144 / 2022-02-26 21:16:40

Lacustrine carbonate depositional model of the Upper fourth Member of Shahejie Formation in Eocene Dongying depression

Lacustrine carbonate, Sedimentary system, Sedimentary model, Geomorphology, East-Asian paleo-monsoon

Thick lacustrine carbonates are developed in the Upper fourth Member of Shahejie Formation (Es4s) in the western Dongying depression, which are always considered as reef deposits overlying on a structural highland, but the details have not been detailedly described and analyzed. Seismic geomorphologic and sedimentologic approaches are conducted to provide a comprehensive understanding of carbonate depositional model in lake basins. Seismic reflection profiles that cut through the tectonic highland reveal that its margins are restricted by major syn-sedimentary faults. To the south, it is directly adjacent with Pingnan fracture zone whose footwall is deep Boxing sag, while the east and west areas contain 1-4 km-width stepped faults as transitional zones transfering gradually into Lijin sag and Lizezhen sag. Based on the interpretation results of seismic horizons, the top surface data of Es4s carbonates are processed via the 3D-Analysis tool of Geomap software in order to obtain the key topographic parameters. Buried depth, topographic slope and their combination with faults are used to analyze the structural highland, and results show that the study area is subdivided into six districts, including two relative flat-topped areas (PFW and SD) and their respective transition zones into deep sags (P1, P2 and S1, S2).

Eight types of lithofacies and five types of their associations are identified based on cores and thin sections, and the sedimentary facies such as reef-shoal, reefal mound, shoal/intershoal, storm deposit and sand/mud flat are recognized in littoral to sublittoral (i.e. shoreface to offshore) environment. Reef-shoal facies is mainly composed of Cloadosiphonia framestone, bafflestone, thrombolite and grainstone to packstone, while shoal/intershoal facies are mainly composed of grainstone-packstone-wackastone, and both of the two facies are mainly distributed in flat topographic areas. Reefal mound facies comprises non-skeleton thrombolite, stromatolite, a minor amount of grainstone and packstone, and a large amount of wackestone and marlstone, which is mainly developed in upper zones of step-fault areas. Storm deposit facies mainly consists of rudstone or floatstone and a large amount of packstone, wackestone and marlstone, and occurs mainly in the step-fault zones facing the east and south. Sand/mud flat facies are featured by arenaceous limestone, marlstone and mudstone that reflect siliciclastic input from the Linfanjia uplift.

Above results show that geomorphology controls the change of sedimentary environment. From NW-SE oriented seismic profiles, the overall landform presents a feature of high - low - high - low, crossing two highlands and three transitional zones. Well cross-sections show that the upper to middle littoral facies are mainly distributed in two flat topped areas of SD and PFW, including reef-shoal and shoal/intershoal facies. In contrast, the lower littoral to sublittoral facies are consistent with step-faulted transitional zones where storm deposits and reefal mounds locally developed. Therefore, the geomorphology-controlled sedimentary model of lacustrine carbonates is established in western Dongying depression. Tectonic activities were relatively inactive during the Es4s period, and differential landforms on the structural highland determined water hydrodynamic conditions and eventually controlled the formation and distribution of lacustrine carbonate facies. In addition, the East-Asian paleo-monsoon climate began to appear in the Eocene period so that the prevailing southeast wind could strengthen the hydrodynamic zoning that formed by previously existed geomorphological zones. The wind-wave effect further promoted the junction of reef-shoal and shoal facies into one larger areas, which will provide important guiding significance for oil and gas exploration.