Thursday, November 14, 2013

A Look Back…5 Years: Valleys…Only in the Finales?

Sequence stratigraphy aims to subdivide the sedimentary record into genetically related units separated by unconformities that represent pronounced temporal gaps. As many unconformities are erosional surfaces, incised valleys might be considered their prototypical expression. Five years ago, Strong and Paola utilized an experimental basin to examine the relation between morphodynamic and stratigraphic expressions of relative changes in sea level on a simulated fluvial-deltaic system. The results revealed complex interactions of erosion and deposition within this system that continuously modified the shape of incised valleys. The results suggested that these dynamics create “stratigraphic” valleys that never had physiographic expression in the fluvial landscape. 

Valleys that Never Were: Time Surfaces VersusStratigraphic Surfaces, by Nikki Strong and Chris Paola, Journal of Sedimentary Research, v. 78, p. 579-593.

Tuesday, November 5, 2013

Highlights—Climate and Fluvial Sand Bodies

Interpretation of controls on sandbody architecture in continental settings is challenging because changes in tectonics, climate, and sea level can produce very similar architecture. Using a newly recognized fluvial style which represents the deposits of strongly seasonal river systems, Allen et al. interpret the record of climate change (variable precipitation and runoff regimes) as manifested by in the internal architecture of sandbodies in Mississippian–Permian strata of the Cumberland Basin, Atlantic Canada. Despite the tectonically-active setting of the basin, a coherent climate signal can be deconvolved from the tectonic and eustatic effects on the stratigraphic architecture. The suggestion that climatic factors exerted a primary control on basinal architecture runs counter to many previous interpretations which have tended to stress tectonic and eustatic controls.

Monday, November 4, 2013

Highlights—Microbes in the Crinkly Bed

Microbial-stromatolitic carbonate units are a characteristic feature of carbonate-evaporite successions throughout the stratigraphic record, and, commonly being dolomitized and porous, can form important hydrocarbon reservoirs. Here, Perri et al. describe with an Upper Permian (Zechstein) stromatolitic deposit (the "Crinkly Bed") that is closely associated with evaporites in northeast England.  The results reveal that this ~ meter-thick unit exhibits a range of cm-scale structures, from stromatolitic domes and cones through to ripple-like features. Despite a complex diagenetic history, it preserves grains and microbial remains, and retains some original geochemical signatures. This distinctive meter-thick stromatolite is interpreted to result from syn-sedimentary and post-sedimentary physical and microbial processes. These results, describing the features, extent and geometry of these stromatolites, highlights numerous challenges in distinguishing the abiotic precipitates from microbialites.

Friday, November 1, 2013

Highlights—Prograding Fluvial Fans

Recent advances in non-marine facies analysis of siliciclastic systems have recognized the deposits of large fluvial systems that exit upland catchment regions into fan-shaped accumulations. Here, Trendellet al. integrate sedimentologic, stratigraphic, paleopedologic, geochronological and petrographic observations to assess the depositional processes and forcing mechanisms of the Chinle Formation at Petrified Forest National Park, USA. Upsection changes in grain size, channel width and depth, mineralogical maturity, and character of paleosols are interpreted to represent deposition in a progradational, large fan. Changing paleosol character is interpreted to reflect progressively more upland deposits resulting from progradation, rather than climate shifts. These results illustrate another large fan system, and show how apparent shifts in climate can result from autogenic processes of fan progradation.