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USGS CMG InfoBank: Change in Environmental Conditions

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Comment: 04:59 - 06:49 (01:50)
Source: Annenberg/CPB Resources - Earth Revealed - 11. Evolution Through Time
Keywords: "Stanley Aramic", fossil, "Alexander Hills", "Eastern California", stromatolite, "sedimentary rock", environment, sand, cyanobacteria, "rate of sedimentation", deposition, precipitation, "calcium carbonate", "Crystal Spring Formation"

Our transcription: Paleobiologist Stanley Aramic specializes in the study of these ancient life forms whose fossils date back as much as three and a half billion years.
This area of the Alexander Hills in Eastern California contains an interesting sequence of stromatolites.
Here in the Crystal Spring Formation we have a sequence of stromatolites with other kinds of sediments that are recording an interesting history on the interplay of microorganisms in sediment.
These columnar stromatolites represent the time when the microorganisms were living in shallow water, trapping and binding the sediment producing the columnar morphologies that are so characteristic of stromatolites.
Then, the environmental conditions change somewhat where coarser sediment, sands, came in and stopped the growth of stomatolite.
Probably the sediments were being deposited too rapidly, and the cyanobacteria just couldn't keep up with this rapid rate of sedimentation.
Then, conditions changed and went back to quieter water again with the precipitation and accumulation of calcium carbonate.
And then, the conditions continued and above here the columnar stromatolites again begin to develop and then higher up these coarser sediments come in and stop stromatolite growth.
So, it's by studying the interplay of the microorganisms with the sediment, and the sequences in which these things occur that a geologist can get an understanding of the environment and hear the history that went on with the deposition of these rocks in the Crystal Spring Formation.

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