Paleo Background | Decision Tree, LLC

Dakota Hogback

The Age of the dinosaurs, in one hill

Stretching parrallel to, and just east, of the Rocky Mountains is a long hil that captures the glory days of the dinosaurs on Earth - the Jurassic and Cretaceous periods from 160 - 80 million years ago

The age of the dinosaurs began in the Triassic almost 243 million years ago and ended in with the Chicxulub impact 65 million years ago. In that time period they colonized almost every ecological niche on Earth from Siberia to Antarctica. Their period of dominance began as the supercontinent Pangea began to split up and ended with the continents approaching their current positions. Though our environment changed remarkably in that time period. For example, the Western US was occupied by a broad Western Interior Seaway which stretched from the Arctic Circle to the Gulf of Mexico.

When the Rocky Mountains began to rise 80 million years ago, they tilted the vast seabed on its side, creating a geological feature known as the Dakota Hogback. The hard limestones left behind by the Western Interior Seaway preserved the softer formations underneath, including the soft grey siltstones of the Morrison Formation below. While the Western Interior Seaway preserves marine reptiles like Mosasaurs of the Cretaceous period, the Morrison includes Jurassic dinosaurs such as Stegosaurus, Allosaurs, and Sauropods like Apatosaurus. The environment of that time would have featured vast alluvial fans similar to what we see in the Mississippi river system today. The transformation of the continent from a series of braided rivers to a sea was complex, but we can find clues to that transformation in the Dakota Hogback.

In early 2018, we went out to one of the best exposures of the hogback, Dinosaur Ridge, located just outside Morrison, Colorado along I-70 headed into the Rocky Mountains. The roadcut was formed artificially as part of the interstate's construction, thus it provides an unusually sharp cross section of a broad geological feature. We used X-ray fluorescence (XRF) to sample it every 40 centimeters, starting from the west in the earliest deposits and ending in the east during the Cretaceous. We are going to broadly treat the eastern half as the Jurassic sediments of the Morrison formation for our purposes, but there are debates as to the chronology and proper association of these early rocks.

Over time, potassium (K) gradually left the system, as you can see from the high K% from right to left. However, less K gets deposited in large bodies of water - could we be looking at the shift from land to sea. If we switch and look at niobium (Nb) to rubidium (Rb), we can see if we can test this idea.

It certainly looks like there is a relationship! As potassium (K) falls, the ratio fo niobium (Nb) to rubidium (Rb) increases. Nb is thought to reflect the circulation of water, and this would confirm that idea. But, these are just wavy lines - can we see if there is a meaningful relationship?

If we plot Nb/Rb vs K, we can see that yes, the difference is meaningful. The Jurassic land deposits show lots of potassium, while the Nb/Rb is much higher in the interior seaway that predominated much of North America. This suggests we can see differences in these two elements to track what kind of environment was present in Golden, Co. XRF can also identify other events.

If you look back to the photo of the formations, you'll see a dark grey layer that doesn't seem to look like sea or land. What element is present there? One useful element to identify these kinds of changes is sulfur (S), which can indicate a very different environment.

Here, we see that the dark grey formation has lots and lots of sulfur in it - as much as 15% in some places! This indicates that there was very low oxygen in these conditions - sulfur tends to precipitate when there is less water circulation. You can see that where the sulfur increase is, the Nb/Rb ratio drops.

If we take a step back, we can see the evolution of Colorado from a complex river system to a sea, the growth of severe anoxic conditions, and then a recover. All just on the side of the highway.

Economic Geology
Understanding the Earth's past requires detailed geochemical analysis This means understanding not only the compositional materials like SiO2 and CaCO3, but also trace elements like Mo and U which can indicate the reduction sequence in marine environments.

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We can use an in-line XRF system to do continues step analysis of cores, enabling us to process data rapidly and accurate. We follow the methods and employ the standards used in Rowe et al. 2012. With a full range of elements, complex paleoclimatic interpretation is possible, which relevance to reconstructing geochemical formation processes.

In this example, a calcium carbonate (Ca~30%) predominates, formed largely by the precipitation of dead plankton on the sea floor. In an ocean with plenty of oxygen, the carbon is recycled to the biosphere via the activity of bacteria; with only remnants of being the shells of the microscopic organisms. However, to the right of the graph, the calcium concentration drops while a trace metal, molybedenum (Mo) increases. Key to understanding this change is knowing that Mo oxide is soluble in water while Mo sulfide is not. In anoxic conditions, the Mo will precipitate out. These same anoxic conditions also prevent the growth of bacteria, thus in turn keeping the carbon from being recycled to the biosphere. These are ideal conditions for the preservation of organic carbon, and can be used to identify fossil fuel deposits.

By pairing XRF data with targeted XRD and FTIR application, we can provide complete analysis to contextualize findings and help guide decision making.