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Devon in the Bakken: A reader writes that he/she received his/her first check from Devon since the deal with Grayson Mill closed. A quick look by the reader says everything looks correct.
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Sharks and Fish: Evolution
Montana: Paleontology
Those following the Bakken are very, very aware of the formations mentioned below.
In the United States, in Montana:
If you want to study dinosaurs, you go to Hell Creek, eastern Montana.
If you want to study the evolution of fish / sharks, you go to Bear Gulch, Fergus County, near Lewistown, Montana.
The Bear Gulch Limestone is commonly considered to be part of the Heath Formation, the youngest formation in the Big Snowy Group of central Montana.
Some authors instead consider the Bear Gulch Limestone to be an early member of the Tyler Formation, a patchy but widespread unit of Carboniferous limestone and terrestrial sediments.
Most of the Heath Formation is represented by black shales and marls, indicative of brackish and salty littoral environments. It developed along a transgressive sequence in a narrow saltwater seaway, known as the Central Montana Trough or Big Snowy Trough. This seaway flowed into the Williston Basin, a shallow inland sea further east. The Central Montana Trough would have also been linked to fully marine basins on the western coastline of Laurussia, but this connection may have been broken by the time of the Bear Gulch Limestone's deposition.
Many distinct limestone lenses (localized sediment packages) are developed in the Heath Formation. They overlap each other in an east-to-west sequence which extends over a distance of 160 km in the Central Montana Trough. The only exposed portions of the sequence are found at Potter Creek Dome, a small uplifted area northeast of the Big Snowy Mountains. The last few limestone lenses form a large portion of the Upper Heath Formation, which is sometimes termed the Bear Gulch Member in recognition of the most well-exposed and fossiliferous lens in the sequence. This lens, the Bear Gulch Limestone, was also one of the last in the sequence, only succeeded by the Surenough Beds immediately west of it. The Bear Gulch Limestone can be observed in numerous outcrops, spread out over an area of more than 20 square miles in Fergus County, Montana.
The creation of limestone lenses in the Heath Formation has been linked to tectonic activity extending the seaway by excavating bays out of the surrounding land. As old bays are filled in and buried by sediment, faulting and seismic events form new bays in a long eastward to westward succession. It may have taken a mere 1000 years for the bay responsible for the Bear Gulch Limestone lens to fill in completely, after only 25,000 years for the entire bay formation sequence to run its course across Montana.
The final limestone deposits in the area were succeeded by freshwater lake sediments of the Cameron Creek Formation, the oldest unit of the early Pennsylvanian-age Amsden Group.
