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Gradual Transition From Net Erosional to Net Depositional Cyclic Steps Along the Submarine Distributary Channel Thalweg in the Rio Muni Basin: A Joint 3‐D Seismic and Numerical Approach
Author(s) -
Li Lei,
Gong Chenglin
Publication year - 2018
Publication title -
journal of geophysical research: earth surface
Language(s) - English
Resource type - Journals
eISSN - 2169-9011
pISSN - 2169-9003
DOI - 10.1029/2017jf004513
Subject(s) - thalweg , geology , sedimentary depositional environment , turbidity current , trough (economics) , geomorphology , hydraulic jump , channelized , geometry , structural basin , flow (mathematics) , sediment , economics , macroeconomics , telecommunications , computer science , mathematics
We integrated measurements of cyclic steps and bankfull hydraulic geometry to estimate turbidity flow properties, yielding two main observations. First, three net erosional cyclic steps were recognized along a distributary channel thalweg based on (1) closed topographic depressions, (2) less asymmetrical cross‐sectional morphologies, (3) disrupted and discontinuous internal architecture revealed from seismic data, (4) erosional basal contacts, (5) stronger up‐current migration, and (6) relatively higher U 1 (flow velocity just before each hydraulic jump), Q 1 (bankfull discharge before hydraulic jump), and Δ E L (energy loss of hydraulic jump) of formative turbidity currents. In contrast, six net depositional cyclic steps were identified based on (1) open topographic depressions, (2) more asymmetrical morphologies, (3) well‐bedded and continuous internal structures, (4) gradual basal contacts, (5) stronger vertical accretion, and (6) relatively lower U 1 , Q 1 , and Δ E L values. These observations may aid in distinguishing between net erosional and net depositional cyclic steps. Second, cross‐sectional asymmetries ( A y ) and width‐to‐depth ratios of the trough ( W 1 / h 1 ) of cyclic steps systematically vary with distance along the distributary channel thalweg from the point of overflow initiation ( D i ) and can be fit by a single relation, which, together with the fact that the two types of topographic depressions separate along these relations, demonstrate a gradual transition from net erosional to net depositional cyclic steps. U 1 , Q 1 , and Δ E L are proportional to the stoss‐side slope angle of cyclic steps ( а ) but are inversely proportional to A y and W 1 / h 1 , indicating that a progressive downstream decrease in U 1 , Q 1 , and Δ E L (i.e., decrease in sediment transport capacity) probably caused the gradual transition from net erosional to net depositional cyclic steps.