Exercise 15 Paleozoic Orogenies of Ancestral North America4231 PART C A Portion

Question

Exercise 15 Paleozoic Orogenies of Ancestral North America4231 PART C A Portion of the Geologic Map of Pennsylvania Scale: 1 inch-4 miles 1. What is the likely source area for the Ordovi- cian Martinsburg Formation and the Devonian Catskill Formation (figure 15.1)? What moun- tain-building events are associated with these formations? Cite your evidence. 2. Using the geologic map found on page 234 (figure 15.4), account for the difference in the structural patterns north and south of the Penn- sylvania Turnpike, which extends east-west through Carlisle, Pennsylvania. 5. What is the reason for the present course of the Susquehanna River cutting across the folds indis- criminately and across all different rock types? marize the evidence. 6. In terms of plate tectonics, what type of plate interaction (extensional, strike-slip, compres- sional) best explains the geology of the map area? Cite evidence. 4. Explain the origin and distribution of the igne- ous rocks in this area. With what episode of mountain building are they associated?

Explanation / Answer

1. The likely source of Martinsburg Formation is anoxic ocean bottom which produces rock formations such as black shales. The likely source of Catskill Formation is river deltas where sandstones formed.

Episode of mountain building associated with the Martinsburg Formation corresponds to the first tectonic cycle when North American Plate subducted in Iapetus Ocean. Catskill Formation corresponds to the second tectonic cycle when Acadian Orogeny occurred resulting in the development of deltas.

2. Differences in structural patterns north and south of Pennsylvania Turnpike is attributed to the transition between two different evolutionary stages of the Appalachian mountain building process. The southern part comprises of rocks from Cambrian to Ordovician signifying the subduction of a continental in an ocean whereas the northern part comprises of rocks formed during and after Silurian characterized by foreland basins and deltas.

3. Appalachian folding took place during Devonian - Mississipian as evident by the large regional scale folds comprising of Dm, Dck, Mp, Mmc formations.

4. Igneous rocks in the area are mainly seen in the form of relatively much younger sills and dikes cross-cutting all the other formations as depicted by the red lines on the map. Such features formed during the late stages of the mountain building process due to the pre-existing fractured conduits connected to a magma chamber.

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