Sediment‐budget modelling of multi‐sourced basin fills: application to recent deposits of the western Adriatic mud wedge (Italy)

We analysed modern mass‐accumulation patterns on the western Adriatic mud wedge (Italy), an elongated belt of shelf mud formed by coalesced prodeltas of the Adige, Po, and Apennine rivers, as part of an integrated strategy aimed at producing a quantitative sediment budget model for muddy continental shelves sourced by multiple compositionally distinct fluvial systems. Sediment provenance and source‐specific accumulation rates of surface sediments were quantified by combining results of grain‐size analysis and geochemical analysis of specific size fractions with bulk mass accumulation rates. Statistical classification algorithms adapted to compositional data were used to partition the total (geochemical) variation of sediment properties into size‐related and provenance‐specific factors. We identified geochemically distinct fluvial end‐member sediment types in two different grain‐size fractions, which were grouped into sediments derived from the Apennine rivers, and sediments derived from the Po and Adige rivers. Compositional fingerprints (end‐member compositions) of each source area were estimated by taking into account relative rates of fluvial sediment supply from rivers as predicted by numerical modelling. The end members allow us to explain geochemical compositional variation of mud‐wedge surface sediments in terms of provenance and size‐selective dispersal, and map mass accumulation rates of sediments from individual source areas (grain size<63 μm), as well as bulk sand accumulation rates (grain size>63 μm) across the western Adriatic mud wedge. The source‐specific rates of fine‐grained sediment supply derived from geostatistical estimates of mass‐accumulation rates were used to calibrate the numerical model of sediment supply to present‐day conditions.     

Genetically meaningful decomposition of grain-size distributions

All spatio-temporal grain-size patterns in sediments can be characterized by a mathematical representation of (un)mixing. This implies that an inverse model of (un)mixing would be ideally suited to obtain genetically meaningful interpretations of observed grain-size distributions (GSDs). GSDs are therefore often decomposed into theoretical end members by parametric curve-fitting procedures. Many researchers have been tempted to use goodness-of-fit measures as a means of justifying such decompositions in the absence of generic process-based models of end-member GSDs. A critical examination of parametric curve fitting through a series of numerical experiments shows that the goodness-of-fit of an approximation may be a poor guide to its genetic significance. The genetic interpretation of GSDs is a poorly constrained problem that cannot be solved without taking into account the geological context of GSDs, which may be captured by the covariance structure of grain-size classes across a series of GSDs sampled in a contiguous area. Curve-fitting methods cannot exploit this geological context, which explains why the geological relevance of curve-fitting results obtained in black-box mode is questionable. The desired genetic interpretation of GSDs can be obtained by applying the end-member-modelling algorithm EMMA to a series of GSDs simultaneously. Many end-member GSDs estimated by EMMA do not conform to one of the popular theoretical GSD models. Consequently, parametric curve fitting with theoretical distributions is more likely to obscure than to reveal the existence of genetically significant grain-size populations in sediments, especially if such populations are present in small proportions.     

Oligocene to Early Miocene sedimentation and tectonics in the southern part of the Calabrian-Peloritan Arc (Aspromonte, southern Italy): a record of mixed-mode piggy-back basin evolution

