Abstract

In central Alberta, Upper Devonian Leduc Formation carbonates comprise a preferred NNE arrangement of linear reef chains, isolated reefs or reef complexes and an extensive shelf complex. Using seismic, potential-field and drillhole data, this study focuses on unanswered questions regarding the influence of the Precambrian crystalline basement on the spatial distribution of these units. In modern carbonate systems, antecedent topographic highs commonly provide favourable sites for reef nucleation. The importance of paleobathymetry on reef initiation and development during Upper Devonian times is examined using compressed seismic displays flattened on datums above and below the level of interest. Cambrian and Devonian sedimentation patterns have been locally influenced by subtle irregularities on the basement surface. Beneath the southern Alberta Leduc shelf margin, for instance, on-structure thinning of Middle Cambrian sediments is associated with low-relief basement arching. In turn, Upper Cambrian paleosurfaces are affected by drape or differential compaction over these pre-existing highs. This process of inheriting underlying topography is transferred upsection into the Devonian system where on-structure thickening of carbonates is discerned. To the west, topographic inheritance in the form of drape of Cambrian strata over a basement hinge line appears to have influenced the western edge of the Bashaw reef complex. On a larger scale of investigation, both examples display only a partial correspondence with magnetic and Bouguer gravity anomaly patterns that mostly originate from the basement. The Homeglen-Rimbey reef, part of the enigmatic Rimbey-Leduc-Meadowbrook reef chain, does not correlate with any significant basement structure and overlies a relatively featureless basement surface, as interpreted from seismic data. However, the possibility of seismically unresolvable undulations existing on pre-Leduc reflectors cannot be ruled out. On a larger scale, a long wavelength, and thus deeply sourced, Bouguer gravity anomaly, interpreted as an extension of the Snowbird Tectonic Zone, coincides with a significant part of this linear chain. Whether this crustal discontinuity played a role in the development of carbonate buildups along this trend is still open to question.

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