© Copyright, 1995 by R.A.Kanen, All Rights Reserved
The Devonian Reefs of the Canning Basin grew in the Middle to Late Devonian around Pre-Cambrian islands of basement rock and along the Pre-Cambrian shore of the Halls Creek Mobile Zone. They occur in a belt 300km long by 60km wide. A period of block faulting is thought to have occurred before reef growth, uplifting basement platforms, upon which the reefs could grow. This gave rise to topography with a relief of several hundred metres onto which the reef grew. The reef belt may have extended several hundred kilometers around the Kimberley Basin to link similar reefs in the Bonaparte Gulf.
Playford (1980) estimates the thickness of the Devonian reefs to exceed 2000m in places but variable due to the relief of the basement surface. Druce and Radke (1979) have determined the reef complexes to be overlain conformably by the Fairfield Group. Throughout most of the outcrop area the Devonian rocks are overlain with angular unconformity by the Upper Carboniferous to Lower Permian Grant Formation.
The reef complexes developed as limestone platforms dipping, with marginal slopes at the edges, to the basin floor. A platform facies, marginal slope facies and basin facies are recognized. Playford (1980) also recognized 5 sub-facies: (1) reef margin, (2) reef flat, (3) back reef, (4) bank, and (6) pinnacle reef sub-facies.
The platform facies was laid down nearly horizontally at sea level, in an interior shelf lagoon, seldom greater than 5-10m thick. A feature of these deposits is that they were lithified at the time of deposition by submarine cementation to form wave resistant platforms. They typically have stromatoporoid, algal, coral, or clastic limestone fabrics.
The marginal slope facies formed on the flanking platform slopes. They consist of platform derived debris bound by algae, and cemented.
The basin floor facies were deposited nearly horizontally on the basin floor between reefs. They consist of shales, siltstones, sandstones and conglomerates with thin beds of limestone and platform derive material.
Pinnacle reefs formed during periods of rapid subsidence during which platforms were drowned and the reefs quickly regressed.
Widespread submarine cementation took place in the reef margin and reef flat deposits, variable cementation in the marginal slope deposits, and virtually none in the back reef and bank deposits. Primary porosity was destroyed in all but the latter two deposits. After burial, stylotization and compaction destroyed most of the primary porosity in the back reef and bank deposits. At present, the best type of porosity is secondary moldic porosity.
