Tuesday, August 9, 2011

Random Notes From The Northern Yellowstone Landscape

The theme of the recent seminar I attended with the Yellowstone Association Institute was geomorphology, the study of landforms – why they look the way they do today and the processes that caused them. For three days we drove up and down and walked all over Lamar Valley in our attempt to understand these processes that have occurred over the past 2.5 million years of Earth history during the Quaternary Period. This more recent slice of geologic time includes the last period of global glaciation along with the advance and retreat of large Northern Hemisphere ice sheets. 

IMG_8178Soda ButteCreekCutting IntoToeOf Landslide
Soda Butte Creek cutting into toe of landslide

However captivating geomorphology might be, nevertheless a constant refrain ran through my head for the entire three days: I wanted see some really old metamorphic rocks on this trip! The Beartooth Mountains of northern Wyoming and southern Montana are Archean terrane, at least 2.5 billion years old, and I knew they were out here somewhere. 

OK so maybe geomorphology does not exactly lend itself to the study of 2.5 billion-year-old rocks. Nevertheless, I would be on the lookout. Perhaps a passing glacier had exposed some nice gneiss or shimmering schist as it scoured its way across the prehistoric landscape.

IMG_8224BusStop InLamarValley
Bus stop for geomorphology in Lamar Valley

In Lamar Valley we were in a high relief area of the Park, outside of the caldera. Some of this relief is due to old, hard resistant metamorphic rocks (there they are!) having been uplifted during mountain–building events while other relief is most likely due to tectonically active faults. The Yellowstone “hot spot” does add additional relief to the landscape from relatively recent uplift and subsidence due to heat emanating from the ginormous magma chamber lurking beneath the surface. 

IMG_8173 MountainsOf Absaroka Volcanism
Mountains of Absaroka Volcanics

The youngest sedimentary rocks seen here in Lamar Valley are the Madison Limestone of the Mississippian Period, about 320 million years old – any younger sedimentary rocks have been eroded away. Additionally, because 50–million-year-old Absaroka Volcanic rocks are sitting directly on top of this Madison Limestone, there is an “unconformity” or gap of around 250 million years here in the geologic record. 

IMG_8205Madison Limestone_PebbleCreek
Cliffs of Madison Limestone at Pebble Creek

The Absaroka volcanism of 50 million years ago occurred as the Beartooth Mountains to the north and east were being uplifted. The cliffs surrounding Lamar Valley (such as Specimen Ridge) are these Absaroka Volcanics – they appear as stratified layers because much of the volcanics occurred as mudflows (similar to Mt. St. Helen’s mudflows). 

Here and there we found boulders of mudflow breccia and fragments of 50 million year old fossilized trees in the alluvial fan deposits coming off the mountains. 

IMG_8209 AbsarokaVolcanicsFromPebbleCreek
Absaroka Volcanics seen from Pebble Creek

IMG_8185MudflowBreccia
Mudflow breccia from Absaroka Volcanics

IMG_8188PetrifiedWoodFrom AbsarokaVolcanics
Fossilized tree section
In a stream bank on Soda Butte Creek we were amazed to see a thin lens of light gray ash deposited from the explosion of Mt. Mazama (Crater Lake, OR) around 6800 years ago.  Click on the images to see if you can find the ash lens – in the third image a red arrow points it out. 

IMG_8229Stream Erosion ExposingMt MazamaAsh


IMG_8230Stream Erosion ExposingMt MazamaAsh


IMG_8231 Erosion ExposingMt MazamaAsh

2 comments:

  1. DID THE SHAPE-EYED GEOLOGIST SPOT THE LIGHT GRAY ASH LENS ON THE STREAM BANK FIRST ??
    SOMEONE WAS REALLY PAYING ATTENTION TO SPOT THAT THIN LAYER ...

    I REALLY LOVE THAT AREA OF THE PARK.

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  2. I did see it (not sure if I was first) but I had no idea what it was.
    The course instructor had been slogging around in the creek bed one day some years ago, and noticed this lens...he had the sample dated and is 99% certain it's from Mt. Mazama.

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