In this study, based on seismic profiles, complemented with boreholes and well logs, two types of deltas are recognized in the Paleogene Qiongdongnan Basin, namely fan deltas and braided river deltas, and their temporal–spatial evolution characteristics and associated controlling factors are studied. The results show the following: (1) The base-level cycle and episodic rifting lead that the characteristics of the basin-marginal deltas vary among different system tracts, third-order sequences, and second-order sequences or rifting stages, i.e. the temporal evolution of the basin-marginal deltas. Among the different system tracts in each third-order sequences, the deltas in the lowstand system tracts are vertically thick with small horizontal extension, while those in the transgressive system tracts present retrogradation with limited vertical thickness and horizontal extension, and those in the highstand system tracts turn to present large extension and develop toplap points. Among the different third-order sequences in each rifting stage, the extension of the deltas presents a pattern “small-medium-large” from bottom to top. Among different rifting stages, the deltas also exhibit different characteristics in terms of extension and thickness, and even the type may change. (2) The paleogeomorphology controls the characteristics of the deltas in lateral space, which is mainly reflected by the fact that the gradient of the paleogeomorphology influences the scale and type of the deltas. This study will enhance our understanding of the relative influence of the controlling factors exerted on fan or braided river deltas, and aid in the exploration of the coal-measure source rocks in the Qiongdongnan Basin, which is a typical basin lacking boreholes, located in the South China Sea.

中文翻译：

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南海西北部琼东南盆地古近系盆地边缘三角洲特征及形成机制

本研究以地震剖面为基础，辅以钻孔和测井资料，在琼东南盆地古近系中识别出扇三角洲和辫状河三角洲两类三角洲，并对其时空演化特征及相关控制因素进行了研究。结果表明： (1) 基底旋回和幕式裂谷导致盆地边缘三角洲的特征在不同体系域、三级层序和二级层序或裂谷阶段之间存在差异，即时间盆地边缘三角洲的演化。在各个三阶层序的不同体系域中，低位体系域三角洲垂直厚，水平伸展小，海侵体系域退积，垂向厚度和水平伸展有限，高水位体系域则转为大伸展，发育顶叠点。在各个裂谷期不同的三阶层序中，三角洲的伸展自下而上呈现“小—中—大”格局。在不同的裂谷阶段，三角洲在伸展和厚度方面也表现出不同的特征，甚至类型可能会发生变化。(2)古地貌控制三角洲横向空间的特征，主要体现在古地貌梯度影响三角洲规模和类型。