Deep Marine Dolomitization, Enewetak Atoll: ABSTRACT

Abstract

Dolomite is present 1,250-1,400 m (4,100-4,600 ft) below sea level in Eocene strata of the Enewetak Atoll. Petrographically, the deep Enewetak dolomite postdates brittle compaction of rigid grains in the host Eocene strata. The 87Sr/86Sr ratios of these dolomites (0.70865-0.70901) indicate that the dolomites formed mainly between middle Miocene and the present. Because the top of the lower Miocene is more than 900 m (2,900 ft) above the deep dolomitic interval, the dolomite must have formed at a minimum burial depth of 900 m (2,900 ft). Stable oxygen isotope determinations suggest dolomite precipitation from cold marine water. Lower Miocene and Eocene carbonate strata on the atoll are currently in open communication with cold, modern ocean water and robably have been since deposition. At a depth of approximately 1,000 m (3,300 ft), modern Pacific Ocean water becomes undersaturated with respect to calcite but is still supersaturated with respect to dolomite. Therefore, it is proposed that the deep Enewetak dolomites precipitated from cold, deep ocean water (undersaturated with respect to calcite) percolating through the atoll at burial depths of more than 900 m (2,900 ft). End_Page 524------------------------------ At least 3 factors appear critical for marine dolomitization: (1) waters undersaturated with respect to calcite, but supersaturated with respect to dolomite; (2) permeable limestones; and (3) hydrologic position of those limestones where numerous pore volumes of undersaturated water will be flowing through. In the past, different oceanographic conditions may have caused some ancient seas to have relatively shallow calcite saturation depths. Many ancient dolomites found in atoll and reef-wall settings may have also precipitated in normal marine water. End_of_Article - Last_Page 525------------