Characteristics and significance of two pre-late-Wisconsinan weathering profiles (Adirondacks, USA and Miramichi Highlands, Canada)

Abstract

The properties of two weathering profiles developed from granitic rocks and covered by deposits of Wisconsinan age were studied in order to characterize saprolites and non saprolite residuum of the glaciated area of northeastern North America. The Warrensburg profile (Adirondacks, New York) is more than 9 m thick and formed on a hornblende granitic gneiss. Total chemical analysis of the sandy saprolite shows slight losses compared to bedrock. Kaolinite is the main weathering product in the < 2 μm fraction, and vermiculite and gibbsite are present. Biotite is altered to kaolinite locally. Hornblende is the most altered mineral and is pseudomorphed by vermiculite. There is 4% of kaolinite and 1.5% of gibbsite in the fine fraction ( < 63 μm) of the saprolite. The Big Bald Mountain profile (Miramichi Highlands, New-Brunswick) is 2 m deep and has developed on a biotite granite. Tors, weathering pits and gruss more than 20 m deep in an alveole at the foot of the mountain were reported. The sandy residuum shows limited geochemical and mineralogic evolution. Most primary minerals appear fresh, biotite and plagioclase being slightly altered. Smectite, chlorite and vermiculite sometimes interstratified with mica, are the dominant clay minerals of the fine fraction, but kaolinite and gibbsite are also present. The two profiles have limited mineralogic evolution even though kaolinite and gibbsite occur in small amounts. Their characteristics do not indicate climatic conditions different from those of the present, thus their age is within the limits of the Plio-Pleistocene. The temperate conditions and the quick drainage of these porous materials can explain the observed properties and their thickness. Saprolites are witnesses of an important process in relief evolution, yet they are overlooked factors in the analysis of landform evolution of formerly glaciated areas. Thorough analysis of weathering profiles is needed in these areas since it can give valuable information on the climate during their formation, even though an exact age cannot yet be obtained on them.