Cenozoic phosphatic deposits in North Patagonia, Argentina: Phosphogenesis, sequence-stratigraphy and paleoceanography

Paleoenvironmental analysis of the Cenozoic marine section cropping out near Gaiman (Chubut Province, Argentina) shows that most of the local succession was deposited in a shallow, storm-dominated marine environment, bearing well preserved Ophiomorpha fossil traces. Sequence-stratigraphic interpretation records the effect of three sea-level oscillations. Phosphatic strata are related to mostly in situ concretions developed within transgressive-early highstand system tracts (Type 1) and to reworked and winnowed lags associated with transgressive surfaces (Type 2) which display a concentration of phosphatic concretions, ooids, vertebrate bones, teeth and shells. Close association of Callianasid bioturbation (Ophiomorpha) and phosphatic levels suggest a genetic link for both, via improved early-diagenetic water circulation and Fe and P early-diagenetic cycling. Phosphogenesis would have taken place after cold and corrosive water, probably similar to the present Antarctic Intermediate Water (AAIW), flooded the continental shelf and mixed with warmer surficial waters. This mixing could explain the contrasting indications from the marine vertebrate fauna together with the general corrosion and replacement processes that are common in the Gaiman strata. The development of the phosphorites would have occurred at times of global climatic transition and increased oceanic circulation, probably during the Late Oligocene–Early Miocene.