Petrography, Diagenetic Aspects, and Isotopes of the An Nagaza Member of the Al Khums Formation, NW Libya

Abstract

This study involves petrographic and geochemical analyses of 33 samples from the An Nagaza Member of the Al Khums Formation in NW Libya. The samples were categorized into five lithofacies types, primarily grain-supported, indicating deposition from a transgression over a Cretaceous and/or Triassic surface. The presence of various fossils and silisiclastic materials suggests a shallow marine environment. The complete succession of the An Nagaza sediments is interpreted as a transgressive sequence over the Cenomanian succession, transitioning from marginal marine to fully marine deposits. Several diagenetic events, including micritization, cementation, compaction, and dissolution, occurred in different diagenetic environments such as marine phreatic, mixed marine and meteoric, meteoric, and shallow burial environments. Low concentrations of Sr and Mn in the samples are attributed to diagenetic effects from meteoric solutions, while high Fe concentrations indicate the presence of clay minerals and quartz grains. Depleted or negative δ18O values suggest sea-level fluctuations during the Miocene, meteoric water influx, and recrystallization of early carbonate cements. Changes in δ13C values indicate shifts from restricted basin conditions to more marine conditions and back. Sr isotope analysis suggests an Early Miocene age for the An Nagaza Member of the Al Khums Formation.