2006 Appalachian Tectonic Studies Group Field Trip

Pennsylvania Piedmont east of the Susquehanna River

Don Wise, University of Massachusetts, dwise@geo.umass.edu
Bob Ganis, bobganis@aol.com

This year’s trip focused on the Pennsylvania Piedmont east of the Susquehanna River. It involves mostly Taconian structures and events but includes a healthy overprint of younger, largely Alleghanian structures.

Click on a link below to view a picture or watch the movieATSG 2006 Proofs

The trip is a two-day version of the shorter one just completed for the NE GSA meetings. That one-day trip included about 60% of the same stops (Previewed and argued over mightily by the likes of Jim Hibbard, Cees van Staal and Bob Wintsch). The litho-tectonic map of the area below is from the 2006 NE GSA guidebook (available from Noel Potter pottern@dickinson.edu).

Tectonic Map

Saturday we saw outcrops with some fancy structures and minor dolomitization superimposed on the transition from carbonate platform to olistostrom and slope carbonates. We continued on into the complex thrust and multiple fold structures along the oft-debated Martic line, and then saw evidence bearing on Faill and Smith’s story of offshore, deep-water deposits of the infamous Peach Bottom slate belt and Peters Creek formation. The day ended at Chickies Rock where paleo-vertical Skolithos “worm” tubes show cleavage formation by Taconian layer-parallel shortening followed by Alleghanian cleavage-piracy that created plunging folds without sympathetic plunge rotation of the internal fabric. (Awwwkk !!). Dinner was at a great brew pub.

Early on Sunday we visited overturned Taconian (mostly) nappe structures of the carbonate platform in the famous Rheems Quarry. Two other quarries showed many types of boudinage formation, an Ordovician ash bed in an overturned nappe limb, plus carbonate mylonite development along a regional scale Allegnainian thrust fault. During the trip we discussed Taconian “foreplay in the foreland” with Ganis’s graptolite-based evidence for the offshore development of several types of allochthons, their subsequent climb onto the platform, and their progressive emplacement into the foreland basin as the Dauphin Formation, a precursor now separated from the traditional Martinsburg Formation. The evidence argues that the famous Hamburg “klippe” as generally known and defined is fiction. ended in the Great Valley a few miles east of Harrisburg on Route 39.

Don Wise, University of Massachusetts

Rev. 2008 May 15