An unique 8 cm thick layer of tektite glass and impact ejecta has been described from Core 21, section 5, from DSDP Site 612 taken on the continental slope off New Jersey (Keller et al., 1987; Thein, 1987, Glass, 1989). Rock fragments along with microtekrites are concentrated in the upper half of the layer, whereas clinopyroxene-bearing (cpx) spherules, similar in texture and composition to those found in the Caribbean Sea and Gulf of Mexico, are concentrated in the lower half of the layer. Previous workers suggested that the tektite-bearing layer at Site 612 is reworked as it lies close to, or on, an unconformity. The main objectives of this study were to determine: 1 ) the nature and provenance of the source rock for the ejecta and tektite debris at Site 612; 2) whether the tektite glass dispersed throughout the lag was from one or two impact events; and 3) the relationship of the microtektires to the cpx spherules concentrated in the lower half of the layer. Other objectives of the study were to define a probable location of the source crater for the North American tektites and to determine whether the tektite-bearing layer at Site 612 has been redeposited. To determine the nature of parent material from which the impact ejecta were derived, X-ray diffraction patterns were obtained for fifty-three rock fragments from the >250 um size fraction. Petrographic and compositional studies were also completed on these rock fragments. The major phases identified by these studies are coesite, quartz, micas, illite, stishovite, glass, feldspar, and pyrite. Almandine garnet and chlorite are minor minerals present in some samples. Aluminum-rich grains (alumina spinel or alumina) were identified by EDS studies. Thin white mullite needles have been tentatively identified by petrographic and X-ray studies. Petrographic studies indicate that the individual minerals are embedded in a very fine-grained matrix (illite). The size and abundance of tektite glass and shocked debris indicate that Site 612 is relatively close to the impact crater. In an attempt to determine the provenance of the source rock and also define a probable location of the source crater, present day literature on the lithology of Eocene rocks along the North American continental margin was reviewed. In the southern portion of the Atlantic Ocean (south of Georgia), upper Eocene rocks are predominantly calcareous, which is not the nature of the rock fragments found in the tektite-bearing layer at Site 612. Rock fragments found at Site 612 have a similar lithology to rocks from the Banquereau Formation, in the northern portion of the Atlantic Ocean (Gulf of Maine, Georges Bank, Scotian shelf). In order to determine if the tektite glass was from two events, the major element composition of glass from different depth intervals within the layer was determined using energy dispersive X-ray analysis. The compositions show no systematic change with depth which indicates that the glass was from a single event. Based on the assemblage and mutual relationship of individual minerals in the rock fragments, it is concluded that the target material was a silty shale or mudstone. Coesite, stishovite, shock metamorphic features, and the presence of glass in some of the rock fragments indicate that the material is impact ejecta. It is concluded that the source crater for the North American tektites is either in the northern portion of the Atlantic Coastal Plain of North America (Gulf of Maine, Georges Bank, Scotian shelf, Southern New England) or as far south as Virginia. No definite conclusion can be made as to whether the tektite-bearing layer at Site 612 was redeposited.

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Expedition: 95

Site: 95-612

Provider: SEDIS Publication Catalogue

Data set link: http://sedis.iodp.org/pub-catalogue/index.php?id=1994-056801 (c.f. for more detailed metadata)

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