The marine palynology of the Upper Cretaceous Abu Roash ‘A’ Member in the BED 2-3 borehole, Abu Gharadig Basin, Egypt

This study presents a high-resolution palynological analysis for the Abu Roash ‘A’ Member in the hydrocarbon-rich Abu Gharadig Basin, North Western Desert, Egypt. Thirty-one rock samples, spanning the upper Coniacian to lower Campanian Abu Roash ‘A’ Member (180 m) from the Badr El Din (BED) 2-3 borehole, were investigated. Highly diverse and well-preserved assemblages of organic-walled dinoflagellate cysts were recorded. A total of 220 species belonging to 123 genera were identified from different levels throughout the studied succession. The marine dinoflagellate cysts represented herein by 160 species allowed three palynological assemblage zones to be recognised. These include the <i>Satyrodinium haumuriense-Isabelidinium belfastense-Odontochitina operculata</i> Assemblage Zone (early Campanian), <i>Dinogymnium acuminatum-Nelsoniella aceras-Odontochitina porifera</i> Assemblage Zone (Santonian), and <i>Cyclonephelium filoreticulatum</i>-<i>Spinidinium echinoideum</i> Assemblage Zone (late Coniacian). Approximately 17 species representing 14 dinoflagellate cyst genera are identified for the first time in the Cretaceous record of Egypt. A comparison to published palynological data from neighbouring areas reveals that several significant events are synchronous throughout the Western Desert, suggesting a common similarity of water masses. Palynofacies and palynomorph analyses of the whole samples led us to define two palynofacies assemblages, which indicate that the Abu Roash ‘A’ Member was deposited during a successive oscillation of sea level from distal inner to middle neritic conditions. A sequence stratigraphic framework assessment was carried out to trace the relative response of the sedimentological organic matter distribution to the changes of sea level. Four third-order transgressive–regressive sequences (SQ2–SQ5) and two incomplete transgressive–regressive sequences (SQ1 and SQ6) are constructed through the Abu Roash ‘A’ Member.

本研究针对埃及西北沙漠富烃阿布格拉迪格盆地（Abu Gharadig Basin）内的阿布鲁阿什‘A’段（Abu Roash ‘A’ Member）开展高分辨率孢粉学（palynological）分析。本次研究对巴德尔丁（Badr El Din）2-3钻孔（简称BED 2-3）中，对应上康尼亚阶（Coniacian）至下坎潘阶（Campanian）、总厚度180米的阿布鲁阿什‘A’段的31件岩石样品展开了系统调查。研究记录了多样性极高且保存完好的有机壁甲藻孢囊（organic-walled dinoflagellate cysts）组合，在所研究沉积序列的不同层位共鉴定出隶属于123个属的220个物种。其中160个物种隶属于海洋甲藻孢囊类群，据此可识别出3个孢粉组合带：分别为<i>Satyrodinium haumuriense-Isabelidinium belfastense-Odontochitina operculata</i>组合带（早坎潘期）、<i>Dinogymnium acuminatum-Nelsoniella aceras-Odontochitina porifera</i>组合带（桑托期（Santonian）），以及<i>Cyclonephelium filoreticulatum</i>-<i>Spinidinium echinoideum</i>组合带（晚康尼亚阶（Coniacian））。另有14个属共17个物种为埃及白垩纪地层记录中首次报道的类群。将本次研究数据与邻区已发表的孢粉学资料对比后发现，西沙漠区域存在多个同步发生的重要生物事件，指示区域内水团具有相似特征。通过对全部样品的孢粉相（palynofacies）与孢粉形态（palynomorph）分析，本研究定义了2个孢粉相组合，表明阿布鲁阿什‘A’段沉积时期海平面经历了从远岸内浅海相至中浅海相的持续振荡。本次研究构建了层序地层（sequence stratigraphic）格架，以探究沉积有机质分布对海平面变化的相对响应；基于该格架，在阿布鲁阿什‘A’段中共识别出4个三级海侵-海退旋回层序（SQ2~SQ5）以及2个不完整的海侵-海退旋回层序（SQ1与SQ6）。