Asymmetry in Holocene river deltas: Patterns, controls, and stratigraphic effects

详细信息    查看全文

• 作者：Jesse T. Korus^a ; ^b ; ^{jkorus3@unl.edu" class="auth_mail" title="E-mail the corresponding author} ; Christopher R. Fielding^a ; ^{cfielding2@unl.edu" class="auth_mail" title="E-mail the corresponding author}
• 关键词：River delta ; Mud wedge ; Longshore drift ; Coastal currents ; Asymmetry ; Coastal processes
• 刊名：Earth Science Reviews
• 出版年：2015
• 出版时间：November 2015
• 年：2015
• 卷：150
• 期：Complete
• 页码：219-242
• 全文大小：5220 K

文摘

Understanding controls on sediment distribution in river deltas is paramount to predicting facies relationships and stratal architecture. Most classifications of deltas have focused on subaerial plan morphology as a simplistic function of river, tide, and wave energy. New work, however, is revealing the importance of subaqueous deposits, and in particular shore-parallel sediment redistribution, in the shaping of delta planform. Delta asymmetry has emerged as an important characteristic reflective of patterns of alongshore sediment dispersal. Asymmetry has been described from onshore and offshore environments from several different types of modern deltas, but aspects of asymmetry have not been fully documented and the degree to which these patterns are recorded in deltaic strata is not yet known. This study is a comprehensive literature review of sediment distributions in modern deltas focusing on studies with high resolution geomorphic, geophysical, and geochronological datasets. We reviewed 27 deltas using over 100 papers primarily from the past 15–20 years. Morphological, facies, and stratigraphic aspects were analyzed across the entire spectrum of deposits from the delta apex to the most distal muds of the prodelta. We define quantifiable indices of asymmetry describing upcurrent vs. downcurrent distribution of sediment volume, sediment area, sediment caliber, and distributary channels. Most deltas are asymmetrical to some degree with respect to one or more of these parameters. Many deltas are increasingly skewed toward the downcurrent side from proximal to distal sub-environments (i.e. delta plain, delta front, prodelta). Some deltas are skewed toward the upcurrent side in one sub-environment and toward the downcurrent side in another sub-environment. Sand is preferentially deposited on the downcurrent side of most deltas, but distributary channels tend to develop toward the upcurrent side. The highest sand:mud ratios are often on the upcurrent side in the lower delta plain, but in the delta front these ratios are highest on the downcurrent side. These complex patterns of asymmetry reflect different combinations of controls including discharge partitioning, lobe abandonment and localized transgression, plume deflection by coastal currents, dominance of longshore current direction, variable subsidence, and other factors. These processes may result in upcurrent–downcurrent variations in clinoform geometry, rates of progradation, and stratal lapping relationships. Asymmetry has multiple aspects, manifestations, and controls – even within a single delta – but the long-term preservation potential of these patterns likely varies, depending on depositional setting and shoreline trajectory. Studies of ancient deltas will be better-informed by recognizing the wide variety of controls on sediment distribution and avoiding the tendency toward a single model of delta asymmetry.