebble-sized, and composed of angular to rounded diabase, gabbro, and basalt clasts with primarily mid-ocean ridge basalt geochemical affinities. Serpentinite-dominated breccias are primarily matrix supported, angular to rounded, and locally contain amphibolite and diabase clasts. The serpentinite breccias are poorly sorted and most contain cobble-sized clasts. Locally, well-rounded sandstone and pebble conglomerate consisting solely of serpentinite clasts exhibit graded bedding and load structures. Massive mudstone is the major component of the sedimentary section. Radiolarian-bearing chert, which is locally graded, is mainly confined to the lower portion of the section. Graywacke is well bedded and displays grading, cross laminae, and load structures typical of turbidite deposits. Interbedded with the graywacke, pebbly conglomerate is clast-supported and composed of primarily ophiolitic detritus. We interpret the association of ophiolitic sedimentary breccias with mudstones and local turbidites to reflect a mixture of locally derived debris and distal deep-marine sediments. The topographic relief adjacent to a nodal basin resulting from fracture zone-oceanic ridge interaction provides a unique environment in which rocks from fault scarps may have been locally shed into the basin forming breccias and olistoliths, interrupting pelagic and local turbiditic sedimentation.