Diagenetic potential for lithification of cool-water carbonate shelf mud

Abstract

Recent studies of cool-water shelf carbonates have emphasized that lithification of fine-grained carbonates is generally delayed (post-mechanical compaction) due to near-surface aragonite dissolution and greatly reduced effective permeability of mud-size sediment. A more complex response may exist. Texture, mineral, and stable (oxygen, carbon) isotope attributes of sediment offshore of northernmost New Zealand are used as input parameters to understand diagenetic pathways preserved in outer shelf (Te Kuiti Group) and equivalent inner shelf (Torehina Formation) carbonate mudstones of Oligo-Miocene age on North Island, New Zealand. Modern seafloor mud is unlithified, and consists of low-magnesium calcite, < 20% fine-grained skeletal aragonite, and no dolomite. Aragonite is absent in the Te Kuiti Group carbonate mudstones that otherwise consist of low magnesian calcite and trace amounts of authigenic and reworked synsedimentary dolomite. Lithification spanned onset of mechanical compaction in the presence of marine-derived pore waters. For comparison, inner shelf (< 50 m) mudstone of the equivalent Torehina Formation accumulated in a slightly brackish water (c. 31‰) setting. Lithification of carbonate mud was rapid, having occurred in the upper tens of meters in the presence of mixed marine and meteoric fluids, whereas lithification of intercalated siliciclastic mud post-dated mechanical compaction. Near-surface sulphate reduction is recorded isotopically (δ 13C = − 8 to − 15 ‰) by shallow-burial concretions and oyster shell calcite. The comparison of modern and Oligocene sediment suggests that diagenetic potential for lithification of cool-water shelf carbonate mud is driven by diagenesis of in situ and allochthonous mud-size aragonite. Lithification rates may approach those associated with tropical deep-water periplatform (shelf-derived) sediment. The Oligocene carbonate mud lithified faster than regionally intercalated coarser-grained skeletal calcarenites, and likely contributes to deeper-burial intrabasinal diagenetic heterogeneity.