A large grained sediment from the same time period as dated by ash fall, 14C dating, etc. This sediment should be found worldwide.
In ice records, we shouldn't find any that go right through this time period. We shouldn't find lake varves and tree ring records that blaze right through this time period without interruption.
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Here is a nice picture illustrating the contrast between normal deposition and flood deposition.
In the middle are a set of layers with alternating color. These are varves from slowly settling and fine sediments. They are different colors because of the different rates of upstream erosion each spring cycle. Notice how smooth these sediments look, and how you can't pick out any grains at all. Compare this to the sediments above and below the varves. These are coarse grained and do not show these finely captured layers. Those are flood deposits. That is how easy it is to tell the difference.