Dave Waltham

Computer modeling the internal architecture of carbonate platforms

A numerical computer model is described that calculates the internal architecture of carbonate platforms in response to varying values of carbonate production, subaerial and submarine erosion, sediment redeposition, and sea-level changes. The computer-generated sections closely resemble large-scale outcrops and interpreted seismic profiles through carbonate platforms. Stillstand and transgressive sequences have prograding and downlapping platform geometries with lagoons developing in transgressive systems. Regressive sequences have downlapping clinoforms and erosional upper surfaces. Glacioeustatic scale cycles have a major control on platform geometry with erosional sequence boundaries developing during low stands and platform drowning occurring during transgressive periods. Lowstand downlapping wedges are minor features when compared with clastic systems, and major progradation and downlap of slope deposits develop with transgressions and flooding of platform tops. Carbonate erosion rates are varied and have an important effect on the morphology of foodback surfaces, which have a major control on platform top production. The computer program contributes to the analysis of carbonate systems in two ways: it gives a visual picture of the quantitative effects of the many parameters controlling carbonate geometries, and it aids quantitative analysis of the architectures and time scales of ancient outcrop or seismic sequences.

Flow Transformations in Particulate Gravity Currents

The nature, thickness, and location of deposits from a particulate gravity current is strongly influenced by whether the flow is concentrated or dilute, whether it is laminar or turbulent, and whether it is supercritical or subcritical. These transitions are causally linked, because there is a clear contrast between a concentrated, laminar flow-type (e.g., pyroclastic flows and debris flows) and a dilute, turbulent flow-type (e.g., pyroclastic surges and turbidity currents). In this paper it is shown that the primary transition is from a dense to a less dense current, leading, in turn, to a transition from laminar to turbulent flow. This density transition can be caused by interface instability or by vigorous entrainment of ambient fluid at a hydraulic jump. It is also shown in this paper that hydraulic jumps occur at Froude numbers significantly different from unity. These concepts are confirmed by previously published data from a gravity current in the San Dimas Reservoir.

Computer modelling of the influence of tectonics on sequence architecture of coarse-grained fan deltas

Abstract Fan delta deposition and extensional faulting are combined in a computer model using a general tectonosedimentary forward modelling equation. Fan delta forms and architectures resulting from tectonic activity are investigated using this model. Fan deltas formed during periods of tectonic quiescence are shown to have a sheet-like form and grow mainly as a result of progradation. Topset development is limited, resulting in the development of oblique to sigmoidal clinoforms. Fan deltas formed during periods of tectonic activity have a more wedge-shaped form and are restricted to a region adjacent to the controlling fault. These fan deltas grow as a result of both progradation and aggradation, resulting in the development of sigmoidal clinoforms. Under conditions of constant tectonic subsidence and sediment supply, fan deltas are shown to undergo a brief period of progradation followed by aggradation and a long period of retrogradation. This is a consequence of an increasing amount of sediment being deposited as topset material as the delta progrades and the position of the offlap break being displaced basinward during hangingwall subsidence. Other controls on fan delta architecture, such as variations in sediment supply and sea/lake level, are also considered. Pleistocene-Recent, Gilbert-type fan deltas from the southern margin of the Gulf of Corinth rift, Greece contain many of the features described above and modelling is used to investigate whether it is possible to identify the controls on their sequence architectures.

Mathematical modelling of growth strata associated with fault-related fold structures

Abstract Previous geometric models of fault-related folding have been limited in their ability to model sedimentation contemporaneous with contractional deformation. Waltham’s 1992 general forward modelling equation allows more realistic models of sedimentation to be combined with mathematical models of fault-bend folding, fault-propagation folding and limb-rotation associated with detachment folding. Using average rates of fault slip and sedimentation derived from the literature, the mathematical models are used to predict stratal geometries associated with each of the major styles of fault-related folding in both sub-marine and sub-aerial settings. Both background sedimentation and local erosion, transport and deposition as a result of fold growth are modelled. Comparison is made with natural examples and the utility of growth strata architectures in the assessment of fault-fold kinematics assessed.

Reservoir Implications of Modern Karst Topography

Tropical karst landscapes exhibit a predominance of positive relief features relative to the negative relief features (i.e., sink holes, dolines, etc.) of temperate karst areas and yet, paradoxically, have never been observed in the subsurface on seismic sections. To understand why this is the case, topographic profiles were made over the positive karst relief features of China, Java, and Jamaica and were used to construct synthetic seismic sections. The results indicate that in most instances the magnitude of the topographic relief is sufficient to be seen on seismic sections. A possibility of misinterpreting karst highs as reefs seems clear, but more important in explaining the paradox is the fact that the magnitude of modern karst relief expression is largely attributable to uplift over an extended period. The probability of preservation of these positive relief solution residuals is dependent on the karstification process being aborted by the deposition of overlying sediment, an event not likely to occur in an uplift area. Uplift of well-lithified carbonate minimally results in an areally extensive runoff surface that controls the development of positive karst landforms. Uplift also commonly induces faulting and fracturing that further control the distribution of karst land forms. Hydrocarbon production from the resulting karst is geared to matrix porosity: the higher the matrix porosity, the greater the rate of deliverability of matrix oil to produced fracture and karst conduit oil. In contrast, the lowering of the base level of erosion due to a fall in sea level is generally less than that induced by uplift and, consequently, minimizes positive karst relief development relative to that of uplift. Subaerial exposure induced by passive sea level fall also minimizes the potential for the related occurrence of fracturing and faulting, which of itself mitigates against the general development of positive karst relief. The subaerial consequences of passive sea level fall are commonly preserved in the geologic record and result in karst reservoirs dominated by extensive moldic and vuggy porosity interconnected by solution channels. These reservoirs lack the production problems commonly attending fractured karst reservoirs with little matrix porosity.

