Oxygen-isotope studies of clastic diagenesis in the Lower Cretaceous Viking Formation, Alberta: implications for the role of meteoric water

Abstract

Summary The oxygen-isotope compositions and paragenetic sequence of diagenetic minerals from the Lower Cretaceous Viking sandstone and conglomerate from south-central Alberta, Canada have been used to identify changes in porewater composition during diagenesis and to relate these changes to major geological events within the western Canada sedimentary basin. Large-scale influx of meteoric water has played an important role in diagenesis of this unit, especially following uplift of the basin in early Eocene time. Diagenetic phases include early kaolinite ( δ 18 O SMOW = + 24.9 to +28.2 ‰), siderite ( δ 18 O SMOW = + 22.0 to + 23.9 ‰; δ 13 C PDB = −6.3 to −1.3 ‰), calcite ( δ 18 O SMOW = + 26.0 ‰; δ 13 C PDB = −0.4 ‰) and chlorite ( δ 18 O SMOW = + 10.7 − + 13.4 ‰), followed in order by dolomite ( δ 18 O SMOW = + 19.3 − + 22.1 ‰; δ 13 C PDB = −5.9 to −2.6 ‰), calcite ( δ 18 O SMOW = + 13.9 − + 15.9 ‰; δ 13 C PDB = −8.4 to −6.1 ‰), ankerite ( δ 18 O SMOW = + 16.3 − + 17.9 ‰; δ 13 C PDB = −12.4 to −3.5 ‰), kaolin group minerals ( δ 18 O SMOW = + 12.0 − + 15.7 ‰), and illite ( δ 18 O SMOW = + 14.2 − + 15.7 ‰) and illite/smectite ( δ 18 O SMOW = + 13.8 − + 16.5 ‰). Quartz overgrowths ( δ 18 O SMOW = + 16.2 − + 27.4 ‰) began crystallizing relatively early during burial diagenesis and continued to form at least up to the onset of late diagenetic formation of clay minerals. The interpretation of these results is that shallow diagenesis, early in the burial history (glauconite, pyrite, calcite, chlorite), occurred largely in the presence of seawater-derived fluids, although a freshwater influence is indicated where siderite and(or) early kaolinite cements are abundant. As compaction and burial diagenesis proceeded (diagenetic chlorite, quartz overgrowths, dolomite), the porewaters became enriched in 18 O due to water/rock interaction. Burial diagenesis was terminated in the early Eocene by uplift related to the major Laramide Orogeny. Recharge of the basin by low 18 O meteoric water occurred at this time. The meteoric water then became involved in the formation of diagenetic quartz, calcite and ankerite (and the dissolution and albitization of feldspar) at or near maximum burial temperatures, and in the crystallization at lower temperatures of kaolin group minerals, illite and illite/smectite as the post-Eocene erosion progressed.