Petrology and paleogeographic significance of Tertiary nannoplankton-foraminiferal limeston, Guam
Abstract The tertiary stratigraphic column on the island of Guam, in the Mariana Island Arc of the western Pacific, is composed largely of volcanic rocks and limestones of shallow-water reef and bank facies. Small amounts of fine-grained limestones, of Eocene, Oligocene and Miocene age, occur interbedded with the volcanic rocks and reef limestones, and consist chiefly of planktonic foraminiferal tests embedded in a matrix of calcareous nannofossils. Although compositionally these pelagic carbonates are nearly identical to nannoplankton-foraminiferal oozes of the deep-sea floor, their deposition probably occurred at comparatively shallow depths — from a few hundred to perhaps 1000–2000 meters rather than several kilometers. The most favorable paleogeographic settings for pelagic deposition of this kind were probably intra-arc basins and shelf areas within the paleo-Mariana Island Arc during periods of volcanic inactivity. The spatial association of these pelagic limestones with shallow-water reef complexes, or with beds of redeposited, reef-derived coarse skeletal material, may serve to differentiate them from their deep-sea counterparts.