EVOLUTION AND EARLY FILL OF THE ORPHEUS GRABEN, 
SCOTIAN BASIN, OFFSHORE EASTERN CANADA

	TANNER, Lawrence H., Dept. of Geography and Earth 	
		Science, Bloomsburg University, Bloomsburg, PA, 
		17815 USA
	BROWN, David E., Canada - Nova Scotia Offshore Petroleum 
		Board, 1791 Barrington Street, Halifax, N.S., B3J 3K9 
		Canada

The Orpheus Graben is an eastward-plunging and widening, fault 
bounded subbasin of the Scotian Basin, located between the Scotarie 
Ridge to the north and the Canso Ridge to the south.  The northern 
boundary is continuous with the Cobequid-Chedabucto Fault Zone, 
which may continue eastward into the Newfoundland-Gibraltar 
Transform.  Formation and evolution of the basin appears similar to 
that of the Fundy Basin to the west in that subsidence was controlled 
by reactivation of the Cobequid-Chedabucto Fault Zone with oblique-
slip movement with a sinistral sense coincident with the initial rifting 
that formed the Scotian Basin.  Up to 10 km of sediment accumulated 
within the graben in two depocenters separated by a structural saddle.
	Initial basin fill consisted of continental red beds comprising 
fluvial-lacustrine conglomerates, sandstones and shales of the Norian 
(possibly Carnian) to Hettangian Eurydice Formation, deposited on an 
irregular basement surface.  Laterally equivalent outcrops along the 
shores of Chedabucto Bay, comprising coarse red beds of Late 
Triassic age informally termed the Chedabucto Formation, represent 
deposition at the western end of the graben.  East-trending 
paleocurrents in the Wolfville Formation of the Fundy Basin indicate 
possible continuity between the Fundy basin and Orpheus Graben 
during the Late Triassic.  Hettangian-age basalt volcanism, 
characteristic of the Newark basins, is absent in the Orpheus Graben.  
The overlying evaporites of the Argo Formation, consisting 
predominately of massive (at the base) to bedded (at the top) halite, 
were deposited during Hettangian to Sinemurian time, as thermal 
subsidence caused a widespread restricted marine incursion.  
Continuing movement on the the Cobequid-Chedabucto Fault, 
responsible for synsedimentary extensional faulting in the Fundy 
Basin, may have caused faulting and salt deformation in the Orpheus 
Graben.  This deformation may be related to a regional Early Jurassic 
unconformity associated with continental breakup.
	Widening of the central rift during Sinemurian to Toarcian 
time caused more widespread transgression, resulting in deposition of 
the overlying Iroquois Formation, comprising dolomite and shallow 
marine clastics and evaporites, shallowing to the west.  This 
deposition was coincident with the formation of salt-withdrawal 
synclines.  Sedimentation continued in the Middle Jurassic with 
deposition of continental to shallow marine clastics of the Mohican 
Formation, in part laterally equivalent to the Iroquois Formation.  The 
onset of active seafloor spreading in the Middle Jurassic, possibly the 
cause of compressive folding and faulting in the Fundy Basin, may be 
responsible for a second regional unconformity.  Subsequent marine 
transgression resulted in deposition of the overlying Western Bank 
Group carbonates and clastics, overlapping the margins of the graben.