Friday, August 29, 2008

Best Owyhee example of lava dam features

The Sand Springs Wash area on the Owyhee retains the best example of a lava dam in the field area. At this site, there are two lava flows from the Saddle Butte vent to the west. The overlying flow is separated from the underlying flow by a spectacularly well-developed lava delta with pillow-laden foresets that dip upvalley. The dam forebay is a turtle-back like feature with a 'carapace' of chilled lava. The surface of the older Saddle Butte lava upstream of the dam is covered with lacustrine mud which is capped with fluvial gravel.

Upstream of this site, the older Saddle Butte lava flow overlies Owyhee River gravel at the site of the Weeping Wall where a spring issues from the contact between the gravel and the lava. Upstream of Weeping Wall, the older Saddle Butte lava contains a sequence of lava deltas and subaerial lavas that suggest a complex interaction with the river.

Downstream, the eastern margin of the younger Saddle Butte flow is wasting away on a grand scale.

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