Posted by: Alan Pitts | May 12, 2010

Structural Geology Trip Part Duex : Swift Run

Our next field area was the Swift Run formation, a neoproterozoic unit which stratigraphically is above the Grenville-age crystalline basement complex  and below the meta-basalts of the Catoctin formation. I especially enjoyed this outcrop, it lay hidden, off the  major roadway and required hiking along a wooded trail that didn’t exist. This was a fun hike, and felt like we were really doing some exploratory science.

At the outcrop we found some arkosic meta-sandstones and conglomerates with intermittent layers of mud. the whole unit showed a  phylitic texture which indicates a slight degree of metamorphic activity.

Here is another one from my field book. I sketched this one as we sat around for a “debriefing” after we finished looking around. Callan for scale:


This sketch illustrates the relationship of bedding and foliation angles. In this outcrop the  foliation dips at a steeper angle than the bedding. Ill show you what I mean:

The red lines indicate the bedding surfaces. The green lines indicate the foliation imprinted on the rocks during metamorphism. The foliation dips at a steeper angle than the  bedding planes, which indicates that this is not an over-turned limb of a fold and we can safely assume that “up” is actually up on these rocks.

Here is a picture by Nik D. of some nice gravel size feldspars:

And here is some normal faulting which is expressed by the offset of this darker mud bed:

Here is a  view of some large pebbles as well as a contact between a conglomeratic bed and muddy bed. picture by Nik D:

The Swift Run formation owes its character to continental rifting and the opening of the proto-Atlantic Ocean. During this time the crust experienced extensional forces and as a result large grabens , or rift valleys formed. These grabens then filled with sediment carried by rivers, which is evident by the rounded pebbles of siliclastic, flesic sediment. The mud beds represent periods when lakes formed and promoted the deposition of fine sediment in an anoxic environment, giving the dark color. Then later periods of flooding  would bring coarser grained material and siliclatics

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