Modern
stratigraphic analysis utilizes unconformity-bounded, genetically related units
to subdivide and then interpret stratigraphic successions. In alluvial plain deposits, an
important challenge represents means to subdivide and date the complex internal
stratigraphy. Here, De Santis et al. use physical sedimentologic characterization, coupled with amino acid
racemization (AAR) and 14C dating, to recognize and date Quaternary units in an Italian alluvial plain to the
scale of marine isotope stages.
Detailing and dating stratigraphy to this level reveals the influences
of subsidence events, climate variability, and sea-level change on the
region. This level of resolution
is interpreted to represent an important advance, compared to other Quaternary alluvial
fan successions that are subdivided and dated using their relative stratigraphic
position alone.
Deltas represent generally lobate sediment accumulations formed by sediment from rivers that empty into standing bodies of water. In these settings, storms and tropical cyclones can represent events of high energy. To assess the role storms in formation of deltas, Garrison et al. compare the timing of the deposition of stratigraphic units, constrained by 137Cs geochronology and historical aerial photographs, and the historical record of storms in the Holocene Gum Hollow Delta (Nueces Bay, Texas Gulf of Mexico). Results indicate that broadly coarsening- then fining-upward bedsets, interpreted to represent waxing- and waning-flow phases interpreted to represent hyperpycnal deposits. Comparison with historical records and aerial photos reveal that these deposits are associated with tropical cyclone landfalls throughout the 80-year historical record. These results suggest that such deposits can be used as a tool to investigate climate records in sedimentary successions in small estuarine and lagoonal deltas.
Syndepositional deformation features can impact depositional geometries, but can they also represent fluid conduits active throughout a strata's entire history? In this paper, Budd et al. address this question using petrographic, geomechanical, and clumped-isotope analysis of diagenetic products in syndepositional fractures (Permian, Guadalupe Mountains, New Mexico, USA). Results show that the complex diagenetic fills in the fractures are multigenerational, and represent numerous, repeated fracture openings, fills, and reactivation over ~260 Ma since deposition. The results highlight the potential for re-activated syndepositional fractures in a carbonate platform to dominate fluid flow in that platform and to drive the diagenetic history of adjacent host rocks.
The shapes of sedimentary particles can be useful for elucidating many aspects of their history, from origin to deposition to diagenesis. Livsey et al. examine the utility of grain shape as a depositional proxy for glacially influenced sediment within the James Ross Basin and Joinville Plateau of the Antarctic Peninsula from the Eocene to the present. The results from Fourier shape analyses of more than 6,000 quartz grains reveal changes through time, associated with climate shifts. It shows that grain roughness increases with the onset of increased glaciation from the Late Oligocene to Middle Miocene, after an Early Pliocene warm period, and from the Pliocene to Pleistocene; in contrast, grain roughness decreases from the Middle Miocene through an Early Pliocene warm period. This study highlights the utility of Fourier grain-shape analysis for understanding the history of grains, and its potential application in polar studies.
Fourier grain-shape analysis of Antarctic marine core: the relative influence of provenance and glacial activity on grain shape by Daniel N. Livsey, Alexander R. Simms, Warren G. Clary, Julia S. Wellner, John B. Anderson, and John P. Chandler
Recently, numerous studies have examined the possible role of bacterially induced mineral precipitation (organomineralization). Laboratory studies represent an important means to explore these processes, but Gallagher et al. reveal how the presence of phosphate in media (needed for microbial growth) may affect rate, morphology and mineralogy of precipitated minerals. The results of this study reveal potential pitfalls that could cause misinterpretation of laboratory biomineralization experiments.
