Monday, December 19, 2016

Highlights: Economic Mineralization—It’s The Fault’s Fault

Pressure and temperature are two fundamental controls on diagenesis and formation of economic minerals. To understand the possible influence of fault-related hydrothermal fluids on uranium mineral deposits of Carboniferous to Jurassic siliciclastic deposits in the Tim Mersoï Basin in Niger, Mamane Mamadou  and others examine the P-T conditions of diagenesis using petrography SEM observations and chemical analyses, supplemented thermometric approaches of chlorite compositions and fluid inclusions in quartz overgrowths. Chlorite thermometry indicates that all Carboniferous to Jurassic section was subjected to elevated temperatures of around 125°C (Carboniferous) and 115°C (Jurassic). These temperatures suggest a strong thermal disequilibrium between incoming fluids and reservoirs, reflecting burial temperatures in excess of those expected at maximum burial depth.  The fault-influenced fluid circulation affecting these strata are interpreted to to be linked to major geodynamic events related to the opening of the Atlantic Ocean, and likely have analogs elsewhere.

Hot fluid flows around a major faultidentified by paleothermometric studies (Tim Mersoï Basin, Niger) by Marah Mamane Mamadou, Michel Cathelineau, Franck Bourdelle, Marie-Christine Boiron, Agnes Elmaleh, and Marc Brouand

Tuesday, December 13, 2016

Highlights: Tibetan Forearc Basin Evolution

Forearc basins are depositionally important tectonic elements of ocean-continent collision zones, and commonly contain strata influenced by subduction and magmatic arc evolution. However, detailed spatial and temporal understanding of the timing and mechanisms of forearc development is limited, in part because these basins are rarely exhumed and easily accessible on land. To better understand tectonics and sedimentation of forarc basins, Orme and Laskowski examine Albian–Santonian strata of southern Tibetan Xigaze forearc basin, a large and well-preserved forearc basins, and which records upper-plate processes active prior to and following the inter-continental collision between India and Asia. Using regional geologic mapping, detailed sedimentologic and facies analysis, modal petrographic characteristics, and U-Pb detrital-zircon geochronologic data, the study documents the relationship between the Yarlung-Tsangpo ophiolite and Xigaze forearc basin and reconstructs the sedimentary environments of the southern margin of Asia during the initial stages of forearc deposition. Results reveal details of the evolution of the southern margin of Asia prior to its collision with India, including diachronous development of the forearc basin prior to India–Asia collision.

Friday, December 9, 2016

Highlights: Ichnology—Mixing It Up

Mixed siliciclastic-carbonate depositional systems occur throughout the geologic record. Although their general sedimentologic aspects are well-constrained, substrate-controlled trace fossil assemblages that can represent discontinuities in these successions are less well understood. By integrating ichnological descriptions and facies analyses of the Baldonnel Formation at Williston Lake, B.C., Canada, Schultz and others explore the ichnological composition of paired Trypanites- and Glossifungites-demarcated discontinuity surfaces, the significance of these surfaces in regional stratigraphy, and the relationship of these two assemblages to each other in individual parasequences sets. Recognition of these surfaces, and an understanding of their character, is paramount to the stratigraphic evolution and architecture of this mixed carbonate–siliciclastic depositional system, and may have analogs in other mixed systems.

Wednesday, December 7, 2016

Highlights: Mud-Dominated Shelf-Edge Rollovers

Although fine sediment represents an important proportion of sediment transferred by rivers, many studies of basin margin progradation emphasize supply and accretion of sand at and beyond the shelf edge. To understand how and when basin margin clinothems prograde in the absence of coarse-grained sediment supply to the shelf edge, Poyatos-Moré and others examine clinforms of the Permian Waterford Formation, in the Karoo Basin (South Africa). In these strata, normal and inverse grading, erosion surfaces, and moderate to low intensity of bioturbation are common in low-gradient (< 0.7°) clinoforms of thick, outer-shelf to upper-slope mudstone overlain by thin sandy shelf topset strata. The study provides a depositional model for shelf-to-slope transitions in fine grained successions, illustrating how progradational mud-rich, shelf-confined deltaic clinothems with muddy shelf-edge rollovers develop.

Mud-dominated basin-margin progradation: processes and implications by Miquel Poyatos-Moré, George D. Jones, Rufus L. Brunt, David M. Hodgson, Richard J. Wild, and Stephen S. Flint

Monday, December 5, 2016

Highlights: All Swell on Mississippian Carbonate Ramps

Clastic shallow marine systems can be grouped into wave- and tide-dominated systems, but carbonate successions are interpreted less commonly in this manner. In this study, Matheson and others document Mississippian-age carbonates with abundant physical sedimentary structures from the Canadian Rocky Mountain Front Ranges. The observations lead to development of a more comprehensive model of sedimentation across a swell-dominated, open-marine carbonate ramp. Comparing and contrasting shelf carbonate and siliciclastic sands suggest two primary differences: 1) in carbonate settings, there may be an increased preservation of deposits created by swell waves, and 2) that swaly cross-stratification in carbonate settings is not limited to the shoreface as it is in siliciclastic settings. These differences are due to the variability in spatial grain-size distributions and aggradation rates between siliciclastic and carbonate environments due in situ grain production in carbonate settings.