Jeroen A. M. Kenter

Origin and Significance of Isotope Shifts in Pennsylvanian Carbonates (Asturias, NW Spain)

ABSTRACT The primary variability of the composition and properties of seawater is much greater in the shallow coastal zones than in the main body of ocean water. An inadequate understanding of this variability, as well as different diagenetic environments, severely limit the interpretation of the stable-isotope record of shoalwater carbonates. In order to investigate this primary and diagenetic variability along a Bashkirian-Moscovian platform-to-basin transect, 13C and 18O analyses have been performed on more than 1000 matrix micrite, carbonate cement, and brachiopod shell samples. In isotope analysis, these different carbonate materials tend to complement each other, inasmuch as they have different advantages and shortcomings. The resulting data reveal spatial trends in 13C and 18O signatures from platform top (lower values) to basin (higher values). In the case of 13C from pristine brachiopods, this trend can be explained by the long residence time (aging) of platform-top water masses. In the case of brachiopod 18O, this variance is interpreted to reflect temperature differences between warm surface and colder bottom water separated by a permanent thermocline at about 150 to 200 m beneath the shelf break. Micrite and marine cement isotopic values from the platform interior were reset (lowered) during pervasive early meteoric diagenesis. In contrast, micrite and marine cement isotopic values from the outer platform, slope, and basin show higher values close to the assumed Pennsylvanian seawater isotopic composition. This implies that isotopic data from shoalwater carbonates (including pristine brachiopod shells) might not necessarily reflect paleoceanographic trends of the open-ocean water masses because of changes in coastal water-mass isotope signature and interaction with early meteoric fluids.

Steep microbial boundstone-dominated plaform margins

Seaward progradation of several kilometers has been documented mostly for leeward margin low-angle carbonate slope systems with a dominant platform top sediment source. However, steep and high-relief margins fronting deep basins can also prograde and as such are somewhat perplexing. Characteristics of two prograding Carboniferous examples provide a model which may apply elsewhere: (1) outcrops in Asturias, northern Spain serve as important analogs for (2) hydrocarbon reservoirs in steep-sided isolated platforms of the North Caspian Basin, Kazakhstan, such as Tengiz. Seismic and well data from Tengiz corroborate outcrop patterns for slope development, showing progradation of up to 5 and more than 10 km, respectively, despite the high-relief (up to 600 m) and steep (~20–358) nature of these margins. The two examples share a highly productive microbial boundstone factory extending from the platform break down the slope to nearly 300 m (or more) depth and a lower slope dominated by (mega)breccias and grain flow deposits derived from the margin and slope itself. The broad depth range of microbial boundstone increases the potential for production during both lowstands and high stands of sea level and thereby facilitates progradation independent from platform-top-derived sediment. Rapid in situ lithification of the boundstone provides stability to the steep slopes, but also leads to readjustment through shearing and avalanching. What controls the microbial cement boundstone formation remains a debate but its presence is a key factor controlling the progradational geometry of these and possibly other margins. This new model of bslopeQ shedding has implications for slope readjustment processes and resulting architecture, sequence stratigraphic interpretation, reservoir characterization, and reservoir modeling. Especially the isotropic character of microbial boundstone will reduce the potential for coherent seismic reflections to develop and possibly invoke, under certain stress regimes, shattering and fracturing thereby generating significant non-matrix permeability. Key considerations are the contrasts with the Bahamian high stand shedding depositional model, slope

Automated separation of touching grains in digital images of thin sections

The determination of textural properties of granular material with image analysis is generally troubled by the fact that touching grain sections merge into single features. Without separation of these touching grain sections, the textural properties derived from the images contain substantial bias. Existing methods for separating touching grains, like erosion-dilation cycles or watershed segmentation, are time-consuming and/or alter the textural properties of the grain sections analyzed. An alternative computer algorithm is presented to separate touching grain sections in binary images of granular material. The algorithm detects characteristic sharp contact wedges in the outline of touching grain sections and creates an intersection after checking if the angle of the contact wedge is smaller than a user-defined threshold value. The performance of the new algorithm is compared to that of the watershed segmentation method. The resulting grain-size distributions after applying automated separation techniques, were verified with the size distribution obtained with a laboratory laser particle sizer. The algorithm shows improved preservation of size and shape characteristics of the granular material over the watershed segmentation method.

