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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. 

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Relief map of northern New Jersey. Note the prominent northeast-southwest trending features.

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.

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Surface geology sketch map of the Franklin-Sterling area. After Frondel and Baum (1974).

A technical discussion of the early, Precambrian rocks of the region, taken from Drake (1990), as cited in Dunn (1995), follows:

"Rocks in the Franklin-Sterling district have a basement of quartz keratophyre, trondhemite, spilite, tonalite, and enderbite interpreted to be an oceanic suite.

"This basement is overlain by a sequence of biotite-quartz-feldspar gneiss (graywacke), marble, amphibolite (pillow basalt in part), and potassic feldspar gneiss that is interpreted to be largely meta-arkose, in part with an exhalative component.

"This association of rocks is only compatible with a rift environment. I suggest that these rocks were deposited in a rift trough on oceanic crust along a continental margin. The ocean lay to the east (present direction) of the New Jersey Highlands as the rocks in the external basement massifs and Adirondacks to the north and south are similar to those here, and metavolcanic rocks core the Baltimore "gneiss domes" to the southeast (Drake et al., 1991).

"Later closing of the ocean resulted in Grenvillian orogenesis and the emplacement of synkinematic granitoids, which were generated in a continental arc at about 1090 Ma.

"Grenvillian orogenesis in this area apparently occurred over a long time-span, as synkinematic granite in the Hudson Highlands of New York has been dated at 1134 Ma (J. N. Aleinkoff, written communication, 1989). Orogenesis had ceased by 1020 Ma when post-kinematic granite related to post-collision uplift was emplaced."

Again, for a good survey of the geological literature on Franklin-Sterling, consult Dunn (1995).

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