At Site 612 (Leg 95), a 20-cm-thick layer containing microtektitic glass spherules separates middle Eocene from upper Eocene siliceous nannofossil oozes. These microtektites presumably are part of the widespread microtektite strewn field of late Eocene age, found in the Gulf of Mexico, the Caribbean Sea, the equatorial Pacific, and the eastern equatorial Indian Ocean. Associated large green tektite or impact glass fragments and shock-damaged silicate minerals demonstrate the impact origin of the Site 612 layer. The age of the layer remains uncertain, as a gap of about 6 m.y. separates the uppermost recovered middle Eocene (Bartonian) from the superjacent upper Eocene (upper Priabonian) sediments. However, geochemical and mineralogical composition as well as micropaleontological dating relate the bed to the lowermost of the three known late Eocene "North American" tektite layers.

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Summary Oolitic ironstones occur in various sedimentary environments: shallow marine to deltaic, lacustrine, fluviatile and pedogenic. Distinction between formational and depositional environment is not always possible. Most of the marine and fluvial minette-type ironstones consist of reworked ferruginous coated grains deposited in agitated water, but there exist also indicative structural features of in situ formation in the supporting medium of lateritic and hydromorphic environments. In the zone of oscillating groundwater repeated leaching and subsequent concretionary precipitation of hydrated ferric oxides take place, according to the prevailing Eh/pH-conditions and microbial activity. The moderate Al substitution of goethite from hydromorphic environments corresponds to the observed range in oolitic ironstones. The authors therefore assume erosion, reworking and subsequent fluviomarine redeposition of soil derived ooids to be the major processes of generating minette-type ironstones. Postdepositional diagenetic changes may convert the aluminous, silica-rich ferric oxides into berthierine in reducing environments if the chemical bulk composition of the primary goethite is similar. Since any aquatic milieu with appropriate fluctuations of Eh and pH can produce ferruginous coated grains, marine iron ooids associated with hardgrounds and areas of low sediment input can also occur. But there, release of ferrous iron, transport in saline interstitial waters and fixation of ferric hydroxides — usually with very low Al-substitution — take place in a much smaller scale, unable of generating the huge iron accumulations of minette-type ore deposits.

Sedimentology and sampling: Sedimentological logs were measured in the N Albert quarry in southern Sweden and in cored sections from deep wells in the Stenlille area in eastern Denmark, as well as the Rødby-1 core in southern Denmark (Figs. 2, DR1-DR2). Similarly, the Schandelah and Mariental cores from Germany, the Grouft core and a temporary road construction outcrop Junglinster Heedhaff in Luxembourg, were measured and logged (Figs. 2, DR3). Drillcore sampling of the Luxembourg material was carried out at the Service Géologique Luxembourg and laboratory analyses at the Steinmann-Institute (University of Bonn). The interpretation of the depositional environments is supported by detailed studies of the palynology, coal petrology, stable isotope geochemistry and wire-line log motifs (

Eocene to Cretaceous siliceous sediments of four Deep Sea Drilling Project sites on the continental margin off eastern North America (Sites 603, 605-Leg 93; Sites 612, 613-Leg 95) were analyzed for their major geochemical and mineralogical composition. Under different environmental conditions (burial depth, in situ temperature, host-rock fades) of the upper slope to lower rise, conversion of biogenous opal-A to opal-CT took place in biosiliceous chalks and marls older than early middle Eocene. Dense sampling revealed the sharp character of the diagenetic silicification front (seismic reflector A c), which demonstrates a widespread, more or less synchronous (51-53 Ma), rapid change of silicoplankton fertility and preservation, which later led to the marked boundary between an opal-A and an opal-CT diagenetic facies. In clay-rich sediments on the lower rise (Site 603), the opal-A opal-CT transformation is largely retarded, whereas clinoptilolite formation is enhanced. Host-rock composition and the, consequent interstitial water chemistry, apparently controlled by palaoclimatology and paleoceanography, appear to be the most important factors influencing the rate and mode of silica diagenesis. Diagenetic alteration of the primary siliceous oozes to porcellanitic chalks and porcellanites took place only in a narrow equilibrium range of the complex geochemical sediment-pore water system. These ideal conditions were realized in our case and at most sites in the Atlantic Ocean in sediments older that 51 m.y.