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.
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
