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RESEARCH PRODUCT

Depositional environments and iron ooid formation in condensed sections (Callovian Oxfordian, South-eastern part of the Paris basin, France)

subject

description

Carbonate platforms across Western Europe were superseded at the Middle–Upper Jurassic (Callovian–Oxfordian) boundary either by alternating marl–limestone and widespread marl deposits or by condensed sections containing iron ooids. The characteristics of marine condensed sections in the south-eastern part of the Paris Basin (France) and their distribution pattern are examined here, and a model of iron ooid formation is developed. Iron ooids are found from the shoreface to the offshore zone. They are most abundant in the median-to-distal offshore transition zone, where they originally formed. They also occur commonly, albeit often as reworked grains, in the proximal offshore zone, to which they were transported. The contemporaneous, thick, predominantly marl sections that occur laterally are devoid of iron ooids and were deposited in deeper settings (distal offshore zone). The iron ooids are composed of goethite. Typically, they have a nucleus made up of a clump of goethite crystals and a laminated cortex. Three distinctive nanostructures are identified in the cortex laminae: (i) a nanograined crystalline structure typical of primary goethite; (ii) a secondary nanoflaked structure thought to have formed mechanically by reorientation of the goethite crystals; and (iii) a coalesced structure acquired by subsequent diagenetic recrystallization. The iron ooids formed successively (i) by lamina growth when goethite precipitated in the surface layer of the sediment (nanograined structure) and (ii) by interruption of growth when the ooids were remobilized by hydrodynamic agents, as reflected by the flaked nanostructure; (iii) these two nanostructures were sometimes transformed into a coalesced structure by recrystallization when ooids were buried.