NEW INSIGHTS INTO DIAGENETIC PROCESSES IN GLACIOGENIC SEDIMENTARY SUCCESSIONS

Understanding the distribution of porosity is critical to evaluating the fluid reservoir potential of sedimentary deposits. In this regard, glaciogenic deposits are among the most poorly understood. This work investigates the origin and distribution of carbonate cement phases distributed through the Cenozoic succession of McMurdo Sound, Antarctica. Key lithologies include diamictite, mudrock and sandstone with minor conglomerate and diatomite. These deposits are arranged in cyclic patterns that record repeated advance and retreat of ice sheets. Thin section observations reveal a complex diagenetic history. Secondary carbonate is present throughout the succession, but its abundance, distribution, and morphology is highly variable. Isotopic data show that these phases are characterized by lower δ¹⁸O values (c. ‒10 to ‒18‰ VPDB) than might be expected in sediments that accumulated under freezing conditions. Such results led previous workers to postulate that connate fluids contain significant contributions of meteoric water derived from glacial melting, implying a fate for meltwater that involves its storage in the subsurface. Our more recent work, however, benefits from analysis of pore water samples recovered during coring. Pore water data show that a body of dense brine resides in the deep subsurface of McMurdo Sound. The brine is characterized by low δ¹⁸O values that are consistent with the compositions of diagenetic calcite. The most likely mechanism for brine generation, involving sea ice formation along ice sheet margins producing dense brines that infiltrated permeable subglacial sediment, suggests that the brine should be regionally present and, perhaps, the most important fluid in the subsurface by volume. Results of this study have the potential to change the way we evaluate burial history and reservoir quality of sedimentary successions that formed in polar environments.