Pre-Devonian Basement

Structure and Stratigraphy of Pre-Devonian Basement in Northeastern British Columbia : Controls on Basement Stratigraphy and Hydrocarbon Entrapment

---

Summary:
Location: Blocks SE: 94-A, NW: 94-N
Strata: Proterozoic to Cretaceous
Year of Study: 1993

---

Introduction

The objective of this study is to determine the nature, origin and distribution of sub-basin crustal structures and their relationship to depositional and structural trends. The analysis incorporates: over 2300 kilometres of seismic reflection profile data (of which 500 kilometres were recorded to 15 seconds); public domain magnetic and gravity data; regional subsurface Paleozoic stratigraphic data; outcrop geological data from the Cordillera; and a comprehensive examination of published research on the composition, structure and age of the Proterozoic crust within and on strike with the study area.

In northeastern British Columbia, the Western Canada Sedimentary Basin is underlain by a “basement” that includes the western margin of the Early Proterozoic North American craton. It is bounded on the west by a crustal-scale ramp system that incorporates Middle Proterozoic metasediments of the Belt-Purcell-Wernecke supergroups. This ancient mountain belt is truncated by a regional Middle Proterozoic unconformity that is overlain by the Middle Proterozoic Muskwa Assemblage of platformal carbonate and clastic sediments more than 6 kilometres thick. Beneath the Foothills belt, the Muskwa Basin is overlain by a westward-thickening wedge of “lower” Paleozoic carbonates.

Profound subsidence occurred during the deposition of Middle Proterozoic Muskwa Assemblage in the southwest along the northeast-trending Great Slave Lake-Hay River Fault System, and northwest of the Clarke Lake area between the Fort Simpson and Nahanni magnetic domains. In the southwest, the embayment is at a high angle to the Cordillera, and is on trend with the Laurier Embayment of the Inner Foothills and Rocky Mountains. Here, Middle Paleozoic carbonate shelf and Triassic shelf abruptly terminate. This continental recess also accommodated Devonian and late Mesozoic strike-slip displacement. The northwestern embayment may be part of the southern margin of the Proterozoic Mackenzie Mountains basin. It is parallel to Cordilleran strike and is associated with collapse of the Middle Devonian shelf.

Contractional and extensional deformation were accommodated along the generally low angle, west-dipping thrust ramp system and overlying pre-Middle Devonian sediments. Periodic uplifts along these features were responsible for localizing Devonian and Cordilleran structural trends, facies transitions, depositional/erosional edges, and possibly diagenetic trends. Structural decoupling between the Antler and Cordilleran folds indicates that prospective Devonian (and possibly Mississippian) facies may not have been tested by wells located on Cordilleran structures, and that regional aquifers, such as the Sulphur Point Formation, may be attractive traps.

Regionally significant “basement”-controlled structural trends that are delineated on the seismic reflection and magnetic data include:

1. Fort Simpson – Hottah boundary
2. Eastern boundary of the Nahanni Domain along the Bovie-Kledo Fault
3. Hay River Fault System
4. Thrust ramps within crystalline basement comprising imbricated Fort Simpson plutonic rocks and ?Wernecke equivalent metasediments
5. The Middle Proterozoic unconformity separating the crustal-scale thrust ramp and the overlying Muskwa Basin
6. Intersections of regional stratigraphic markers within the Muskwa Basin and Middle Devonian strata
7. Structures delineated by regional stratigraphic markers within the Muskwa Basin
8. Escarpments within the Lower Paleozoic subcrop

Exploration models and concepts documented in this study include:

1. Middle Proterozoic carbonate subcrop and structural traps
2. Lower Paleozoic carbonate subcrop and structural traps
3. Elk Point sandstones forming stratigraphic and structural traps
4. controls on Keg River reefs and Slave Point bank margins
5. Mississippian carbonate shoals on Antler-age structures
6. Upper Triassic coquina submarine channel complexes
7. Late metasomatic thermal dolomitization
8. basement-controlled structural trends