Parameters controlling sonic velocities in a mixed carbonate-siliciclastics Permian shelf-margin (upper San Andres formation, Last Chance Canyon, New Mexico)

We have measured the acoustic properties and mineralogic composition of 48 rock specimens from mixed carbonate-siliciclastic outcrops of the Permian upper San Andres formation in Last Chance Canyon, New Mexico. The goals were:(1)identify and model the parameters controlling the sonic velocities; (2) assess the influence of postburial diagenesis on the acoustic velocities. The variation in sonic velocity in the 0 to 25 % porosity range is primarily controlled by porosity, and secondly by the ratio of carbonate-siliciclastic material. Linear multivariate fitting resulted in a velocity-porosity-carbonate content transform that accurately predicts sonic velocity at different effective stresses. The slope of the velocity-porosity transform steepens with increasing carbonate content, which may be explained by the higher velocity of carbonate minerals. Another reason may be the property of carbonate minerals to form more perfect intercrystalline boundaries that improve the transmission properties of acoustic waves and are less sensitive to changes in effective stress. The velocity ratio VP / VS is an excellent tool to discriminate between predominantly calcitic lithologies (ratio between 1.8 and 1.95) and predominantly dolomitic and quartz-rich lithologies (ratio between 1.65 and 1.8). Gardners experimental curve overestimates, and the velocity-porosity transforms by Wyllie and Raymer underestimate, the observed sonic velocities, probably because they do not account for variations in texture, carbonate mineralogy, and pore geometry. Petrographic observations show that postburial diagenesis is minor and does not seem to significantly affect porosity. Therefore, the outcrop data set can be regarded as a proxy for the subsurface analog. These findings underline the significantly more complex acoustic behavior in mixed carbonate-siliciclastic sedimentary rocks than in pure siliciclastics where mineralogic composition explains most of the observed relationships between porosity and sonic velocity.

Sedimentation on a Lower Jurassic carbonate platform flank: geometry, sediment fabric and related depositional structures (Djebel Bou Dahar, High Atlas, Morocco)

Abstract Along the southern margin of the High Atlas, Morocco, carbonate platforms of Lower Jurassic age developed in a transtensional basin, the South Atlas Trough. Complete exposure and preservation of the platform margin, flank and basin of the Djebel Bou Dahar, allowed the subdivision of the slope into an upper, mid-, and lower slope, based on geometry, sediment fabric and depositional structures. At ca. 50 m below the platform margin, coral-algal patch reefs and ooid sand shoals grade into the depositional slope. The straight upper slope, showing discontinuous bedding at angles up to 29°, extends down to ca. 250 m and consists mainly of poorly sorted mixtures of sand and gravel, almost devoid of mud. Avalanching, rockfall and shearing of the coarse gravelly sediment are the dominant processes readjusting the upper slope. The middle and lower part of the slope profile, extending down to ca. 500 m below the margin, is separated from the upper slope by a distinct slope break. Slope angles range from 19° near the slope break down to 4–8° where the slope merges with the basin-plain. The irregularly bedded mid slope is characterized by scouring and deposition of muddy sands and non-cohesive gravelly debris flows. Sedimentation on the, dominantly muddy, concave-upwards lower slope can be separated into evenly bedded shales, muddy sands and, mud-free, gravel-sand mixtures, deposited as debris sheets. The results of this study emphasize the relationship between sediment fabric and the slope geometry of a carbonate platform flank. The overall geometry, internal heterogeneity and depositional structures are suggested to be strongly related to the original character of the sediment supplied to the Djebel Bou Dahar platform flank.

Lithofacies character and architecture across a Pennsylvanian inner-platform transect (Sierra de Cuera, Asturias, Spain)

