16-40 Petrographic Analysis of the Octoraro and Peter’s Creek Schists, Lancaster County Pennsylvania

Session: From Thin Section to Outcrop: Exploration of Undergraduate Research (Posters)

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Abstract:

Lancaster County, Pennsylvania contains several schist units that narrate the region’s history. This study examined two units, the Octoraro and Peter’s Creek Formations on a petrographic level and compared them based on compositional and textural differences. These units were described by Knopf and Jonas (1929) followed by Valentino et al. (1994) and Blackmer et al. (2010). The Glenarm Series contains the Peters Creek Formation, which is described by Knopf and Jonas (1929) as a series of chlorite and sericitic quartzites interbedded with chlorite-muscovite schist and grades upward into quartzose and conglomeratic sediments. Blackmer et al. (2010) describes the Peter’s Creek as a light-gray metasandstone and minor conglomerate with sand- to pebble-sized detrital quartz and feldspar in a fine-grained quartz matrix with foliation defined by aligned muscovite laminae. The Octoraro Schist, as described by Blackmer et al. (2010), is a gray to green-gray schist that ranges from psammitic schist to muscovite-chlorite schist.  Foliation is defined by muscovite and chlorite compositional layers.

This study consisted of a North-South transect along the Susquehanna River with sites located at Wissler’s Run Park, Safe Harbor Dam, and multiple points along railroad cuts in Drumore, PA. Samples were collected at each location and thin sections were made to study compositional variations along the transect. Both the Octoraro and Peter’s Creek are of similar metamorphic grade with abundant chlorite and muscovite defining foliation.  The Octoraro unit (Wissler’s Run and Safe Harbor Dam) consists of quartz, muscovite, and chlorite as well as plagioclase porphyroblasts.  Accessory minerals include pyrite and tourmaline. The Peter’s Creek member (Drumore Railroad) samples are greater than 85% recrystallized quartz, with chlorite and muscovite laminae defining the foliation.  Both idioblastic pyrite and magnetite occur as accessories in Peter’s Creek.  These compositional differences reflect the protolith composition. 

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