Michael Walsworth, Ryan Sullivan, Simi Odueyungbo, William Budd Estuarine Environment At first (Pritchard, 1967), an estuary was defined by the salinity of the water. Then by Clifton (1982) as “an inlet of the sea reaching into a river valley as far as the upper limit of tidal rise.” By definition also includes barrier lagoons and deltaic regimes. Refined by Dalrympal (1992) Basic definition still applied today “The seaward portion of a drowned valley system which receives sediment from both fluvial and marine sources and which contains facies influenced by tide, wave and fluvial processes. The estuary is considered to extend from the landward limit of tidal facies at its head to the seaward limit of coastal facies at its mouth” (Dalrympal, et al, 3). Fluvial Upstream of transition zone Purely fluvial processes Fluvial-Tidal Transition Zone (“that part of the river which lies between the landward limit…”) Mixture of fluvial and tidal/wave processes Estuarine Downstream of transition zone Feels no effects of fluvial processes Transition Zone is defined by Van den Berg as “that part of the river which lies between the landward limit of observable effects of tidally induced flow deceleration on fluvial cross-bedding at low river discharge, and the most seaward occurrence of a textural or structural fluvial signature at high river stage.” Extends past measurable dilution of salt water Includes river-sea relationships in deltas Studies on Rhine and Meuse River System (Netherlands/Germany) A mesotidal estuaries with a medium-large river system in a temperate climatic zone Quaternary and Pliocene-aged sediments Large-scale dune X-bedding Results in foresetting, tend to be approximately regularly spaced Ripple and Transitions Longitudinal Sequences sorting from sands upstream to clays downstream Dm-scale successions in the vertical Cyclic Neap-Spring Successions Herringbone X-bedding and alternating sand and mud interbedding, indicators of flow reversal Inclined linsen and flaser bedding Wave-Dominated Tidal-Dominated Fluvial-Dominated Grey area, as at this point the area could be defined as a delta rather than an estuary Not affected by fluvial processes Fine sediments accumulate No distinct vertical sequence at meter scale, except for the top one to two meters which transition into clays and intertidal salts In estuarine environments, discontinuities are draped with clays in dune X-bedding as opposed to peats in transition zone Ebb flow is concentrated in channels Flood can cross shoal morphology Ebb channels dictate the large morphological changes Tidal signatures in the stratigraphy of fluvial sediments allows for the determination of the maximum flood surface Transition zone is characterized by a single meandering channel that guides the main flow of both tides Results in thick cosets displaying the dominant ebb-direction Hydraulic and morphological characteristics make transition zone differ from other fluvial/tidal environments • As flow strength increases the hydraulic roughness of fluvial dunes decreases, whereas dune height remains relatively the same • Fluvial-tidal zones contain characteristic Herringbone X-bedding - Due to alternating periods of ebb and flood dominance Regular vertical alternations of flood and ebb directed X- lamination present The landward limit of the transition zone is dictated by the turbidity maximum Just upstream of the salt intrusion (salinity .1-.5%) is • Deposits where the landward • Stepwise fining-upward transition zone marker sequence from decimeter to This marker is a function of discharge rate of the river meter scale in outcrops • Mud drapes of silty nature In fluvial dominated systems, moves to be more seaward • Peat lump-dominated than in tidal dominated regimes. Typically, tidal influence is small 2 maxima 1 at the mouth caused by wave energy and 1 at the head produced by river currents Called the “Central Basin” Energy minimum in central portion of estuary Contains extensive salt marshes and muds Mouth Mouth of system experiences relatively high wave energy, accumulating marine sand Causes the sediment to form a submerged barrier at the mouth, preventing wave energy from entering the estuary Head Sand and gravel deposited at head forming a bay-head delta Best examples are Macro-tidal, i.e. Salmon River They occur at much smaller tidal ranges if wave action is limited and/or tidal prism is large Tidal-current exceeds wave energy at mouth Elongated sand bars develop and dissipate the wave energy that does exist causing it to decrease in cross-sectional area up the estuary Energy minimum (Central Basin) is not as pronounced as in wave- dominated systems Site of finest channels sands Deposits Regular, vertical repetitions due to alternating spring and neap tides Two or more lithologic units with different sedimentary structure and texture (one for neap tide, one for spring tide) Largest particles are clay pebbles Dalrymple, Robert W., Brian A. Zaitlin, and Ron Boyd. "Estuarine Facies Models: Conceptual Basis and Stratigraphic Implications." Journal of Sedimentary Petrology 62.6 (1992): 1130-146. Print. Van Den Berg, J. H., J. R. Boersma, and A. Van Gelder. "Diagnostic Sedimentary Structures of the Fluvial-Tidal Transition Zone - Evidence from Deposits of the Rhine and Meuse." Netherladns Journal of Geosciences 86.3 (2001): 287-306.
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