Mendocino Fracture Zone Cruise - 2001
August 20 - 30, 2001
Over 300 km off the California-Oregon Coast

August 23, 2001: Day #4

Debra Stakes at the rock saw on the back deck of the Western Flyer.

Log Entry: The wind is still blowing at 25 knots but Tiburon started its’ second dive smoothly at 0628 local time. Today’s dive was more successful, and Tiburon landed were it was meant to land. We are now on the eastern part of the Mendocino Transform Fault, and followed the uplifted face of the Gorda Escarpment in order to collect as many samples of sedimentary rock as possible. The sediments will be used to study the subsidence and uplift history of the Escarpment in addition to microfossils, such as forams and radiolaria, present in the sediments. Today’s dive plan was put together from seismic evidence for exposed sedimentary strata.

The dive began at 2,500 m where igneous basement rocks protrude through the sediments. Above this we observed the contact between the igneous basement and the overlying sedimentary units as shown on the seismic section. The sedimentary rock types collected include sandstones, mudstones and carbonates. Sandstone deposition takes place at shallow depths, mudstones reflect deep-sea and/or continental slope deposition, and carbonates generally reflect deep-sea deposition. Therefore the lithological variations (see picture) we observed today reflect the position of the Gorda Escarpment in the past relative to sea-level. The fossils present in these rock units can be used to constrain the timing deposition. Radiolaria collected during last years’ cruise indicates that most of the deposition occurred during the Pleistocene era, around 10,000 years ago.

We also want to relate the sedimentary units to reflectors as observed in the seismic data collected across the Gorda Escarpment in 1999, and also to the units present in Ocean Drilling Program Hole 1022C. This drill hole is located just south of the Gorda Escarpment, and consisted of mudstones, sandstones and carbonates sitting on top of a horizon of chert. This horizon would be a good marker that we could recognize and sample with the ROV.

We also collected some interesting deep sea fauna living on a log (see picture) at a depth of 2,440 m. Wood falls in the deep sea often provide increased organics and a hard substrate that deep sea creatures can exploit. Biologists on board picked over the wood and found abundant flatworms, snails, anemones, and polychaetes, including a very large worm with sticky long tentacles. The fauna was collected and preserved for further study back at MBARI.

The rocks collected yesterday were cut in half using the rock saw (see picture above) on the deck. Most of the rocks turned out to be volcaniclastic fault breccia. Some of them showed stockwork sulfide mineralization (see picture). The first conclusion we can make from these rocks is that the Mendocino Ridge is likely to be tectonic mélange, and the rocks have undergone significant high temperature fluid flow that has resulted in sulfide mineralization. This is likely to have occurred before the rocks were tectonically removed from their original environment. This is drawn from the fact that not all the rocks collected from a single locality show mineralized features. There is also evidence for low temperature fluid flow. A low temperature alteration product, celadonite, is present in some of the rocks.

We look forward to the cold seep dive scheduled for tomorrow.


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