Stratigraphy, petrology, and geochemistry of a mid-oceanic carbonate platform: Niue Island, South Pacific.

摘要

Mid-oceanic carbonate platforms offer a simple setting to study the essentials of dolomitization. One such platform is Niue Island, a former coral atoll capping a Miocene-age seamount in the South Pacific. Samples from 6 wells (≤303 m depth) and 3 outcrops reveal two dolomite bodies at -300 to -165 m (Lower Dolomite Unit, LDU) and -30 to +50 m (Upper Dolomite Unit, UDU) elevation.;Paleomagnetism and strontium isotope chronostratigraphy place the depositional age of the uppermost 340 m at 8.6--1.7 Ma. Generally decreasing accretion rates (62--19 m/Ma) are consistent with subsidence of an extinct seamount. The distribution of early meteoric diagenesis points to 15 sea-level lowstands between 8.5 and 1.5 Ma. Nine occurred at 6.49--5.33 Ma. Lowstand 1--8 falls were brief (115 ka) and ranged in magnitude 30--36 m, whereas Lowstand 9 fell 14 m and lasted 50--178 ka. The six Plio-Pleistocene lowstands lasted 15--34 ka and ranged 5--13 m. The record agrees well with the Haq third-order eustatic curve.;UDU dolomitization is characterized by near-complete replacement by three dolomite textures which are mixtures of three dolomite phases with distinctive crystal, elemental, and stable isotope geochemistries. Texture distribution is related to the diagenetic evolution of precursor porosity and permeability. LDU dolomitization is marked by partial replacement by one texture composed of one phase. There is no relationship between lithofacies and degree of LDU dolomitization.;The LDU dolomites formed ≥3 Ma after deposition at 5.3, 3.9, and 1.2 Ma on the basis of their 87Sr/86Sr ratios. Lithofacies, stable isotopes, and elemental geochemistry point to seawater-mediated dolomitization at temperatures of 20--25°C. The prolonged post-depositional delay and deep burial (≥10--200 m) suggest dolomitization through Kohout or thermal convection. UDU dolomitization occurred 600 ka after deposition during a series of brief events (3.4--1.5 Ma). Each dolomitized only a thin interval, but overlapped thus thoroughly replacing the UDU. Lithofacies and geochemistry point to seawater-mediated dolomitization near 25°C. The saucer-shaped dolomite bodies with their sharp contacts and platform-wide extent indicate lateral fluid flow. Together with the shallow burial depth (≤80 m), the evidence indicates dolomitization by mixing zone-entrainment of seawater during sea-level lowstands.