FAN Caiwei,born in 1973,is a professor-level senior engineer. He is mainly engaged in integrated geological studies of oil and gas,as well as technical management in hydrocarbon exploration and development. E-mail: fancw@cnooc.com.cn.
The Zhujiang Formation(23-16 Ma) in Zhu Ⅲ Depression of Pearl River Mouth Basin developed during the rapid marine transgression in Early Miocene,documenting a critical depositional transition between the marine-continental transitional deposits of the Oligocene Zhuhai Formation and shallow-marine deposits of the Hanjiang Formation. However,sedimentary facies types,depositional evolution,and controlling factors during this transgression remain poorly constrained,limiting regional sedimentological interpretation and hydrocarbon exploration. This study integrates seismic,well-log,core,thin-section,and paleontological data to analyze sedimentary facies,depositional environments,and their controlling factors. Low foraminiferal abundance and diversity indicate a coastal sedimentary environment of the Member 2 of Zhujiang Formation,where as a gradual increase in abundance and diversity in the lower part of Member 1 reflects a shift to inner-shelf settings. Higher abundance and diversity of foraminifera in the upper part of Member 1 further indicate a transition to middle-outer shelf environments. Although water depth increased during the upper Member 1 deposition,strong sediment supply resulted in tidal-influenced deltaic systems prevailing in Member 2 and the lower part of Member 1. These deltas exhibit progradational patterns oriented from northwest to southeast,with distinct distributary-channel,mouth-bar,and interdistributary-bay microfacies. Abundant bioclasts and significant suspended mud indicate tidal rather than wave dominance,as wave dominated deposits typically show uniform grain size,good sorting,high rounding,and minimal mud content. Nevertheless,mouth bars exhibit evidence of both wave and tidal processes,highlighting complex hydrodynamic conditions. In the upper part of Member 1,depositional environment transitioned fully to middle-outer shelf settings characterized by tidal sand ridges and muddy shelf deposits dominated by silt and clay,reflecting significantly reduced sand input. The evolution of these sedimentary systems was primarily controlled by steadily rising sea levels,gradually decreasing sediment supply,and paleotopography,causing the depositional environment to shift from sand-rich deltaic systems to silt-and mud-dominated tidal sand-ridge and shelf-mud systems. Tidal sand ridges developed extensively but mainly occupied local paleotopographic highs. The findings provide insights into depositional evolution and controlling factors of the Zhujiang Formation around the Qionghai uplift,enhancing the understanding of sedimentary processes during rapid marine transgression.
| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |