5 Time Slices Across the Appalachians: Tectonic and Depositional Settings of Cambro-Ordovician Rocks in the Appalachian Orogen I

Monday, 20 March 2006
8:00 AM - 12:00 PM

NEW INSIGHTS ON THE ORIGIN OF THE MEGUMA GROUP IN SOUTHWESTERN NOVA SCOTIA, CANADA

Paper number: 5-5
Program: THEME SESSIONS
Time: 9:25 AM – 9:45 AM

Authors

Abstract

The easternmost tectonic element of the northern Appalachian orogen, the Meguma terrane of Nova Scotia, includes the Goldenville and Halifax formations (Meguma Group) and the younger White Rock and Torbrook formations, intruded by mainly Devonian plutonic units and overlain by Carboniferous and younger rocks. New mapping in the southern part of the terrane has resulted in subdivision of the Goldenville Formation into a lower metasandstone-dominated Green Harbour member, a middle metasandstone/slate Government Point member, and an upper metasiltstone Bloomfield/Moshers Island member. The Green Harbour Member contains a distinctive metasiltstone unit with abundant trace fossils including the early Cambrian deep-water ichnofossil Oldhamia, suggesting that the Goldenville Formation below the fossiliferous member likely extends into the Neoproterozoic. The upper part of the Government Point member has yielded a Middle Cambrian trilobite faunule of Acado-Baltic affinity. The overlying slate-rich Halifax Formation has been divided into the Cunard and Bear River members. The upper part of the Bear River member locally contains the graptolite Rhabdinopora flabelliformis and acritarch species that are Early Ordovician, suggesting that underlying Cunard, Bloomfield, and Moshers Island members are of Late Cambrian age, and that a significant unconformity exists between the Halifax Formation and the overlying late Ordovician - Early Silurian White Rock Formation. Both the Goldenville and Halifax formations are locally intruded by swarms of syn-depositional mafic sills of within-plate chemical character. A revised minimum thickness for the Meguma Group is 10 km.

Protoliths of the Goldenville Formation were predominantly immature feldspathic wackes, whereas protoliths of the Halifax Formation were mainly mudstone. Preliminary whole-rock geochemical data show that most of the clastic material in the Meguma Group was deposited on an active continental margin and (or) oceanic island arc, not on an Atlantic-style passive continental margin as previously assumed. These new chemical data will provide additional constraints on the position of Meguma terrane in lower Paleozoic continental reconstructions.