21 March 2010

Transect debrief 2: weathering the Grenvillian landscape

Posted by Callan Bentley

From the basement complex, the next unit up in the Blue Ridge province’s stratigraphic sequence is the Swift Run Formation. It rests atop an erosional unconformity. After the Grenville Orogeny (~1.1 Ga) added a swath of new crust along the margin of the North American continent, the landscape began to weather and erode. Eventually, an episode of rifting broke open rift valleys and a new ocean basin, the Iapetus. The Neoproterozoic rift valleys filled with sloughed-off detritus from the exposed Grenvillian rocks (granitoids, mainly), resulting in arkosic sediment. This arkose is mixed in with muddy layers: it looks very much like the much-younger rift valley sediments in the Culpeper Basin (Triassic rifting for those, not Neoproterozoic). This is the principle of uniformity at work. The same tectonics yield the same signature, even though they happen at different times. Same as it ever was, same as it ever was.

Here’s a reposted iPhone photo of some of the Swift Run, showing rip-up clasts of mudstone in the arkose:

Some of it is conglomeratic, with rounded quartz pebbles surrounded by immature-composition sand (reposted iPhone photo):

Later, during Paleozoic mountain-building (Alleghanian Orogeny), the Swift Run developed a penetrative cleavage. Here’s a photo showing bedding and cleavage intersecting in the Swift Run:tt_3


This is a cool outcrop: In spite of being polka-dotted with lichens, it shows primary bedding truncations (a primary geopetal sedimentary structure that tells us that up is “up” in this photo) as well as a small S-fold (top to the left) that probably resulted from Paleozoic Alleghanian deformation:tt_4


In spite of small folds and well-developed cleavage, I was shocked when someone on the field trip noticed this:tt_2

That’s two recumbent isoclinal folds! Annotated:

These folds may be just a local phenomenon formed as one layer of the Swift Run slipped over its neighbor… but they also may hint that deformation is more pervasive in this unit than a cursory glance would indicate. Quite interesting, if you ask me.

Take home lessons: (1) The Swift Run Formation is a post-Grenville rift-related sedimentary deposit. It is compositionally and texturally immature. (2) The Swift Run, like everything else in the Blue Ridge province, got deformed millions of years later during the Alleghanian phase of Appalachian mountain-building.