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Geosciences Students lead NIGEC Field Trips

 

Geosci grad student Ryan Deasy led a field trip with 70 participants from 8 states at the 2015 New England Intercollegiate Geological Conference (NIGEC) October 9-11 at Wesleyan University in Middletown, CT.

Grad Bryan Wathen and undergrad Chris Helou led a field trip of 26 people for the same NEIGC conference. Brian and Chris reported on their senior and junior research projects. They got serious cheers of applause from the participants at the end of their trip.

Trip A1. Tectonic Slivers of Oceanic (MORB) Terrane(s): Sheeted Dikes and Sheared Gabbros Hiding in the Hills just West of New Haven, Connecticut
Leaders: Ryan T. Deasy, Robert P. Wintsch, Bryan Wathen, Ryan McAleer

Description: This trip reexamines the eastern part of the Orange Milford Belt of "low grade"rocks. Existing interpretations hold that these rocks are low-grade, Ordovician to Devonian metasediments and metavolcanics. But hidden among the suburban lawns and golf courses of New Haven, West Haven and Woodbridge are fault slivers of oceanic amphibolites with complicated high- and low-grade metamorphic fabrics, cut by a swarm of sheeted porphyritic basalt dikes, all weakly re-metamorphosed. The excursion will feature a long walk around the Maltby Lakes Recreation Area, where outcrops include high-grade mylonites and low-grade foliated breccias. Other stops include a chlorite mylonite that marks the southernmost exposure of the Monroe thrust, a swarm of sheeted dikes believed to correlate with the Silurian Comerford dikes in Vermont, and the nonconformity where the basal conglomerate of the Triassic New Haven arkose overlies Ordovician greenstones. For the history buffs we will include a visit to B. Silliman’s 1810’s verd antique quarry in a lens of obducted Paleozoic oceanic crust. Discussions at each stop will be supported by major and trace element geochemistry, photomicrographs, Xray diffraction patterns, microprobe data on mineral compositions, 40Ar/39Ar age spectra, and structural analysis based on detailed mapping at the scale of 1:12,000.

Deasy, R.T., Wintsch, R.P., Wathen, B., McAleer, R., Bish, D., Meyer, R. 2015. Tectonic Slivers of oceanic terrane(s?): Sheeted dikes and sheared gabbros in south-central Connecticut, in, Gilmore, M.S., and Resor, P.G., Guidebook to field trips in Connecticut and Massachusetts, Wesleyan University, p. 1-24

Trip B3. 200 Million Years of Deformation along the Peri-Laurentian (Pumpkin Ground Orthogneiss) Buttress and the East Derby Shear Zone, South-Central Connecticut
Leaders: Bryan Wathen, Chris Helou, Bob Wintsch, Ryan Deasy, Keewook Yi, Ryan McAleer, and Jessica Matthews

Description: New field and analytical evidence show that the Ordovician Beardsley and Pumpkin Ground Orthogneisses acted as a backstop for Paleozoic deformation. A sequence of outcrops along a west-to-east transect will show increasingly higher strain and progressively lower grade conditions overprinting Acadian anatectic metamorphic fabrics in the west, and kyanite grade rocks in the east. Stops in the Beardsley and Pumpkin Ground Orthogneisses, with new U-Pb zircon ages, will demonstrate their relative and absolute age relationships in the Late Ordovician, and major and trace element analyses will demonstrate the much wider distribution of these rocks (in the Collinsville Fm) than indicated on Rodgers (1985) bedrock map of Connecticut. The eastern limit of these buttressing orthogneisses is marked by the ductile East Derby Shear Zone (EDSZ), derived primarily from the argillaceous Wepawaug schist on the east. Transects through the EDSZ will show the localization of deformation along this boundary, producing both mylonites and phyllonites. The late Paleozoic evolution of the EDSZ will be discussed in the context of new 40Ar/39Ar data on syntectonic muscovites. Photomicrographs of textures and fabrics from each stop will supplement the discussion of the roles of geochronology, as well as reaction and textural softening throughout the trip. Over the trip as a whole, we will document a long, tortuous history along the Laurentian margin as it acted as a buttress over 200 million years of deformation.

Wathen, B., Helou, C., Wintsch, R.P., Deasy, R.T., Yi, K., McAleer, R., Matthews, J. 2015. Deformation spanning 200 million years along the peri-Laurentian (Pumpkin Ground Orthogneiss) Buttress and the East Derby Shear Zone, South-central Connecticut, in, Gilmore, M.S., and Resor, P.G., Guidebook to field trips in Connecticut and Massachusetts, Wesleyan University, p. 153-172