Introduction to local geology
Detailed references and an overview of geologic studies for the area can be found in Dunn (1995). Only a brief description and selected excerpts from source documents are provided here.
Franklin and Sterling Hill are located in the Reading Prong of the New England physiographic province.
The oldest rocks of the region are Precambrian gneisses, of mixed metasedimentary and metavolcanic origin.
The Franklin Marble, the host rock for both Zn-Mn-Fe orebodies, was deposited within this Precambrian sequence, along with metaliferous sediment that would later become the Zn-Mn-Fe deposits we know today.
All these early metasediments and metavolcanics were deeply buried and strongly metamorphosed to the sillimanite grade later in the Precambrian. Ages for this metamorphism range from 800 to 1150 Ma.
The rocks, now folded and recrystallized, were then uplifted. From the late Precambrian through early Cambrian, their uplifted surface was extensively eroded.
On this erosion surface the Hardyston Quartzite was unconformably deposited. The Hardyston Quartzite was in turn overlain by the Kittatinny Limestone in Cambrian-Ordovician time.
The rocks were subject to uplift and subsequent erosion from the late Mesozoic through to the present. The current topography reflects this stage of uplift and erosion. The region was heavily glaciated during the Pleistocene.
Again, for a good survey of the geological literature on Franklin-Sterling, consult Dunn (1995).
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