“Xenoconformity”
The term “unconformity” refers to a part of a sequence of sedimentary rocks where there is no sediment deposited. There are several types of unconformities – a nonconformity refers to a contact where sediments drape on top of igneous or metamorphic rocks, an angular unconformity refers to a boundary where rocks were tilted and partially eroded before new sediments were deposited, and a disconformity refers to a time within layers of sediment where deposition stops, but no changes can be easily recognized (https://tmblr.co/Zyv2Js1JtNUXH).
A recently published paper has proposed a new term for spots like this. This point is a GSSP, a “Global Boundary Stratotype Section and Point” where a single spot has been marked as containing a boundary on the geologic column. This point is at the start of the cap carbonate for the last proposed Snowball Earth glaciation, 640 million years ago. This GSSP point marks the start of the Ediacaran, a distinct period on the geologic timescale. The new paper proposes that a site like this should be called a “Xenoconformity”
Unconformities are, by definition, local changes. Rocks must be faulted or eroded or facies must change in a single area for it to be an unconformity. However, when big things happen – when global glaciations collapse, or there is an asteroid impact, sedimentation patterns change worldwide. When the asteroid struck the area that is now the Yucatan Peninsula at the end of the Cretaceous, sediment patterns changed literally on the opposite side of the world; the boundary has been recognized in places including Europe and North Africa.
That’s the type of boundary that scientist Alan Carroll at the University of Wisconsin proposes should be called a “Xenoconformity”. This term would distinguish local unconformities, as can be caused by small changes within a basin, from global events.
-JBB
Image credit: http://geology.geoscienceworld.org/content/45/7/671 Original paper: https://doi.org/10.1130/G38952.1