Abstract
Abstract Previous studies to recognize a ‘palaeoweathering’ signature in the framework composition of ancient sandstones have been undertaken in study areas where ancient climates are inferred to have been more humid than the present climate. Unfortunately, in studies in which the ancient climate was more humid than the modern, reduction of modal unstable-grain abundance by ancient weathering cannot be distinguished from unstable-grain destruction by burial diagenesis. Reinterpretation of data from Triassic Chatham Group sandstones of the Deep River basin (North Carolina) and modern sands derived from similar source rocks shed new light on the matter, as the Triassic sediments were generated under more arid conditions than the modern wet climate. If diagenesis were the more important factor, the sandstones would have lower contents of unstable grains. If weathering were more important, the ancient sandstones would have higher contents of unstable grains. Chatham Group sandstones have higher unstable grain contents, indicating that the compositional signature imposed on the sandstones under the Triassic climate has survived without being obliterated by diagenesis. Only ‘long-term average’ palaeoclimatic conditions are preserved in sandstone detrital framework modes, and several specific criteria must be met before these conditions can be inferred from sandstone framework composition. The framework-petrographic signatures of palaeoclimate can be discerned only in unglaciated sequences from mature extensional tectonic settings on continental plates, only in sedimentary basins with mild late-diagenetic histories, and only for those modern-ancient pairs where the modern climate is more humid than the inferred ancient climate.
Published Version
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