ABSTRACT Seismic-scale outcrops of lower Pennsylvanian steep-margined carbonate platforms in northern Spain (Sierra de Cuera, Cantabrian Mountains) provide the opportunity to link depositional facies to platform-interior architecture. Lithofacies character, vertical stacking patterns, and spatial distribution of depositional facies and shallowing-upward cycles were investigated to provide a semiquantitative depositional model for inner-platform strata. A 70-m-thick succession of eight shallowing-upward cycles (2.5-15 m thick) was studied across a 2-km-wide transect. Cycles consist of algal bioherms and skeletal packstone (lithofacies association B) deposited in an open marine and sub-wave-base environment, which are locally lateral to crinoid-dominated packstone (lithofacies association D). Lithofacies association B developed on a transgressive, one-meter-thick high-energy coated-grain grainstone (lithofacies association A) and is overlain by open marine to restricted lagoonal deposits (lithofacies association C) indicative of decreased paleo-water depth. Cycle boundaries are marine flooding surfaces occurring at the base of lithofacies A and the top of lithofacies C. Petrographic and outcrop evidence of subaerial exposure is conspicuously lacking. Cycle and lithofacies thickness varies laterally, and all the lithofacies can be either continuous for the 2 km width of investigation or discontinuous, terminating within tens to hundreds of meters. In most of the cycles, the lithofacies succession changes laterally because of lateral facies transitions (between lithofacies B and D) and pinching out of lithofacies A and C. Meter-scale variations in depositional topography are related to the stacked lens-shaped bioherms assigned to lithofacies B and appear to control the thickness and lateral continuity of the overlying facies. Sierra de Cuera strata do not resemble other Pennsylvanian cycles driven by high-frequency, high-amplitude glacio-eustasy, which are commonly characterized by subaerial exposure surfaces developed on subtidal deposits, except for the absence of peritidal facies and the presence of low-relief algal bioherms on the platform top. The necessary accommodation to enhance the growth of bioherms on the platform top was probably due to high-frequency (100-240 ky) moderate-amplitude (nearly 40 m) sea-level fluctuations combined with subsidence rates of 70-140 m/My. Tectonic subsidence might have been responsible for the lack of evidence of subaerial exposure surfaces in the examined Moscovian strata. The studied inner-platform lithofacies character and architecture were controlled by the interplay of high-frequency changes in accommodation and the presence of meter-scale depositional topography attributed to lateral variations of carbonate accumulation.

Influence of Organic Matter in Soils on Radar-Wave Reflection: Sedimentological Implications

ABSTRACT Soils are excellent reflectors of ground-penetrating radar (GPR) signals because of the ability of organic matter to hold water. In this paper, GPR profiles of an eolian sedimentary succession are combined with textural, dielectric, and moisture-retention characteristics to illustrate the influence of soil moisture on radar-wave reflection. Organic matter in this succession varies strongly, from ;lt 0.15% for clean sand to 7% for the most prominent soil, whereas grain-size distributions are comparable. Moisture-retention curves show a complex relationship between suction potential (pF) and volumetric water content (). As a result of their uniform pore-size distribution, clean sand and weakly developed soils with ;lt 1% organic matter experience a sudden loss of water between pF 1.5 and pF 1.8, going directly from saturated to almost dry conditions. In contrast, the most prominent soil shows a more gradual decrease in with increasing suction potential. It follows that the dielectric contrast between clean sand and this soil increases sharply above pF 1.5, reaches a maximum value at field-capacity conditions, and then decreases slowly. Synthetic GPR images for different suction potentials show that field-capacity conditions, when reflection coefficients are high, are favorable for tracing one single soil. Dry sediments are preferable when imaging widely spaced soils, whereas saturated sediments are best when imaging closely spaced soils.

Seismic models of a prograding carbonate platform: Vercors, south-east France

Abstract The southern Vercors area (south-east France) displays superb exposures of a Lower Cretaceous prograding carbonate platform. These exposures allow a geometrical sequence analysis to be combined with detailed outcrop observations. Two-dimensional seismic modelling techniques were used to determine the seismic response of three of these exposures: the Montagnette, the Archiane Valley and the Rocher de Combau. The outcrops display prograding and retreating series of clinoforms interfingering with fine-grained basinal sediments. Photographs from different angles of view, combined with detailed field observations, were used to construct lithological profiles with a resolution of f few metres. P-wave velocities and bulk densities were measured from cored hand specimens and selected values were assigned to lithostratigraphic units in each profile. The vertical incidence modelling technique was then used to compute migrated time sections of reflectivity. These were convolved with source wavelets of different frequencies to produce the final synthetic seismic profiles. None of the modelled outcrop geometries is correctly portrayed seismically at 25–50 Hz frequencies. Not only do the synthetic seismic profiles show less detail, they also show misleading geometries. In most seismic models, for instance, pseudo-unconformities are present that correspond to rapid changes in dip and in facies in outcrop. Furthermore, the seismic tool is not able to resolve between stratal patterns punctuated by unconformities and those with gradual shifts in facies belts accompanied by complex interfingering relationships. This implies that an unconformity recognized on a real seismic line should not automatically be assumed to represent a true stratal unconformity. Large outcrops are often used to show the validity of sequence stratigraphy, a concept mainly based on the analysis of seismic lines. This study, however, shows that there is no one to one relationship between geometries observed in outcrop and in seismic lines. Seismic modelling of outcrops therefore provides a means for evaluating outcrop geometries in terms of seismic stratigraphy.

