[[What on earth does layers of ash from a volcano got to do with varves?]]
Quite a bit actually, as it turns out:
“Creationist suspicions about the validity of the varve interpretation were confirmed in a study by two geologists published in 1988.9 Near Kemmerer in Wyoming the Green River Formation contains two volcanic ash (tuff) layers, each about two to three centimetres thick.
A volcanic ash layer is an example of what geologists call an ‘event horizon’, because it is laid down essentially instantaneously by a single event, in this case a volcanic eruption. The two ash layers are separated by between 8.3 and 22.6 centimetres of shale layers.
If the standard interpretation is correct, then the number of shale layers between the ash layers should be the same throughout the Green River basin, since the number of years between the two eruptions would be the same.
However, the geologists found that the number of shale layers between the ash beds varied from 1160 to 1568, with the number of layers increasing by up to 35% from the basin centre to the basin margin! The investigators concluded that this was inconsistent with the idea of seasonal ‘varve’ deposition in a stagnant lake.
So how were the great thicknesses of finely laminated shale in the Green River Formation laid down? Creationist geologists need to investigate the issue more closely, but there seems to be great potential for developing a catastrophic model for the origin of these sediments. There is a large body of experimental and observational data that shows that varve-like sediments can build up very rapidly under catastrophic conditions.10, 11, 12, 13, 14 For instance, in 1960 Hurricane Donna struck the coast of southern Florida and deposited a blanket of thinly-laminated lime-mud six inches thick.15 Another example comes from a Swiss lake, in which up to five pairs of layers were found to build up in a single year, deposited by rapid underflows of turbid water.16
Given the right conditions, thinly-laminated muddy sediments can and do form by rapid sedimentation. Contrary to claims by old-earth proponents, long periods of time are not demanded.”
http://www.answersingenesis.org/creation/v19/i3/greenriver.asp
“If the standard interpretation is correct, then the number of shale layers between the ash layers should be the same throughout the Green River basin, since the number of years between the two eruptions would be the same.”
—I haven’t studied the Green River basin in any detail, but I do know that not all the layers at the basin are believed to be varves. The formation of varves requires specific conditions and there were times when the varves weren’t being created but other types of layers were. I believe (iirc) that the shale layers between the two volcanic events are not varves.
“So how were the great thicknesses of finely laminated shale in the Green River Formation laid down? Creationist geologists need to investigate the issue more closely, but there seems to be great potential for developing a catastrophic model for the origin of these sediments. There is a large body of experimental and observational data that shows that varve-like sediments can build up very rapidly under catastrophic conditions.10, 11, 12, 13, 14 For instance, in 1960 Hurricane Donna struck the coast of southern Florida and deposited a blanket of thinly-laminated lime-mud six inches thick.15 Another example comes from a Swiss lake, in which up to five pairs of layers were found to build up in a single year, deposited by rapid underflows of turbid water.16
Given the right conditions, thinly-laminated muddy sediments can and do form by rapid sedimentation. Contrary to claims by old-earth proponents, long periods of time are not demanded.”
—I couldn’t find any details of the examples given of layers quickly laid down, but I’d wager that, as with the volcanic ash layers, that any geologic undergrad could spot immediately that they are layers that were created quickly.