Abstract

A joint surface-subsurface study of the Devonian Hare Indian-Ramparts (Kee Scarp) Formations examined the nature of basin-fill and platform-reef development in the Mackenzie Mountains and Norman Wells area, Northwest Territories.

The Givetian-Frasnian (?) Hare Indian and Ramparts (Kee Scarp) strata consist of repeated shoaling-upward sequences. These sequences or cycles of sedimentation were initiated in response to accelerated rates of relative sea-level rise. Two major first-order cycles (each greater than two hundred metres thick) were discerned. The lower cycle consists of prograding shale banks of the Hare Indian Formation and the immediately overlying “shale ramp” sequence of the Ramparts Formation. The upper cycle commenced with deposition of the widely correlatable, dark argillaceous, carbonaceous limestones of the “Carcajou Marker”. Overlying platform and reefal limestones of the Ramparts (Kee Scarp) Formation, off-reef and fondothem shales of the Canol Formation and interbedded clinothem shales and sandstones of the Imperial Formation make up the remainder of the upper cycle.

The first-order cycles consist of a number of smaller second-order cycles. These cycles are best defined in shallow-water platform and reef complexes where they can be traced across the entire complex, but also are recognized in the Hare Indian shale clinothem. In platforms and reefs, these second-order cycles contain a wide range of facies, varying from shallow reef and platform to deeper foreslope limestones that can be traced, in certain locations, even further out into adjacent basinal limestones. In reef interiors, these second-order cycles are made up of even smaller third-order cycles consisting of subtidal and tidal-flat limestones.

This cyclic arrangement of strata is well-defined in both the Mackenzie Mountains and the subsurface Norman Wells field, enabling correlation of time-equivalent growth stages between both complexes over a distance in excess of one hundred kilometres. This and the small-scale periodicity of the second and third-order cycles argue for eustatic sea-level control. The following eight major growth stages are recognized in both complexes: (1) drowning of the Hume limestone, (2) progradation of Hare Indian shale banks, (3) drowning of Hare Indian shale banks, and deposition of the “Carcajou Marker”, (4) carbonate platform inception, initial growth and subsequent localized upbuilding, (5) reef inception, progradation and subsequent aggradation, (6) reef backstepping and subsequent aggradation, (7) drowning of the Norman Wells reef complex and backstepping of the Mackenzie Mountain reef complex and (8) drowning of the Mackenzie Mountain reef complex.

Interpretations presented in this paper are significantly different than those in other papers, especially in regard to the stratal and temporal relationships of the Ramparts (Kee Scarp) and Canol Formations. Authors of these other papers have interpreted a pre-Canol unconformity, making the Canol shale entirely post-Ramparts limestone. We have found no evidence to support this interpretation, and instead observe the Canol shale to intertongue with, onlap onto and drape over (at Norman Wells) the Ramparts and Kee Scarp reef complexes. These stratal relationships indicate that the Canol is both time-equivalent to and postdates the Ramparts (Kee Scarp) Formation.