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Polygenetic (Polyphase) Karsted Hardground Omission Surfaces In Lower Silurian Neritic Limestones: Anticosti Island, Eastern Canada
Exquisitely preserved and well-exposed rocky paleoshoreline omission surfaces in Lower Silurian Chicotte Formation limestones on Anticosti Island, Québec, are interpreted to be the product of combined marine and meteoric diagenesis. The different omission features include: 1) planar erosional bedding tops, 2) scalloped erosional surfaces, 3) knobs, ridges, and swales at bedding contacts, and 4) paleoscarps. An interpretation is proposed that relates specific styles of omission surface to different diagenetic–depositional processes that acted in separate terrestrial–peritidal–shallow neritic zones. Such processes were linked to fluctuations in relative sea level with specific zones of diagenesis such as: 1) karst corrosion, 2) peritidal–shallow subtidal erosion, 3) subtidal seawater flushing and cementation, and 4) shallow subtidal deposition. Most surfaces are interpreted to have been the result of initial extensive shallow–water synsedimentary lithification forming hardgrounds that were, as sea level fell, planed by inner-neritic abrasion, successively altered by exposure and subaerial corrosion, only to be buried by sediments as sea level rose again. This succession was repeated several times, resulting in a suite of recurring polyphase omission surfaces through many meters of stratigraphic section. Synsedimentary cloudy marine cements are well preserved and are thus interpreted to have been calcitic originally. Aragonite components are rare and thought to have been dissolved just below the Silurian seafloor. Large mollusks that survived such seafloor removal were nonetheless leached and the resultant megamolds were filled with synsedimentary calcite cement. These Silurian inner-neritic–strandline omission surfaces are temporally unique. They are part of a suite of marine omission surfaces that are mostly found in early to middle Paleozoic neritic carbonate sedimentary rocks. These karsted hardgrounds formed during a calcite-sea time of elevated marine carbonate saturation and extensive precipitation of marine cement. The contemporaneous greenhouse atmosphere was supercharged with CO2, leading to profound surface karst under strongly acid rain. Younger peritidal omission surfaces, although potentially formed during aragonite-sea or calcite-sea times, would have been subject to very different terrestrial diagenetic process with lower atmospheric pCO2 values but increasingly complex biogenic soils producing dissimilar alteration features.
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