PRELIMINARY RESULTS OF PETROGRAPHIC AND SCANNING ELECTRON MICROSCOPE ANALYSIS OF CHARLESTON BUMP PHOSPHORITE COATED ROCKS, ATLANTIC OCEAN

The Charleston Bump is a bathometric shallowing from about 2000 feet to 1200 feet over which the Gulf Stream flows. During the Neogene, the ion carrying capacity of the seawater was reduced, precipitating phosphate and other dissolved ions forming a pavement that is very resistant to weathering. Returned samples from the Charleston Bump are rare and offer insight into phosphorite precipitation processes during the Neogene.

We analyzed eleven samples collected by submersible in 2001 using transmitted and reflected light to study mineralogy and texture, along with a Scanning Electron Microscope equipped with an EDAX to determine composition. Megascopically, samples consist of phosphate-coated vesicular conglomerate composed of carbonate clasts and ooze, angular layered phosphorite clasts, and minor chert. Goethite appears to fill some of the vesicles. Microscopically, carbonate clasts contain abundant foraminifera and rounded glauconite. The older layered phosphorite has lower vesicularity and is more manganese and iron rich. Texturally, both generations of phosphorite have complex intergrowths of at least two phosphate minerals. Some of the samples also contain an authigenic fibrous and radiating orange mineral that is very rich in aluminum and silicon. Many of the observed vesicles are filled with the authigenic orange mineral. The younger phosphorite that coats most of the rocks also exhibits colloform intergrowths and impregnations with carbonate ooze. Rounded pyrite clasts were detected in some of the samples.

The compositional and textural distinction between these two types of phosphorite appears to indicate either a change in precipitation conditions (e.g., bedded Fe-Mn-rich older phosphate), or chemical modification of phosphorite with time.