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Mt. Rainier from the Pinnacle Peak trail.
Sheepbed science
This outcrop was the featured rock suite described in the majority of an entire session on Sunday at the annual meeting of the Geological Society of America 2013 #GSA125 in Denver. This rock suite are the rocks found at the Yellowknife Bay position on Mars by the Curiosity Rover. Yellowknife Bay was a position located from orbital images where 3 different rock units came together; the Bradbury Rise suite that the rover landed on meets 2 other units here; the Glenelg unit and the Sheepbed Unit.
Many of these results have been published previously and highlighted at the TU and TES pages but there were definitely some new debates worth noting here today.
The rover has been able to assemble an impressive, detailed stratigraphy of these units based on the elevations. The lowest unit in the sequence is the Sheepbed unit. That is overlain by the Glenelg unit, and finally the Bradbury Rise units sit at higher elevations.
As Dr. Stack presented in her talk, the stratigraphy doesn’t exactly show the age relationships because there could be unconformities between the units, but the elevation does suggest that the Sheepbed unit sits underneath Glenelg. Bradbury Rise could be an earlier unit that these 2 units were deposited on, or it could sit on top of the other 2 units. The rocks aren’t 100% clear which units sit on top, but I would bet based on the chemistry that Sheepbed is oldest, Glenelg sits on top of it, and Bradbury Rise is the topmost, youngest unit.
If that’s the case, then Sheepbed, the location of the drill sites, could be the oldest unit in the entire Gale Crater region; it might well extend the entire way beneath Mount Sharp. We might run into this unit again as we climb the slopes of Mount Sharp.
Sheepbed was the unit that was drilled and a number of fascinating results were presented on Sunday. One MSL team member showed results from the Chemcam instrument (the gigantic laser beam) that were amazing. Chemcam was able to target analysis spots within the drill sites and analyze down the drill holes. For comparison, he dropped a U.S. dime on the ground, stood back 20 feet, and said “basically we’re analyzing spots 1 millimeter apart from 7 meters away within a hole with the diameter of this dime.” (He left the dime on the podium after his talk, it was still there when I gave mine).
The rocks in Sheepbed also have some remarkable properties. Dr. Schieber of Indiana University (Hoo-Hoo-Hoo Hoosiers!) showed that the rocks the rover is sitting on in this image are loaded with fractures and veins. The vein minerals were formed by fluids flowing through the rocks that deposited dissolved elements as they passed through. Dr. Schieber noted that the fractures really don’t penetrate upwards into the Glenelg unit; he argued that the fractures were produced by natural hydrofracking. That’s right, read that again; natural martian hydrofracking.
If fluids were flowing through the lower unit, they could have been capped by the Glenelg unit and unable to penetrate upwards. That impermeable layer would have caused any fluids in the lowest Sheepbed unit to build up pressures…until eventually the pressures caused the rocks to break. Literally, this process would have been naturally-occurring martian hydrofracking.
There is also a single line in this image known as “the snake”. That set of rocks might be a sedimentary dike; a rock formed when sediments at pressure were pushed upwards into one of the cracks, from the same mechanism Dr. Schieber described.
Finally, do you see the polygonal fracturing pattern in the rocks in this image? There were 2 talks about that topic. First, Dr. Lewis showed that these polygonal plates actually moved as a consequence of the rover; when the drill sites (shown in this image) were created, by the 33 kilogram (73 pound) arm pressing down on the plates and drilling 7 centimeters into it…the plates actually moved. They settled downwards due to the rover itself; the plates are loose. The 1-ton rover actually broke through a couple of them as it drove across.
On top of that, Dr. Sletten from the University of Washington argued that the plates themselves could have been produced by desiccating lake sediments. When water evaporates from lake sediments, the remaining sediments contract, forming mudcraks. Mudcracks on earth are similar sizes and at spaced at similar distances from the cracks observed within these plates on mars; that could mean these plates were produced when the lake where the sediments were deposited dried up. The Glenelg unit would then have been deposited as a fresh unit on top of the dried-up lake sediments.
-JBB
Source: facebook.com
earthstory reblogged
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