Abstract
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, T.J., 2002. Field relations, structural geology, and
geochemistry of the Jonestown Volcanic Field, Lebanon County,
southeastern Pennsylvania.
Unpublished MSc. thesis, State University of New York at
Albany. 111 pp., +ix; 1 folded plate (map)
University at Albany Science Library call number: SCIENCE
Oversize (*) QE 40 Z899 2002 A84
MS thesis digital
text pdf (14.2 MB)
Plate 1 - Geologic
Map of the Jonestown Volcanics, Lebanon County, Southeastern
Pennsylvania
(coloured
geological outcrop map and cross-section, scale 1:12,000) - 0.8MB
pdf file
Return to MS Theses completed in the
Geological Sciences Program, University at Albany