October 23. Morrison Formation

The Morrison Formation, named for Morrison, Colorado, west of Denver, has been the most prolific source of Jurassic dinosaur fossils in North America. The original stegosaurus and many others came from the Morrison. 

Camarasaurus skull in wall of Dinosaur Quarry,
Dinosaur National Monument, Utah. USGS photo.

The Morrison is mostly reddish and greenish mudstones and shales reflecting different oxidation states of iron, together with some channel sandstones, thin lake beds that include limestones, and swamps in some places. Taken together, the rocks portray a vast, relatively flat flood plain of a complex river system. The Morrison strata contain the sediments that were being eroded from the uplifts in the west – the mountains resulting from the Nevadan Orogeny in eastern California, where subduction was creating a magmatic arc. We talked about it October 10 – the Sierra Nevada Batholith today is essentially the roots of the volcanoes that developed above that subduction zone. 

Rivers flowing to the east from the mountains spread sediments across a zone from Alberta to New Mexico. Deposition spanned about 9 million years, from 156 to 147 million years ago, in the late Jurassic. The rocks include volcanic ash – no surprise, since the mountains providing the sediment were volcanoes. The situation must have been somewhat like Patagonia today, a wide sloping plain east of a high volcanic mountain chain. But unlike the Andes and Patagonia today, the Morrison was laid down in a warm, wet setting. North America in late Jurassic time was subtropical, or at least in the warmer parts of the wide temperate zone.

The remnants of the Sundance Sea were still around too, so parts of the Morrison include possible marine sediments, but for the most part it was a low-relief terrestrial flood plain. Dinosaurs apparently loved it. 

One of the best places to see Jurassic dinosaurs in their original position is the Dinosaur Quarry at Dinosaur National Monument, Utah. A steeply tilted rock face exposes hundreds of fossils within a coarse sandstone bed of the Morrison Formation. The interpretation is that the bodies of many dinosaurs were washed onto a sandbank in a large river, perhaps during a flood. They collected there and were entombed by later deposits. The titling that helped expose the layer took place about 80 million years later, during the Laramide Orogeny, which we’ll get to late next month.

In addition to its famous dinosaur fossils, the Morrison Formation has been one of the primary sources of uranium in the United States. The uranium minerals occur in lenses and layers a few feet thick, concentrated in the sandstones of the Morrison. Their shape in cross-section gives these ore bodies the name C-roll deposits, which probably form when mineral-bearing waters flowed through the porous rock. The Uravan area – for uranium and vanadium – lies in western Colorado and eastern Utah. This mineral belt supplied half the world’s supply of radium in the 1910s, and uranium mining continued at Uravan, Colorado, until 2009 when low prices closed the last operating mine. In 2008, the United States imported 85% of the uranium it consumed for nuclear power, mostly from Canada, Australia, Russia, Kazakhstan, Namibia, Uzbekistan, and South Africa.

The geologic map symbol for the Morrison is Jm – capital J for Jurassic, little m for Morrison. Legions of geology students learning about the stratigraphic section in the Rockies have referred to it as the Jim Morrison formation.

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Today is the day the earth was created, according to Bishop James Ussher of Dublin, Ireland. His analysis of the Old Testament was published in 1654. In that work, he determined that creation took place on October 23, 4004 B.C. The exact time of day is somewhat debated.

—Richard I. Gibson