k, so if you¹re not a geo-nerd, but you think this might be an interesting thing to be able to yap at your folks about, what they¹re saying is this: The Grand Canyon Supergroup rocks, some 13,000 feet in all, represent some of the very long rock record that is missing between the Vishnu Schist and the Tapeats Sandstone. The Great Unconformity between those two layers is 1.25 billion years‹that¹s the biggie in terms of missing time in the Grand Canyon.

But there are actually many unconformities within the Supergroup, all of which help add up to the biggie. There are unconformities between the Unkar Group and the Vishnu Schist, between the Unkar Group and the Nankoweap Formation, within the Nankoweap Formation, between that and the Chuar Group, between that and the Sixtymile Formation, within the Sixtymile Formation and between that and the Tapeats Sandstone. Meaning what? Well, that these layers weren¹t deposited continuously on top of one another, that something happened in between all of these little ³packages² of layers. Those somethings that happened are the interesting part of the story. This story is hypothesized, based on evidence from a lot of different sources.

A little over a billion years ago, our continent was part of a giant supercontinent called Rodinia (that other more famous supercontinent, Pangea, assembled later, at about 245 million years ago). While Rodinia was coming together, the collisions with other bits and pieces of continents caused mountain building further east. (Some folks think that our little corner of the world actually ended up in the middle of this continent, smashed up against western Australia and eastern Antarctica on the west and the rest of the u.s. on the east.) This movement broke the crust in the Grand Canyon region along long faults. Compression from mountain building happening in Texas caused ne-trending reverse faults to develop, such as those seen in Bright Angel, Bass, Vishnu and Red Canyons. This compression may also have caused stresses that pulled the crust along nw-trending faults in our region, causing normal faults like the Palisades Fault at Palisades Canyon (where the Morning Star Mine is) to develop.

So the result is that these faults generally cross each other in a big, regional ³X²: compression in one direction and extension in the other. The stress opened big basins that were the perfect places for sea water to invade, and loads of sediment to collect, hence the Unkar Group. The layers in the Unkar Group were deposited between 1.25 and 1.1 billion years ago, in rivers and near-shore environments of the coast. These sediments were all deposited either just below, just at or slightly above sea level, which moved up and down periodically, causing the shoreline to move in and out. The Cardenas Basalts erupted near the close of this time, covering the nearly 7,000 feet of sediment already deposited. The Unkar Group sediments are all those in the Furnace Flats, Phantom Ranch, and Bass Camp areas, and Bedrock to Deer and Tapeats Creeks.

Then there¹s a long period of about 300 million years when no one knows what was happening here, because there are no rocks to record it in this region. But at perhaps about 800 million years ago Rodinia began to split apart. As we pulled away from our neighbors on the west, a large basin began to form and the Chuar Group sediments were deposited into this, even while it kept deepening. The Chuar Group is a lot like the Unkar Group: sands, shales and limestones deposited in a shallow marine and near-shore environment, but the Chuar Group has a lot more organic material in it, stuff like algae and stromatolites‹apparently life, even these simple, single-celled forms of life, had taken off by this time and was going great guns along this warm, shallow shoreline. You can see the Kwagunt and Galeros formations of the Chuar Group from the ³Brain Rock² at the top of Carbon Creek, and if you look up Basalt Canyon as you race through Furnace Flats you can see the Nankoweap Formation as a prominent purple cap on top of the Cardenas Basalts.

By the time the Sixtymile Formation (the very top of the Supergroup) was being deposited around 700 million years ago, slumps and conglomerates in that formation show that the Butte fault system was active. It had to have been for several hundred million years in order to make space for all those Chuar sediments to be deposited. (The west side of the fault dropped down, creating a low spot for the stuff to collect.) Movement on this fault may have resulted in as much as 3 kilometers (1.8 miles!) of displacement in Proterozoic times alone. The Butte fault was also active in Laramide times, 60 or 70 million years ago. So the Butte fault started as an extensional fault, reversed movement in Laramide times to become a compressional feature, then may have had extensional movement again in the last 20 million years, similar to many of the faults in Grand Canyon. Pretty cool.

Christa