The seaward end of modern rivers is characterized by the interactions of marine and fluvial processes, a tract known as the fluvial to marine transition zone, which varies between systems due to the relative strength of these processes. To understand how fluvial and tidal process interactions and the fluvial to marine transition zone are preserved in the rock record, large‐scale outcrops of deltaic deposits of the Middle Jurassic Lajas Formation (Neuquén Basin, Argentina) have been investigated. Fluvial–tidal indicators consist of cyclically distributed carbonaceous drapes in unidirectional, seaward‐oriented cross‐stratifications, which are interpreted as the result of tidal modulation of the fluvial current in the inner part of the fluvial to marine transition zone. Heterolithic deposits with decimetre‐scale interbedding of coarser‐grained and finer‐grained facies with mixed fluvial and tidal affinities are interpreted to indicate fluvial discharge fluctuations (seasonality) and subordinate tidal influence. Many other potential tidal indicators are argued to be the result of fluvial–tidal interactions with overall fluvial dominance or of purely fluvial processes. No purely tidal or tide‐dominated facies were recognized in the studied deposits. Moreover, fluvial–tidal features are found mainly in deposits interpreted as interflood (forming during low river stage) in distal (delta front) or off‐axis (interdistributary) parts of the system. Along major channel axes, the interpreted fluvial to marine transition zone is mainly represented by the fluvial‐dominated section, whereas little or no tide‐dominated section is identified. The system is interpreted to have been hyposynchronous with a poorly developed turbidity maximum. These conditions and the architectural elements described, including major and minor distributary channels, terminal distributary channels, mouth bars and crevasse mouth bars, are consistent with an interpretation of a fluvial‐dominated, tide‐influenced delta system and with an estimated short backwater length and inferred microtidal conditions. The improved identification of process interactions, and their preservation in ancient fluvial to marine transition zones, is fundamental to refining interpretations of ancient deltaic successions.
18Fluvial discharge fluctuations are a fundamental characteristic of almost all modern rivers 19 and can produce distinctive deposits that are rarely described from ancient fluvial or mixed-
Discharge fluctuations in modern rivers
77Of the 1534 present-day rivers in the database of Milliman and Farnsworth (2011), almost 78 95% show variations in discharge through the year (Fig. 1). In most of the rivers in low-and
Crevasse subdeltas develop on modern river-dominated delta plains, and may be affected by the interaction of river currents and marine processes. However, their sedimentology and stratigraphic architecture is poorly constrained, leading to simplistic depositional models of delta-plain systems in the ancient record. Extensive exposures of the Middle Jurassic Lajas Formation permit the architecture, main stratigraphic surfaces, and lateral and vertical facies variations of crevasse subdelta deposits to be constrained. Lower-delta-plain successions studied in the Lajas Formation consist of up to 5-m-thick distributary channels and interdistributary-bay deposits, interpreted as crevasse subdeltas. Crevasse subdelta deposits consist of small-scale lenticular units (, 1-2 m thick) interpreted as crevasse channels and upward-coarsening and upward-thickening packages (, 2 m thick) with clinothems interpreted as crevasse mouth bars. These deposits preserve interbedding of coarser and finer sediments that are interpreted as river flood and interflood couplets associated with variations in river discharge. River flood beds are commonly structureless and erosionally based, and show little evidence of tidal action and brackish-water conditions. Interflood deposits show rhythmically distributed mudstone drapes, bimodality, and brackish trace fossils. This study highlights an important but largely undocumented component of interdistributary deposits consisting of tide-influenced, but strongly riverdominated, prograding depositional bodies. An implication is that some coarsening-upward, forward-accreting units previously interpreted from the rock record as interchannel ''tidal bars'' may instead represent minor mouth bars of tide-influenced crevasse subdeltas. Furthermore, present-day crevasse subdeltas are restricted to river-dominated delta systems that flow into semi-enclosed or enclosed seas and lakes with microtidal conditions and limited wave action, which is comparable to paleogeographic reconstructions for the Neuquén Basin during the Middle Jurassic.
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