Peter Adams

M. Sc. Thesis

A Depositional and Diagenetic Model for a Carbonate Ramp: Iroquois Formation (Early Jurassic), Scotian Shelf, Canada

(PDF -  Mb)

The Early Jurassic Iroquois Formation is a thick, areally extensive carbonate wedge that underlies most of the Scotian Shelf. The late-rift Iroquois sediments were deposited prior to complete separation of the North American and African continental plates, on a transgressive, shallow marine ramp. Deposits of semi-restricted tidal flats of sabkha grade offshore to successively lagoonal, inner shelf, and finally open marine, oolitic environments. On the basis of downhole logs and lithology the Iroquois is vertically divisible into three lithosomes that reflect a transition from a semi-arid, alluvial plain or salt pan, to an open marine environment, and then back to a marginal marine environment. The Iroquois can be correlated with similar, coeval, transgressive carbonate sequences on both margins of the present North Atlantic Ocean, confirming, in part, the sea level fluctuation models of Vail et al. (1977) and Hallam (1978) for Early Jurassic time.

The Iroquois Formation carbonates have undergone intense diagenesis involving states of cementation, replacement, and porosity modification. Dolomite and early calcite cement combined to obliterate porosity. Only the advent of fracturing and associated vuggy porosity allowed migration of hydrocarbons through the formation. Fractures and vugs were plugged by macrocrystalline dolomite and anhydrite. Late diagenetic anhydrite filled remaining porosity and replaced earlier mineral phases. The Iroquois Formation was pierced by salt domes resulting in caprock reactions that further altered the nature of the rocks.

Sucrosic and microcrystalline dolomite destroyed much of the primary texture. Dolomitization is strongly fabric selective, being controlled by composition and permeability of the original sediment, and by previous diagenetic events. Dolomitization occurred by a combination or succession of mechanisms similar to the models of seepage reflux and mixing.

Keywords:
Pages:
Supervisors: L. Jansa / Paul E. Schenk