DinoByte Wednesday: Rock Formations of the DIG Field School

Why do paleontologists care about rocks? Rock formations house the secrets of the past! Let’s take a walk through time, starting with the oldest formation preserved at our field site, the Bearpaw Shale, and work our way forward through the Fox Hills Sandstone, to the Hell Creek and Tullock formations, the two formations that are the focus of our DIG research.

A simplified stratigraphic section of the formations visited during the DIG Field School. The lowest (and oldest) formation is the Bearpaw Shale, while the Tullock Formation is the highest in section (and the youngest) in the area. The meters at the left indicate the approximate stratigraphic position relative to the Hell Creek–Tullock formational contact (0 meters), which also coincides in this area with the K/Pg mass extinction. Modified from Johnson et al. (2002).
A simplified stratigraphic section of the formations visited during the DIG Field School. The lowest (and oldest) formation is the Bearpaw Shale, while the Tullock Formation is the highest in section (and the youngest) in the area. The meters at the left indicate the approximate stratigraphic position relative to the Hell Creek–Tullock formational contact (0 meters), which also coincides in this area with the K/Pg mass extinction. Modified from Johnson et al. (2002).

Bearpaw Shale

The lowest (and oldest) formation exposed near the DIG field camp is called the Bearpaw Shale (or Bearpaw Formation). This formation formed ~74-70 mya (million years ago) as a fine-grained layered mudstone (or shale) in a shallow sea, the Western Interior Seaway, that ran through the United States from the Gulf of Mexico to Canada. This formation was deposited just before the sea began to recede near the end of the Cretaceous Period. Can you believe there was once a sea cutting North America in half, and living in this seaway were sharks, giant marine reptiles like the long-necked plesiosaurs, and extinct molluscs called ammonites? All of these creatures (and others!) went extinct with the dinosaurs 66 mya.

A paleoreconstruction map of Late Cretaceous North America 75 mya depicting the Western Interior Seaway that separated North America (left), and a shark and plesiosaur (right) that inhabited this sea. In addition to these giants, other twenty foot long swimming reptiles like mosasaurs lived in the sea and fed on fish and ammonites, relatives of squids and octopi. We often find the remains of the straight-shelled ammonites, called Baculites, in these shale deposits. *Ron Blakey has produced many maps like this of North America and the world through time that can be found online.
A paleoreconstruction map of Late Cretaceous North America 75 mya depicting the Western Interior Seaway that separated North America (left), and a shark and plesiosaur (right) that inhabited this sea. In addition to these giants, other twenty foot long swimming reptiles like mosasaurs lived in the sea and fed on fish and ammonites, relatives of squids and octopi. We often find the remains of the straight-shelled ammonites, called Baculites, in these shale deposits. *Ron Blakey has produced many maps like this of North America and the world through time that can be found online.

Fox Hills Sandstone

As the Western Interior Seaway receded, eastern Montana went from being covered by a shallow sea to being near shore and beach, with the seaway still present but located further to the south and east. The sediments that preserve this ancient beach make up the Fox Hills Sandstone Formation, and were deposited 70-68 mya. These yellow/tan, plain sandstone beds are very thick and contain few fossils in our field area, so we most often use the resistant, ledge-forming sediments at the top of this formation as a “marker bed” to help identify the  overlying Hell Creek Formation. Scientists think the Fox Hills Sandstone would have been home to a community of dinosaurs, mammals, reptiles, and early birds that would have come down to the shores of the shallow sea to drink and feed.

In Reid Coulee (northeastern MT), the Fox Hills Sandstone and Hell Creek Formation are exposed in one stratigraphic section. This sandstone is “concreted” or “well-indurated,” meaning it’s resistant to weathering and forms steep cliffs like the one pictured here. Photo courtesy of Dave DeMar, 2012.
In Reid Coulee (northeastern MT), the Fox Hills Sandstone and Hell Creek Formation are exposed in one stratigraphic section. This sandstone is “concreted” or “well-indurated,” meaning it’s resistant to weathering and forms steep cliffs like the one pictured here. Photo courtesy of Dave DeMar, 2012.

