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Geochemistry and palynology of the upper Albian at the Abu Gharadig Basin, southern Tethys: Constraints on the oceanic anoxic event 1d
Author(s) -
Mansour Ahmed,
Gentzis Thomas,
CarvajalOrtiz Humberto,
Tahoun Sameh S.,
Wagreich Michael
Publication year - 2020
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3810
Subject(s) - total organic carbon , geology , anoxic waters , palynology , organic matter , sedimentary depositional environment , sedimentary rock , geochemistry , palynofacies , paleontology , weathering , structural basin , oceanography , environmental chemistry , chemistry , pollen , ecology , biology , organic chemistry
The mid‐Cretaceous period was a time of abnormal polar temperature warming and consequent global oceanic anoxia that enhanced the widespread accumulation of organic matter‐rich marine sediments. The current contribution focuses on the late Albian oceanic anoxic subevent 1d (OAE 1d) that represents a period of increased perturbation of the global carbon cycle. Palynological and geochemical investigations, including TOC/Rock‐Eval pyrolysis and stable carbon isotope of organic matter ( δ 13 C TOC ), were conducted for the upper part of the Kharita Formation from the Abu Gharadig Basin, north Western Desert, Egypt, to (a) confirm the age of this unit, (b) assess the depositional environments and prevalent redox conditions, and (c) confirm the occurrence of the OAE 1d within the sedimentary record in this part of the southern Tethys. Based on the TOC content and Rock‐Eval pyrolysis parameters, the organic matter in the upper Kharita Formation is expected to have a limited hydrocarbon generation potential consisting of gas only. The sedimentation patterns and role of continental weathering and runoff influx were assessed through trace element ratios and relationships. Redox conditions were interpreted based on S total –TOC relationship, S total –TOC–Fe total ternary plot, Rock‐Eval S 2 , trace element proxies and relationships (including Cr vs. V + Ni, V/Cr vs. S total , and S total /Fe vs. Mn/S total ). The ratio of TOC/P total was implemented to assess both marine primary productivity and oceanic anoxia during this period. Biostratigraphic constraints showed that the upper part of the Kharita Formation spanned a late Albian age ( Cretacaeiporites densimurus Interval Zone). Results showed that the OAE 1d was characterized by organic matter deposition during dysoxic‐suboxic to anoxic redox conditions in a fluvio‐deltaic to shallow marine depositional environment in this part of the Tethys. A low to moderate primary productivity was also prevalent during a fairly high rate of sediment accumulation. Regional correlation of positive δ 13 C TOC excursion during the OAE 1d throughout the Tethys and Atlantic oceans matched well, in several parts, with the current study interval that confirm the global distribution of the OAE 1d. In this region of the southern Tethys, an overall collapse of water column stratification during the OAE 1d principally related to decreased terrestrial input at low relative sea level, as evidenced from freshwater algae Pediastrum and pteridophyte spore Crybelosporites pannuceus ; however, orbital forcing likely induced monsoonal activity at low‐latitudes in the SE, N, and W Tethys.