Digital rocks: linking forward modelling to carbonate facies

Forward stratigraphic models usually display sediment types on simulated stratigraphic profiles as ‘facies’ defined only by their depth of deposition. More recently, ‘facies’ have been defined and displayed in terms of the dominant processes of deposition (e.g. in situ growth, pelagic production, turbidite deposition). Standard carbonate facies; that is, the Dunham classification, are defined by rock textures and grain composition that imply that a combination of processes acted together to generate a facies. For example, a bioclastic wackestone is a matrix‐supported rock containing up to 90% matrix and > 10% shelly grains. In terms of modelled processes, the muddy matrix could be generated by: (i) reworking of the shallow platform sediments, (ii) from pelagic deposition, or (iii) in situ production. A traditional depth of deposition process display would not be able to distinguish such a wackestone from any other facies deposited at this water depth and a majority process display would not combine reworked, pelagic muds and in situ contribution in one simulated ‘facies’. This paper introduces a new scheme that enables forward models to output simulated facies defined by a range of values for each of the controlling processes and thereby predicts rock textures within simulated stratigraphies. This approach has been applied to the Jurassic carbonate ramps of the Iberian Basin in northeastern Spain. It is shown to provide more accurate information about the processes that are being simulated, allowing more direct comparisons to be made with the facies observed in the field and providing potential for a more rigorous method for assessing the ‘goodness of fit’ of a simulated stratigraphy.     

A theoretical and experimental comparison of the anisotropies of magnetic susceptibility and remanence in rocks and minerals

Summary. Susceptibility, thermo-remanent magnetization (TRM) and isothermal remanent magnetization (IRM) anisotropy ellipsoids have been determined for several rock samples. The results indicate that the ellipsoid of initial susceptibility is less anisotropic than the TRM and low field IRM ellipsoids which are found experimentally to be of identical shape. This suggests that palaeomagnetic data for anisotropic rocks may be corrected by using the anisotropy ellipsoid determined from magnetically non-destructive low field IRM measurements. Such IRM measurements can also be used to obtain anisotropy axes of samples which are inherently anisotropic but which have a susceptibility which is too weak to be accurately measured. The results for a series of artificial anisotropic samples containing magnetite particles of different sizes (in the range 0.2–90 μm) were very similar to those for the rocks. In contrast, a comparison of the susceptibility and IRM ellipsoids for anisotropic samples containing particles from a magnetic tape gave very different results in accordance with theory. Such results imply that susceptibility and IRM ellipsoids could be used to determine whether anisotropic rocks contain uniaxial single-domain particles (magnetization confined to the easy axis) or whether the particles are essentially multidomain.     

Changes in direction of the remanence of rocks produced by stationary alternating field demagnetization

Summary. A theoretical investigation of the way in which an isotropic rock containing single-domain particles acquires both IRM and ARM (or TRM) has indicated that stationary single-axis alternating field (af) demagnetization with the af axis at an angle to the remanence vector should produce progressive angular changes in a single-component remanence as demagnetization proceeds. Just before the remanence is completely removed it should lie at 90° to the af axis irrespective of the original orientation of the remanence (apart from 0°). Experimental observations on a rock sample support these deductions.
This analysis has been extended to investigate the way in which ARM (or TRM) and IRM are demagnetized by static three-axis demagnetization methods which are used by some workers in palaeomagnetism. Theory, in conjunction with the use of a numerical model, predicts that an ARM or TRM should not undergo significant direction changes when these methods are applied but an IRM should undergo progressive direction changes as demagnetization proceeds, usually moving until it makes an angle of cos⁻¹ (1/3) with each of the three af axes just before it is removed. Confirmation that such changes do occur have been obtained by experiments on a rock sample. The relative merits of static and tumbling af demagnetization methods are also discussed.     

