Abstract: |
The Jurassic–Early Cretaceous was a time of variable organic carbon burial associated with
fluctuations of marine primary productivity, weathering intensity, and redox conditions in the pore
and bottom water at paleo-shelf areas in north Egypt. This time interval characterized the deposition
of, from old to young, the Bahrein, Khatatba, Masajid, and Alam El Bueib Formations in the north
Western Desert. Although several studies have been devoted to the excellent source rock units, such
as the Khatatba and Alam El Bueib Formations, studies on paleoenvironmental changes in redox
conditions, paleoproductivity, and continental weathering and their impact on organic carbon exports and their preservation for this interval are lacking. This study presents organic and inorganic
geochemical data for the Jurassic–Lower Cretaceous sediments from the Almaz-1 well in the Shushan Basin, north Western Desert. A total of 32 cuttings samples were analyzed for their major and
trace elements, carbonates, and total organic carbon (TOC) contents. Data allowed the reconstruction of paleoenvironmental conditions in the southern Tethys Ocean and assessment of the changes
in paleo-redox, paleo-weathering, and marine primary productivity, and the role of sediment supply. Additionally, factors that governed the accumulation of organic matter in the sediment were
interpreted. Results showed that the Khatatba Formation was deposited during a phase of enhanced
marine primary productivity under prevalent anoxia, which triggered enhanced organic matter
production and preservation. During the deposition of the Khatatba Formation, significant terrigenous sediment supply and continental weathering were followed by a limited contribution of
coarse clastic sediment fluxes due to weak continental weathering and enhanced carbonate production. The Bahrein, Masajid, and Alam El Bueib Formations were deposited during low marine primary productivity and prevalent oxygenation conditions that led to poor organic matter production
and preservation, respectively. A strong terrigenous sediment supply and continental weathering
predominated during the deposition of the Bahrein Formation and the lower part of the Alam El
Bueib Formation compared to the limited coarse clastic supply and continental weathering during
the deposition of the carbonate Masajid Formation and the upper part of the Alam El Bueib Formation. Such conditions resulted in the enhanced dilution and decomposition of labile organic matter, and, thus, organic carbon-lean accumulation in these sediments
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