ORDOVICIAN SEDIMENTARY BRECCIA AND MAGNETITE-COTICULE METASILTSTONE, NORTHEAST KINGDOM, VERMONT

Metasiltstone and metasandstone of the Arenig Albee Formation underlie most of the Stone Mountain quadrangle. Near Burnside Mountain, sedimentary breccia, as thick as 200 m, forms the limbs of a NNE-trending syncline in the Albee. Basal breccia consists of Albee clasts, up to a meter long, in a matrix of Albee-like siltstone and sandstone. Higher in the breccia the matrix is black pelite or micaceous siltstone. Although clasts of Albee dominate, the suite includes black slate, coticule-bearing siltstone, and sparse quartz pebbles; no volcanic clasts were observed. An Fe-rich unit of siltstone, chert, and ironstone forms the trough of the 1 x 8 km syncline. All lithologies may be magnetite-rich and coticule-bearing. The unit produces a positive magnetic anomaly of about 1000 nT (USGS Map I-1898-E). The breccia/Fe-rich unit contact may be gradational through interlayering over a few meters. The forearc basin, in which the Albee was deposited, deepened and marginal slopes became unstable producing debris flows. As water depth increased or the source became more distal, breccia matrix changed from sand to mud. The Fe-rich coticule unit, in an outlier to the east, is overlain by the Ordovician Partridge Formation and may be coeval with volcanism farther east in the Ammonoosuc arc.

The breccia and magnetite-coticule unit were mapped as Clough and Littleton Formations, respectively, by Johansson in 1963. The western breccia was shown by Moench and others in 1995 as Silurian Smalls Falls Formation with admixed volcanic rocks; the magnetite-coticule unit was shown as Devonian Ironbound Mountain Formation. The eastern breccia was interpreted by Moench in 1999 as marking the base of the Piermont allochthon. An Ordovician age for all rocks of the syncline is supported by these observations in the Upper Connecticut Valley: 1) coticules characterize some sedimentary rocks of the Ammonoosuc Volcanics; coticules are absent in Siluro-Devonian rocks, 2) magnetite is locally present in the Albee and some sedimentary rocks of the Ammonoosuc, but not Siluro-Devonian rocks.