David Waltham

Carbonate production and offshore transport on a Late Jurassic carbonate ramp (Kimmeridgian, Iberian basin, NE Spain): evidence from outcrops and computer modelling

Abstract The complex interplay between shallow-water carbonate production, pelagic sedimentation, and sediment erosion and redeposition on a Kimmeridgian carbonate ramp is analysed from field data and computer modelling. Field data come from reconstructed cross-sections near Zaragoza and near Teruel, NE Spain. Inner ramp areas are dominated by coral patch reefs, ooid shoals and bioclastic packstones and grainstones, whereas middle ramp areas are typified by tempestites and pinnacle reefs. Outer ramp areas are dominated by carbonate muds and marls. The large-scale stratigraphic features of the ramp are simulated using the forward modelling computer program carbonate 6. A good match is found between real and simulated stratigraphic thicknesses along the two sections, large-scale internal stratigraphic geometries, amounts of aggradation and progradation, and the location of sequence boundaries, flooding surfaces and systems tracts in the two modelled cross-sections. To investigate the origin of the offshore carbonate mud, two synthetic stratigraphies, which both closely replicate the overall ramp geometries, were generated by computer simulation. These two simulations are based on two different hypotheses that had not been resolved from field investigation: (1) that most of the outer ramp mud was derived from erosion and redeposition from inshore areas; (2) that most of this mud originated from pelagic sedimentation. Detailed comparison between the simulated occurrence of sedimentary facies (characterized in the program by depth and process of deposition) and the real facies occurrence indicates that the closest match is given by hypothesis (1) where most of the offshore mud is produced by resedimentation from inshore areas. Offshore transport and resedimentation is thought to have been favoured by the windward orientation of the ramp, and offshore transport was highest during periods of sea-level highstands.

WHY DOES SALT START TO MOVE

Abstract This paper concerns mechanisms of salt (and ductile shale) movement. It investigates salt flow due to differential loading, folding of the overburden during compression and drag by a moving overburden. The approach is to compare the salt flux caused by these processes to that generated by buoyancy. It is demonstrated that overburden folding and drag by the overburden can, under commonly encountered conditions, result in greater amounts of salt movement than that produced by buoyancy or differential loading. These conclusions apply during the early stages of salt anticline, salt pillow and salt roller formation but not during the later stages of salt diapir and salt wall growth when buoyancy dominates. The quantitative significance of these alternatives to buoyancy is determined by considering an elastic plate overlying a viscous fluid. This is the simplest mathematical model that can reproduce the processes considered. The model shows that: (1) Under certain conditions, these mechanisms produce more salt movement than buoyancy. Differential loading dominates when the surface slopes become more than a small fraction of the slope of the salt top. Overburden buckling dominates if the in-plane stress exceeds a critical value. Drag dominates when the salt layer is thinner than a few hundred metres. (2) The strength of the overburden inhibits formation of salt diapirs, even those due to buoyancy, on wavelengths less than about 12 km.

COUPLED TWO-DIMENSIONAL DIAGENETIC AND SEDIMENTOLOGICAL MODELING OF CARBONATE PLATFORM EVOLUTION

We have used a new coupled two-dimensional diagenetic and sedimentological model to investigate the evolution of a simple aggrading carbonate platform subject to glacio-eustatic sea-level fluctuations. The model employs variable hydrological zones, internally defined from platform exposure and climate, to provide a framework within which diagenetic processes are simulated at specified rates. Simulations show a clear stacked sequence of diagenetic zones, which are readily recognizable because of their lateral continuity and distinct trends in diagenetic evolution from platform interior to margin. However, spatial associations between unconformity surfaces and underlying diagenetic zones that suggest causality are misleading. The interaction between high amplitude sea-level fluctuations and subsidence results in substantial overprinting and a complex diagenetic history that could not be unraveled by traditional stratigraphic and sedimentological methods.

