强制水退背景下湖盆古水深变化特征探讨: 以涠西南凹陷西部渐新世早期湖盆为例

强制水退在地质历史时期普遍存在, 但由于物源供应不匹配, 地质记录较少, 研究案例更少。以南海西北部北部湾盆地涠西南凹陷渐新世早期涠四段湖盆为研究对象, 利用地震反射结构法开展了湖盆古水深恢复研究, 明确了强制水退背景下古水深变化特征, 并进一步探讨了利用三角洲前积结构法研究古水深的恢复方法。研究表明, 渐新世早期涠四段沉积时, 涠西南凹陷物源充足, 周缘发育大型下切河谷, 凹陷内发育大型斜交前积反射结构, 具有典型的强制水退特征, 形成了较大规模的强制水退体系域。通过对典型前积剖面的精细解剖, 识别出了强制水退体系域中的6个典型前积反射层。结合地震、钻井与砂泥岩压实系数, 恢复出湖盆古水深介于111.2~286.5 m, 平均为218.5 m。自湖盆边缘向中心方向, 前积层的高度和倾角呈先缓慢增大后快速减小特征, 表明湖盆古水深具有相似的变化特征。其中水深方面早期在230 m左右, 至中晚期逐步加深至280 m左右, 末期快速下降至110 m, 相较最大水深变化幅度达61.5%;前积层倾角早期在10°左右, 至中期达14.5°后逐步下降, 末期快速变化至2.9°, 倾角变化幅度达79.8%。结合区域地质研究认为涠四段沉积时经历的地质时期小于2 Ma, 古水深与前积层倾角在不到2 Ma内快速剧烈变化, 与强制水退特征基本一致, 印证了地震反射结构的响应特征。从理论模型和恢复参数优化方面探讨了利用三角洲前积结构进行古水深恢复的可行性与未来研究方向。上述研究成果为丰富和完善强制水退背景下湖盆古水深变化特征的认识提供了重要参考。 

Discussion on the variation characteristics of paleo-water-depth of lake basin under the background of forced regression: An example in the Early Oligocene of west of Weixi'nan Sag

Forced regression is common during geological history, but due to mismatched sources andsupplies, there are fewer geological records and fewer research cases. Taking the 4th member of Weizhouformation in the Early Oligocene of Weixi'nan Sag in the Beibu Gulf Basin in the northwest of the South China Sea as the research object, the restoration of paleo-water-depth is carried out by using the seismic reflectionstructure method, the variation characteristics of paleowater depth under the background of forcedregression are clarified, and the restoration method of paleowater depth by using the delta progradationstructure method is further discussed. The research shows that during the deposition of 4th member of Weizhou Formation in the Early Oligocene, the provenance of Weixi'nan Sag was sufficient, a large incisedvalley was developed around it, and a large oblique progradation reflection structure was developed in thesag, which has typical Forced Regression characteristics, forming a large-scale forced regression systemtract. Through the fine dissection of typical progradation sections, six typical progradation reflection layersare identified. Combined with seismic, drilling and compaction coefficient, the paleo-water-depth of thelake basin is restored to be 111.2-286.5 m, with an average of 218.5 m. From the edge of the paleo-laketo the center, the height and dip angle of the progradation layers increase slowly and then decrease rapidly.In terms of water depth, it is about 230 m in the early stage, gradually deepened to about 280 m in themiddle and late stage, and rapidly decreased to 110 m in the last stage, with a change range of 61.5% comparedwith the maximum water depth; The dip angle of progradation layer is about 10° in the early stage, gradually decreases after reaching 14.5° in the middle stage, and changes rapidly to 2.9° in the final stage, with a change range of 79.8%. Combined with the regional geological research, it is considered that thegeological period experienced during the deposition of 4th member of Weizhou Formation is less than 2 Ma, and the paleowater depth and paleodip angle of progradation layers change rapidly and violently in lessthan 2 Ma, which is basically consistent with the characteristics of forced regression, which confirms theresponse characteristics of seismic reflection structure. Further, the feasibility and future research directionof paleo-water-depth restoration using delta progradation structure are discussed from the perspectiveof theoretical model simplification and restoration parameter optimization. The research results provide animportant reference for enriching the understanding of the variation characteristics of paleowater depth underthe background of forced regression.