Episodic Yo-Yo Movements (Epeirogeny) on Continental Platform Intracratonic Basins: Need for Reinterpretation of Paleogeography, Faunal Extinctions, and Source Rock Maturity

Gerald M. Friedman

The concept of G. K. Gilbert's term "epeirogeny" relating to broad vertical movements of the earth's crust unaccompanied by crumpling of the strata has been commonly explained in terms of glacial rebound. More recently, the recognition of unexpectedly large-scale vertical uplift and erosion bringing deeply buried strata to the present land surface explains the renewed interest in epeirogeny. These drastic tectonic changes represent crustal unroofing with widespread implications for paleogeography of a kind unrecognized at present. Crustal epeirogeny is episodic; it may or may not be explained by reference to plate-margin processes.

In the Appalachian basin and Mississippi Valley, dates for epeirogeny cluster between 250 and 300 Ma, with the completion of the uplifts at about the Permian-Triassic boundary. In the Fennoscandian shield and elsewhere, uplifts appear to be of comparable age. This was the time when the continents had collided to become supercontinent Pangea which, as a result of uplift, stood high above sea level; environments became stressful, weather patterns changed, and faunal extinctions occurred. Large-scale epeirogeny began again at about 100 Ma, with some dates at about 60 Ma marking Cretaceous-Tertiary boundary extinctions. Precambrian basements, such as the Adirondacks, the Canadian shield, and the Arabo-Nubian shield suffered domal uplift during the Oligocene-Miocene, especially in the Miocene to Holocene interval. Oceanic sedimentation rates were elevated in the Miocene to accommodate this increased continental erosion. Active Holocene uplift in the Arabo-Nubian shield involving several meters r even tens of meters occurred as recently as between 3,405 ± 90 years and 2,465 ± 155 years ago.

Depending on technique and computation, crustal uplift has been inferred as amounting to several kilometers; even 10 km and larger amounts of uplift have been quoted. The deeply buried and subsequently uplifted strata remained undeformed. Additional epeirogenic episodes have been recognized, but are not discussed here. Such large-scale uplifts resulting from regional or global epeirogeny resulted in relative sea level drops, and stressful conditions which may have caused faunal extinction. From an economic point of view, rocks that are now at shallow depth may have lain within the window of maturity for hydrocarbon source rocks before the removal of thick sequences of strata, although the present depth of burial may be too shallow.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.