Shallow-marine sequences as the building blocks of stratigraphy: insights from numerical modelling

Two types of depositional sequences can be defined within the sequence stratigraphic framework: the parasequence and the high‐frequency sequence. Both sequences consist of stacked regressive and transgressive deposits. However, a parasequence forms under conditions of overall sea‐level rise, whereas a high‐frequency sequence forms as the sea level oscillates which results in typical forced regressive deposits during sea‐level fall. Both depositional sequences may develop over comparable temporal (10–100 kyr) and spatial (1–20 km wide and 1–40 m thick) scales. Numerical modelling is used to compare the architecture, preservation potential, internal volumes, bounding surfaces, condensed and expanded sections and facies assemblages of parasequences and high‐frequency sequences. Deposits originating from transgression are less pronounced than their regressive counterparts and consist of either preserved backbarrier deposits or shelf deposits. Shoreface deposits are not preserved during transgression. The second half of the paper evaluates in detail the preservation potential of backbarrier deposits and proposes a mechanism that explains the occurrence of both continuous and discontinuous barrier retreat in terms of varying rates of sea‐level rise and sediment supply. The key to this mechanism is the maximum washover capacity, which plays a part in both barrier shoreline retreat and backbarrier‐lagoonal shoreline retreat. If these two shorelines are not balanced, then the retreat of the coastal system as a whole is discontinuous and in time barrier overstep may take place.     

Geomorphological and sequence stratigraphic variability in wave‐dominated,shoreface‐shelf parasequences

Abstract Physical stratigraphy within shoreface‐shelf parasequences contains a detailed, but virtually unstudied, record of shallow‐marine processes over a range of historical and geological timescales. Using high‐quality outcrop data sets, it is possible to reconstruct ancient shoreface‐shelf morphology from clinoform surfaces, and to track the evolving morphology of the ancient shoreface‐shelf. Our results suggest that shoreface‐shelf morphology varied considerably in response to processes that operate over a range of timescales. (1) Individual clinoform surfaces form as a result of enhanced wave scour and/or sediment starvation, which may be driven by minor fluctuations in relative sea level, sediment supply and/or wave climate over short timescales (10¹?10³ years). These external controls cannot be distinguished in vertical facies successions, but may potentially be differentiated by the resulting clinoform geometries. (2) Clinoform geometry and distribution changes systematically within a single parasequence, reflecting the cycle in sea level and/or sediment supply that produced the parasequence (10²?10⁵ years). These changes record steepening of the shoreface‐shelf profile during early progradation and maintenance of a relatively uniform profile during late progradation. Modern shorefaces are not representative of this stratigraphic variability. (3) Clinoform geometries vary greatly between different parasequences as a result of variations in parasequence stacking pattern and relict shelf morphology during shoreface progradation (10⁵?10⁸ years). These controls determine the external dimensions of the parasequence.     

Assessing the worldwide developments of national spatial data clearinghouses

Many countries have spent considerable resources over the past few years debating optimal national spatial data infrastructures. One of the (main) elements of these infrastructures is the national spatial data clearinghouse, which facilitates access to required spatial data and provides complementary services. With this in mind, in April 2000, 2001, 2002 and December 2000, 2001, 2002, a web survey was carried out to assess systematically the developments of these national clearinghouses worldwide. Regarding the development in the number of implementations, it can be considered a worldwide success. However, of concern are the declining trends in use, management and content. One of the main reasons for these negative trends could be the dissatisfaction of the spatial data community with the functional capability of current clearinghouses. The functional capabilities of clearinghouses should likely be changed from a data-oriented to a user and application-oriented focus. This is in accord with the objectives of the second generation of spatial data infrastructures. The main factors, therefore, that will have positive impacts on developments in this field are the inclusion of web services, stability of funding and creation of user-friendly interfaces.     

Spatial data infrastructures as complex adaptive systems

Many researchers throughout the world have been struggling to better understand and describe spatial data infrastructures (SDIs). Our knowledge of the real forces and mechanisms behind SDIs is still very limited. The reason for this difficulty might lie in the complex, dynamic and multifaceted nature of SDIs. To evaluate the functioning and effects of SDIs we must have a proper theory and understanding of their nature. This article describes a new approach to understanding SDIs by looking at them through the lens of complex adaptive systems (CASs). CASs are frequently described by the following features and behaviours: complexity, components, self-organization, openness, unpredictability, nonlinearity and adaptability, scale-independence, existence of feedback loop mechanism and sensitivity to initial conditions. In this article both CAS and SDI features are presented, examined and compared using three National SDI case studies from the Netherlands, Australia and Poland. These three National SDIs were carefully analysed to identify CAS features and behaviours. In addition, an Internet survey of SDI experts was carried out to gauge the degree to which they consider SDIs and CASs to be similar. This explorative study provides evidence that to a certain extent SDIs can be viewed as CASs because they have many features in common and behave in a similar way. Studying SDIs as CASs has significant implications for our understanding of SDIs. It will help us to identify and better understand the key factors and conditions for SDI functioning. Assuming that SDIs behave much like CASs, this also has implications for their assessment: assessment techniques typical for linear and predictable systems may not be valid for complex and adaptive systems. This implies that future studies on the development of an SDI assessment framework must consider the complex and adaptive nature of SDIs.     

