Environmental Reconstruction in the Orangeville Moraine

Primary sedimentary structure of topographic features in a selected portion of the Orangeville Moraine is examined for the purpose of deriving paleoenvironmental conditions existent during the formation of that feature. Five different environments could be recognized: 1. Sub-glacial tunnel, 2. Pro-glacial outwash, 3. Lacustrine, 4. Sub-glacial, and 5. Pro-glacial sub-aerial outwash. Sub-glacial tunnels were represented topographically by discontinuous ridges and sinuous depressions, and sedimentologically by massive matrix-supported gravels. Pro-glacial outwash was represented typographically by elongated hills and ridges, and sedimentologicallyby various sediment complexes that revealed both rapid downstream reductions in flow competency, and a downstream change from fluvial to lacustrine characteristics. These sediment complexes compared favorably to pro-glacial outwash models already derived by Davis (1890), Salisbury (1892), Banerjee and McDonald (1975) and Hjulstrom (1952). Lacustrine environments were represented sedimentalogically by rhythmically bedded and massively bedded sands, silts and clays characteristics of turbidity flows and massive settling from suspension. Topographically these sediments covered those features already described below the 1612 ft. contour (±), thus establishing an approximate surface level for the implied lake. The sub-glacial environment was represented by a massively muddy gravel and formed a covering blanket to all sediments hereto described. The pro-glacial sub-aerial environment consisted of local ice-front pondings interconnected by braided streams. Local pondings were represented topographically by low swampy plains and implied sedimentologically by a sediment complex similar to that of deltaic nature derived by G.K. Gilbert (1885). The surface of this delta plain was marked by circular depressions, and upper gravel classts revealed reorientation characteristic of arctic polygon formation. Braided streams followed valley depressions and displayed corase gravels indicative of high energy environments, arranged in complexes similar to braid bar models established by Eynon & Walker (1974), and Smith (1974). The vertical sequence of environments suggests the building of the moraine’s topography began with sub-glacial tunnel discharge into a lacustrine environment during a period of ice-front retreat. This building was followed by a period of ice-front readvance, during which time a till blanket was applied. Meltwater from final deglaciation erosively modified those structures previously formed. This erosion would seem to mark the final major stage of the moraine’s formation, although some minor periglacial modification by frost action is indicated