Devonian strata

in the late silurian and Devonian a major regression affected most of the craton, exposing the underlying rocks to subaerial erosion, except for a few deep basins and narrow seaways. This major unconformity is overlain by a new transgressive sequence known as the Kaskaskia sequence, which, like the two preceed-ing sequences, is marked by a basal quartz sandstone, overlain in turn by a thick carbonate sequence. Much of the continent was again covered by carbonate and shale deposition, and areas that saw enhanced subsidence during the Tippecanoe sequence deposition also experienced greater than normal subsidence during the evolution of the Kaskaskia sequence. These areas became deep basins, including the Michigan and illinois basins in the midcontinent region.

maps of the upper Devonian sedimentary facies show a >1-mile (2-km) thick sequence of rocks on the eastern side of the craton known as the Catskill clastic wedge. This group of rocks provides evidence for another episode of mountain building or orogenesis in the Appalachians. The Catskill delta consists largely of molasse (conglomerate, red beds, mud-stones) deposited by river systems in an alluvial fan/ braided, meandering river complex, with the rocks derived from uplifted mountains in the east during the Devonian. The Catskill clastic wedge formed in front of the Acadian orogen, which was active (as shown by the ages of igneous rocks) from 360 to 330 million years ago. The Acadian orogeny is thought to have been caused by closure of an ocean basin between North America and Avalonia (part of northwest Africa), along two subduction zones that dipped beneath both continental masses. The convergent margins collided, forming thick clastic wedges, then the collision continued as mostly sideways or strike slip motions through the late Devonian. The Catskill clastic wedge has an equivalent, but slightly older, sequence of rocks in Europe, known as the old Red sandstone, which also represents rapid erosion of a mountain range that formed by the north-south closure of an ocean basin in the Devonian. At this time North America and Europe were still connected in one landmass, and the ocean that closed between the combined North American/European continents and the colliding masses to the south (largely thought

A Late Llanvirnian

Shelf

N. America

Rise

Outer swell

Ammonoosuc Arc

Shelf

Rise

Outer swell

Ammonoosuc Arc

B Llandeilian

Post-Chazy unconformity

Indian River

Ophiolites

B Llandeilian

Post-Chazy unconformity

Indian River

Ophiolites

C Early Caradocian (lower D. multidens zone)

WGBS

Orwell Walloomsac | Pawlet Flysch

WGBS

Orwell Walloomsac | Pawlet Flysch

D Late Early Caradocian (upper D. multidens zone)

Glens Falls Snake Hill Austin Glen Moretown -«-Tuffs-

E Medial Caradocian (C. americanus zone)

Whipstock and Snake Hill Austin Glen Moretown

-Tuffs-

-Tuffs-

F Late Medial Caradocian (O. ruedemanni zone)

Austin Glen

Tuffs-

Felsic volcanism?

Austin Glen

Tuffs-

Felsic volcanism?

6 Infobase Publishing

Melting?

6 Infobase Publishing

Melting?

Geological evolution of the Taconic arc-trench collision during the Ordovician Taconic Orogeny in the Appalachian Mountains (modified from D. Rowley and W. S. F. Kidd, 1981)

to be African and South America) was the remaining open part of the Iapetus Ocean that did not close during the Middle Ordovician Taconic Orogeny.

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