October 15. The Sundance Sea and an Island in Montana

I’ve had some inquiries about assembling the podcasts into bigger packages. Assuming I manage to get all the year’s daily episodes done, the podcast will continue next year but not on a daily basis – I’m not sure if it will continue to follow a chronological format or not, but there will be more posts – just not every day. And yes, I do plan to edit the existing episodes into collections, probably one for each month as well as a complete collection of all episodes on DVD or whatever is appropriate. Thanks for your interest and support – I appreciate it very much.

So now back to the Jurassic.

Although much of Pangaea was breaking apart during much of the Jurassic, there were of course places where compression was happening, too. We talked about the Jurassic collision in western North America that produced the Sierra Nevada Batholith and began a long series of accretions that added to the western margin of North America. As the mountains were lifted up in what is now California and points north, they began to constrain a seaway to the east, in what is now much of the Rocky Mountains in Wyoming, South Dakota, eastern Montana and adjacent areas.

The Sundance Sea, named for the Sundance Formation and the town of Sundance, Wyoming, in the northern Black Hills, transgressed across an unconformity surface that represents either non-deposition or erosion or both, so that the Sundance Sea’s sediments were laid down typically on top of the Triassic Chugwater Formation that we talked about on September 4.

Map of Jurassic sedimentation patterns (USGS map I-175). Purple is the Belt Island where Phosphoria chert was exposed and eroding into the Rierdon Limestone sea.

The Sundance Formation in the Black Hills area is mostly marine sandstones and shales, sediment eroded from the land off to the southeast in what is now central South Dakota and Nebraska. The Hulett sandstone member of the Sundance is a 60-foot-thick resistant bed that makes ridges in the Black Hills area, including a ridge that nearly encircles Devil’s Tower. In many places it contains excellent ripple marks, a testament to the shallow water in which it was deposited. Both the Hulett member and other parts of the Sundance Formation contain abundant belemnites (see the episode for October 6) that date it to late Jurassic time. Parts of the Sundance are also noted for pterosaur tracks.

While this relatively quiescent deposition was going on in the Black Hills and adjacent areas, further west, the first impacts of the tectonic activity, the collision, in the Cordillera were beginning to be felt. In central western Montana, the Jurassic sea was interrupted by a rising island, called Belt Island because it is more or less centered on the town of Belt, Montana.

Evidence for this island can be seen clearly in the sedimentation patterns. The sandy rocks of the Sundance Formation become limestones, called the Rierdon Formation, as you head into central and western Montana. The Rierdon rocks north of the Tobacco Root Mountains contain grains, pebbles, and cobbles of chert within the limestone, increasing in size as you go north within the Rierdon. This indicates you’re getting closer and closer to an erosional source area for those chert pebbles – and eventually, the Rierdon rests upon not the underlying Triassic, but the Permian Phosphoria formation, which we talked about August 10. The Phosphoria contains abundant, thick bedded chert, and it’s clearly the source for the chert pebbles deposited in the Rierdon limestone. How can that happen? The Phosphoria had to be exposed, above sea level, for this erosion to happen. The Phosphoria was in the core of the rising Belt Island during Jurassic time, with its debris, the chert pebbles, washing into the Jurassic sea where the Rierdon limestone was forming.

Other Jurassic strata record the initial rise of Belt Island, and as the Jurassic progressed, later sediments nearly overtopped the low-lying island. We’ll come back to this in about 10 days when we talk about the Morrison Formation. For now, just take the Belt Island as early evidence of increasing tectonic activity in the west. The Rockies are starting to form.

—Richard I. Gibson

LINKS:
Sundance Formation 

Enigmatic tracks in the Sundance Formation

Belt Island   

Map modified from Paleotectonic maps, Jurassic system, 1956, by McKee, Edwin D.; Oriel, Steven S.; Swanson, Vernon E.; MacLachlan, Marjorie E.; MacLachlan, James C.; Ketner, Keith B.; Goldsmith, June Waterman; Bell, Ruth Young; Jameson, Dolores J.; Imlay, Ralph W. USGS IMAP: 175