Chemostratigraphy of the Upper Cretaceous-Paleogene organic-rich succession on the southern Tethys margin in Egypt

Abstract

During the Late Cretaceous-Paleogene time, a widespread sedimentary organic-rich succession developed throughout the middle latitudes of Egypt. Since the study area conserves the organic-rich intervals three shallow boreholes were drilled through the Upper Cretaceous-Paleogene succession on the southern Tethys margin of Egypt. This work employs complete unpublished inorganic and organic geochemical data of 368 metric core samples to construct the chemostratigraphic framework of the studied sedimentary sequence. The paleoenvironmental conditions that prevailed at the time of deposition were discussed. A total of 368 samples were analyzed for 52 major and trace elements by Inductively Coupled Plasma Mass Spectrometry and for organic carbon concentrations by LECO C230 combustion furnace.The statistical treatment suggested five influential controlling factors during the deposition of the studied sedimentary succession. These factors express the influence of the terrestrial controls, phosphorite, anoxia, argillaceous marine sediments and/or marine shale and earliest diagenesis, in descending order of their relative strength. According to the microscopic examination supported by the chemical composition of the major oxides, seven lithofacies were defined; carbonate (limestone), dolomitized rocks, argillaceous limestone (marlstone), calcareous shale, siltstone-mudstone, phosphorites, and phosphatic rocks. Moreover, the diagnostic trace element content of each geological formation was assessed. Some of the obtained geochemical features were adopted as reliable chemostratigraphic indicators and to discriminate among the different depositional environments of the studied sequence.