Friday, April 10, 2009

Imbricated thoughts on gravel, pt. 2

Now the high gravels. Maybe I haven't shared this with the entire group, but there are very, very sparse gravels in very few locations near the western edge of the Bogus Rim lava (Qbbr). Cooper and I spotted some in October 07 and I saw them again in Oct 08 and got a precise GPS reading on them of 1194 m. This is the level of the highest gravels upstream (skirting Little Owyhee Butte), but I haven't gotten a gps reading on them to really pin it down yet (May trip). Thus, these most likely represent the effect of damming by the Qbbr. The edge elevation of the Qbbr approaches 1204 m at the narrowest gap between it and the west edge of the Gorge at Iron Point. Note that the copious (6 m thick) volcanic-clast dominated overflow gravel appears downstream from the likley point of blockage at an elevation of 1160 to 1140. Suggests clearwater pour-over of the blockage to me.

Other high gravel elevations of note include the gravels on the Ice-Axe (1148 m), the low Rim Gravels on the tip of Bogus Point ( 1114 m), and the lone gravel deposit on the West Crater Kipuka (1050 m). More on them later.

Imbricated thoughts on gravel, pt. 1

Have lately been thinking about gravel deposits along the Owyhee. Using the LiDAR data, it is possible to rapidly extract precise elevation data from any point within the swath. This allows for a quick appraisal of the potential significance of the positions of various gravel deposits. In this image, I have highlighted some interesting spots. Of particular interest here are the gravels at the 993-995 m level ('perimeter gravels'). These are roughly concordant with the height of the west edge of the the West Crater flow and probably define the position of the first established channel of the river on its way through the lava. These surfaces stand well above the highest gravels at Dogleg, but that gradient is controlled by the gradient of the lava flow. What of the large difference between the elevation of the perimeter gravels and the next level down at Dogleg...a 33 m difference over only 3 km? Steep.

Also of note in the figure are the gravel deposits in the 1017-1025 m range (need to step on these sites before too long). These deposits must pre-date the West Crater flow and would have flanked the paleochannel that it followed. I was pleased to notice that the 1017 m gravel coincides with a prominent scarp-ish feature that is fairly obvious on the east side of the valley. Suggestive of gooey paleovalley margin for sure.

More posts on other gravel issues to come. Feel free to participate.

Wednesday, April 8, 2009

Bogus Rim lava now ~solidly in Quaternary!

According to the new geologic time scale being promulgated by GSA as well as the IUGS: