Mexico's seismological agency estimated its preliminary magnitude at 6.8 and said its centre was east of the city in the state of Puebla.
People remove debris as they search for possible victims after a quake rattled Mexico City on September 19, 2017.
There is a good picture of an actual unconformity in America here.
Unconformity between Popotosa Formation playa deposits and post-Santa Fe Group (Pleistocene or younger) alluvium, San Lorenzo Canyon area, north of Socorro, New Mexico.
The youngest rocks below the unconformity are about 140 million years old (from fossil evidence).
The coal above the unconformity is about 100 million years younger – so this ‘gap’ represents a huge period of time in which New Zealand eroded away to a level, swampy plain (hence the coal) and then sank below the sea (forming the limestone).