Tidal Influences from the Deposition of Gravel: Examples from the Megatidal Bay of Fundy, Canada

Shahin E. Dashtgard¹ and Murray K. Gingras^2
1Earth Sciences, Simon Fraser University, Burnaby, BC, Canada
²Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada

Comparison of two gravel deltas and a gravel beach in the megatidal (>8 m tidal range) Bay of Fundy, Canada 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 sedimentological indicators of tides are bidirectional bedforms developed where onshore-directed 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.

In the rock record the reliable recognition of tidal control on the deposition of conglomerates requires identifying at least two or three sedimentological characteristics that may be tidally generated. 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 salt-marshes deposits; and (4) a possible increase in the amount and extent of mud deposition in conglomeratic systems.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas