Ooids and Shallow-Water Debris in Mesozoic Sediments from Seamount 853, Western Pacific

C. R. Orban, J. A. Haggerty

Oolitic limestones were recovered from Seamount 853 in the Pacific. Ooids have not been observed forming in modern Pacific atoll environments, and the only other occurrences in the equatorial and North Pacific are limited to Cretaceous strata.

Seamount 853 is located approximately 200 mi east-southeast of Guam on the western rim of the East Mariana basin. This seamount is 45 km long and 20 km wide, oriented with the long axis north-south. The seamount rises more than 4,000 m above surrounding sea floor; the shallowest point is 1,303 m below sea level. From bathymetric and seismic data, the morphology of the seamount is a platform topped by a pelagic cap from 2,000 to 1,500 m below sea level; some additional pinnaclelike features project upward.

Rocks dredged in 1981 by the R/V Kana Keoki from the upper slope of Seamount 853 include oolitic packstones, fossiliferous peloidal packstones and wackestones, and conglomerates. A hand specimen of an oolitic packstone has cross-stratification that suggests deposition in a high-energy environment, perhaps a shoal. The fossiliferous peloidal packstones and wackestones contain dasycladacean algae, oncoids, echinoderms, gastropods, Inoceramus, benthic foraminifera, peloids, and other micritized bioclasts. The conglomerates contain Cretaceous planktonic foraminifera, shallow-water debris, and altered glass shards.

We interpret this association of shallow-water fossils and peloids to represent deposition in a protected lagoonal setting during the Mesozoic. The presence of Cretaceous foraminifera associated with shallow-water debris and altered glass indicate the existence of a volcanic edifice supporting a shallow-water environment prior to, or during, the Cretaceous. The oolitic deposits may represent the terminal facies associated with drowning of the platform.

AAPG Search and Discovery Article #91039©1987 AAPG Mid-Continent Section Meeting, Tulsa, Oklahoma, September 27-29, 1987.