Geosciences Seminar: Karl Karlstrom

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"Origin of the Colorado River from the Rockies through Grand Canyon" seminar will be held in BGYM 219 at 3 PM. There will be coffee and cookies in the Lobby of ERB prior to the talk at 2:40 PM. We will be holding a Pizza Lunch at 12pm in ERB 1161! Title: Origin of the Colorado River from the Rockies through Grand Canyon

Abstract: For >150 years, scientists have debated when and how the Colorado River first emerged as a continental-scale river system and carved its way through the Grand Canyon. This study combines dating of ancient sand grains (sanidine) and fish DNA to reveal that the river developed gradually from north to south between 8 million and 4.8 million years ago. Tectonically driven integration of the 4.8 Ma Colorado River USA tracked with detrital sanidine and fish genetics. The research team analyzed about 10,000 volcanic sanidine crystals and discovered a distinctive set of tracer grains dating to between 60 million and 40 million years old. Using single-crystal 40Ar/39Ar geochronology, the researchers tracked these grains as they traveled downstream from volcanic sources in Idaho, to Brown's Park Formation near the Uinta Mountains in Utah, to the Bidahochi Formation in present-day Arizona between 8 and 7 million years ago, moved through the Grand Canyon between 6 and 5 million years ago, and ultimately reached the Gulf of California by 4.8 million years ago. The geological evidence for this downward integration is reinforced by an independent data source—DNA preserved in modern fish species linked to fossil fish. Molecular clock data match the timing revealed by the sand grains. Native Colorado River fish species, including pikeminnow and chub species, diverged from their northern ancestors after ~12 million years ago, diverged from each other in the Bidahochi Formation in Arizona between 7 and 6 million years ago, and passed through Grand Canyon between 6 and 5 million years ago. The mechanism for downward river development involved tectonic forcings that influenced the pathway and tempo of river integration. Volcanic activity associated with the Yellowstone hotspot and uplift of the western Rocky Mountains tilted the Colorado Plateau southwestward toward the subsiding Gulf of California, and localized mantle drips created basins such as pre-8 Ma Brown's Park and 8-7 Ma Hopi Buttes volcanism that temporarily trapped sediments and fish before basin uplift associated with drip detachment allowed both to continue downstream. Old debates renewed, the findings challenge the hypothesis that the Colorado River formed rapidly through catastrophic lake spillover, river piracy or headward erosion. (link below) The ~ 3 Ma timeframe favors tectonic forcings. Short-lived river events likely occurred but there is currently no convincing evidence showing whether, when, or where lake spillover or river piracy events played key roles in Grand Canyon’s origins. Thermochronologic data indicate portions of the Grand Canyon had already been partially carved and the Kaibab uplift partially breached before the modern Colorado River became fully integrated.