LES ORTSTEINS DU QUÉBEC: RÉPARTITION GEOGRAPHIQUE, RELATIONS GÉOMORPHOLOGIQUES ET ESSAI DE DATATION

Ce travail porte sur I'extension géographique des orts-teins au Québec et vise à déterminer les causes de leur répartition principalement pour la Côte-Nord du Saint-Laurent. Les données ont été puisées en partie dans des rapports pédologiques publiés sur le Québec, mais aussi observées dans des sites d'échantillonnage sur la Côte-Nord du Saint-Laurent, l'île d'Anticosti et en Gaspésie. Par ailleurs, pour les datations au ¹⁴C, six sites d'echantillonnage ont été choisis sur la Côte-Nord. Les résultats indiquent que les ortsteins se développent sous toutes les latitudes au sud de 55e parallèle et sous différentes conditions physiographiques et géologiques. On les retrouve aussi bien sur des silts sableux que sur des graviers grossiers et leur présence est favorisée lorsqu'il y a fluctuation de la nappe d'eau. Les datations au ¹⁴C ont donné des âges relativement jeunes pour les ortsteins, soit moins de 420 ans bp, indiquant qu'il y a une migration active de la matière organique dans les horizons concrétionnés. The present paper concerns the geographic delimitation of ortsteins in Québec and deals with the causes of their distribution mainly for the North Shore of the St. Lawrence River. Data was gathered in part in published geological reports in Quebec, and also acquired in the field along the North Shore of the St. Lawrence River, Anticosti Island, and in the Gaspé Peninsula. Furthermore, six study sites were selected for dating by ¹⁴C along the North Shore. Results indicate the ortsteins develop under all latitudes south of the 55th parallel and under different physiographic and geological conditions. They can be found on both sandy silts and coarse gravels, and their presence is enhanced whenever the water level fluctuates. ¹⁴C datings give relatively young ages for ortsteins, less than 420 years BP, indicating that an active migration of organic matter occurred in the indurated horizons.     

Raised relief on nephrite jade artifacts: observations,explanations and implications

Mineralogical analyses of nephrite jade artifacts have been used to identify likely causes of alterations in which some specimens exhibit raised relief above the original polished surfaces. In all of the specimens examined, the raised areas consist of either large diopside ± large tremolite crystals, or, in a few cases, only fibrous tremolite, within a non-raised matrix of nephrite (fibrous tremolite). Apparent relief may be cause by a number of different effects, either mechanical (abrasive), chemical (e.g., dissolution), or a combination (mineralogical alteration leading to formation of relief). The most enigmatic examples occur where the relief formed after the object was polished and can not be easily attributed to chemical dissolution or mechanical abrasion. One possible explanation for these is that minor amounts of secondary growth of hydrous minerals, such as clays along grain boundaries, produced a volume increase which caused the crystals to rise. Carbon-14 analysis on one vessel that displays raised relief has produced an age of 3010 ± 30 years before present for organic material that coated the alteration.