Champlain clay micropaleontology and micrography
The Champlain Sea, an inland sea, invaded the Saint Lawrence River valley in Southern Quebec at the end of the last glaciation, from 12 000 to 10 000 years ago. During this marine transgression, thick clay deposits were emplaced in Southern Quebec. The thickness of these Champlain clay deposits can reach 100 m at the center of the basin.
In the 10 000 years that followed the withdrawal of the Champlain Sea, the saline pore water in the clay was replaced by fresh water. This leaching rendered the clay structure metastable. It resulted in clay sensitivity, a reduction of the clay remoulded shear strength.
Other than the current pore water salinity, several parameters are known to influence the degree of sensitivity resulting from salt leaching. For example, models describing the formation of sensitive clays often include the influence of monovalent and divalent cation concentrations, grain size distribution and mineralogy. The salinity of the clay pore water at the time of deposition is also inferred to have an impact on clay sensitivity, but it is difficult to estimate. Nevertheless, the Champlain Sea salinity is known to have varied in both time and space. Clay deposited in saline water is expected to become more sensitive with leaching than clay deposited in brackish water.
The study of microfossil assemblages in clay specimens is one of the few methods that allow the pore water salinity during deposition to be estimated. By comparing microfossil assemblages found in Champlain clays with assemblages in modern environments, the salinity at the time of deposition can be estimated. During the summer of 2013, microfossils were concentrated from 28 clay specimens from the Lachenaie area, Quebec. The main tasks for the intern would be to participate in the microfossil identification and inventory, and to document the identification process with a scanning electron microscope.
This research project is part of a series of projects on the Lachenaie clay deposit. Since 2006, data have been gathered for the clay geotechnical and geochemical properties. The main objectives of these research projects was to model the geochemical evolution of the clay pore water since the end of the last glaciation and to relate the clay geotechnical properties with its geology and the geochemistry of its pore water. The main results for the micropaleontology study will be initial salinity profiles for the Lachenaie clay deposit. These profiles will be used to define the initial conditions for a numerical model of the pore water geochemical evolution since the Champlain Sea episode. The initial salinity data will also be added in our clay property database to model the influence of deposition salinity on clay sensitivity.
The sensitivity of Champlain clays is responsible for the large flow-type landslides that occur periodically in the Saint Lawrence River valley. These landslides have claimed more than 70 lives since the beginning of the 20th century. A better understanding of the formation of sensitive clay would help anticipate large landslides and identifythe most sensitive parts of Champlain clay deposits.
