Tuesday, July 31, 2012

Atop Grand Mesa in Western Colorado

Grand Mesa looms high above the junction of two rivers - the Gunnison and the Colorado (formerly known as the Grand River before 1922). At over 11,000 feet Grand Mesa is a cool oasis above the steamy flats down near Grand Junction.

The mesa is capped with basalt lava flows that are almost 11 million years old (radiometrically dated at 10.76 Ma). Beneath this caprock, lies unmistakable Colorado River pebbles and cobbles that are about 10 to 20 feet thick. This suggests  that the fluid lava likely flowed in and was confined in a valley of the ancestral Colorado River. Since 11 million years ago, the river has dissected a new course to the north of Grand Mesa and this lies a little less than 5,000 below the old gravels. This gives an average incision rate for the Colorado River in the last 11 million years of about 460 feet every 1 million years. Of course, averages in the Southwest are sometimes meaningless, since climate and tectonic uplift rates may vary throughout geologic time. But it gives a sense of how fast canyons have been carved on the Colorado Plateau.

In July, I took a nine day trip through Colorado to see some new geologic sights with my colleague Jack Share. Here are some pictures and I will be posting more Colorado geology stories in the days to come.

We traveled up the road from Delta and Orchard on the south side of the mesa. As we climbed higher, we could see the great San Juan Mountains to the south. The slanted mesa in the middle distance rises up to the east (left) to become the south rim of the Black Canyon of the Gunnison.

Another view of the lower Gunnison River Canyon with the high San Juan's in the distance

A view of the top of Grand Mesa. It was a very hot day ion Delta but up here we cooled off in the alpine air.

The flowers were spectacular but this little drainage caught my eye. I wondered if maybe this drainage had formed just after the lava flows filled the ancestral valley of the Colorado River. The emplacement of the lava would have interrupted the flow of water and perhaps the river tried to establish it's old course on top of the newly formed surface?

More likely though, these little drainages are the result of more recent Ice Age conditions on top of Grand Mesa. But it is an intriguing thought top consider how the Colorado River responded to having its bed filled with a hundred feet of basalt lava.

The valley definitely looks underfit for the amount of water in it today. These grasses filled the floor of the valley and we watched dragonflies dance in this green paradise.

As we began our descent off of the north side of Grand Mesa, we had this spectacular view of the western terminus of the lava flow - Land's End. The valley of the modern Colorado River is just out of view on the right, showing the depth to which a new valley has been carved since the lava flows were emplaced.

A view farther north than the previous pictures exposes a butte carved into quite young Cenozoic age sediments. These were the rocks that likely "contained" the ancestral valley of the Colorado River some 11 million years ago. This is a perfect example of inverted topography, whereby a former valley is filled with har rocks and then becomes a high standing mesa (Grand Mesa) when the softer rocks become eroded down.

Wild turkeys seen on the northern flank of Grand Mesa.

Below are a few pictures of Miocene age animals that might have been roaming around when the Grand Mesa lava flow was emplaced 11 Ma.

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