Phase-lagged amplitude modulation of hemipelagic cycles: A potential tool for recognition and analysis of sea-level change

Many ancient rhythmic hemipelagic sequences have been interpreted to record orbital variations, but the exact nature of the climatic and depositional transfer functions responsible for this link remains poorly understood. Two-dimensional numerical simulations were used to explore selected aspects of orbital signal distortion in linked siliciclastic and hemipelagic systems. The models suggest that transfer of multiorder (e.g., 20, 100, and 400 k.y.) oscillations in relative sea level into the hemipelagic record produces an inherent amplitude distortion of the shorter-period (e.g., 20 k.y.) cycle. This distortion gives rise to amplitude modulation (AM), which is qualitatively similar to AM of orbitally driven changes in insolation (e.g., eccentricity modulation of precession-driven cycles). However, unlike the orbitally driven AM, synthesized AM is distinctly phase shifted relative to the stratigraphic record of the long-period (e.g., 100 k.y., 400 k.y.) cycle as a result of sealevel‐driven changes in the storage capacity of nearshore through alluvial parts of the source siliciclastic system. Hence, multiorder changes in sea level can leave a distinct AM signature in dilution-affected hemipelagic records, thus making hemipelagic rhythms due to eccentricity-forced sea-level changes distinguishable from other types of orbitally driven hemipelagic cyclicity.

Stratigraphic modelling of turbidite prospects to reduce exploration risk

This article presents an integrated workflow to model the evolution of ancient turbidity currents on a 3D structurally reconstructed palaeo-seafloor, allowing ancient turbidite sediment distributions to be estimated. Effective use of such approaches requires efficient model-inversion procedures so that model parameters (e.g. flow dimensions, densities etc.) can be estimated from any available data. It is shown that a directed Monte Carlo approach (i.e. a simple genetic search algorithm) is very effective. A case study of a Mesozoic prospect in the UK North Sea shows the power of these methods to discriminate between potentially attractive sediment-source locations. The main power of this approach lies in its ability to exclude many, otherwise attractive, sedimentation scenarios.

Particulate gravity currents on Venus

Canali are moderately sinuous channels, typically a few kilometers wide and hundreds of kilometers long, that occur principally on the plains of Venus. Plausible hypotheses for their formation include the following: open channels cut by exotic, low-viscosity lavas; roofed-over basaltic lava channels; or water on a cooler, wetter ancient Venus. Although it is accepted that a fluid cut these channels, none of these hypotheses are entirely satisfactory. It is therefore prudent to investigate other explanations. A particulate gravity current is a rapidly moving, sediment-laden flow that moves downslope as a result of its high density compared to the ambient fluid. This high density is produced by suspension of dense particles in a lower-density fluid. As these flows are largely driven by slope, rather than by momentum, they are potentially capable of traveling great distances, producing extensive channel systems. We apply this process to Venus, exploring its channel-forming potential via mathematical modeling and morphological comparison of submarine channels on Earth to canali on Venus. Results of our modeling show that atmospheric particulate gravity currents are physically and geologically plausible on Venus. The potential of this process to form channels of great length is such that particulate gravity currents can be considered as an alternate explanation for canali genesis.

Decoupled flexure in the South Pyrenean Foreland

Complex subsidence patterns during the mid-late Eocene of the easternmost South Pyrenean foreland are well explained by a decoupled flexure model in which upper crustal buckling is accommodated by ductile flow in the mid-lower crust while, simultaneously and independently, the lower lithosphere flexes above a ductile asthenosphere. The resulting predicted subsidence values agree with observation to well within the estimated uncertainties. The resulting lithospheric effective elastic thickness (EET) estimate (10±3 km) is low but similar to values obtained in the South Pyrenean foreland by other authors. The separate EET obtained for the upper crust (1.5±0.8 km) also agrees with tentative values given by other workers.

Error Estimation in Decompacted Subsidence Curves

Subsidence curves, based on decompacted sediment thicknesses, are generally displayed without indicating depth or age errors. This makes their interpretation ambiguous, leading to incorrect identification of subsidence pulses and incorrect identification of periods of steady subsidence. Error analysis should also form part of thermal maturation studies based on such subsidence curves. Formal error analysis of subsidence curves is a complex problem because of the iterative and nonlinear nature of the decompaction calculations. This paper presents an alternative approach, based on Monte Carlo simulation, that allows subsidence curves to be calculated along with error estimates. We also show that identification of subsidence pulses and identification of periods of steady subsidence cannot be made on the basis of such subsidence curves, even when errors are shown, because errors on successive points on the subsidence curves are correlated. Instead, an ensemble of subsidence rate curves must be calculated at the same time as the ensemble of subsidence curves so that a mean subsidence rate curve can be calculated along with associated error estimates.