Transgressive surfaces of erosion as sequence boundary markers in cool-water shelf carbonates

摘要

Although sequence stratigraphy was first conceptualized in, and applied to, siliciclastic successions, its methodology and concepts are applicable to the carbonate regimen, provided that potential modifications to the existing models are appreciated that take into account the specificities of carbonate systems. We demonstrate that some properties inherent to isolated (i.e. continent-detached), shallow-water, temperate carbonate systems, including their typical low diagenetic potential due to the prevalance of calcitic mineralogies, the critical role exerted by storm waves and tidal currents on sediment dynamics (e.g. shedding of sediments at wave abrasion depth during highstand and lowstand conditions), and uneven sea-bed topographies cause both the genesis and preservation of subaerial unconformities to be uncommon, thereby rendering their use as sequence boundaries inappropriate. Instead, transgressive surfaces of erosion (TSEs) that formed as wave base shifts upwards during early rise of relative sea level, are readily identifiable in the field, extensive and, by their very nature, are preserved in the rock record, typically at the base of upward-deepening stacked facies. In Pliocene cool-water carbonate successions from New Zealand, the TSEs are characterized by starved sedimentation and burrowed network development, and are overlain by coarse bioclastic shoreface sediments and shell beds. In most cases, the surfaces top-truncate shoaling-upward deposits, but also deepening-upward stacking facies in places. Emersive events rarely correspond to actual physical surfaces, but diagenetic evidence preserved beneath TSEs indicates that occasionally subaerial surfaces had previously developed and were later modified into TSEs. Therefore, we propose that TSEs are the most adequate unconformities to define high-order depositional sequences (20-250 ka) in the studied successions because, by contrast with emersion surfaces, their formation and preservation are not dependent upon cool-water sedimentary facies and mineralogy, and are favoured by the dynamics of the setting (i.e. wave base shift and accommodation creation during relative sea-level rise).