Geochemistry of carbonate cements in the Sag River and Shublik Formations (Triassic/Jurassic), North Slope, Alaska: implications for the geochemical evolution of formation waters

• Published in: Sedimentology
• Main Authors: MOZLEY, PETER S., HOERNLE, KAJ
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1990
• Subjects: north slope; Alaska
• Online Access: http://dx.doi.org/10.1111/j.1365-3091.1990.tb01827.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1365-3091.1990.tb01827.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3091.1990.tb01827.x

ABSTRACT Carbonate cements (calcite, siderite, dolomite, and ankerite) formed throughout the diagenetic history of the Sag River and Shublik Formations. The trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation. Earliest calcites, formed prior to significant compaction of the sediment, are relatively enriched in Mg (up to 4·4 mol%), and have 87 Sr/ 86 Sr values (mean = 0·707898) compatible with the original marine pore waters. Later calcites are relatively Fe‐rich (up to 5·0 mol%) and are characterized by increasing 87 Sr/ 86 Sr values (up to 0·712823) and Sr content with decreasing age. The Fe content of zoned siderite and dolomite/ankerite rhombs increases towards the outside of the rhombs (i.e. increasing Fe content with decreasing age). These geochemical variations appear principally to result from changes in pore‐water chemistry during diagenesis. The increase in 87 Sr/ 86 Sr and Sr content of the cements is most likely due to interaction between pore waters and 87 Sr‐rich clay and possibly feldspar in Ellesmerian mudrocks (whole rock 87 Sr/ 86 Sr signatures for the mudrocks are > 0·716). Pore‐water Fe 2+ concentration was probably controlled by diagenetic alterations involving Fe‐bearing minerals (e.g. pyrite precipitation). A reconnaissance examination of carbonate cements in the overlying Kingak Shale indicates that similar alterations occurred in the Kingak. The low δ 18 O value of some calcite cements (‐11·96% PDB) suggests that an influx of meteoric water may have occurred in the mid‐Neocomian, though the low value could also result from an abnormally high geothermal gradient associated with mid‐Neocomian rifting.