CRINOIDS AS KARST AND DIAGENETIC INDICATORS IN DEVONIAN LIMESTONE BENEATH THE ALBERTA OIL SANDS

The Cretaceous oil sands of northeast Alberta, Canada disconformably hides the complex diagenetic and karst history of underlying Devonian limestone, shale, and evaporites. In the past decade, the effects of karst and diagenesis in Devonian strata have become a critical focus for identifying potential environmental and safety concerns during oil sands mining. Some karst and diagenetic processes are evident in core and outcrop through conspicuous preservation and alteration of crinoid columnals.

Throughout most of the oil sands surface mining region, the Givetian Keg River Formation is fully dolomitized, but locally, dolomitization of this limestone was not complete. Thin sections of partly dolomitized rock indicate that (a) dolomitization followed the fluid flow pathways of Thalassinoides burrow networks and (b) crinoids were usually the first allochems to dolomitize in the unaltered areas around dolomitization fronts.

In some places beneath the oil sands, the Frasnian Waterways Formation limestone was altered to iron-rich rock, with burrows or entire beds often heavily infiltrated by hematite and occasionally replaced by siderite. In these rocks, crinoids alone often remain unaltered as bright white allochems in a rust- to black-colored rock. Fully altered limestones at the Devonian-Cretaceous contact, sometimes the product of basement-originating hydrothermal fluids, are often discerned from iron-rich concretions in the basal oil sand only by the presence of crinoid columnals.

Epigenic karst beneath the oil sands is a mix of collapse from evaporite dissolution at depth and dissolution of near-surface Waterways Formation limestone, all of which resulted from Cretaceous and Pleistocene episodes of surface recharge. Crinoids are not indicators for collapse-related karst features, but columnals were highly resistant to decalcification in near-surface limestone. The mud and silt of Cretaceous paleosols and lacustrine deposits lack crinoids, whereas the shaly residue of fully dissolved limestone contains conspicuous, white crinoid columnals. Thus, crinoids are important evidence for (1) discerning whether a shale beneath the oil sands might be a Mesozoic terrestrial mud accumulation versus karsted limestone, and (2) most critically, the evidence of possible nearby karst-related geohazards.