Northeastern Section - 56th Annual Meeting - 2021

Paper No. 17-6
Presentation Time: 3:10 PM


LAPORTA Jr., Philip, Department of Geochemistry, Lamont Doherty Earth Observatory of Columbia University, Gary C. Comer Geochemistry Building, Room 127, 61 Route 9W, Palisades, NY 10964; The Center for the Investigation of Native and Ancient Quarries, P.O. Box 2266, Middletown, NY 10940 and BREWER-LAPORTA, Margaret, The Center for the Investigation of Native and Ancient Quarries, P.O. Box 2266, Middletown, NY 10940; Department of Chemistry and Physical Sciences, Pace University, 861 Bedford Road, Pleasantville, NY 10570

Within the New York Recess of the Appalachian fold-thrust belt, there exists thousands of prehistoric bedrock quarries housed within the Cambrian and Ordovician carbonates of the Great Valley Physiographic Province. The prehistoric quarries, along with the bedrock stratigraphy and structural geology, have been mapped at scales of 1:24,000 over the course of decades of research. Analysis of the spatial distribution of the quarries has permitted classification based on stratigraphic and structural constraints. In the region, the fold-thrust belt ranges from a normal faulted section cropping out along the eastern part of the Province, to thrust-ramp and duplex sections exposed at the western boundary.

The normal faulted section hosts quarries within the Neoproterozoic through Lower Cambrian succession; namely the Hardyston and Leithsville formations. The rocks are down faulted and preserved in graben-like depressions, with dip orientations ranging from 10⁰ to 30⁰ west. Such field relationships are ideally suited to the development of prehistoric quarries.

The thrust-ramp section supports quarries cropping out within the Limeport and Allentown formations of Middle Cambrian age. Attitude measurements are steeper in the thrust-ramp section, ranging from 30⁰ west to almost vertical at some locations. The differences in dip orientation between the normal faulted and thrust-ramp sections impacts the style of quarrying, the instruments required to extract chert, and the resulting chain of operation of production events.

Rocks are folded into tight, plunging antiforms and synforms in the duplex section. Additionally, the section is repeated by synthetic and antithetic faults. Dip orientations are steep and change from west to east through the vertical. Quarries developed within the highly deformed Upper Cambrian through Lower Ordovician Stonehenge, Rickenbach and Epler formations are smaller, and more complex, than those in the less strained Hardyston through Allentown formations. Exceptionally large quarries are found in the Lower to Middle Ordovician Ontelaunee Formation due to the unit’s greater stratigraphic thickness; thus providing more extensive exposures of chert.