Phosphatic Sediments and Rocks Recovered from the Peru Margin during ODP Leg 112

Coring during Ocean Drilling Program (ODP) Leg 112 recovered phosphatic materials from six sites in forearc basins on the shelf and upper slope west of Peru. Three types of phosphates occur. F-phosphates are friable, light colored micronodules and peloids of carbonate fluorapatite (CFA); these formed by precipitation of CFA in laminated diatom muds deposited within the oxygen-minimum zone. Phosphatic sands, termed P-phosphates, consist of phosphatic peloids, coated grains, and fish debris, often having an admixture of fine siliciclastic grains. These sands occur in thin layers and burrowed beds up to 2 m thick; they may record high energy conditions and, in places, they occur above unconformities. Most abundant are dark and dense phosphates, herein called D-phosphates, that occur as nodules, gravels, and hardgrounds. These phosphates formed through complicated cycles of CFA precipitation during early diagenesis, erosion and exhumation, and reburial and rephosphatization processes associated with changing energy conditions that may reflect the effects of changes in sea level on the Peru shelf. CFA cements in Pand D-phosphates often replaced microbial structures, but our data do not reveal whether this microbial involvement was passive or active. The three phosphate types record a time and energy spectrum, with F-phosphates at the lower end, D-phosphates at the upper. Phosphates along the Peru margin occur in sediments as old as middle Miocene but are most abundant in Pliocene and especially Quaternary upwelling sediments. F-phosphates are most common in deeper water, outer-shelf/upper-slope sites, whereas Dand P-phosphates tend to predominate at shallower shelf sites more subject to episodic high energy conditions, especially during lowstands of sea level. Although lowstands of sea level probably led to concentration of phosphatic particles by winnowing, most CFA precipitation and cementation may have occurred, at least during Quaternary time, during interglacial highstands.