ABSTRACT: Evaluation of sediment spectral gamma measurements as a tool for regional stratigraphic correlation of Pleistocene deposits in southern Ontario.

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Abstract

Many aquifers in Southern Ontario are located within complex Pleistocene sediments deposited during multiple ice readvances and retreats. Current knowledge of the regional continuity of Pleistocene strata remains limited due to the difficulty of identifying diagnostic, chemical, physical, or textural attributes that can be used in regional correlation. Previous studies have employed lithologic and downhole geophysical attributes for regional correlation and determination of sediment provenance. This paper presents the results of one of the first studies that has utilized spectral gamma as a regional stratigraphic tool.All rocks and sediments include the natural radioactive elements uranium (U238), thorium (Th234) and potassium (K40). Each radioactive element emits gamma rays with characteristic energy levels. The amplitude of the signal at the various energy levels allows discrimination of the relative contribution of the three elements. The mobility of, and the minerals associated with, each radioactive element are unique. These differences lead to fractionation that is dependent on primary source rock, depositional environment, transport and sorting mechanisms, sediment reworking and diagenesis. The summation of these processes leads to a radioactive abundance for each sedimentary unit at each location. If all portions of a stratigraphic unit were formed under similar depositional processes then it should be identifiable by a unique spectral gamma signature.A hand-held Exploranium GR-320 gamma spectrometer was used to obtain more than 350 measurement on outcropping Pleistocene tills and deltaic deposits in the Greater Toronto Area. Crossplots and three-component plots show that the most sedimentary units have a unique spectral gamma signature. Clay-rich glaciolacustrine strata (e.g. Sunnybrook diamict, Thorncliffe and Scarborough clays) are characterized by more abundant Th, K and U when compared to sandy deltaic units and subglacial tills (e.g. Newmarket Till). The element ratios for each unit show a regional consistency, except for the Halton Till, which has a variable spectral signature that reflects the composition of underlying deposits. This association reflects the sub-glacial reworking and incorporation of pre-existing deposits into the till during ice readvance. The results show that many Pleistocene units have unique spectral gamma signatures and that gamma is a useful attribute for regional stratigraphic studies in Southern Ontario.

Bibliography of Canadian Geomorphology