Basement Rocks from the Cocos Ridge
Today is the second of the basement sampling party. No this is not like a sale in the basement of Filene’s. This is expensive ignenous rock that we hope will tell us many things.
Last night, I asked at dinner what the story of the day was and Akito’s eyes lit up. Basement rock! Same conversation this morning with Udo. These rocks will confirm (or refute, oh no!) his age model. Basement rock is igneous rock. This is a term geophysicists use to describe the lowest strong, coherent reflector on seismic reflection profiles.
Just 8 days ago we had another basement sampling party. It was at the first coring site – U1379. We had cored through about 900 m of sediment, at a shallow site (138 m seafloor depth) and then got into the basement rock. Site U1379 is on the continental slope and the sedimentary material was terrigenous (from the land). Only 10 miles or so from land. Greenish – grayish, sand, silt, clay, with shells and wood. Very young nanofossils (about 1 million years old). At the bottom of the sediment was basement rock. A mixed up group of rocks. A mélange type of rock containing, among others, basaltic clasts with large, bright-green, pristine olivine. We have lots of mélange in the San Francisco Bay area – a mixture of rocks.
We returned to our very first site U1378 where we had drilled a hole and logged data by sending instruments down on the drilling pipe. So, we sorta knew what we were getting into. This was a deeper site, farther offshore, but still on the slope. More terrigenous sediments of sand, silt, and clay with some ash layers. Same old stuff with some variation. From the logging data, it was not a surprise when it was time to stop drilling. I still don’t understand all the lingo and different drilling techniques, but I did know that this was not the day to go learn about the fine art of coring.
Next site was a day later, maybe. I’m losing track. The co-chief scientists want the basement rock. It is part of the scientific objective of CRISP Expedition 334. The basement rock will provide evidence on the stress within the seismogenic zone. We had moved just a half of a mile away for site U1380, at about the same seafloor depth of 500 m or so.
In order to be able to core for the basement rock, the decision was made to drill down the first ~400 m at this site and then start coring. According to the seismic data from a previous cruise, the boundary between sediment and basement at this site (U1380) is at around 550 m +/- 50 m. The plan was to drill about 100 m of sediment before the basement rock and then into the basement rock. This amount of sedimentary rock would allow a correlation of the data with the sediments from the last station, just a half mile a way. Then we get stuck at the same depth or so. More decisions had to be made.
For our last site (the clock is ticking down), we moved about 20 miles to the west, following the same transect line, moving offshore. We are on the Cocos Ridge which has never been drilled before. This is the subucting plate, that is moving under Costa Rica. At site 1381, we are now over about 2000 m of water. We’ve recovered sediment cores (about 100 m depth) and basement core. Still drilling more basement rock and that make the petrologists happy. They will be conducting experiments on the rocks to measure strain, frictional properties and deformation properties. The sedimentologists and chemists are happy to have this record of what is being subducted. The sediment is very different material than from the slope. Brown sediments, the same color as Stephan’s pants. Oceanic origin.