چينه نگاري رسوبات كنياسين-مائستريشتين و نگرش ويژه بر مرز كرتاسه- پالئوژن در تاقديسهاي لار و زردشهنه، زون ايذه (جنوب غرب ايران)

Stratigraphy of the Coniacian–Maastrichtian sediments an‎d special attention to Cretaceous–Paleogene boundary in Lar an‎d ZardShehneh anticlines, Izeh zone (southwest of Iran)

زمين شناسي

The Gurpi Formation, deposited during the Late Cretaceous, is a pelagic succession composed of shale an‎d marly limestone rich in planktonic foraminifera. Within parts of the Zagros forelan‎d basin, this formation is of particular significance as it serves both as a cap rock an‎d as a hydrocarbon source rock. The occurrence of the Cretaceous-Paleogene boundary in its uppermost interval further underscores its scientific importance. Accordingly, detailed investigations of its biostratigraphy, bioevents, an‎d paleoenvironmental variations are essential. This study focuses on planktonic foraminiferal assemblages from the Zardashneh an‎d Lar anticlines in southwestern Iran. Analysis of 125 samples, using thin sections an‎d washed residues, led to the identification of 30 genera, 88 species, an‎d 10 biozones within the Gurpi Formation, as well as 7 genera, 12 species, an‎d one biozone in the basal part of the Pabdeh Formation at the Zardashneh section. At the Lar section, 34 genera, 104 species, an‎d 9 biozones were recognized in the Gurpi Formation, alongside 6 genera, 11 species, an‎d one biozone in the basal Pabdeh Formation. Key bioevents identified in the Gurpi Formation include: 1) the appearance of Globotruncana linneiana as an indicator of the Coniacian-Santonian boundary at the base of the formation; 2) the extinction of Dicarinella asymetrica an‎d most marginotruncanids at the Santonian-Campanian boundary; 3) the early appearance of Globotruncana aegyptiaca; 4) recognition of the Campanian-Maastrichtian boundary within the lower part of the Gansserina gansseri Zone; an‎d 5) five abundance events. The extinction of keeled taxa began in the upper part of the Abathomphalus mayaroensis Zone during the Late Maastrichtian, coinciding with the proliferation of elongate-chambered species. The high-resolution biostratigraphic framework identified in this study allows more precise age determination of the Gurpi Formation an‎d facilitates correlation of paleoenvironmental changes an‎d depositional sequences between the Zardashneh an‎d Lar sections, as well as with other regional an‎d global records. Sedimentological an‎d micropaleontological analyses enabled recognition of four paleoenvironmental intervals: 1) Santonian to early Campanian, characterized by oligotrophic conditions under warm an‎d arid climate; 2) early to late Campanian, marked by cooling, increased turbidity, an‎d tectonic influences; 3) late Campanian, characterized by transgressive sedimentation under intensified Tethyan currents; an‎d 4) Maastrichtian, dominated by mesotrophic to eutrophic conditions, oxygen-deficient conditions, an‎d ocean acidification, largely attributable to upwelling an‎d volcanic activity of the Deccan an‎d Ninety-East Ridges. In the studied sections, the Cretaceous-Paleogene boundary, as the upper limit of the Gurpi Formation, is characterized by an unconformity an‎d sedimentary condensation. Possible controlling factors for this unconformity an‎d the lack of preservation of the Chicxulub impact ejecta layer in the Zardshehneh an‎d Lar sections include low sedimentation rates, carbonate dissolution, erosion by oceanic currents during global cooling intervals, gravity currents, the absence of thick sedimentary successions after the Cretaceous-Paleogene boundary, an‎d tectonic activity linked to the closure of the Tethys. Overall, the sequence boundaries recognized in the studied sections correlate with global sequence boundaries, emphasizing eustasy as the principal control on sedimentary dynamics in the Zagros Basin.

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