Colorado Sketches: a musical day trip into the Mountains

You are listening to a musical installation based on the sonification of a Transient Electromagnetic sounding, collected by USGS over the Crested Butte Mountains (AEM, see http://crestedbuttenews.com/2017/12/u…). The performance is running at the Crested Butte Heritage Museum (map: https://goo.gl/maps/pQNyFtWgQAEj4ZuD8) and has been designed by EMusic Association (https://emusic.world) and USGS (https://www.usgs.gov), with the support of RMBL (https://www.rmbl.org). For more info about the EMusic project visit: https://www.emusic.world/emusic-liste… . We have expanded the effective acquisition times of the geophysical instrument, of 500,000 times, so that 1 microsecond of data corresponds to 0.5 seconds of EMusic. The track represents a travel into the Crested Butte Mountains, riding the eddy currents, i.e. the Earth signal. It begins from the surface and reaches a depth of about 500 m. After 2.5 hours it comes back to the surface, ending after 5 hours. Any time is referred to the MTD reference. The first 19 seconds are the sounds of the shallow thin Glacial deposits; during this short time, the signal has crossed about 20 m. It testifies the effect of the Ice Age during the Pinedale glaciation in this area, which occurred about 30,000 to 10,000 years ago. In the following 18 seconds, i.e. until 1:00:37 p.m., the sounds are drawn from the top layer of Mancos shale, that is from 20 to 50 m depth. Due to the low resistivity of the formation, the decay rate is very slow, hence the pitches show a clear chromatism. The age of this rock is Upper Cretaceous (about 70 – 100 million years ago) and was deposited as marine sediments when this area was submerged beneath the Western Interior Seaway that cut across North America. From this time and until 1:02 p.m., we cross the Granodiorite sill. This intrusive rock is younger (Tertiary age, , approximately 30 million years old) than the Mancos shale and testified a huge volcanic activity that caused the uprising of heated rocks from depth, so that now we can find them embedded within the older formations. Due to its high resistivity, this travel occurs very fast, in spite of the remarkable thickness of the sill (about 100 m). If you are listening to the track between 1:02:28 and 1:26 p.m., you are once again into the Mancos shale, for about 80 m: the low resistivity has greatly slowed down the EM signal. From 1:26 p.m. and until 3:30 p.m. you are travelling into a thick resistive complex composed by Jurassic-Cretaceous formations (Morrison and Dakota formations, composed by sandstone, claystone, siltstone). The Jurassic Morrison formation (about 150 million years ago) is well known in Colorado for hosting many dinosaur fossils (http://dinoridge.org). The Dakota formation represents nearshore sediments deposited as the Western Interior Seaway expanded into Colorado. Just below our depth of exploration are much older Pennsylvanian and Permian rocks (320 – 250 million years old) that formed the ancestral Rocky Mountains, which are now tilted upwards and can be found at the surface just to the East of our location. The reversed trip follows: from the maximum depth to the surface: between 3:30 and 5:30 p.m., you are listening to the sounds produced by the Sandstone complex. Between 5:30 and 5:56 p.m., we enter the Mancos Shale formation. At 5:56 p.m. we have reached a depth of about 160 m and you are hearing the sounds extracted from the Granodiorite sill. At 5:59 p.m. we have reached the top of the sill (resting at a depth of about 50 m) and we enter another Mancos shale layer for only 22 seconds. The final 19 seconds let hear the sound of the Glacial deposits, occurring from a depth of about 20 m until the surface. If you wish to learn more about the geological evolution of Colorado click here: https://vimeo.com/143654356 This project has been supported from an American Geophysical Union (AGU) Centennial Grant, see https://centennial.agu.org

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