Thursday, March 12, 2015

Highlights—Abiotic Needle-Fiber Calcite

Needle-fiber calcite (NFC) is common diagenetic cement that has been found globally in many different types of limestone. The origin of NFC has been widely debated, and both abiogenic and biogenic processes have been invoked to explain its origin. In recent years, the origin of NFC has been closely allied to fungi. Jones and Peng document NFC in the spring deposit at Shiqiang, Yunnan Province, China, where NFC is common—but never associated with fungi. The study explores the different possible mechanisms and concludes that these NFC formed by abiogenic processes, driven largely by evaporation. These results highlight the critical problems in trying to ascertain the origin of carbonate cements, and the dangers of trying to ascribe all NFC to biogenic processes.

Monday, March 2, 2015

Highlights—Drainage Area of Distributive Fluvial Systems

By its very nature, the production of siliciclastic sediment destroys much of the direct evidence of its source region, so geoscientists are left with indirect tools to assess metrics such as catchment basin size. In this paper, Davidson and Hartley relate the areal extent of existing large (> 30 km in length) distributive fluvial systems (DFS) formed in endorheic basins to contributing drainage basin area. Regression analyses indicate a strong positive relationship between drainage area and DFS area; additionally, drainage basin relief influences sediment supply in terms of volume or caliber, which in turn affects the depositional gradient of the DFS surface and resultant channel planform. Application of these regression relationships to examples in the rock record shows that DFS area can be used as a proxy to predict the surface area of fluvially transported sediment deposited in a sedimentary basin from the contributing catchment. The modern regression relationships suggest a measurable link between source and sink in the sedimentary rock record, and provide a potential tool for more accurate prediction of preserved fluvial architecture within basin-scale climatic and tectonic contexts.

A quantitative approach to linking drainage area and distributive-fluvial-system area in modern and ancient endorheic basins by Stephanie K. Davidson and Adrian J. Hartley