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.

Tuesday, December 23, 2014

Highlights—Settling for Maërl

Although the effects of shape and roughness on particle settling velocity are broadly understood, settling velocities of biogenic particles that have the quality of grain-size dependent roughness are less well constrained. This work by Joshi et al. addresses that gap in knowledge by examining the properties of maërls. The results indicate that maërl is suspended at lower current speeds than equant sediment, and how unsorted siliciclastic and maërl sediment can be advected differentially to deposit unsorted maërl gravel and fine sand. These results reveal how the hydrodynamic behavior of maërl may explain its propensity to form spatially well-defined "beds" or beaches with very low amounts of sand.

Settling velocity and grain shape of maërl biogenic gravel by Siddhi Joshi, Garret P. Duffy, and Colin Brown

Monday, December 22, 2014

Highlights—Mixed Deposits of the Sego

Siliciclastic systems are lumped into wave- or tide-dominated classes, and many detailed facies and sequence-stratigraphic analyses interpret pronounced changes to a shift from one to the other caused by major (commonly external) perturbations. To explore fundamental assumptions of some of these studies, Legler et al. document distal deposits near the pinchout of the widely studied lower Sego Sandstone in the Book Cliffs, western Colorado, a unit generally regarded as the type example of an ancient tide-dominated delta. The analysis of facies and stratigraphic architecture suggests that coeval waves and storms were equally prominent in controlling its deposition. The re-interpretation of the lower Sego Sandstone as a mixed, tide- and wave-influenced delta is important in three ways: (1) the prevailing, widely used archetype of an ancient tide-dominated delta should be viewed as recording a wider mixture of depositional processes; (2) the recognition that wave- and tide-dominated deposits can be coeval; and, as a result, (3) sequence stratigraphic framework of the lower Sego Sandstone and other ancient tide-dominated/influenced, regressive strata may need to be re-assessed.

Facies relationships and stratigraphic architecture of distal, mixed tide- and wave-influenced deltaic deposits: lower Sego Sandstone, western Colorado, U.S.A. by Berit Legler, Gary J. Hampson, Christopher A.L. Jackson, Howard D. Johnson, Benôit Y.G. Massart, Marcus Sarginson, and Rodmar Ravnås