John R. J. Letson

Contemporary shallow marine sediments in southwestern Mahone Bay have been studied to determine processes of sediment erosion, transportation and deposition. The bay is a structurally controlled glacially modified inlet on the South Shore of Nova Scotia in which erosion of drumlin islands are virtually the only sediment source. The glacial debris is unmixed into discrete particle populations in the nearshore. These are dispersed and recombined in response to the modern hydraulic regime to form 9 sediment lithofacies occupying three storm dominated sedimentological environments: the wave cut platform, the basin margin and the basin floor.

Erosion of islands creates flat lying wave cut platforms characterized by lag boulders and cobbles set in a thin, sandy, gravelly matrix, which becomes muddy toward the seaward edge of the platform. The complex offshore-onshore movement of sediment over the platform produces seaward fining of boulders, cobbles and matrix.

Storm and fair weather waves coupled with an offshore flow (caused by onshore build up of water by shoaling waves) results in offshore movement of sand and mud in a series of resuspensions and onshore bedload movement of larger clasts. A sharp slope break where the platform meets the basin margin causes a sudden lessening of wave energy resulting in storm deposition of sand on the margin. However, during fair weather most sand does not reach the margin but is deposited on the lower platform which suspended clay, silt and some fine sand trapped in flocs by-pass the margin to collect on the basin floor.

The fine sand, silt, and clay, delivered to the basins during both storm and fairweather conditions, settle out to form the flat lying muds of the basin floors. Within these muds storm deposited silt and sand form laminae that can be correlated locally but which are discontinuous over an entire basin.

Recent development of the study area has been controlled by the presence of a sill at the bay mouth, a rising Holocene sea level and the high erodability of the drumlin till. Modern development dates from the time the sill was breached (6,000 to 8,000 years B.P.) and fully marine conditions were established within the bay. The ensuing marine erosion has caused the rapid planation of drumlin islands creating wave cut platforms, shoals and large volumes of sediment.

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Supervisor: D. J. W. piper