『Abstract
The Napo phosphorites were deposited at the edge of a stable
marine shelf during the Upper Cretaceous (Coniacian) oceanic anoxic
event (OAE3) at the transition from bioclastic limestone to organic-rich
shale facies. Phosphogenesis was triggered in the shelf margin
environment by a number of factors including strong upwelling
currents, high biological activity, plankton blooms, and large
amounts of organic matter production and subsequent accumulation.
Dissolved phosphate levels increased in the sediment from a combination
of anoxic conditions and microbial activity. Once dissolved phosphate
concentrations were high enough, apatite began to form around
nucleic sites including mineral grains, shells, wood fragments,
and foraminifera tests forming peloidal fluorine rich carbonate
fluoroapatite (francolite). As the peloids formed, sedimentation
continued and dissolved phosphate concentrations diminished. A
period of minor winnowing ensued, and as dissolved phosphate concentrations
remained low, shale layers were deposited separating the various
phosphate layers.
Keywords: Paleoenvironments; Origin; Napo; Phosphorites; Late
Cretaceous; Ecuador』
1. Introduction
2. Materials and methods
3. Geological background
4. Napo phosphorites
4.1. Petrology
4.2. Geochemistry
4.3. Depositional environment
5. Conclusions
Acknowledgements
References