深水砂体成因研究新进展

New perspectives on deep-water sandstones:Implications

在深水重力流研究中,传统浊流理论正受到砂质块体搬运和底流再改造砂体概念的冲击。陆架边缘沉积物滑塌是重力流驱动沉积过程最普遍的诱因,块体搬运过程(包含滑动、滑塌以及碎屑流)由于具有较高沉积物浓度(体积分数为25%～100%),表现为弹性和塑性特征。浊流因具有较低的沉积物浓度(体积分数为1%～23%)表现为黏性流动,不属于块体搬运的范畴。除重力流外,底流也是重要的深海搬运沉积方式。4种最常见的底流分别为:温盐流、风力驱动底流、深海潮汐及斜压流,牵引构造可用来区分这4种底流形成的再改造砂体,其中内波和内潮汐引起的斜压流再改造砂体的特征仍需深入研究。仅通过地震剖面与地质体形态无法识别块体搬运和底流再改造砂体类型,岩心和露头观察是唯一能鉴别其沉积相特征的方法。短期事件,如地震、海啸、热带风暴等,可能只持续几小时或者几天,却是沉积物滑塌的重要触发机制,比长达数千年低水位沉积期更为重要。

The emerging new concepts of sandy mass-transport deposits （SMTD） and bottom-current reworked sands （BCRS） have made a big impact on conventional turbidite concepts. Sediment failures near the shelf edge are the common cause of gravity-driven downslope processes. Mass-transport processes, which include slide, slump, and debris flow, exhibit elastic and plastic behaviors due to high sediment concentration （25%-100% by volume）. Turbidity currents which represent viscous fluid behavior with low sediment concentration （1%-23% by volume） are not mass-transport processes. Four common bottom currents are thermohaline, wind-driven, deep tidal, and baroclinic types. A distinctive attribute of BCRSs is their traction structures. However, depositional aspects of baroclinic currents associated with internal waves and tides are poorly understood. Seismic facies and geometries are unreliable for distinguishing types of SMTDs and BCRSs in the ancient record. The only reliable method of distinguishing a specific depositional facies is by detailed bed-by-bed examination of sedimentological features in core and outcrop. Short-term events that represent only a matter of hours or days （e.g., earthquakes, tsunamis, tropical cyclones, etc.） are more important in triggering sediment failures than periods of sea-level lowstands that represent thousands of years.