368 research outputs found
Microfacies and paleoenvironments of Donezella accumulations across an Upper Carboniferous high-rising carbonate platform (Asturias, NW Spain)
No full textDonezella 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. Though Donezella 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 of Donezella 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 as in 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 contain Donezella and a diverse fossil assemblage of calcareous algae, bryozoans, and foraminifers. Donezella's delicate network appears fortified by the in situ precipitation of peloidal micrite. On the upper slope in situ precipitated peloidal micrite, abundant radial fibrous cement, and fenestellid bryozoans are the major components of the boundstone facies, along with Donezella and a skeletal community similar to the outer platform one. On the slope, in situ Donezella were observed down to paleo-water depths up to 200 m. Sedimentologic, petrographic, and microfacies analysis of Donezella 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 either Donezella 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 of Donezella's skeleton and its sedimentological occurrences are not exclusively indicative of an affinity with Chlorophyta. Therefore, it is suggested that Donezella should be considered as a microproblematicum organism. The data presented in this study contribute to the interpretation of comparable Donezella accumulations in carbonate depositional systems where limited outcrop exposures do not allow correct evaluation of the geometry and facies distribution
Depositional facies and stratal geometry of an Upper Carboniferous prograding and aggrading high-relief carbonate platform (Cantabrian Mountains, NW Spain)
No full textSeismic-scale continuous exposures of an Upper Carboniferous (Bashkirian-Moscovian) carbonate platform (N Spain) provide detailed information about the lithofacies and stratal geometries (quantified with differential global positioning system measurements) of microbial boundstone-dominated, steep prograding and aggrading platform margins. Progradational and aggradational platform-to-slope transects are characterized by distinct lithological features and stratal patterns that can be applied to the understanding of geometrically comparable, high-relief depositional systems. The Bashkirian is characterized by rapid progradation at rates of 415-970 m M
Steep microbial boundstone-dominated plaform margins
No full textSeaward 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 (∼20-35°) 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 debat e but its presence is a key factor controlling the progradational geometry of these and possibly other margins. This new model of "slope" 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 progradation rates that range from 450 to > 1500 m/My, and net growth rates of in situ boundstone of ∼1000 m/My, comparable to or higher than accretion rates for metazoan skeletal reef growth. © 2005 Published by Elsevier B.V
Depositional Architectures and Lithofacies of a Pennsylvanian High-rising Carbonate Platform (Sierra del Cuera, Cantabrian Mountains, N Spain)
No full textAn alternative model for positive shifts in shallow marine carbonate ¿13C and ¿18O
No full textPositive shifts in global seawater
Investigating controls on Carboniferous high-relief carbonate platform margin (Cantabrian Mountains, N Spain) using a forward stratigraphic modeling program (Carbonate 3D)
No full textHigh-resolution ground verification, cluster analysis and opticalmodel of reef substrate coverage on Landsat TM imagery (Red Sea, Egypt)
No full textA combination of high-resolution ground verification, cluster analysis using Landsat Thematic Mapper (TM) data, and optical modelling, was applied to Red Sea reef substrate. Ground verification, in an area of 3 by 20 pixels (90 by 600 m) with one metre scale resolution, identified the presence of 30 different bottom types that were later reduced to twelve dominant bottom types. A combination of bispectral plots and principal component analysis using spectral bands 1, 2 and 3 confirmed the presence of nine bottom types. The identified clusters were separated and used as a training set to classify substrate. Optical modelling, using literature radiance values and coverage of the original twelve dominant bottom types and a simple model for atmospheric and water column absorption, revealed a difference of up to 60 W m-2 between predicted substrate radiance and the satellite sensor values in the reef top area. Considering the simple atmospheric correction model, the lack of in situ radiance measurements and the uncertainties with respect to possible changes in bottom type distribution since the acquisition of the 14 year old image, the results show the potential use of satellite imagery for reef research in both biological and geological analysis through very precise and semi-quantitative ground verification, including in situ reflectance measurements
Lithofacies character and architecture across a Pennsylvanian inner-platform transect (Sierra de Cuera, Asturias, Spain)
No full textSeismic-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
Late Palaeozoic platform margins dominated by microbial cement-rich boundstone: revising the carbonate factory-depth paradigm
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