Sequence Stratigraphic Context of Syndepositional Diagenesis in Cool-Water Shelf Carbonates: Pliocene Limestones, New Zealand

Abstract

Abstract Pliocene limestones in eastern North Island, New Zealand, are cool-water deposits composed of barnacle–epifaunal bivalve–bryozoan dominated calcitic biota formed in shoal areas about the margins of a tectonically active forearc basin seaway. Vertical changes in the thickness and depositional facies of the limestones, and fundamentally the position and nature of internal bounding discontinuities within them, have enabled identification of high-order (likely 41 ka) transgressive (deepening-upwards) and transgressive–regressive (deepening-then shallowing-upwards) sequences within the formations. Syndepositional (or sea-floor or pre-compaction) diagenesis in the limestones involves a spectrum of destructive to constructive features, including skeletal abrasion, bioerosion, encrustation, dissolution, and cementation. Upward changes in the type and extent of these pre-compaction diagenetic features within the discontinuity-bounded sequences relate closely to the stacking patterns of depositional facies and their associated skeletal and mineralogical composition. Consequently, it has been possible to develop a series of predictive diagenetic models for cool-water shelf carbonates deposited upon and about upthrust antiforms in the forearc that integrate their patterns of syndepositional diagenesis into a sequence stratigraphy based framework driven ultimately by relative sea-level changes. Marine constructive diagenesis, characterized by bioencrustation, turbid cements, and interstratified internal sediments, occurs most commonly in those facies close to the sequence-bounding discontinuities, within the lower levels of transgressive systems tracts (TSTs) and the uppermost levels of regressive systems tracts (RSTs). These sediments equate to the highest-energy facies upon antiform tops, close to the wave abrasion base, where sedimentation rates were low. In contrast, destructive diagenetic processes typically dominate over constructive ones, often exclusively so, in the upper part of TSTs, in highstand systems tracts (HSTs), and in the lower portion of RST carbonates, where the sediments were not so influenced by active fluid circulation or strong reworking for prolonged periods, but were more rapidly buried within accommodation traps below wave abrasion base about the antiforms. The link between the facies motifs and synsedimentary diagenesis in these cool-water Pliocene limestones helps unlock their otherwise often cryptic, high-order sedimentary architecture. Such an interpretive opportunity is not afforded by the subsequent post-compaction diagenetic features in the limestones because these are no longer closely related to the sedimentation realm.