Paleo-Oceanographic and Tectonic Settings of Early Miocene Reefs and Associated Carbonates of Offshore Southeast Asia

Abstract

The late Oligocene to early middle Miocene was a period of widespread reef and associated carbonate deposition over a wide latitudinal range in the offshore region of southeast Asia. A generally rising eustatic sea level during this period allowed the accumulation of thick reefal units. An expansion of the coral-algal regional facies belt north to Japan and south to New Zealand occurred during the early Miocene as the result of a reduced oceanic latitudinal temperature gradient. An episode of enhanced organic carbon burial is reflected in a positive ^dgr13C excursion from 17.5 to 13.5 Ma. Increasing paleo-oceanographic equability may have resulted in reduced ocean circulation with the resultant development of an expanded oxygen-minimum zone. Marine oil-prone or anic carbon was deposited in silled anoxic basins both bounded by, and within, island-arc systems. A combination of tectonic and paleo-oceanographic factors, therefore, created conditions that favored contemporaneous deposition of organic carbon-rich source beds and reef reservoirs during the latest early to early middle Miocene. Paleogeographic and facies reconstructions for an 18-m.y. interval indicate that deposition occurred in three tectonic settings: (1) passive continental margins, (2) convergent plate boundaries, and (3) obliquely convergent plate boundaries. In all these settings, the vertical-motion histories of fault-bounded blocks and of larger tectonic elements within which they are contained directly controlled the geographical distribution of carbonate buildups, their scal and morphology, duration, diagenesis, and relationship to siliciclastics.