Limestone diagenesis of Upper Devonian Nisku carbonates in the subsurface of central Alberta

Abstract

Limestone diagenesis of the Upper Devonian Nisku Formation in the subsurface of central Alberta began syndepositionally. Most early calcite cements are fibrous, bladed, and columnar and form isopachous precipitates in primary voids. Partial syndepositional induration of the lime matrix (sand and mud) is indicated by angular limestone intraclasts in deposits flanking some buildups, hardgrounds, and neptunian dikes in the reefal facies. Some buildups contain intertidal fades, possible vadose cements, and some early dissolution molds and vugs in their upper parts, suggesting shallow influence of meteoric pore waters towards the end of Nisku time. Postdating these features, precipitation of fibrous and subequant calcite cements, neomorphism, mechanical compaction, and incipient stylolitization resulted in complete induration and lithification of the buildups and associated marly strata.Trace-element compositions of the various calcites are variable within individual petrographic groups and may have originated from meteoric and (or) marine-phreatic diagenesis. However, isotopic and petrographic data of most cements suggest marine-phreatic diagenesis. Therefore, most cements were probably formed and (or) recrystallized in pore waters of marine parentage. The oxygen-isotope data suggest that limestone diagenesis and lithification were essentially completed at a depth of about 300 m. This burial depth was achieved near the end of Famennian or in Early Mississippian time.