Abstract: Facies Model for Macrotidal Environment, Cobequid Bay, Nova Scotia

G. V. Middleton, R. J. Knight, R. W. Dalrymple

Cobequid Bay, the eastern extension of the Bay of Fundy, is reputed to have the highest tides in the world. At Burntcoat Head the mean tidal range is 11.6 m, springs at lunar perigee average 15.4 m and the largest range ever measured directly was 16.3 m. Several other areas in the world have tides almost as large, and there seem to be no major differences in morphology and sediment distribution between areas of extreme tidal range and those with ranges of about 5 to 6 m. These macrotidal environments are widespread, but have been studied little compared to areas with smaller tidal range. Cobequid Bay thus may serve as a general model for macrotidal bays.

The bay is about 30 km long, and 10 km across at the connection with the Minas basin. Two rivers enter at the east end, but the high tidal range precludes vertical temperature or salinity stratification. In spite of the high tidal range, tidal currents generally range from 0.5 to only 2 m/sec. There is a high concentration of suspended sediment in the water but thick deposits of mud are present only in sheltered areas at the head of the Bay. The shoreline generally is marked by a cliff and a gravel beach at the high-tide level. A broad, sloping wave-cut platform is common and may have a thin veneer of gravel and mud. The Bay encloses a single major sand body, divided into several bars by a system of tidal channels. Channels generally are dominated by one phase of the semidiurnal tidal cycle. Most flood-dominated channels are close to shore, whereas ebb channels are closer to the axis of the bay. At the seaward end of the bay the depth of the channels, and relief and thickness of the sand bars is greatest and the sand body is mainly subtidal. Toward the head of the bay, tidal currents reduce in strength and sand bar thickness and relief decrease, and the bars become mainly intertidal. Most bars are covered by megaripples, but near the head of the bay bars are rippled or plane and show a "braided" pattern.

The present sand body has resulted from reworking of Pleistocene sands and coastal erosion as sea level (and especially, high-water level) has risen and the tidal range has increased during the last 3,000 years. If subsidence continues over the Bay of Fundy-continental-shelf system that produces present tidal resonance, tidal amplitude should decrease, leading to drowning of the bars and a sequence passing up into subtidal muds. If there is stability or slight uplift, slow infilling of the Bay should take place: this also will decrease resonance from the present optimum, leading to progradation of intertidal and supratidal muds over the present sands.

AAPG Search and Discovery Article #90972©1976 AAPG-SEPM Annual Convention and Exhibition, New Orleans, LA