Abstract

The Nisku Formation in the Alberta subsurface displays three distinct coeval facies along the Outer Shelf. These are: basinal/slope facies, reefal facies, and bank facies. The reefal Zeta Lake Member occurs as isolated, roughly circular buildups on the slope, and as a more elongated body fringing the bank-edge. Most Zeta Lake buildups are coral-bearing mudmounds. They were initiated in moderate water depth, probably below wave base on a carbonate platform. It is not clear what determined the loci of the reefal deposition.

The off-reef Lobstick, Bigoray, and Cynthia/Wolf Lake couplet represent three shallowing-upward cycles in the Nisku. In some buildups the Zeta Lake facies displays upward shallowing in the uppermost part, but exposure can rarely be demonstrated. The overlying Calmar and Blue Ridge represent an initial transgression with a subsequent shallowing-upward sequence. The Blue Ridge sediments in the study area are marine, in parts even reefal, and do not represent repeated sabkha cycles as previously thought.

The green clays (shales) which occur in the Nisku and in many other Devonian strata of Western Canada are not residual soils but marine sediments. They represent non-carbonate background sedimentation and/or are storm deposits.

The Nisku contains at least four types of dolomite. The most widespread and abundant type is the (grey) matrix dolomite. Matrix dolomization selectively affected the lime mud matrix, but various calcite fossils and cements survived this phase of replacement. Where dolomization went to completion, the dolomite forms tight crystal mosaics with almost no intercrystalline porosity. Significant porosity in matrix dolomites did not result from mole-per-mole replacement of calcite, but from later dissolution of calcites that had survived dolomization.

Matrix dolomization is most pronounced in the structurally down-dip part of the study area. Even here, notable amounts of limestone occur in a number of off-reef wells, and three pools are preferentially dolomitized on their down-dip side. Petrographic and geochemical evidence suggest that the matrix dolomitization of the Nisku took place at intermediate burial depths (perhaps between 300 to 1500 m), possibly influenced by the regional structural tilting which began in Pennsylvanian time. Trace element trends indicate that the dolomitizing fluid flow was mainly upwards. The exact nature of these fluids is not known.