Alternating Authigenic and Carbonate Factory Production within a Cool-water Carbonate Sequence Stratigraphic Framework

Glaucony and phosphate production is characteristic of low sedimentation rates within low energy, oxygen-depleted environments, and is often associated with condensed sections and unconformities. Glaucony- and phosphate-rich rocks are often interpreted to be associated with sea-level highstands and maximum flooding surfaces, especially in siliciclastic systems. In contrast, our research shows that in the cool-water carbonate realm glaucony and, to an extent, phosphate are connected to lowstands and transgressions, requiring a reversal in the way they are interpreted within a sequence stratigraphic context. The Mid-Tertiary rocks of the Waitaki Basin, South Island, New Zealand, contain a cool-water carbonate and greensand succession formed on the eastern passive margin of the Zealandia continental fragment, and include two major sequence boundaries within a broad regional transgression. The palaeobasin contained an eastern outer shelf volcanic-induced high that acted like a platform rim, from where the basin deepened towards the west. The basal sequence contains a bryozoan grainstone facies that formed a shoal on the eastern volcanic seamount. This grades westward to a quartzose impure wackestone containing terrigenous material derived from low-lying islands of Zealandia far to the west. The overlying sequence boundary forms a karst surface associated with the high in the east, and a firmground in the west, and developed as a result of sea-level fall and lowstand conditions. During these lowstand conditions the terrigenous supply of silt and clay material was moved closer to the Waitaki Basin and this was then available for glauconitisation. Calcareous glaucony- and phosphate-rich greensands accumulated during the subsequent transgression, with the glaucony and phosphate content decreasing through this second sequence to form pure packstones during highstand and early regression. The second sequence boundary overlying these packstones shows similar karst and firmground distribution to the first, and has a similar greensand transgressive sequence deposited above it. In this cool-water setting, where the carbonate factory is located on an offshore volcanic-induced high, authigenic mineral production occurs at lowstand settings. During lowstand, mud-sized terrigenous clay minerals are introduced to the basin from the west, smothering the carbonate factory and supplying the building blocks for glaucony production. Through the transgressive phase this terrigenous clay supply is progressively shut down and the system returns to carbonate production at highstand. Sea-level rise in such an environment encourages the development of the cool-water carbonate factory, and thus a decrease in production from the 'authigenic factory'.