Diagenetic carbonate cementation of clastic sediments near the sediment-water interface on the lower continental slope off Washington and northern Oregon.

Dredging of lower continental slope ridges off Washington and northern Oregon (water depths of 1000-2200 m) has recovered numerous pebble to boulder-sized rocks that consist of clastic sediments firmly cemented by microcrystalline (<5-10 μm diameter) carbonate. The carbonate content of these samples ranges from 27-82 weight percent, with nearly two-thirds of them containing >50 percent carbonate. Magnesian calcite is the predominant cementing agent, but dolomite is conunon; aragonite occurs rarely. The framework detrital sediments of these high-carbonate samples are texturally diverse; silty clays, silts, silty sands and conglomeratic sediments are all quite common. The carbonate content of these rocks was found to show a strong positive correlation with decreasing mean detrital grain size and a strong negative correlation with the sand-to-mud ratio of the framework grains. Carbonate content is limited by the porosity of the detrital sediment at the time of cementation. Comparison of the carbonate content values to the textural parameters of the framework grains indicates that the host sediments were largely uncompacted at the time of cementation. The carbonate-cemented sediments are mostly turbidite sand/silt layers apparently deposited and cemented in Cascadia Basin near the base of the slope. Some hemipelagic lutites are also cemented, but the majority of these remain uncemented. These uncemented lutites are tectonically dewatered and compacted to form low-carbonate (<20 percent) mudstone which is the dominant lithology on the lower slope ridges...