MINERALOGICAL AND O-ISOTOPE STUDIES OF DIAGENESIS AND POREWATER EVOLUTION IN CONTINENTAL SANDSTONES, CRETACEOUS BELLY RIVER GROUP, ALBERTA, CANADA

The mineralogy and O-isotope geochemistry of diagenetic phases from continental sandstones of the Upper Cretaceous Belly River Group, west-central and south-western Alberta, have been determined. Diagenesis of the continental sandstones is dominated by crystallization of clay minerals, including Fe-rich chlorite (delta-O-18 = +3.3 to +9.3 parts per thousand), I/S, C/S and illite (delta-O-18 = +9.5 to +10.7 parts per thousand), kaolinite (delta-O-18 = +5.9 to +12.7 parts per thousand), S/I and smectite (delta-O-18 = +11.0 to +14.0 parts per thousand). Other diagenetic phases include quartz overgrowths (delta-O-18 = +12.0 to +21.0 parts per thousand) and calcite (delta-O-18 = +8.5 to +12.9 parts per thousand; delta-C-13 = -14.3 to -2.0 parts per thousand). Chlorite generally crystallized first in the paragenetic sequence, followed by quartz, I/S, calcite and kaolinite, and then S/I and smectite. Interpretation of these data suggests that early diagenesis occurred in the presence of low-O-18 meteoric water whose delta-O-18 value rose moderately during subsequent burial. Highest burial temperatures (almost-equal-to 110-degrees-C) were probably achieved in the south-western portion of the study area, and are reflected by the appearance of R1-ordered I/S in this locality. Post-Laramide influx of meteoric water subsequently resulted in a substantially lower porewater delta-O-18 value. Values of delta-O-18 for diagenetic minerals from the continental sandstones and immediately underlying deltaic and shoreline, basal sandstones are systematically lower (by +1 to +6 parts per thousand) than equivalent phases from the Lower Cretaceous marine sandstones of the Viking Formation. Relative differences in the oxygen isotopic compositions of connate waters (freshwater vs sea water) have been preserved, surviving delta-O-18-enrichment during burial and delta-O-18-depletion during post-Laramide influx of meteoric water. While substantial meteoric dilution of the porewater has occurred, flushing of (evolved) connate waters has not been complete.