maybe that it is possible that conditions at some points in the past might have no modern analogs.
There are certainly some ancient geologic processes that don't happen much, or at all, today, but that's not to say that they don't have analogs. Oceanic and atmospheric chemistry changed down through time, and sediments like banded iron formations and primary dolomites don't form now, as far as I know. But well-(or sorta-)understood mechanisms are responsible for the formation of these sorts of things, and they can be made in the lab. It's just sort of hard on the wildlife to, say, make the Pacific Ocean anoxic just so you can finish your thesis.
™ Version 4.2