Lithostratigraphy And Geochronology Of The Quaternary Deposits Of The Pierreville And St-pierre Les Becquets Areas, Quebec

For three decades, a Traditional Concept of stratigraphy involving one glaciation with two major ice advances separated by one brief interstade has been invoked to explain the distribution of Pleistocene deposits in the St-Lawrence Lowland. In this thesis, field work and sedimentologic analyses document the lithostratigraphic units in the Pierreville and St-Pierre les Becquets areas. A new stratigraphic nomenclature is proposed. Isotopic studies of calcareous concretions found in situ in glacio-lacustrine sediments and as striated clasts in tills provide evidence that those carbonates are the product of an early diagenetic process. Stadial and interstadial sediments as well as modern silt collected in the St-Lawrence River have been dated by thermoluminescence using the R-Gamma method. The major problems encountered were: (a) anomalous fading of the polymineralic fraction, (2) estimation of the TL growth curve, (3) determination of the past water content and (4) laboratory overbleaching of the inherited TL in recent sediments.;The data reported, herein, provide evidence that the succession of geologic events may be as follows: (1) An Illinoian glacial event: the St-Lawrence Stadial, represented by the Odanak Formation, older than 135 ka; (2) an early Wisconsinan glacial event, the Nicolet Stadial, represented by glaciolacustrine sediments and tills (Riviere aux Vaches Formation; Levrard Formation), dated at 70 to 86 ka by TL; (3) A middle Wisconsinan nonglacial event: the St-Pierre Interstadial, represented by organic-rich sediments (St-Francois du Lac Formation), dated at 60 to 75 ka (Pierreville) and 28 to 65 ka (St-Pierre les Becquets) by TL and ('14)C; and (4) a late (?) Wisconsinan glacial event: the Trois-Rivieres Stadial, represented by the Gentilly Formation; the timing of the onset of this glacial advance is problematic and ranges from 66 to 30 ka. Three formations, Les Becquets, Deschaillons and Pierreville, have an equivocal stratigraphic position. This framework is presented as an alternative to the Traditional Concept of earlier workers.