The Sixtymile formation: last of the Supergroup
On top of the shales of the Chuar group sits one last unit of sedimentary rocks before things change enormously. The Sixtymile formation is a fairly thin unit named for Sixtymile canyon (it is not 60 miles thick) Where observed, at most it is about 60 meters thick.
The Sixtymile formation is only exposed in a few places at the top of the Chuar group. The rocks of this group sit up against a large fault and are folded into a gentle syncline; the Sixtymile formation’s thickness actually seems to follow the pattern of this fold, indicating that it was being deposited as the rocks were actively folding.
The last few units we saw were shales, deposited in quiet water. Over a few meters at the top of the Kwagunt formation, the unit changes conformably into the Sixtymile formation. This formation starts as a mixture of siltstones and sandstones and grades upwards into quartzites, sandstones, and even a conglomerate/breccia layer.
A couple things have clearly changed. The sediments of the Chuar group beneath were deposited offshore in quiet water; here we see sandstones reappear and sandstones often represent environments close to beaches. After that, there are conglomerates like those that are deposited in rivers.
The rocks of the Grand Canyon area have gone from being below the waters to being above the waters, and they’re about to go a whole lot higher.
One other interesting thing potentially happened at this time; the waters may have receded as well. There are no firm ages within the Sixtymile formation, so all we know is that it is younger than the 742 million year old rocks below it. The Sixtymile formation could also overlap with the time of the great glaciations that covered Earth around this time; some of which may have even frozen the entire planet in a phase nicknamed Snowball Earth.
If glaciers that big did form, the sea level would have dropped dramatically. There are locations in Death Valley and elsewhere that contain deep channels, filled with conglomeratic sediments, which have been proposed as forming when the waters receded during the great glaciations. The Sixtymile formation has a similar sequence; conglomerate rocks deposited in channels as the rocks moved above the ocean. There were tectonic forces acting to move the rocks upward as well, making this a complicated area, but this formation could very well record the sea level changes associated with the start of the Sturtian glaciation.
Image credits: http://www.nature.nps.gov/geology/parks/grca/age/index.cfm