Hell Creek Formation

The Hell Creek Formation overlies the Fox Hills Sandstone, and is one of the two focus formations for the DIG researchers. This is one of the more famous and widely exposed formations from the Mesozoic Era in the state of Montana. This formation was deposited 68-66 mya and is primarily composed of “drab” and “somber” colored beds of tan sandstones, gray siltstones, and purple mudstones, with little to no coal. These sediments were deposited by freshwater and brackish rivers flowing from the proto-Rocky Mountains into the Western Interior Seaway. Paleontologists have used these sediments to infer an environment that looked something like the picture below. During this time, the environment was composed of large rivers that had rocky shores. Fossils from many animals are found here including invertebrates (like clams and snails), fishes, amphibians, mammals, turtles, crocodiles and dinosaurs. In fact, the first T. rex skeleton, discovered in 1902 by Barnum Brown, was found in the exact region of the Hell Creek Formation that the DIG Field School takes place!

The Hell Creek Formation during the late Cretaceous (left), and in 1902 when Barnum Brown, of the American Museum of Natural History, found the first Tyrannosaurus rex skeleton in the Hell Creek area. Note the preferred attire of the earliest paleontologists: a fur coat and bowler hat.
The Hell Creek Formation during the late Cretaceous (left), and in 1902 when Barnum Brown (right), of the American Museum of Natural History, found the first Tyrannosaurus rex skeleton in the Hell Creek area. Note the preferred attire of the earliest paleontologists: a fur coat and bowler hat.

Tullock Formation

The second focus formation, and the highest we find at the DIG Field school preserves the first Paleogene sediments, and was formed just after the K/Pg mass extinction event. This earliest Paleogene formation was deposited 66-64 mya and is known as the Tullock Formation (also known as the Tullock Member of the Fort Union Formation in some areas). This formation consists of thinner, vibrant and colorful beds with yellow, orange, and tan sandstones, siltstones, and mudstones, and lots of large coal seams (low-grade coals known as “lignites”). These beds are so thin, that from far away the different sediments look like stripes on the outcrop, and they have been dubbed “pajama beds” by someone who must have had striped pj’s! During this Era, rivers carried sediment from the mountains to the inland sea causing a swampy vegetative environment. Here, we find remnants of the mammals, reptiles, amphibians, fish, and birds that succeeded the dinosaurs, some of whom survived, and others who immigrated to the area shortly after the mass extinction.

The Tullock Formation of Montana during deposition in the early Paleocene (left), and as the pajama beds seen today (right). The Paleocene environment included sequoia trees, with a dense undergrowth of shrubs such as tea and laurel, with the addition of ferns and horsetails. Pictured above on the ground is Chriacus, a racoon-like omnivore. On the tree is Ptilodus, a surviving member of the multituberculates, primitive mammals often termed the "rodents of the Mesozoic. " Higher up in the tree is Peradectes, an early opossum-like marsupial. Figure and caption revised from The Book of Life: An Illustrated History of the Evolution of Life on Earth, by Stephen Jay Gould.
The Tullock Formation of Montana during deposition in the early Paleocene (left), and as the pajama beds seen today (right). The Paleocene environment included sequoia trees, with a dense undergrowth of shrubs such as tea and laurel, with the addition of ferns and horsetails. Pictured above on the ground is Chriacus, a racoon-like omnivore. On the tree is Ptilodus, a surviving member of the multituberculates, primitive mammals often termed the “rodents of the Mesozoic. ” Higher up in the tree is Peradectes, an early opossum-like marsupial. Figure and caption revised from The Book of Life: An Illustrated History of the Evolution of Life on Earth, by Stephen Jay Gould.

When you compare the deposits of the Hell Creek and Tullock formations, they look totally different, as do the inferred landscapes they represent! Can you differentiate the drab, somber, mudstones of the Hell Creek Formation from the more finely striped beds of the coal-bearing Tullock Formation in the photo below? Knowing where you are in time when you’re standing on the outcrop is a critical paleontological skill.

Actual Hell Creek and Tullock formation rocks that were formed during the Cretaceous and Paleogene Periods, respectively, in the northeastern Montana.
Hell Creek and Tullock formation rocks formed during the Cretaceous and Paleogene Periods, respectively, in the northeastern Montana, at a site the DIG Field School visits for fossil plants.

Next week we DIG into the fossils we will find during the DIG Field School!

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