Tidal controls on the morphology and sedimentology of gravel-dominated deltas and beaches: Examples from the megatidal Bay of Fundy, Canada

摘要

Comparison of two gravel deltas and a gravel beach in the megatidal Bay of Fundy provides insights into tidal influences on gravel-dominated coastlines. Tidal effects on the morphology and sedimentology of the three systems are manifest as: a consistent onshore to offshore topography of the intertidal zone; deep incision of fluvial and tidal channels at the landward end of each system; and the occurrence of these deposits with salt marshes. Indirect morphological and sedimentological manifestations of tides include control on the height of surf-zone bars developed within the intertidal zone; extensive mud deposition (passive association with surf zone bars); and the potential preservation of thick gravel deposits, particularly in transgressive systems. The only direct sedimentological indicators of tides are bidirectional bedforms developed where onshoredirected high-angle cross bedding and offshore-directed trough cross bedding are interbedded. The presence of mud in relation to these bidirectional bedforms provides further evidence of tidal influence within conglomerate systems. Interestingly, no sedimentological characteristics of megatidal conditions are recognized. The results of this research suggest that, although there are a few morphological and sedimentological characteristics that are indicative of tidal deposition, the reliable recognition of tidal control on the deposition of conglomerates requires identifying at least two or three sedimentological characteristics that may have developed as a result of tidal depositional processes. These include: (1) a sharp change upwards in bedding dips, in a vertical succession of upper-shoreface to foreshore deposits; (2) anomalously thick gravel sequences, particularly in transgressive settings; (3) association of gravel beaches and deltas with saltmarsh deposits; and (4) an increase in the amount and extent of mud deposition in conglomeratic systems.