Prograding distributive fluvial systems—geomorphic models and ancient examples

Recent work indicates that most modern continental sedimentary basins are filled primarily by distributive fluvial systems (DFS). In this article we use depositional environment interpretations observed on Landsat imagery of DFS to infer the vertical succession of channel and overbank facies, including paleosols, from a hypothetical prograding DFS. We also present rock record examples that display successions that are consistent with this progradational model. Distal DFS facies commonly consist of wetland and hydromorphic floodplain deposits that encase single channels. Medial deposits show larger channel belt size and relatively well-drained soils, indicating a deeper water table. Proximal deposits of DFS display larger channel belts that are amalgamated with limited or no soil development across the apex of the DFS. The resulting vertical sedimentary succession from progradation will display a general coarsening-upward succession of facies. Depending on climate in the sedimentary basin, wetland and seasonally wet distal deposits may be overlain by well-drained medial DFS deposits, which in turn are overlain by amalgamated channel belt deposits. Channel belt size may increase upward in the section as the DFS fills its accommodation. Because the entry point of rivers into the sedimentary basin is relatively fixed as long as the sedimentary basin remains at a stable position, the facies tracts do not shift basinward wholesale. Instead, we hypothesize that as the DFS fills its accommodation, the accommodation/sediment supply (A/S) ratio decreases, resulting in coarser sediment upward in the section and a greater degree of channel belt amalgamation upward as a result of reworking of older deposits on the DFS. An exception to this succession may occur if the river incises into its DFS, where partial sediment bypass occurs with more proximal facies deposited basinward below an intersection point for some period of time. Three rock record examples appear to be consistent with the hypothesized prograding DFS signal. The Blue Mesa and Sonsela members of the Chinle Formation at Petrified Forest National Park, Arizona; the Tidwell and Salt Wash members of the Morrison Formation in southeastern Utah; and the Pennsylvanian–Permian Lodéve Basin deposits in southern France all display gleyed paleosols and wetland deposits covered by better-drained paleosols, ultimately capped by amalgamated channel belt sandstones. In the Morrison Formation succession, sediments that represent the medial deposits appear to have been partially reworked and removed by the amalgamated channel belts that show proximal facies, indicating that incomplete progradational successions may result from local A/S conditions. The prograding DFS succession provides an alternative hypothesis to climate change for the interpretation of paleosol distributions that show a drying upward succession

Archean to Recent aeolian sand systems and their preserved successions: current understanding and future prospects

The sedimentary record of aeolian sand systems extends from the Archean to the Quaternary, yet current understanding of aeolian sedimentary processes and product remains limited. Most preserved aeolian successions represent inland sand-sea or dunefield (erg) deposits, whereas coastal systems are primarily known from the Cenozoic. The complexity of aeolian sedimentary processes and facies variability are under-represented and excessively simplified in current facies models, which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour, and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes. Archean and Neoproterozoic aeolian successions remain poorly constrained. Palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts. During the Triassic, widespread desert conditions prevailed across much of Europe. During the Jurassic, extensive ergs developed in North America and gave rise to anomalously thick aeolian successions. Cretaceous aeolian successions are widespread in South America, Africa, Asia, and locally in Europe (Spain) and the USA. Several Eocene to Pliocene successions represent the direct precursors to the present-day systems. Quaternary systems include major sand seas (ergs) in low-lattitude and mid-latitude arid regions, Pleistocene carbonate and Holocene–Modern siliciclastic coastal systems. The sedimentary record of most modern aeolian systems remains largely unknown. The majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes, whereas successions representing linear and star dunes remain under-recognized. Research questions that remain to be answered include: (i) what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology; (ii) what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation; (iii) can sophisticated four-dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation; and (iv) is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of Earth history when large-scale aeolian systems were active, or has the evidence to support the existence of other major desert basins been lost for many periods throughout Earth history

Developmental Reaction Norms: the interactions among allometry, ontogeny and plasticity

How micro- and macroevolutionary evolutionary processes produce phenotypic change is without question one of the most intriguing and perplexing issues facing evolutionary biologists. We believe that roadblocks to progress lie A) in the underestimation of the role of the environment, and in particular, that of the interaction of genotypes with environmental factors, and B) in the continuing lack of incorporation of development into the evolutionary synthesis. We propose the integration of genetic, environmental and developmental perspectives on the evolution of the phenotype in the form of the concept of the developmental reaction norm (DRN) The DRN represents the set of multivariate ontogenies that can be produced by a single genotype when it is exposed to environmental variation. It encompasses: 1) the processes that alter the phenotype throughout the ontogenetic trajectory, 2) the recognition that different aspects of the phenotype are (and must be) correlated and 3) the ability of a genotype to produce phenotypes in different environments. This perspective necessitates the explicit study of character expression during development, the evaluation of associations between pairs or groups of characters (e.g., multivariate allometries), and the exploration of reaction norms and phenotypic plasticity. We explicitly extend the concept of the DRN to encompass adjustments made in response to changes in the internal environment as well. Thus, ‘typical’ developmental sequences (e.g., cell fate determination) and plastic responses are simply manifestations of different scales of ‘environmental’ effects along a continuum. We present: (1) a brief conceptual review of three fundamental aspects of the generation and evolution of phenotypes: the changes in the trajectories describing growth and differentiation (ontogeny), the multivariate relationships among characters (allometry), and the effect of the environment (plasticity); (2) a discussion of how these components are merged in the concept of the developmental reaction norm; and (3) a reaction norm perspective of major determinants of phenotypes: epigenesis, selection and constraint