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Only about 10% of the geologic time is imprinted in sedimentary strata, and...
Only about 10% of the geologic time is imprinted in sedimentary strata, and the rest is hidden in non-deposition or erosion surfaces called unconformities. Stratigraphic unconformities (disconformities) are principal bounding surfaces in sequence stratigraphy which a geologist would easily identify in the outcrop but dubiously in the subsurface unless core is available. Proportion of mis-identified and overlooked disconformities in subsurface stratigraphy is quite large, which puts a warning sign on simplistic sequence stratigraphic models. Time imprinted in disconformities can be evaluated with relative weathering maturity of the subaerial profile, cyclostratigraphic calibration, absolute dating, and biostratigraphy. However, using biostratigraphy alone is never enough as biostratigraphic gaps tend to fill with increasing data coverage. Identification of paleo-vadose zones and subaerial exposure profiles is regarded critical for finding stratigraphic unconformities and is the only approach in strata where geophysically mappable fluvial systems are absent. Drowning unconformities are carbonate-platform drowning surfaces that usually produce distinct reflection horizons and have better stratigraphic value in the subsurface than platform-embedded subaerial unconformities. This discussion is supported by examples of subaerial disconformities from the Devonian, Carboniferous, and Permian of Canada and Russia and with an example of a geographically extensive mid-Devonian drowning unconformity from Northwestern Canada.
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