Tuesday, December 16, 2014

Highlights—Reef Sand Apron Hydrodynamics and Sediment

Reefs are prolific sediment sources; once produced, however, the sediment can be transported on-platform, and produce reef—reef sand apron—lagoon facies transitions. Although this general pattern is evident on many carbonate platforms, the details concerning the sedimentologic variability within and among these geomorphic elements and the processes controlling the sedimentologic differentiation are poorly constrained. To better understand the depositional variability of reef sand aprons, Wasserman and Rankey combine field, petrographic, and granulometric data from Holocene sediment with hydrodynamic observations and modeling of Aranuka Atoll, Kiribati. Results provide a predictive conceptual model for the depositional variability and processes operative in reef sand apron systems, which may be comparable to ancient reservoir analogs, many of which host prolific hydrocarbon resources.




Monday, December 15, 2014

Highlights—Large-Scale Fluvial Seismic Geomorphology

Although it is widely recognized that fluvial systems can change markedly down depositional dip, the details of variability are difficult to assess and quantify in the subsurface. In this study, Klausen et al. use data from six seismic volumes to trace and quantitatively analyze heterogeneity in fluvial strata in the Snadd Formation (Triassic, offshore Norway) across several hundred km. The results suggest that much of the deltaic depositional environment is mud-rich and channel width and aspect ratios (but not thickness) diminish in size downstream of a proximal apex point close to an interpreted delta-paleovalley transition apex. The conceptual framework provided by these data may provide a large-scale process, geomorphic, and stratigraphic context for more detailed studies of parts of this and other large-scale fluvial systems in the geologic record.


Spatial and temporal changes in geometries of fluvial channel bodies from the Triassic Snadd Formation of offshore Norway by Tore Grane Klausen, Alf Eivind Ryseth, William Helland-Hansen, Robert Gawthorpe, and Inger Laursen


Wednesday, December 10, 2014

Highlights—Neoproterozoic Impressions: Hail, Yes!

Throughout geologic history, it may have poured when it rained, but the record of hailstorms is nearly absent. In this study, Remin et al. explore the possible presence of hailstone impressions in strata of the Neoproterozoic-Cambrian transition of western Africa. Integrating experimental analyses with field observations, they describe the hailstone impressions and show features that clearly distinguish them from other sedimentary structures. Comparing these systems with modern atmospheric and sedimentologic processes can provide insights into the physical and chemical properties of the atmosphere that promoted or hindered the formation of hailstones in the past, and, indirectly, the paleoclimate and paleogeography of ancient continents.




Tuesday, December 9, 2014

Highlights—Tethyan Cretaceous Dolomite

Although dolomitic strata can form expansive and prolific reservoirs, dolomitization of shallow-water carbonates of the southern Tethyan platform is extremely rare. This study by Sena et al. seeks to evaluate “why” by assessing the processes of early dolomitization and the controls on the dolomite bodies distribution in the Lower Cretaceous Arabian carbonate platform. Integrating clumped isotopes, geochemistry, and isotopes in a framework of petrography and field observations, the results indicate early dolomite facilitated by the presence of microbial mats was re-equillibrated at shallow burial depths. They suggest that the study has implications on the definition of predictive rules on early dolomite occurrences this platform and on epeiric carbonate platforms in general.




Monday, December 8, 2014

Highlights—Cretaceous Carbon and Climate

Although the presence of a greenhouse climate is well established for the Cretaceous, documenting the details of the timing and magnitude of climate and oceanographic change is challenging due to the combination of local, regional, and global influences that shape the geologic record. This report by Joo and Sageman documents new δ13C data from three cores from the Late Cretaceous of the central Western Interior Basin to construct a composite δ13C reference curve that spans middle Cenomanian to early Campanian time, and which is linked to new radioisotopic and astrochronologic data. The new δ13C framework allows revised and improved age control to time equivalent sections with δ13C records, but which may lack in situ biostratigraphy and geochronology. Collectively, these data should provide insights to regional and global changes in carbon cycling and sea-level change, and deposition of organic-rich sediment.


Interior Basin. U.S.A. by Young Ji Joo and Bradley B. Sageman


Friday, December 5, 2014

Highlights—Asymmetric Orogenic Sands

Although critical wedge theory predicts asymmetrical exhumation, peak metamorphism and erosion of the pro- and retro- sides of an orogen, the implications for sandstone attributes are not well understood. Here, Nagel et al. use sandstone petrography, clay mineralogy, and heavy-mineral assemblages in Plio-Pleistocene sandstone from the Western Foothills of Taiwan to unravel the record of a rapidly subsiding foreland basin that records the collision between a volcanic arc and the Asian passive margin. Comparison of the provenance record from the western and eastern basins illustrates the opposing signature of unroofing and recycling, variability interpreted to be characteristic of an asymmetric orogenic wedge. The results illustrate how sandstone petrography could be used in ancient sedimentary basins to complement other indications of the polarity of subduction.


Provenance evolution duringarc–continent collision: sedimentary petrography of Miocene to Pleistocenesediments in the western foreland basin of Taiwan by Stefan Nagel, Sébastien Castelltort, Eduardo Garzanti, Andrew T. Lin, Sean D. Willett, Frédéric Mouthereau, Mara Limonta, and Thierry Adatte