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Recognition Criteria and Facies Models
The Post-Glacial Evolution of Chignecto Bay, Bay of Fundy, and Its Modern Environment of Deposition
Chignecto Bay is a macrotidal estuary situated in the upper part of the upper Bay of Fundy. Its physiography is bedrock controlled though it is blanketed in the upper reaches by transgressive, silty, glacial and postglacial unconsolidated sediments. This sequence is largely preserved in the sublittoral portion of the bay. It is complicated by rapid changes in sea level (55 m above present sea level at 15,000 BP, 25 m below present sea level circa 7,000 BP) and large changes in tidal range (varying from less than 2 m to more than 12 m over the last 13,300 years). The fining-upward macrosequence and expansive sand flats, which typify wide, shallow, mesotidal and macrotidal estuaries such as Minas Basin, are largely absent due to the major changes in sea level and tidal range. The depositional character and trapping efficiency of the estuary has also changed dramatically. From 13,300 to 10,000 BP, the estuary was characterized by seabed erosion, by the dominance of macrotidal sand and gravel facies throughout the estuary, by a low trapping efficiency of fine grained sediment and by a net export of this material. Between 9,000 and 6,000 BP, the estuary was characterized by the rapid accumulation of sand/mud laminae in its central, eastern side, possibly within a mesotidal lagoon, and by a high trapping efficiency and a probable import of material suspended in the Bay of Fundy. Wave activity during this time was suppressed, possibly as a result of a barrier island across the mouth of the paleoestuary. The estuarine depocentre moved to the western side of the central and inner estuary between 6,000 and 3,000 BP. The dominant facies also changed to macrotidal lenticular and flaser bedding. From 3,000 BP to the present, the estuary returned to conditions interpreted as having prevailed during late Pleistocene. The depocentre for these Holocene sediments is along the eastern part of the central sublittoral estuary. Recent sublittoral sediments are dominated by sand and gravel facies. Recent littoral sediments form fringing mud flats and salt marshes in the inner estuary. The modern estuary is subject to a mean spring semidiurnal tidal range of 11.3 m and associated tidal currents in excess 1.5 m/s. These currents are eroding the previously-deposited muddy sediments and creating a transgressive sand unit dominated by large-scale bedforms (sand waves, 2-D and 3-D megaripples) and sand ribbons. The bay is presently exposed to a peak significant wave height of 2 m. Waves thus play a major role in reworking and resuspending seabed sediments and in releasing fine sand and silt (1.03×106 m3/a) through cliffline erosion (up to 1.0 m/a). A total of 7.3×106 m3 of sediment is mobilized each year; 99 per cent of this volume is in the silt-clay size range and is transported in suspension. Residual circulation near the estuary mouth is headward along the eastern margins (0.1 m/s) and seaward along the western margin. Suspended sediment is transported out of the system as a result of this residual circulation (6.3×106 m3/a) and is subsequently deposited in the outer Bay of Fundy.
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