Deltas constitute complex depositional systems formed when a land-derived gravity-flow (carrying water and sediments) discharges into a marine or lacustrine standing body of water. However, the complexity of deltaic sedimentary environments has been oversimplified by geoscientists over the years, considering just littoral deltas as the unique possible type of delta in natural systems. Nevertheless, a rational analysis suggests that deltas can be much more complex. In fact, the characteristics of deltaic deposits will depend on a complex interplay between the bulk density of the incoming flow and the salinity of the receiving water body. This paper explores the natural conditions of deltaic sedimentation according to different density contrasts. The rational analysis of deltaic systems allows to recognize three main fields for deltaic sedimentation, corresponding to (1) hypopycnal (2) homopycnal and (3) hyperpycnal delta settings. The hypopycnal delta field represents the situation when the bulk density of the incoming flow is lower than the density of the water in the basin. According to the salinity of the receiving water body, three different types of hypopycnal littoral deltas are recognized: hypersaline littoral deltas (HSLD), marine littoral deltas (MLD), and brackish littoral deltas (BLD). The basin salinity will determine the capacity of the delta for producing effective buoyant plumes, and consequently the characteristics and extension of prodelta deposits. Homopycnal littoral deltas (HOLD) form when the density of the incoming flow is roughly similar to the density of the water in the receiving basin. This situation is typical of clean bedload-dominated rivers entering freshwater lakes. Delta front deposits are dominated by sediment avalanches. Typical fallout prodelta deposits are absent or poorly developed since no buoyant plumes are generated. Hyperpycnal deltas form when the bulk density of the incoming flow is higher than the density of the water in the receiving basin. The interaction between flow type, flow density (due to the concentration of suspended sediments) and basin salinity defines three types of deltas, corresponding to hyperpycnal littoral deltas (HLD), hyperpycnal subaqueous deltas (HSD), and hyperpycnal fan deltas (HFD). Hyperpycnal littoral deltas are low-gradient shallow-water deltas formed when dirty rivers enter into brackish or normal-salinity marine basins, typically in wave or tide-dominated epicontinental seas or brackish lakes. Hyperpycnal subaqueous deltas represent the most common type of hyperpycnal delta, with channels and lobes generated in marine and lacustrine settings during long-lasting sediment-laden river-flood discharges. Finally, hyperpycnal fan deltas are subaqueous delta systems generated on high-gradient lacustrine or marine settings by episodic high-density fluvial discharges.

中文翻译：

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Deltas：扩展 Bates 概念的新分类

当陆源重力流（携带水和沉积物）排放到海洋或湖泊静水体中时，三角洲构成了复杂的沉积系统。然而，多年来，地球科学家将三角洲沉积环境的复杂性过度简化，将沿海三角洲视为自然系统中独特的可能类型的三角洲。然而，理性分析表明增量可能要复杂得多。事实上，三角洲沉积物的特征将取决于流入流的体积密度和接收水体的盐度之间复杂的相互作用。本文根据不同的密度对比探讨了三角洲沉积的自然条件。三角洲系统的合理分析允许识别三角洲沉积的三个主要领域，对应于 (1) 低密度 (2) 高密度和 (3) 高密度 delta 设置。低密度 delta 场代表流入流的体积密度低于盆地中水的密度时的情况。根据受纳水体的盐度，识别出三种不同类型的低密度滨海三角洲：高盐度滨海三角洲（HSLD）、海相滨海三角洲（MLD）和微咸滨海三角洲（BLD）。盆地盐度将决定三角洲产生有效浮力羽流的能力，从而决定前三角洲沉积物的特征和延伸。当流入水流的密度与接收盆地中水的密度大致相似时，就会形成同密度滨海三角洲 (HOLD)。这种情况是进入淡水湖的以清洁床负荷为主的河流的典型情况。三角洲前缘沉积物以沉积物雪崩为主。由于没有产生浮力羽流，典型的沉降前三角洲沉积物不存在或发育不良。当流入流的体积密度高于接收盆地中水的密度时，就会形成高密度三角洲。流动类型、流动密度（由于悬浮沉积物的浓度）和盆地盐度之间的相互作用定义了三种类型的三角洲，对应于高密度滨海三角洲 (HLD)、高密度水下三角洲 (HSD) 和高密度扇三角洲 (HFD)。高密度滨海三角洲是当肮脏的河流进入微咸或正常盐度的海洋盆地时形成的低梯度浅水三角洲，通常在波浪或潮汐主导的大陆表海或咸水湖中。高密度水下三角洲是最常见的高密度三角洲类型，在长期充满沉积物的河流洪水排放过程中，在海洋和湖泊环境中形成了河道和裂片。最后，超重扇三角洲是在高梯度湖泊或海洋环境中由间歇性高密度河流排放产生的水下三角洲系统。