The results presented in this paper by Patrick Lajeuesse reveal for the first time that many large river and lake valleys of northeastern North America are underlain by deep bedrock inner gorges and V-shaped valleys. Geological evidence provided by excavation and drilling work at many dam sites as well as seismic data collected on lakes and at river mouths indicates that these deep bedrock trenches were not eroded by glaciers, but were rather incised by streams or rivers that flowed before the ice ages when sea level was much lower than today. These valley profiles share striking similarities with those of many fjord-lakes of other glaciated landscapes, suggesting that many other valleys previously interpreted as glacial in origin may in fact enclose preserved preglacial fluvial channels at their bottom. Because they have survived glacial erosion beneath the Laurentide Ice Sheet through successive glaciations, valleys of northeastern North America have a great potential for containing a long term record of sedimentation and environmental change. During glacial episodes, these fluvial valleys also formed preferential pathways that influenced glacial ice flow and the delivery of sediments to ocean basins.
Replacement origin for hematite in 2.5 Ga banded iron formation: Evidence for postdepositional oxidation of iron-bearing minerals
Birger Rasmussen et al., Department of Applied Geology, Curtin University, Kent St, Bentley WA 6102, Australia. Posted online ahead of print on 3 Feb. 2014; http://dx.doi.org/10.1130/B30944.1.
Banded iron formations (BIFs) are widely used to infer early microbial processes and the composition of the ancient ocean. Those inferences are based on assumptions of the mineralogy of the original sediment and how it was de
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Geological Society of America