Lower Cretaceous magnetic stratigraphy in Umbrian pelagic carbonate rocks

Summary. A record of geomagnetic field polarity for the Barremian, Aptian and Albian stages of the Early Cretaceous has been derived in three over-lapping sections of pelagic carbonate rocks in the Umbrian Apennines of northern Italy. The remanence carrier in the greyish-white Majolica limestone and Fucoid Marls is magnetite, with haematite also an important constituent in a zone of 'couches rouges' within the Fucoid Marls. The weak remanent magnetizations were measured with a cryogenic magnetometer. Alternating field or thermal demagnetization was used to isolate the characteristic remanent magnetization (ChRM) in 655 specimens from 248 stratigraphic levels. The samples respond positively to a tectonic fold test, indicating that the ChRM predates the Late Tertiary folding of the Umbrian sequence. The magnetic stratigraphy derived from variations of virtual geomagnetic pole latitude clearly defines the recognizable reversal pattern associated with Mesozoic marine magnetic anomalies M0 to M4. The sections have been zones palaeontologically on the basis of planktonic foraminifera and calcareous nannofossil assemblages. The ages of magnetic anomalies M0 to M4 determined in this way are somewhat older than those in the reversal time scale of Larson & Hilde (1975). Anomaly M0 is located in the Early Aptian, close to the Aptian/Barremian boundary. A long period of normal polarity in the Aptian and Albian corresponds to the early part of the Cretaceous magnetic quiet zone. It is interrupted in the Late Aptian by a reversal which we find in only one of the Fucoid Marl sections, and which has not been reported in oceanic magnetic anomaly investigations.     

The large-scale dynamics of grain-size variation in alluvial basins, 1: Theory

We study the interplay of various factors causing vertical grain-size changes in alluvial basins using a simple coupled model for sediment transport and downstream partitioning of grain sizes. The sediment-transport model is based on the linear diffusion equation; by deriving this from first principles we show that the main controls on the diffusivity are water discharge and stream type (braided or single-thread). The grain-size partitioning model is based on the assumption that the deposit is dominated by gravel until all gravel in transport has been exhausted, at which point deposition of the finer fractions begins. We then examine the response of an alluvial basin to sinusoidal variation in each of four basic governing variables: input sediment flux, subsidence rate, supplied gravel fraction, and diffusivity (controlled mainly by water flux). We find that, except in the case of variable gravel fraction, the form of the basin response depends strongly on the time-scale over which the variation occurs. There is a natural time-scale for any basin, which we call the ‘equilibrium time’, defined as the square of basin length divided by the diffusivity. We define ‘slow’ variations in imposed independent variables as those whose period is long compared with the equilibrium time. We find that slow variation in subsidence produces smoothly cyclic gravel-front migration, with progradation during times of low sedimentation rate, while slow variation in sediment flux produces gravel progradation during times of high sedimentation rate. Slow variation in diffusivity produces no effect. Conversely, we define ‘rapid’ variations as those whose period is short compared with the equilibrium time. Our model results suggest that basins respond strongly to rapid variation in either sediment flux or diffusivity; in both cases, deep proximal unconformities are associated with abrupt gravel progradation. This progradation occurs during times of either low sediment flux or high diffusivity. On the other hand, basin response to variation in subsidence rate gradually diminishes as the time scale becomes short relative to the equilibrium time. Each of the four variables we have considered - input sediment flux, subsidence, gravel fraction, and diffusivity - is associated with a characteristic response pattern. In addition, the time scale of imposed variations relative to the equilibrium time acts in its own right as a fundamental control on the form of the basin response.     

The large-scale dynamics of grain-size variation in alluvial basins, 2: Application to syntectonic conglomerate

