Eocene to early Miocene growth of Enewetak Atoll: Insight from strontium-isotope data

The margin of Enewetak Atoll contains 1,260 to 1,405 m of Cenozoic carbonates above a volcanic basement. Previous biostratigraphic studies on two deep wells (E1 and F1) yielded conflicting results on the ages of Tertiary strata on Enewetak, and on the presence of Oligocene strata. Strontium-isotope ratios support relatively continuous carbonate sedimentation from late Eocene to early Miocene (23 Ma). Carbonate deposition on the atoll margin started with rapid aggradation during much of the late Eocene followed by gradual seaward progradation of depositional facies during the latest Eocene, Oligocene, and early Miocene. The only distinct break in sedimentation from late Eocene to early Miocene was observed at ∼850 m in the E1 well. Here an unconformity formed by subaerial exposure separates backreef, upper Eocene grainstones from lower Oligocene carbonates. No contemporaneous subaerial exposure surface and unconformity were observed in the F1 well because correlative rocks were deposited in a slope environment.

Extensive diagenesis occurred in upper Eocene, Oligocene, and lower Miocene strata at the atoll margin where sea water circulated through in response to thermal convection and tidal pumping. The main diagenesc processes are (1) aragonite dissolution, (2) radiaxial calcite cementation, (3) compaction in slope deposits, and (4) dolomitization. Aragonite dissolution is pervasive below 375 m in the F1 well. Radiaxial calcite cement is common at 375-825 m in the F1 well. Compaction-related fracturing and pressure solution are common in poorly cemented packstones and grainstones deposited in a slope environment and now buried more than 1,100 m deep. Dolomite is common in slope carbonates at ∼1,320 m in the F1 well and in reefal carbonates at 1,245 m in the E1 well.

Stable carbon- and oxygen-isotope data support formation of radiaxial calcite and dolomite in cool sea water. Radiaxial calcite cement and dolomite have distinctly higher 87Sr/86Sr ratios than does adjacent depositional carbonate. Strontium-isotope ratios of radiaxial calcite cements indicate precipitation at burial depths of 100-350 m. The absence of meteoric cements and association of radiaxial calcite cement with aragonite dissolution suggest that aragonite dissolution also occurred in moderately deep sea water. Strontium-isotope ratios in dolomites indicate dolomitization by sea water at burial depths greater than 950 m. Marine diagenesis is dependent on the saturation state of sea water relative to aragonite, calcite, and dolomite. Shallow sea water, supersaturated with respect to aragonite and high-magnesium calcite (HMC), precipitates aragonite and HMC. Moderately deep sea water (undersaturated with respect to aragonite and HMC, super-saturated with respect to low-magnesium calcite—LMC) dissolves aragonite, converts HMC to LMC, and precipitates LMC (including radiaxial calcite). Deep sea water (under-saturated with respect to calcite, supersaturated with respect to dolomite) dissolves calcite and precipitates dolomite. The intensity of marine diagenesis depends on the amount of sea water circulating through the platform or atoll margin.

Field of study: Geology