A Network Perspective on Spatial Data Infrastructures: Application to the Sub-national SDI of Flanders (Belgium)

In order to provide a basis for quantitative and qualitative assessment of SDI-performance, a clear definition and a theoretical framework for SDI are needed. From the various definitions and frameworks of SDI available in the literature, the productional and the geographic information process perspective were selected, combined and extended. From the productional perspective, SDI-development is a dynamic process in which suppliers and users of spatial data interact to add value to the data by using them in applications and processes. The geographic information process perspective incorporates the "information acquisition – delivery – usage" chain. The extension proposed in this article leads to a network perspective on SDI. The nature of this perspective is described for the Flemish SDI. Its applicability to characterise and underpin the assessment of SDI is tested using a Social Network Analysis (SNA) focusing on the network structure parameters "density", "distance" and "centrality". The SNA confirms the applicability, usability and extensibility of the network perspective to characterize the SDI, to describe the data flows between stakeholders and to analyse the behaviour of the different (types) of stakeholders within the network. We conclude that SNA should likely be complemented by an impact analysis of different SDI setups on business processes which will provide a good basis for a holistic SDI-performance assessment.     

Preservation of cross-sets due to migration of current ripples over aggrading and non-aggrading beds: comparison of experimental data with theory

An experimental study of the preservation of cross-sets during the migration of current ripples under aggrading and non-aggrading conditions was conducted in order to test the modified Paola–Borgman theory for distribution of cross-set thickness as a function of distribution of bed-wave height. In a series of flume experiments, the geometry and migration characteristics of the ripples did not vary systematically with aggradation rate and are comparable to other flume and river data.
Mean cross-set thickness/mean formative bed-wave height is less than 0·4, and mean cross-set thickness/mean bed-wave height is less than 0·53. In the present experiments, the primary control of cross-set thickness is the variability of ripple height. Aggradation rate accounts for only 1–7% of the total cross-set thickness.
A two-parameter gamma density function was fitted to histograms of ripple height to determine the value of parameter a needed for the modified Paola–Borgman model. This model underestimates cross-set thickness because of its assumption that bed-form height spreads evenly above and below the mean bed level, which is not the case in reality. Mean cross-set thickness is predicted quite well if the model constant is increased to 1·3.     

Oblique aggradation: a novel explanation for sinuosity of low‐energy streams in peat‐filled valley systems

Low‐energy streams in peatlands often have a high sinuosity. However, it is unknown how this sinuous planform formed, since lateral migration of the channel is hindered by relatively erosion‐resistant banks. We present a conceptual model of Holocene morphodynamic evolution of a stream in a peat‐filled valley, based on a palaeohydrological reconstruction. Coring, ground‐penetrating radar (GPR) data, and ¹⁴C and OSL dating were used for the reconstruction. We found that the stream planform is partly inherited from the Late‐Glacial topography, reflecting stream morphology prior to peat growth in the valley. Most importantly, we show that aggrading streams in a peat‐filled valley combine vertical aggradation with lateral displacement caused by attraction to the sandy valley sides, which are more erodible than the co‐evally aggrading valley‐fill. Owing to this oblique aggradation in combination with floodplain widening, the stream becomes stretched out as channel reaches may alternately aggrade along opposed valley sides, resulting in increased sinuosity over time. Hence, highly sinuous planforms can form in peat‐filled valleys without the traditional morphodynamics of alluvial bed lateral migration. Improved understanding of the evolution of streams provides inspiration for stream restoration. Copyright © 2016 John Wiley & Sons, Ltd.     

Delayed delivery from the sediment factory: modeling the impact of catchment response time to tectonics on sediment flux and fluvio‐deltaic stratigraphy

The sediment flux from a catchment is driven by tectonics and climate but is moderated by the geomorphic response of the landscape system to changes in these two boundary conditions. Consequently, catchment response time and the non‐linear behavior of landscapes in response to boundary condition change control the downstream propagation of climatic or tectonic perturbations from catchments to neighboring basins. In order to investigate the impact of catchment response time on sediment flux, we integrated a spatially‐lumped numerical model PaCMod, with new routines simulating the evolution of landscape morphology and erosion rates under tectonic and climatic forcing. We subsequently applied the model to reconstruct the sediment flux from a tectonically perturbed catchment in central Italy. Finally, we coupled our model to DeltaSim, a process‐response model simulating fluvio‐deltaic stratigraphy, and investigated the impact of catchment response time on stratigraphy, using both synthetic scenarios and a real world system (Fucino Basin, central Italy). Our results demonstrate that the differential response of geomorphic elements to tectonic and climatic changes induces a complex sediment flux signal, and produces characteristic stratigraphic architectures and shoreline trajectories. Copyright © 2014 John Wiley & Sons, Ltd.     