Synthetic seismic reflection profile through the Ivrea zone–Serie dei Laghi continental crustal section, northwestern Italy

A geologic cross section, restored to its original horizontal orientation in Permian-Triassic time, has been constructed for the middle and lower continental crustal rocks of the Ivrea-Verbano zone and the adjacent Serie dei Laghi of northwestern Italy. Seismic P-wave velocities of a representative suite of rock samples were measured to high-pressure and high-temperature conditions. A synthetic seismic reflection profile, ∼76 km long and 30 km thick, was computed to compare what can be deduced from the seismic profile with what is known in much more detail from geologic mapping. Imaged features correspond closely to those seen on many present-day profiles, and the broad features of the tectonic evolution would be correctly interpreted, but important recumbent fold structures would be missed, and relationships between intrusive bodies and their country rocks would be unclear.

Mathematical modelling of sedimentary basin processes

Abstract Mathematical models of various sedimentary basin processes including hanging wall deformation, evaporite deposition, carbonate platform sedimentation and sequence development on active domino-style fault blocks are presented. Many of these processes may be formulated using the equation of continuity of an open system as a starting point. Sedimentary and tectonic processes may be combined into a single mathematical formulation, provided that an Eulerian coordinate system is used, and this ensures that tectonics and sedimentation are modelled as simultaneous rather than sequential processes. The resulting algorithms are fast, robust and applicable to many, geologically very different, situations, but may be limited to relatively simple examples. The conditions under which the models are numerically stable are also easily found. The abstract nature of the mathematical models requires that simply determined properties such as palaeo-water depth and palaeo-slope must be used as proxy facies markers. The main limitation of the method is that it is restricted to modelling processes which change slowly with time and space and the method can therefore not cope properly with episodic or chaotic processes.

Physical and seismic modelling of listric normal fault geometries

Abstract Scaled analogue models of listric fault systems display a characteristic rollover anticline and crestal collapse graben features. Faults nucleate in sequence into the hangingwalls of the crestal collapse graben and towards the footwall of the master fault. Superposition of crestal collapse grabens produces complex fault arrays. When the basal detachment surface is tilted a characteristic panel of unfaulted synfault stratigraphy forms adjacent to the detachment. These experiments produce structures with close similarities to listric growth fault systems on prograding delta slopes. Synthetic seismic sections of the experimental models have been produced using Kirchhoff integral techniques. These illustrate the complexities in trying to image complex structures in listric fault systems but also show good agreement with natural seismic examples.

The velocity description of deformation. paper 1: theory

Abstract There are several advantages in using velocities to describe the deformation of the crust and lithosphere induced by extension, compression, isostasy and other processes. (1) The method is very general. Any deformation style can be described in this way. Simple examples are: pure shear, inclined or vertical simple shear, bulk rotation; Airy or flexural isostasy and compaction. (2) Velocity fields can be combined to model more complex deformation. (3) The syn-tectonic evolution of parameters such as temperature, pressure, hydrocarbon maturity and porosity are easily modelled once the deformation velocities are specified. (4) The syn-tectonic evolution of sedimentary architectures can be modelled using a general tectono-sedimentary forward modelling equation which incorporates the deformation velocities. These advantages are illustrated by a computer model which simulates clastic sediments prograding into an active half-graben formed by extension above a listric fault. Processes included in this model are compaction of the sediments, hanging wall deformation by inclined simple shear and simultaneous footwall deformation. To avoid space problems, the velocity field for the hanging wall block above the non-rigid footwall must obey a simple contact condition.

