The fluvial to marine transition zone in the rock record.

GUGLIOTTA, M.; VEIGA, G.D.; FLINT, S.S.; HODGSON, D.

Congreso; 32nd IAS Meeting of Sedimentology; 2016

International Association of Sedimentologists

published examples, including the Late Jurassic Lourinhã Formation (Portugal), the Early Cretaceous McMurray Formation (Canada) and Tertiary to Holocene deposits of the Rhine and Meuse Rivers (Netherlands).Unidirectional, seaward-oriented cross-stratifications with cyclically-distributed carbonaceous drapes have been interpreted as the result of tidal modulation of the fluvial current in the inner, unidirectional fluvially-dominated, tidally-modulated subzone of the FMTZ. This facies is similar to the classic tidal bundle described since the late 1960s, but is instead controlled by the fluvial process. Additional expressions of tidally-modulated fluvial facies can be represented by cyclical changes in grainsize, cross-bed thickness, brink point height and bottomset thickness in cross stratification. This type of facies can also show non-cyclical reactivation surfaces and opposing directed ripples, but in this case the facies might have formed in the bidirectional fluvially-dominated, tidally-influenced subzone, still dominated by the fluvial process. Dm-scale alternations of coarser- and finer-grained beds with mixed fluvial and tidal affinities have been interpreted as the result of seasonal fluctuations in fluvial discharge and subordinate tidal influence. This facies suggests that the relative importance of the depositional processes is changing through time with stronger fluvial process during high river stage and temporal tidal dominance during low river stage.Application of the FMTZ concept to ancient successions provides a useful framework for improved identification of process interactions and suggests that not all facies commonly interpreted as tidal are the results of tidal process only. Differentiating different types of tidal or fluvial-tidal facies is fundamental to refining interpretations of tide-influenced deposits and understanding the spatial and temporal evolution of tide-influenced shallow-marine successions.