Post-caldera dacites from the Santorini volcanic complex,Aegean Sea,Greece: an example of the eruption of lavas of near-constant composition over a 2,200 year period

The post-caldera Kameni islands of the Santorini volcanic complex, Aegean Sea, Greece are entirely volcanic and were formed by eleven eruptions between 197 B.C. and 1950. Petrographic, mineral chemical and whole-rock major and trace element data are presented for samples of lava collected from the products of seven eruptive cycles which span the entire period of activity. The main phenocryst phases are plagioclase, clinopyroxene, orthopyroxene and titaniferous magnetite, which are weakly zoned (e.g. plagioclase — An₅₅ to An₄₂). The lavas are typical calc-alkaline dacites and show a restricted range of composition (from 64.1 to 68.4 wt. % SiO₂). The phenocrysts were in equilibrium with the melts at temperatures of 960–1012 °C, pressures of 800–1500 bars and oxygen fugacities of 10^–9.6-10^–9.9 bars. The pre-eruptive water content of the magmas was 3–4 wt. % but since the lavas contain only 0.1–0.4 wt. % H₂O, a considerable amount (about 0.01–0.015 km³) of water was lost prior to or during eruption. This indicates that the magmas rose to the surface gradually allowing the (largely) non-explosive loss of volatiles. The lavas were probably extruded initially from more or less cylindrical conduits which developed into fissures as the eruptions proceeded. The post-caldera lavas evolved from more mafic parental magmas (basalt-andesite) via fractional crystallization. The small range of compositional variation shown by these lavas can be explained in terms of near-equilibrium crystallization. Analyses of samples of lavas belonging to single eruption cycles and to individual flows indicate that the underlying magma chamber is compositionally zoned. The average composition of erupted magma has remained approximately constant since 1570 A.D. but that fact that the 197 B.C. magma was sligthly richer in SiO₂ provides additional evidence that the magma chamber is compositionally zoned. Crystal settling has not affected the composition of the magma over a 2,200 year period of time which indicates that the melts do not behave as Newtonian fluids. Zonation was thus probably established prior to the 197 B.C. eruption though it is possible that it is developed and maintained by crystal-liquid differentiation processes other than crystal settling (e.g. boundary layer crystallization). The data indicate that there has been no significant cooling during 2,200 years; the maximum amount of cooling is <50 °C and is probably less than 30 °C. Two hypotheses are considered to explain the thermal and chemical buffering of the post-caldera magma chamber: (i) The magma chamber is large and heat losses due to conduction are largely compensated by latent heat supplied by thick, partially crystalline cumulate sequences. (ii) Periodic influx of hot mafic magma, which does not mix with the dacitic magma, inhibits cooling. The second alternative is favored because the post-caldera lavas differ geochemically from the pre-caldera lavas which signifies that a new batch of magma was formed and/or emplaced after the catastrophic eruption of 1390 B.C., and hence that mafic magmas may still be reaching upper crustal levels.     

Evaluating alluvial stratigraphic response to cyclic and non‐cyclic upstream forcing through process‐based alluvial architecture modelling

Formation of alluvial stratigraphy is controlled by autogenic processes that mix their imprints with allogenic forcing. In some alluvial successions, sedimentary cycles have been linked to astronomically‐driven, cyclic climate changes. However, it remains challenging to define how such cyclic allogenic forcing leads to sedimentary cycles when it continuously occurs in concert with autogenic forcing. Accordingly, we evaluate the impact of cyclic and non‐cyclic upstream forcing on alluvial stratigraphy through a process‐based alluvial architecture model, the Karssenberg and Bridge (2008) model (KB08). The KB08 model depicts diffusion‐based sediment transport, erosion and deposition within a network of channel belts and associated floodplains, with river avulsion dependent on lateral floodplain gradient, flood magnitude and frequency, and stochastic components. We find cyclic alluvial stratigraphic patterns to occur when there is cyclicity in the ratio of sediment supply over water discharge (Q_s/Q_w ratio), in the precondition that the allogenic forcing has sufficiently large amplitudes and long, but not very long, wavelengths, depending on inherent properties of the modelled basin (e.g. basin subsidence, size, and slope). Each alluvial stratigraphic cycle consists of two phases: an aggradation phase characterized by rapid sedimentation due to frequent channel shifting and a non‐deposition phase characterized by channel belt stability and, depending on Q_s/Q_w amplitudes, incision. Larger Q_s/Q_w ratio amplitudes contribute to weaker downstream signal shredding by stochastic components in the model. Floodplain topographic differences are found to be compensated by autogenic dynamics at certain compensational timescales in fully autogenic runs, while the presence of allogenic forcing clearly impacts the compensational stacking patterns.