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Sr Isotopic Age Analysis of Co-Occurring Miocene Phosphate Grain Types on the North Carolina Continental Shelf
Stanley Riggs (1), Peter Stille (2), Dorothea Ames (1)
Four phosphate grain types and associated foraminifera were analyzed for their 87Sr/86Sr ratios to test the hypothesis that different authigenic grain types have specific times and processes of formation in response to changing conditions during one sea-level transgression. The samples were collected from one phosphate-rich depositional sequence (FPS-1) of the Miocene Pungo River Formation in Onslow Bay, North Carolina continental shelf. The hand-picked grains were from three cores representing upsection changes from lowstand, to transgressive, and highstand systems tracts. The phosphate grain types carried significantly different strontium isotopic signatures and included the following: peloidal grains (spherical to oval, highly rounded and polished grains with inor inclusions and common coatings), intraclastic grains (irregular-shaped rip-up grains from preexisting crusts and nodules), microsphorite (laminated crusts and bored hardgrounds associated with unconformities and containing common inclusions), and skeletal grains (formed directly by organisms). Four different phosphate skeletal grains were analyzed (brachiopods, vertebrae, teeth, and bone splinters); however, the results were not significantly different and therefore they were all included as one grain type.
Analyses of phosphate grain types and associated foraminifera support the interpretation that they formed in response to specific chemical and physical processes and at different times during phosphogenesis, as follows. (1) Peloids cluster tightly and have Sr isotope ages contemporaneous with the associated foraminifera. They are interpreted to represent the time of phosphogenesis and primary sedimentation during early- to mid-stage transgression. (2) Intraclasts are significantly older than other associated phosphate grain types with Sr isotope ages similar to the microsphorites. They are interpreted to have formed as microsphorite on unconformity surfaces during the lowstand, were ripped up from these surfaces during the subsequent transgression, and reworked into the overlying sedi ent sequence. (3) Skeletal grains have quite variable Sr isotope ages, but in general they are younger than the other phosphate grain types. Most skeletal grains are interpreted to be contemporaneous with the peloids and foraminifera. However, their younger ages are interpreted to be in response to secondary geochemical processes associated with (a) recrystallization from carbonate hydroxyapatite to carbonate fluorapatite and (b) subsequent precipitation of apatite in the pore spaces characteristic of porous vertebrate skeletal material. Some skeletal material is older and probably has been reworked from the underlying unconformity surfaces.
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