Abstract

An iron-rich crust separates terrigenous hemipelagic lutites and turbidites from overlying pelagic foraminiferal oozes and lutites on the Amazon abyssal fan and adjacent continental rise and abyssal plains. The crust marks a sharp change in color and oxidation state of the sediments; the terrigenous sediment below is generally gray and reduced, while the pelagic lutite above it is tan and oxidized. The crust formed on top of hemipelagic lutite and turbidites after the postglacial sea-level rise shut off the supply of terrigenous sediment to deep water. Decaying organic material reduced the iron in the terrigenous minerals. The reduced iron dissolved in the interstitial water and was expressed upward during compaction. Upon reaching the sediment-water interface, the iron was oxidized and precipitated, forming an iron-cemented crust in the sediment that has been buried by postglacial foraminiferal lutite. This crust is found over wide areas of the western equatorial Atlantic as well as in many other regions of the world. Its formation marks the end of terrigenous hemipelagic and turbidite deposition. The crust is always postglacial, and occurs at or very near the Pleistocene-Holocene boundary.

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