基于沉积过程分析的扇三角洲沉积模式研究:以滦平盆地桑园剖面下白垩统西瓜园组为例

Depositional model of fan deltas based on analysis of sedimentary processes:a case study of the Lower Cretaceous Xiguayuan Formation in Sangyuan outcrop of Luanping Basin

洪水类型与流量变化是影响扇三角洲地貌形态与沉积特征的主要因素,但不同类型洪水作用下扇三角洲沉积单元发育规律研究薄弱。论文以滦平盆地桑园剖面下白垩统西瓜园组为例,通过剖面测量和岩相划分,恢复了沉积流体类型、沉积过程及其沉积响应单元,建立了不同洪水作用下扇三角洲沉积模式。根据岩相组合将该剖面的主控沉积过程解释为碎屑流、洪流与片流3种洪水,分别对应于碎屑流建造期、洪流建造期与片流建造期。结合沉积过程与砂砾岩沉积形态特征,将目的层解释为8种沉积单元,具体包括扇三角洲平原环境的陆上碎屑流舌状体、洪流主控辫状河道和扇三角洲前缘的水下碎屑流舌状体、高密度浊流水道、牵引流水下分流河道、高密度浊流朵体、水下片流朵体及滑塌体。在扇三角洲碎屑流建造期,发育多期垂向上叠置的、向湖盆进积的碎屑流舌状体;在洪流建造期,平原中形成洪流主控辫状河道,而进入湖盆后洪流转化为高密度浊流,发育高密度浊流水道与朵体;在片流建造期,则以水下快速沉积形成的水下片流朵体为特征。洪水期扇三角洲前缘沉积物快速沉积和沉积物坡失稳易于形成滑塌体,而间洪期则以平原上辫状河道和前缘中低流态水下分流河道的发育为特征。基于洪水类型及其沉积过程的扇三角洲沉积规律的研究,深化了对其砂体形态与结构规律的认识。

Flood flow types and variable discharge are the critical controls on fan delta morphology and depositional characteristics.However,characteristics of depositional units formed by variable flow types are still poorly understood.This study focuses on the well-exposed Sangyuan outcrop of the Lower Cretaceous Luanping Basin,which was measured in detail.Based on the classification of lithofacies,flow types and depositional processes are interpreted,with corresponding depositional units determined.Deposi-tional models are constructed in response to various types of flood flow.Three types of flood flow are identi-fied based on the classification of lithofacies associations,containing debris flow,hyperconcentrated flood flow and sheet flood flow.According to the processes and geometry,eight types of depositional units are i-dentified,including subaerial debris-flow tongue deposits,hyperconcentrated flood flow-dominated brai-ded channels,subaqueous debris-flow tongue deposits,high-density turbidite channels,subcritical-flow dominated subaqueous distributary channels,high-density turbidite lobes,subaqueous sheet flood domi-nated lobes,and slump deposits.During the debris flow-dominated phase of fan delta development,multi-storey debris-flow tongue deposits gradually prograded into the lacustrine basin.In the hyperconcentrated flood flow-dominated phase,braided channels were formed in the fan delta plain,while high-density tur-bidity currents,transformed from hyperconcentrated flood flow,generated high-density turbidite channels and lobes in the fan delta front.During the sheet flood flow-dominated phase,unconfined sheet flood flow is developed when flood flows exit the incised channels,generating subaqueous sheet flood dominated lobe in the fan delta front.The rapid deposition of sandstones in the fan delta front during flood flow stage leads to the instability of sediment and the development of slumps.During base flow stage,subaerial braided channels and subaqueous distributary channels dominated by subcritical-flow structures are developed.This study sheds light on the sandbody geometry and depositional architecture of fan delta formed by variable types of flood flow.