Friday, January 23, 2015

A Look Back…25 Years: Ireland Ice Sheet Collapse

The nature and rate of decay of ice sheets is central to understanding possible impacts of climate change. Twenty-five years ago, McCabe and Dardis documented the character and distribution of Late Pleistocene drumlins, deposited in association with the melt of the ice sheet in western Ireland. The results illustrated several lithofacies associations, interpreted in the context of depositional environment in association with the ice sheet. These insights provided a conceptual model for ice-distal to ice-contact subaqueous deposition, modified by till deposition and drumlinization. The results provided a detailed documentation of rapid facies changes associated with a dynamic ice margin, and how these details “may not therefore be linked directly to climatic change.”

Tuesday, January 6, 2015

A Look Back…50 Years: Analcime in the Record

Sedimentary rocks are mineralogically diverse, the result of depositional and diagenetic variability. Fifty years ago, Teruggi reported a new occurrence of sedimentary analcime, which he suggested was “one of the most important so far recorded in the geological literature.” His results, based on the study of a single section, revealed “considerable quantities of sedimentary analcime” in Argentina. He interpreted the analcime to be a diagenetic alteration of “altered vitroclastic matrix.”

Tuesday, December 30, 2014

A Look Back…75 Years: Devils Lake Sediment: Sludgy, Soupy

Lacustrine sedimentary systems are spatially and temporally variable, but can include considerable quantities of organic-rich sediment. Seventy-five years ago, Twenhofel and McKelvey examined the sediment of Devils Lake, a small closed lacustrine system in Wisconsin. Their documentation of factors such as composition, color, and “bacteria” revealed shore-parallel zonation of sedimentary attributes. They illustrated abundant bacteria in the deeper waters, which they interpreted to be the result of anoxic conditions in the poorly circulated lake.

Monday, December 29, 2014

Highlights—Grain Size Related to Shoreface Setting

Granulometric characterization is a fundamental tool for sedimentologic analyses, and ideally provides unique insight into sedimentary dynamics. Here, Barusseau and Braud show that many sands of the recent deposits of the shoreface of the Golfe du Lion (Mediterranean Sea, France) represent mixtures of four sediment types. Each sediment type is interpreted to be distinct, but that these can be mixed to create asymmetric grain size distributions. Areas with exclusive or dominant transport processes include mixtures that tend toward a log-normal grain-size distribution. These findings provide insights into possible mechanisms underlying the deposition of sediments in the coastal environments, results that may have use in paleogeographic reconstructions of the coastal zone and basin borders in geologic analogs.