Dr. Kathleen Surpless Dr. Kathleen SurplessAssociate Professor, Geosciences
Trinity University (San Antonio)
Grant 45737-B8
Detailed Provenance Analysis of the Hornbrook Formation, Oregon and California: Testing Paleogeographic Models of the Cretaceous Cordillera in the Western United States

Dr. Kathleen Surpless, an Associate Professor in Trinity University’s (Texas) Department of Geosciences, received an ACS PRF grant to focus research on the Hornbrook basin of the North American Cordillera. The basin straddles the Oregon-California border, north of Surpless’ prior work in the Great Valley Group in central and northern California.

Kathy Surpless in the field.Kathy Surpless in the field.

The North American Cordillera is a complex mountain chain that extends the entire length of Western North America. One of the best-studied mountain belts in the world, this massive chain presents unique challenges and opportunities for scientists.

What makes the North American Cordillera particularly intriguing are large blocks of crust that, due to tectonics, have moved either from where they originally formed or from where they were initially added to North America. Add to that the fact that these blocks of crust have been deformed, intruded, and eroded over time, and you have a challenge that’s almost impossible for a geoscientist to resist.

In essence, Dr. Surpless and her team are attempting to paint a picture of what this rocky, difficult terrain might have looked like about 100 million years ago. They examine sedimentary material that, through erosion, has collected in basins in the Cordillera. (Or, in what would have been basins in previous times, but may now be covered by younger rocks or deformed and uplifted into modern mountains.) With her current focus on the Hornbrook region, she tries to piece together where this material came from and how it developed over time. “We’re essentially trying to reconstruct what is no longer there.”

“From my Ph.D. work, I’ve been interested in Cordilleran geology. It’s a fascinating place to work. People have generated a lot of great and intriguing ideas about how this mountain belt may have developed through time.”

The collected evidence indicates that millions of years ago, current “pieces” were likely joined in a single, continuous formation, but were later separated by environmental changes and tectonic shifts.

This idea is not necessarily new or novel. Previous research had concluded that the sediment within both the Hornbrook Formation and the Great Valley Group came from the Klamath Mountains (directly west of Hornbrook), suggesting that the two basins may once have been joined.

New Direction and Innovations

Surpless and her team are employing new tools and techniques to sample and analyze mudstone on the Hornbrook Formation for geochemical analysis. What they are discovering from these more advanced analyses is that, while earlier sedimentary material did, in fact, likely come from Klamath Mountains, later materials sampled from the Hornbrook Formation could not have come exclusively from Klamaths.

The implication of this research indicates that at some point, there was a geological shift. Samples indicate that three now-separate systems—the Ochoco asin, Hornbrook Formation, and Great Valley Group—may have been connected during Cretaceous time and received sediment from the Klamath Mountains to the west, as well as the Sierra Nevada Mountains and the Blue Mountains, south and east of the basin.

In the future, Dr. Surpless plans to investigate causes for the apparent breaking up of the Ochoco Basin, Hornbrook Formation, and Great Valley Group. “If they were connected, why aren’t they anymore?”

Dr. Kathleen Surpless is an Associate Professor at Trinity University’s Department of Geosciences. She received her Ph.D. from Stanford University and conducted post-doctoral research there for an additional three years.

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