Posted by: paulgarner | February 7, 2012

Feedback on cyclicity in the GRF

Further to yesterday’s post on sedimentary cyclicity, a reader sent me the following comment. Although I generally don’t accept comments on blog posts, I do appreciate receiving feedback and thought this comment was worth sharing. If anyone else wants to contact me on this or any other subject, just drop me a line.

In October last year, I listened to a lecture on the Green River Formation and the lagerstätten from two horizons from Fossil Lake. Of the 21 ways suggested for explaining the mass mortality events, all except one have been rejected: algal blooms involving dinoflagellates. This hypothesis is currently being tested – looking for biomarkers across a large range of stratigraphic horizons.

It was interesting to find that the word “varve” was not mentioned once when discussing the sedimentology. The laminated layers were interpreted as successive algal mats. In the question time afterwards, I asked about this. The lecturer acknowledged that varves had been a popular interpretation in the past. However, she said, the layer counts in the centre and the edge of the depositional basin had revealed significant differences. Thus, the layers cannot be varves.

This is relevant to M-cycles, because there are several papers arguing that orbital cyclicity characterises the Green River Formation. If the sediment layers are not annual, the researchers were coming up with false positives.

This did stimulate some further thought. I was aware of the work of Buchheim and Biaggi (1988), but this appeared only as an abstract at a GSA conference. I have often wondered whether it ever appeared as a formal paper (but have never found one). I was impressed by the way the lecturer handled the information – if there is a variance between the “varve” count at the centre and at the edge of the basin, they cannot be varves! I surmise they have repeated this work and confirmed it. I hope it will be formally published one day.



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