Geochemistry, mineralogy, and paleoenvironments of deposition of the Oil Shale Member in the Negev

The Oil Shale Member has been defined as the organic-rich lithofacies of the lower part of the Upper Campanian-Maastrichtian Ghareb Formation (Shahar, 1967). Study of hundreds of samples, mostly from subsurface, many of them composite, from the Oil Shale Member in three large Negev basins (Mishor Rotem-Mishor Yamin, Oron, and Nahal Zin) indicates a great chemical and mineralogical similarity in the organic-rich sediments of these three basins, implying that good connections prevailed among the Negev synclinal basins and between them and the open sea during the Oil Shale Member deposition. Factor analysis of some 120 chemical analyses (majors, minors, and traces) led to 4 significant factors which explain 77% of the variance. The predominant factor, clays and total organic carbon (TOC), explains 37% of the variance. Two other factors (2 and 4) relate to the disoxic conditions which prevailed during the sedimentation of this member. Discriminant analysis was carried out on two database, a larger one (∼350 samples) for major elements and a smaller one (∼120 samples) for major, minor, and trace elements. The results of both point to a division of the Oil Shale Member into (1) a lower unit rich in TOC (≥10%), high in authigenic minerals, such as apatite and quartz, and low in detrital ones (clays); and (2) an upper unit lower in TOC (less than 10%), richer in detrital minerals (clays), and poorer in authigenic minerals (apatite, quartz). The calcite content remains more or less constant along the whole member. There also appears to be a biostratigraphic control to this division. The main reason for the differences between the lower and upper units of the Oil Shale Member appears to be a decrease in surface productivity, coupled with an increase in detrital components in the upper unit. The changes were probably the result of variations in the Tethys currents during the Early Maastrichtian. We interpret the upper Mishash-lower Ghareb transition mainly as the result of a relative sea level rise, combined with local tectonics which created a stronger relief with deeper basins and shallower highs. The deeper bathymetry led bottom water anoxia-dysoxia to expand and thus enhanced organic matter preservation together with an increase of benthic regeneration of phosphorus from the organic matter. This caused a considerable increase in organic matter and a sharp decrease of P in the sediments of the lower Ghareb. The Oil Shale Member was deposited only in the basins (synclines) because it is a "high stand" sediment. Gradually the relief diminished and normal oxic open marine conditions were reestablished.