过渡型流体转换对洪水型重力流沉积研究的启示及地质意义

洪水型重力流是重力流沉积学的研究热点，以往研究认为洪水型重力流具有紊流支撑的流体性质，对于其流体性质转化及其沉积记录的识别不够深入。近年研究揭示重力流沉积过程中可形成多种过渡型流体，具有特殊流体转换机制和沉积特征。通过调研国内外最新文献，系统介绍了过渡型流体基本特征、沉积机制方面的研究进展，并讨论了其对洪水型重力流沉积研究的启示及地质意义。研究结果表明：在少量黏土矿物影响下，沉积物重力流流体的性质即可由紊流向层流转化，形成特殊的过渡型流体。转化过程主要取决于黏土矿物含量和类型控制的流体内聚力和流速控制的紊流应力二者之间的相互作用。过渡型流体可以产生大型流水沙纹（large current ripple）、砂质纹层—泥质纹层间互形成的低幅度沙波（low amplitude bed wave）等独特的底床类型。尽管实验研究揭示了过渡型流体可能形成的沉积底床特征，针对洪水型重力流沉积记录中过渡型流体的解释仍十分缺乏，尤其是过渡型流体转换机制及其沉积响应仍亟待深入探索。过渡型流体的沉积底形是研究洪水型重力流沉积动力机制的重要载体，可为深入理解洪水型重力流沉积过程提供新视角，同时可能具有更广泛的沉积学研究价值。

Transitional flow transformation mechanism:Implication for hyperpycnal flow sedimentary research and geological significance

Flood- generated gravity flow (hyperpycnal flow) is the research focus of gravity flow sedimentology in recent years. In the past, the sedimentary dynamic interpretation of hyperpycnal flow was mainly based on the fluid properties supported by turbulence. The possible transformation of its fluid properties and identification of related sedimentary records is still a question for us. Recent studies have revealed that a variety of transitional flows can be formed during the deposition process of gravity flow, with special flow transformation mechanisms and deposition characteristics. Methods:Through the investigation of the literature review, the research progress on the basic characteristics and depositional mechanism of transitional flows is introduced. And, its geological significance for the study of flood- generated gravity flow is also discussedResults: The results show that: under the influence of a small amount of clay, the nature of sediment gravity flow can be transformed from turbulent flow to laminar flow. The transformation of transitional flow mainly depends on the interaction between the clay mineral content related fluid cohesion and the turbulent force controlled by the velocity. During this process, a variety of transitional flows could be formed. Unique bedform types such as large current ripple, low amplitude bed wave with sandy laminae and clay laminae could be generated. Although the experimental study reveals the possible bedform characteristics of transitional flows, the interpretation of transitional flows in the subsurface ancient hyperpycnal flow sedimentary records is still lacking. The transformation mechanism of transitional flows and their sedimentary response in hyperpycnites need to be further explored. The latest research reveals that the special sedimentary bedform related to the transitional flows can be identified in the sedimentary records. The transitional flow deposition could give implications for the sedimentary mechanism of flood - generated gravity flows. Conclusions: Transitional flow sedimentology could provide a new perspective for further understanding the fluid transformation and sedimentary response in the process of gravity flow deposition. And, transitional flow may have a broader range of significance in sedimentary geology research.