Computer Modeling a Miocene Carbonate Platform, Mallorca, Spain

This paper describes a computer model based on algorithms that simulate processes of carbonate sediment production, deposition, erosion and redeposition, and sequence stratigraphy of carbonate platforms. We use the exceptionally well-exposed cliff sections through the reef-rimmed Miocene carbonate platform of Mallorca, Spain, to test the ability of our program to simulate an outcropping stratigraphy. When an outcrop-derived sea level curve and the average Holocene rates of production and erosion are used in our model, we can precisely match the natural and the synthetic stratigraphies. In addition, we can reconstruct the nonoutcropping parts of the platform. This modeling indicates the ability of our program to simulate the sequence stratigraphy, stratal geometries, and facies of a real carbonate platform. In addition, this modeling strongly suggests that the processes for which we have no field evidence and that we are not modeling, i.e., compaction, differential subsidence, and three-dimensional sedimentary processes, are unimportant factors in the development of the Llucmajor Platform of Mallorca. The model is then used to explore and bracket some of the rates of different processes considered to be important in controlling the development of this late Miocene platform. Carbonate production and a decrease in production from the reefal margin into lagoonal areas have important controls on slope progradation rates and drowning of lagoons. When model-matching stratal geometries and sea level curves, one can establish a precise relationship between rates of sea level rise and rates of production. However, the solution is not unique in that production rates may be doubled and the sea level curve periodicity halved, and the same result is achieved. Both outcrop and modeling data indicate that rates of sea level rise never exceeded production rates. Therefore, production and accommoda ion were limited by sea level. One consequence of this is that erosion rates are very important in controlling the carbonate stratigraphy. With a constrained sea level curve, erosion rates are refined from the modeling of the Mallorca outcrops by the levels of erosion surfaces generated during sea level falls. The outcrops and the modeling also indicate that the bulk of the carbonate production and platform progradation occurs during the transgressive and highstand periods rather than during periods of falling sea level. Condensed sections characterize lowstands and not transgressions. This and other features of the Mallorca platform illustrate the inapplicability of many published carbonate sequence stratigraphic models. Modeling has contributed to three advances in our analysis of this carbonate platform. First, this method is probably the best way to study the many effects that the different controlling parameters have on platform evolution. Second, if some parameters can be quantified from field analysis, then other parameters can be bracketed from modeling. Third, this method strongly suggests that the parameters we are not modeling have no important control on platform evolution. Accurate forward modeling of carbonate stratigraphies allows petroleum geologists to independently test sequence stratigraphic interpretations, reconstruct partially exposed or imaged carbonate stratigraphies, locate and quantify the cross sections of likely reservoir facies, illustrate the development and likely interconnections of reservoir facies, and predict stratigraphies around a bas n margin. End_Page 247------------------------------

The impact of fire on the Late Paleozoic Earth system.

Analyses of bulk petrographic data indicate that during the Late Paleozoic wildfires were more prevalent than at present. We propose that the development of fire systems through this interval was controlled predominantly by the elevated atmospheric oxygen concentration (p(O2)) that mass balance models predict prevailed. At higher levels of p(O2), increased fire activity would have rendered vegetation with high-moisture contents more susceptible to ignition and would have facilitated continued combustion. We argue that coal petrographic data indicate that p(O2) rather than global temperatures or climate, resulted in the increased levels of wildfire activity observed during the Late Paleozoic and can, therefore, be used to predict it. These findings are based upon analyses of charcoal volumes in multiple coals distributed across the globe and deposited during this time period, and that were then compared with similarly diverse modern peats and Cenozoic lignites and coals. Herein, we examine the environmental and ecological factors that would have impacted fire activity and we conclude that of these factors p(O2) played the largest role in promoting fires in Late Paleozoic peat-forming environments and, by inference, ecosystems generally, when compared with their prevalence in the modern world.

Sediment geometries and domino faulting

Abstract A general forward modelling scheme is used to produce a model of sediment geometry in half-grabens formed during extension of domino fault blocks. The model includes the effects of erosion, sediment transport, sedimentation and isostasy and simulates both syn- and post-rift sedimentary sequences. The geometries and relationships produced by the model are broadly similar to natural examples seen on North Sea seismic sections. This similarity is emphasized by the generation of synthetic seismic data using the numerically modelled geometry as a template. The unconformable nature of the boundary between syn- and post-rift seismic sequences is clearly visible on the synthetic data in the region of the fault block crest.