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Taconic events in Pennsylvania: Datable phases of a 20 m.y. orogeny

Consulting Geologist, P. O. Box 6128, Harrisburg, Pennsylvania, 17112
Department of Geosciences, University of Massachusetts at Amherst, Amherst, Massachusetts 01003

^* bobganis{at}aol.com

^** dwise{at}geo.umass.edu

A suite of well-dated graptolite and conodont locations allows division of about 54 m.y. of Late Cambrian through Late Ordovician events into three time segments in Pennsylvania: 1) Pre-Taconic, offshore deepwater sediments including olistostromes were deposited and deformed in several offshore domains in Iapetus (sensu lato) from 497 to 459 Ma; 2) Taconic Phase I began as these rocks were translated onto the foreland as allochthons, some with piggyback basins from 459 to 450 Ma. This was accomplished first by thrusting onto the shelf edge, creating a foreland peripheral bulge and local depositional basins, then by depressing the larger foreland region to allow emplacement of allochthons of the Dauphin Formation (the erroneous "Hamburg klippe" of past literature), and finally with deposition of the thick, widespread fill of the traditional Martinsburg Formation; 3) Taconic Phase II advanced diachronously across the foreland with regional overturning and nappe formation (the classic "Taconic" orogeny in Pennsylvania), driven as the ongoing collision overrode and incorporated the platform and its foreland basin into alpine-scale nappes. This phase started at 459 Ma at the shelf edge and lasted from 450 to 443 Ma in the present location of the Great Valley foreland and eastward. This series of well-dated events may be a model for timing of a "simple" orogenic cycle in a convergent margin with application to other parts of the Appalachians and beyond.