The concept of‘syntectonic’ conglomerate is based on the idea that gravel progradation is mainly generated by an increase in tectonic uplift and erosion of a source area with attendant increase in sediment flux supplied to a basin. However, other mechanisms, such as changes in basin subsidence rates, sorting of supplied sediment, and capability of transporting streams, can also lead to progradation and be difficult to distinguish from a syntectonic origin. Here we use our previously developed model to help understand the origin of gravel progradation in three Neogene alluvial basins - the Bermejo Basin of Argentina, the Himalayan Foreland Basin, and the San Pedro Basin of southern Arizona - all of which have available high-resolution magnetostratigraphy. Interpretation of the origin of gravel progradation in these basins begins with calculation of basin equilibrium time, which is the time-scale required for the streams to reach a steady-state profile, assuming constant conditions. We then compare the time-scale of the observed changes in the basin with the equilibrium time to determine if and how the model can be applied to the stratigraphic record. Most of the changes we have studied occur on time scales longer than the equilibrium time (‘slow variations’), in which case the key to interpretation is the relationship between overall grain-size change and sedimentation rate in vertical sections. Of the three examples studied only one, the Bermejo Basin, is consistent with the traditional model of syntectonic progradation. Overall progradation in the two other basins is most consistent with a long-term reduction in basin subsidence rates. In addition, short-term variation in diffusivity or sediment flux, probably climatically driven, is the most likely control of small-scale progradation of gravel tongues in the San Pedro Basin. These results, along with observations from other basins, suggest that subsidence is clearly an important control on clastic progradation on ‘slow’ time scales (i.e. generally a million years or more). If subsidence rates are directly linked to tectonic events, then subsidence-driven progradation marks times of tectonic quiescence and is clearly not syntectonic in the traditional sense. These examples show that the model can be useful in interpreting the rock record, particularly when combined with other traditional basin-analysis techniques. In particular, our results can be used to help discriminate between clastic progradation due to tectonic origin and progradation resulting from other mechanisms in alluvial basins.     

Nature and origin of magnetic minerals within the Middle Jurassic shallow-water carbonate rocks of the Paris Basin, France: implications for magnetostratigraphic dating

The Middle and Upper Jurassic Bathonian-Oxfordian shallow-water carbonate rocks from the Paris Basin, France, consist mainly of oolitic and bioclastic limestones that are hydrocarbon reservoirs in the subsurface. Despite a preliminary positive study, these deposits have been considered to be largely remagnetized (Rochette, private communication), and hence not amenable to palaeomagnetic dating. To establish their magnetic mineralogy and test this remagnetization hypothesis, we have used an integrated investigation combining petrographic, geochemical, rock-magnetic and palaeomagnetic measurements on samples extracted from five cores from the Paris Basin and from outcrops in Burgundy. Magnetic minerals in the Bathonian-Oxfordian carbonates include: (1) primary biogenic single-domain magnetite and detrital multi-domain Ti-magnetite and their oxidized form, maghemite; (2) authigenic spheres of magnetite probably related to hydrocarbons; and (3) goethite, either restricted to ferruginous ooid layers or resulting from surficial alteration, notably replacement of pyrite framboids. Rock-magnetic experiments carried out on 68 samples reveal H _cr/ H _c and M _rs/ M _s ratios ranging from 1.88 to 5.58 and 0.017 to 0.314, respectively. These values are clearly distinct from diagnostic values for a chemical remagnetization. Pyrrhotite was not identified within these sediments. Moreover, the average H _cr/ H _c ratio of 3.14 is significantly different from the value of 1.333 for natural pyrrhotite (Dekkers 1988). These results have a direct implication for the preservation of the primary magnetization; consequently, these deposits are selectively amenable for magnetostratigraphic dating and possible regional correlations.     

Current-scattered (?) detrital remanence directions in the Zn-rich Pennsylvanian Stark black shale, USA

Palaeomagnetic results are reported from the metalliferous Stark black shale in the Upper Pennsylvanian (Missourian/Kasimovian) Kansas City Group. Palaeomagnetic analysis of 400 specimens from 28 sites gives a characteristic remanent magnetization in 17 sites of the shale that yields a Late Mississippian to Middle Pennsylvanian palaeopole at 32.2°N 128.5°E (dp = 4.7° and dm = 8.8°). The observed palaeomagnetic age is slightly older than the host rock, indicating that the mineralization of the Stark Shale has, excluding recent alteration, a primary sedimentary or syngenetic origin. The reason for the slightly older age is likely due to trace modern hematite that slightly steepens the remanence inclination. The large oval of 95 per cent confidence is interpreted to be caused by clay–magnetite aggregates that formed during sediment transport and the biasing effect of the gentle palaeocurrent at each site acting on the large aggregates. Therefore, the scattered distribution of the site mean remanence declinations found for the Stark Shale is evidence of a detrital remanent magnetization that is formed by primary sedimentary processes with an enriched metallic content and not remagnetization with mineralization by secondary hydrothermal processes.     

