Comment: 25:34 - 26:56 (01:22)
Source: Annenberg/CPB Resources - Earth Revealed - 17. Sedimentary Rocks: The Key to Past Environments
Keywords: "James Sadd", "sedimentary rock", "clastic sediment", "Grand Canyon", "Colorado River", sand, mud, "volcanic ash", "lava flow", "ocean basin", deformation, mountain, orogeny, "Mazatzal Mountains", erosion, uplift, limestone, shale, "sea level rise", flood, volcanism, deposition, unconformity, ocean, preservation
Our transcription: The oldest rocks in the Canyon were deposited as sediment about 2 billion years ago.
A great pile of sand and mud interbedded with volcanic ash and lava flows filled a rapidly subsiding marine basin.
These rocks were deformed in a mountain building episode into an ancient mountain range, the Mazatzal Mountains about 1.5 billion years ago.
The mountains were deeply eroded and then covered with a mixture of limestone and shale as sea level rose and flooded the region creating an unconformity about 1 billion years ago.
Mountain building and volcanic activity were then reactivated followed by yet another rise in sea level and deposition of more marine sediment.
In fact, the rocks of the Grand Canyon record no fewer than four more repetitive episodes of structural uplift and erosion followed by deposition of more sediment.
As sea level encroached on the land a nearby mountain shed sediment into the region, and this process continues today.
The Colorado River is now cutting an extraordinary unconformity surface which will eventually be covered with sediments once again.
I sometimes wonder what the Grand Canyon will look like when that happens.
Will it be covered by an ocean?
Will the sediments be shed from mountains yet to be formed?
No one yet knows, but one thing is for certain: The answers will be recorded in the sedimentary rock.
Geology School Keywords