THE POST-KNOX UNCONFORMITY: PRODUCT OF GLOBAL, NOT REGIONAL PROCESSES

The post-Knox unconformity has been touted by several Appalachian geologists and geophysicists as the product of loading of the Ordovician Laurentian margin by obducted volcanic arcs and ophiolites. Amounts of Knox Group (and equivalents: Beekmantown, St. George, Arbuckle, Ellenberger, Prairie du Chien Groups) strata removed along the unconformity range from zero (in N VA to PA) to >150 m elsewhere in eastern Laurentia. This unconformity is recognizable from the Appalachian region through the Mid-Continent into the Rocky Mountains region. In the clastic section of the Ouachitas, the Blakely Ss. is present in the Knox-Middle Ordovician interval, separating deeper water black shales (Lower Ord. Mazarn Sh. and Middle Ord. Womble Sh.). Suggestions of the unconformity exist as far west as NV and CA. This wide a distribution of the Knox unconformity negates the possibility that the unconformity has a solely regional origin related to tectonic loading affecting eastern Laurentia during the Early to early Middle Ordovician. Withdrawal of the Early to early Middle Ordovician seas from much of Laurentia had to be the origin of the unconformity. Coeval unconformities and/or shallowing episodes are reported on other continental blocks as well, e.g, Siberia, SE Asia, S Baltica (Estonia). The cause of this withdrawal is difficult to pinpoint: there was no known major glaciation until the Ashgillian, but there were Ordovician compressional tectonic events at this time in several locales (e.g., Pacific margin of Gondwana, southern margin of Siberia, W Baltica, and paleo-Variscides). These tectonic events mostly involved arc accretion to the margins of existing continents, so the likelihood of them collectively producing global changes in sea level is unlikely. Withdrawal of Early and early Middle Ordovician seas from Laurentia may just be related to mantle-driven general uplift of Laurentia prior to initiation of the early Paleozoic orogenies, but presence of the unconformity on other continents may reflect major reorganization of the upper mantle at this time.