Date of Award
Master of Science (MS)
The Jonestown Volcanic Field is a five kilometer by fifteen kilometer area of volcanic and hypabyssal rocks, basalt and diabase, located in southeastern Pennsylvania. These igneous rocks presently occur within an allochthonous belt of Ordovician deep water sedimentary rocks and Taconic flysch rocks known collectively as the Hamburg Klippe. Detailed field mapping during this project has revealed that the contacts between the igneous rocks and the flysch are not conformable. The volcanic rocks are associated with the Ordovician limestone that is adjacent to these volcanics. This limestone also is not conformable to the Hamburg Klippe sediments, rather it shares outcrop characteristics of some Laurentian platform carbonates in the region. The association between the igneous rocks and the limestone suggests that the igneous rocks were emplaced on a carbonate platform. Trace element data gathered from whole rock geochemical analysis suggests that the volcanic and hypabyssal rocks formed from different melts. They could, however, have formed in the same magmatic province. The volcanic rocks share geochemical characteristics with rocks emplaced on continental forelands, while the hypabyssal rocks show evidence of continental lithospheric influence on the magma, suggesting the Jonestown igneous rocks were emplaced on a carbonate platform on the Laurentian foreland, not a seamount. Their origin may be related to the approach of the Taconic arc. The structural geology indicates the flysch rocks and the igneous rocks were originally deformed and juxtaposed during the Taconic orogeny. Detailed mapping has also shown that a sandstone unit in the Bunker Hills region previously mapped as part of the Hamburg Klippe sequence is more likely an outlier of the Silurian Tuscarora Formation, and it is probably not conformable to any of the other rocks in the field area. The geometry of the deformation of the sandstone in the Bunker Hills region suggests there was again thrusting in the region during the Alleghanian orogeny.
Ashcroft, Tristan J., "Field relations, structural geology, and geochemistry of the Jonestown Volcanic Field, Lebanon County, southeastern Pennsylvania" (2002). Geology Theses and Dissertations. 6.