BUILDUPS UNDER THE INFLUENCE: DEVONIAN AND PENNSYLVANIAN HYBRID REEFS/SEEPS IN WESTERN AND ARCTIC CANADA

A priori, the carbonate-forming seep environment differs much from the reef environment. Seeps, and the communities they are associated with, are fed at the lowest trophic level by chemical compounds seeping or venting from below. In contrast, reefs are fueled by light penetrating from above, feeding autotrophs that provide the energy to a variety of heterotrophs, from microbes to higher life forms. Product-wise, however, the distinction between seep and reef carbonates is vague. Both types of carbonates can form rigid or coherent constructions that may or may not stand above the surrounding sea floor where they are associated with rich biota, large cavities, cements, internal sediments, as well as chemical, physical or biological breakdown. The processes of organic growth, cementation, sedimentation and destruction operate equally in both the reef and seep environments resulting in similar products that do not easily betray their origin, especially in absence of an isotopic smoking gun. So how many shallow water carbonate buildups, interpreted as reefs, actually were under the influence of chemical seepage during their growth or early diagenetic history? Some reefs may well have started as seeps only to be later colonized by reef-forming organisms that took advantage of well-lit shallow-water conditions and a hard substrate provided by the seep carbonates. In contrast, some seeps may have started as reefs, localized on faults or other fractures, which later provided the conduits for seeping or venting fluids. Some reefs may have been fed simultaneously by seepage from below while receiving a healthy dose of sunlight (or rain down of light-fed plankton) from above. We present two compelling examples of hybrid carbonate buildups with a complex depositional and early diagenetic history that was driven by both seepage from below and radiation from above: a Pennsylvanian shelf margin Nansen buildup (Arctic Canada) and the Devonian Swan Hills Buildup (Western Canada).