Facies patterns and subsidence history of the Jumilla-Cieza region (southeastern Spain)

Abstract A detailed analysis of the facies patterns and subsidence history since the Late Cretaceous (100 Ma) of the Jumilla-Cieza region in southeastern Spain, has implications for the debate on the mechanisms causing third-order changes in sea-level. The local subsidence history shows periods characterized by specific trends in tectonic subsidence, each with a typical concomitant facies pattern. The successive periods are considered to illustrate the transition of the southern Iberian plate boundary from a passive margin (108-40 Ma), through an extensional phase (40-20 Ma) to a thrust-and-fold belt (20 Ma to Recent). Timing and changes in relative sea level derived from the sedimentary record, closely match with the regional tectonic events. The order of magnitude of these sea-level changes exceed those predicted by the “global” curve. Therefore, we suggest that, in the studied area, third-order sea-level fluctuations are of relatively minor magnitude and only enhance the effect of tectonic-induced vertical movements.

Microfacies and paleoenvironments of Donezella accumulations across an Upper Carboniferous high-rising carbonate platform (Asturias, NW Spain)

SummaryDonezella is a problematic organism that during the mid-Carboniferous (latest Serpukhovian to Moscovian) characterized carbonate depositional systems in Europe, North Africa, Russia, Kazakhstan, and North America. ThoughDonezella is generally included in the green calcareous algae, it has been attributed to different systematic groups and its classification and paleoecology still remain controversial.This work focuses on the distribution ofDonezella across a carbonate platform (Sierra de Cuera) of Upper Carboniferous (lower Bashkirian-lower Moscovian) age located in the Cantabrian Mountains (Asturias, NW Spain). Sierra de Cuera exhibits a well-exposed cross-section from the horizontal platform through a steep slope (30°) to the basin floor. This unique feature allows reliable estimates of paleo-water depth and distance from the platform margin.Donezella specimens are interpreted asin situ when they form a network supporting cement-filled primary cavities and the ramified skeletons are surrounded by micrite coatings, often with a peloidal fabric, or by early marine cement. In the platform interior,Donezella is associated with phylloid algae and occurs in mud-rich low-relief bioherms. Towards the platform margin, massive units of boundstone are characterized by clotted peloidal micrite and radial fibrous cement-filled primary cavities. They containDonezella and a diverse fossil assemblage of calcareous algae, bryozoans, and foraminifers.Donezellas delicate network appears fortified by thein situ precipitation of peloidal micrite. On the upper slopein situ precipitated peloidal micrite, abundant radial fibrous cement, and fenestellid bryozoans are the major components of the boundstone facies, along withDonezella and a eskeletal community similar to the outer platform one. On the slope,in situ Donezella were observed down to paleowater depths up to 200 m.Sedimentologic, petrographic, and microfacies analysis ofDonezella accumulations in the different facies belts of Sierra de Cuera strongly suggest that this problematic organism was able to thrive over a large depth range, in low-energy but also in moderately agitated environments or in settings with temporary increase in current action, and in organic, physical-chemical and oceanographic conditions that enhanced the precipitation of peloidal micrite. The interval of water depth inferred from the well-exposed slope geometry of Sierra de Cuera suggests that eitherDonezella might not have belonged to the green calcareous algae or, alternatively, this depositional system was influenced by particular paleo-oceanographic conditions that extended the euphotic zone below the average depth. The morphology ofDonezellas skeleton and its sedimentological occurrences are not exclusively indicative of an affinity with Chlorophyta. Therefore, it is suggested thatDonezella should be considered as amicroproblematicum organism.The data presented in this study contribute to the inter-pretation of comparableDonezella accumulations in carbonate depositional systems where limited outcrop exposures do not allow correct evaluation of the geometry and facies distribution.