Spatial and temporal variations in the grain-size characteristics of historical flood plain deposits, Blue River, Wisconsin, USA

This study examined vertical, lateral, and downstream variations in the grain-size characteristics of historical (post-1830) overbank deposits in a watershed that has experienced high rates of accelerated flood plain sedimentation. More than 800 samples were collected from 53 cores along nine flood plain transects. Overbank deposits exhibit a coarsening-upward sequence attributed to historical changes in the sand content of source materials. The erosion of loess-capped soils increased the exposure, erosion, and transport of sandy parent materials. The average sand content of near-channel cores increases moderately downstream along two of the reaches because sandy source materials are increasingly exposed in larger main valleys in the northern part of the watershed. The two northernmost reaches are coarser overall, but do not display significant downstream trends. The sand content of surface and early historical overbank deposits generally decreases laterally as an exponential function of distance from the channel, suggesting transport by turbulent diffusion. The presence of sand throughout the transects and lateral coarsening at two of the transects, however, suggests that sediment transport by convection is also important.     

黄河口不同粒度泥沙沉积与扩散分析

本文采集和收集大量黄河三角洲沉积物剖面和钻孔泥沙粒度资料以及黄河口来沙粒度组成数据,定量研究了黄河口泥沙的沉积与扩散特征.结果 显示黄河口来沙以粉砂为主,黏土次之,砂最少,年均中值粒径无长期变化趋势.黄河三角洲平原相泥沙与来沙的黏土、粉砂和砂含量无显著差异;前缘相泥沙比来沙的黏土含量较低,砂含量较高;前三角洲相泥沙比来...     

Till fabric and grain-size analysis of glacial sequences in the Upper Sil River Basin,Cantabrian Mountains,NW Spain

Fabric and grain-size analysis of eight deposits present in the Upper Sil River Basin palaeoglacial system of northwest Spain were used to interpret till types and to reconstruct the glacial paleoprocess history of this region. They are represented by lodgement, deformation, and melt-out tills. The first, representative of glacial advance stages, present cluster fabrics and are generally composed of poorly sorted and finer sediments. Melt-out and deformation tills, representative of stages of glacial stability followed by fast recession, present transitional to girdle fabrics and are composed of poorly sorted coarser sediments. In any case, multiple criteria, including lithological, clast-shape, or structural data, support these directional and grain-size observations. These sequences, most of them located in the snout area of the Sil palaeoglacier, are important in the reconstruction of glacial dynamics due to poor preservation of landforms related to the maximum glacial advance. This palaeoglacial system was formed by multiple tributaries and had an extension of over 450 km², with the Sil paleoglacier being 51 km long during its maximum glacial advance.     

Cloudburst floods in mountains: State of knowledge,occurrence, factors of formation

A detailed analysis is made of the current ideas concerning floods of a special type. We examine short-lasting torrential flash floods, one of the most widespread and hazardous natural phenomena in the world characterized by a high rate of development, and by a short duration. It is established that such floods are in a primitive stage of study, which is testified by the fact that there is no general consensus as to what should be treated as a flash flood. It is pointed out that a special term designating them is also absent in many countries. It is determined that the key formation conditions for flash floods include intense short-lasting cloudbursts, the occurrence of a river basin in mountainous regions and a small drainage area; on this basis, it is suggested that they be termed flash floods. It is shown that such floods are of the most widespread occurrence in the northern hemisphere in regions with a temperate and subtropical climate. We suggest the scheme of natural factors for formation of flash floods and their differences from debris flows and floods of other types. It is determined that the main problems of investigating the formation mechanisms and forecasting the aforementioned floods are associated with the small spatiotemporal scale of these phenomena.