Stable isotopic composition of the Upper Paleocene Tarawan Chalk, Eastern Desert, Egypt

The evaluation of the different diagenetic phases recognized in the Tarawan Chalk is based on a combined isotopic/petrographic approach. The chalk matrix (consisting primarily of pelagic biogenic and authigenic microcrystalline calcites) displays a wide range of relatively low delta(18)O values (+0.5 to -4.3parts per thousand PDB). These more negative oxygen isotope values than expected for marine Paleocene sediments, are related to early diagenesis that resulted in the formation of authigenic microcrystalline calcites and hence reducing the diagenetic potential of the studied chalk before suffering shallow burial. The low 813 C values (+1.6 to -4.3parts per thousand PDB) of the chalk matrix, coupled with the presence of pyrite, favors the assumption that the authigenic microcrystalline calcites were formed within a sulfate reduction zone. The diagenetic system during this early diagenetic stage had a high degree of openness with respect to both oxygen and carbon. Mild mechanical compaction, however, during a subsequent shallow burial stage resulted in the presence of frequent broken coccoliths. The low variability of the extremely low delta(18)O values (-9.0 to -11.1) parts per thousand PDB) of calcite mosaics filling fractures is consistent with the formation of these mosaics from meteoric waters in a diagenetic system with a high water/rock ratio. The low delta(13)C values of these fracture filling mosaics (-2.4 to -5.0parts per thousand PDB) indicate that light carbon was introduced into the system from the decay of organic matter.