The Calabrian-Peloritan Arc (southern Italy) represents a fragment of the European margin, thrusted onto the Apennines and Maghrebides during the Europe-Apulia collision in the late Early Miocene. A reconstruction of the pre-Middle Miocene tectono-sedimentary evolution of the southern part of the Calabrian-Peloritan Arc (CPA) is presented, based on a detailed analysis of the Stilo-Capo ?Orlando Formation (SCO Fm). Deposition of the SCO Fm occurred in a series of mixed-mode piggy-back basins. Basin evolution was controlled by two intersecting fault systems. A NW-SE oriented system delimited a series of sub-basins and fixed the position of feeder channels and submarine canyons, whereas a NE-SW oriented system controlled the axial dispersal of coarse-grained sediments within each of the sub-basins. From base to top, sedimentary environments change from terrestrial and lagoonal to upper bathyal over a timespan of approximately 12 Myr (late Early Oligocene-late Early Miocene). During this interval, extensional tectonic activity alternated with oblique backthrusting events, related to dextral transpression along the NW-SE oriented faults. This produced a characteristic pulsating pattern of basin evolution. Oligocene-Early Miocene evolution of the W. Mediterranean basin was dominated by ‘roll back’ of the Neotethyan oceanic lithosphere. Considerable extension in the overriding European Plate gave rise to the formation of a back arc-thrust system. The initial stages of Calabrian Basin evolution are remarkably similar to the evolution of rift basins in the back arc (Sardinia). The Calabrian basins, which are inferred to have originated as thin-skinned pull-apart basins, were subsequently incorporated into the Apennines-Maghrebides accretionary wedge by out-of-sequence thrusting, and became decoupled from the back arc. Periodic restabilization of the accretionary wedge, resulting in an alternation of backthrusting and listric normal faulting, provides an explanation for the structural evolution of these mixed-mode basins. The basins of the southern part of the CPA may be termed ‘spanner’ or ‘looper’ basins, in view of their characteristic pulsating structural evolution, superimposed upon their migration toward the foreland. This new term adequately accounts for the occurrence of tectonic inversions in long-lived piggy-back basins, as expected in the light of the dynamics of accretionary wedges.     

The life cycle of crevasse splays as a key mechanism in the aggradation of alluvial ridges and river avulsion

Accommodation space in the unconfined distal part of low‐gradient fluvial fans facilitates abundant floodplain deposition. Here, the development of crevasse splays plays a key role in the aggradation of alluvial ridges and subsequent river avulsion. This study presents an analysis of different stages in the evolution of crevasse splays based on observations made in the modern‐day Río Colorado dryland fluvial fan fringing the endorheic Altiplano Basin in Bolivia. A generic life cycle is proposed in which crevasse‐splay channels adjust towards a graded equilibrium profile with their lower‐lying distal termini acting as a local base level. Initial development is dominantly controlled by the outflow of floodwater, promoting erosion near the crevasse apex and deposition towards the splay fringes. When proximal incision advances to below the maximum level of floodplain inundation, return flow occurs during the waning stage of flooding. This floodwater reflux leads to a temporary repositioning of the local base level to the deeper trunk‐channel thalweg at the apex of the crevasse‐splay channels. The resultant decrease in the floodplainward gradient of these channels ultimately leads to backfilling and abandonment of the crevasse splay, leaving a subtle local elevation of the floodplain. Consecutive splays form an alluvial ridge through lateral amalgamation and subsequent vertical stacking, which is mirrored by the aggradation of their parent channel floor. As this alluvial ridge becomes increasingly perched above the surrounding floodplain, splay equilibration may cause incision of the levee crevasse down to or below its trunk channel thalweg, leading to an avulsion. The mechanisms proposed in this study are relevant to fluvial settings promoting progradational avulsions. The relatively rapid accumulation rate and high preservation potential of crevasse splays in this setting makes them an important constituent of the resultant fluvial stratigraphy, amongst which are hydrocarbon‐bearing successions. Copyright © 2018 John Wiley & Sons, Ltd.     

End-member modeling of compositional data: Numerical-statistical algorithms for solving the explicit mixing problem

Linear mixing models of compositional data have been developed in various branches of the earth sciences (e.g., geochemistry, petrology, mineralogy, sedimentology) for the purpose of summarizing variation among a series of observations in terms of proportional contributions of (theoretical) end members. Methods of parameter estimation range from relatively straightforward normative partitioning by (nonnegative) least squares, to more sophisticated bilinear inversion techniques. Solving the bilinear mixing problem involves the estimation of both mixing proportions and end-member compositions from the data. Normative partitioning, also known as linear unmixing, thus can be regarded as a special situation of bilinear unmixing with (supposedly) known end members. Previous attempts to model linear mixing processes are reviewed briefly, and a new iterative strategy for solving the bilinear problem is developed. This end-member modeling algorithm is more robust and has better convergence properties than previously proposed numerical schemes. The bilinear unmixing solution is intrinsically nonunique, unless additional constraints on the model parameters are introduced. In situations where no a priori knowledge is available, the concept of an “ optimal ” solution may be used. This concept is based on the trade-off between mathematical and geological feasibility, two seemingly contradictory but equally desirable requirements of the unmixing solution.