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Lower Jurassic sponge-microbial mounds and reefs in the Djebel Bou Dahar ramp to high-relief platform (High Atlas, Morocco)
No full textLower Jurassic automicrite-sponge mounds and coral-sponge reefs in the Djebel Bou Dahar ramp to high-relief platform (High Atlas, Morocco)
No full textThe Djebel Bou Dahar (DBD) carbonate platform (High Atlas, Morocco) developed in an intracratonic rift basin during the early Jurassic. From Hettangian?-Sinemurian to Pliensbachian, the DBD evolved from an extensive low-relief depositional system into a high-relief platform as a result of progressive extensional tectonics. Platform growth was terminated by drowning in the earliest Toarcian. Six major stages of DBD platform evolution have been distinguished on the basis of geometry, character of depositional facies and internal architecture. This study focuses on the automicrite-rich bioherms accumulated during the Upper Sinemurian stage III and the Pliensbachian stages IV-VI and documents variations in composition, morphology and depositional setting of the sponge-automicrite boundstone.
During the late Sinemurian stage III, siliceous sponge-automicrite mounds, up to 15 m-thick and tens of metres in width developed surrounded by skeletal coated grain packstone. These beds were deposited in a middle-ramp to gentle dipping slope setting, below wave base and include crinoids, echinoid spines, brachiopods, bryozoans, bivalves, siliceous demosponges and hexactinellids, foraminifers (Involutina liassica, Lenticulina, lagenids and nodosarids) and the problematic Radiomura cautica. The mounds consist of automicrite displaying homogeneous, clotted peloidal and laminated fabrics, gravity defying structures and isolating stromatactis-like cavities filled by radial fibrous cement. The automicrite is generally associated with sponge spicules, demosponges and hexactinellid sponges. The complex fabrics and the rapidly lithification at the seafloor suggest biologically-induced precipitation for the origin of the micrite.
The Pliensbachian Stages IV to VI were characterized by steep slopes (up to 450-600 m relief and dip angles up to 30°) formed as a result of fault-controlled adjustment of the originally low-relief topography. Sponge-automicrite boundstone accreted on the upper slope down to a depth of 140 m below the platform break, while coral-sponge-automicrite boundstone occurred from the platform-break and outer platform down to depths of 70-100 m. Boundstone alternated with redeposited grainstone, packstone and rudstone beds. The deeper sponge-automicrite boundstone accumulations were devoid of corals and dominated by homogeneous to clotted peloidal micrite embedding sponge spicules of demosponges and hexactinellids. Irregular mm to cm wide stromatactis-like cavities were filled by radial fibrous cement. Sponges can be attributed to lithistid demosponges and lyssakine hexactinellids encrusted by Radiomura cautica, Terebella and serpulids.
The margin and uppermost slope were characterized by the growth of metre-scale colonies of phaceloid corals surrounded by a patchy distribution of sponge-automicrite boundstone associated to stromatoporoids, chaetetids, Bacinella ordinata, Baccanella floriformis and Tubiphytes.
The DBD platform developed in a setting suitable for the biologically induced precipitation of microcrystalline carbonate in association with the presence of sponges. These favourable conditions might have been facilitated by the availability of substrates following the end-Triassic extinction event and the specific chemico-physical properties of the adjacent basin seawater. The narrow rift basin configuration and arid climate setting may have promoted highly saturated and possibly mesotrophic waters that appear necessary for the biologically induced precipitation of calcium carbonates during degradation of biofilms and sponge-derived organic substrates. In addition, the character, composition and geometry of the sponge-automicrite boundstone were affected by the evolution of the geometry of the depositional system driven by extensional tectonics and eustatic sea level
Control of Lower Jurassic microbial reef communities on carbonate platform geometry (Djebel Bou Dahar, High Atlas, Morocco)
No full textThe Djebel Bou Dahar (DBD) carbonate platform (Early Jurassic, High Atlas, Morocco) was deposited on the footwall high of an active marine rift. It contains six depositional sequences bounded by footwall unconformities and correlative flooding surfaces along the adjacent hanging wall. The DBD evolved from a low-relief ramp profile (Sequences I-III) to a high-relief steep-fronted platform with slopes up to 30° and 600 m relief (Sequences IV-VI). The architectural evolution was controlled by fault-block rotation, regional subsidence in an extensional tectonic setting, third order eustatic sea level and sediment production and dispersal rates, while the margin geometry was, at least in part, controlled by changing reef communities on the slope and margin.
Sequence III consisted of siliceous sponge microbial mud mounds associated with coated grain skeletal packstone and grainstone in middle and outer ramp regions. This deep-water carbonate factory did not build into wave-agitated shallow-water and lacked the capability to construct a high-relief margin geometry.
During sequences IV (retrogradational) and V (progradational) the growth of a highly productive coral stromatoporoid microbial reef at the platform margin and on the slope (10-60 m depth), adjacent to deeper water siliceous sponge microbial lenses (60-140 m), promoted the accretion of a high-relief and steep slope and the development of a productive shallow-water platform top.
Sequence VI record increased accommodation space creation and retrogradational patterns prior to final platform drowning. Coral stromatoporoid microbial boundstone similar to Sequence V slope lithofacies extend on the outermost platform, 200-500 m inward of the platform break.
The DBD Lower Jurassic carbonate platform demonstrates the influence of various carbonate factories and microbialites in building and stabilizing a high-relief geometry. It also shows, in contrast with the generally accepted belief that Lower Jurassic reefs are dominated by platform bivalve bioherms, that similar age reef systems can have substantial contributions by microbialite components
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Stratal and Facies Anatomy of a Lower Jurassic High-rising Carbonate Platform (Jebel Bou Dahar, High Atlas, Morocco)
No full textSemi-quantitative analysis of platform interior facies mosaic (Lower Jurassic, Jbel Bou Dahar, High Atlas, Morocco)
No full textStratal patterns and lithofacies of an intact seismic-scale Carboniferous carbonate platform (Asturias, NW Spain)
No full textPore structure, porosity and permeability of continental carbonates: A case study of Pleistocene travertine (southern Tuscany, Italy)
No full textTravertines are continental carbonates precipitated from hot water (>20°C) issuing from hydrothermal vents. They display a large variety of growth fabrics and associated pore structures. This variety reflects their origin (by interplay of biotic and abiotic processes) and subsequent diagenesis. Samples from a Pleistocene–Holocene travertine bodies located in southern Tuscany (central Italy) were examined for reservoir properties. Seventy horizontally and vertically drilled plugs were measured for porosity and permeability. The results were integrated with petrographic observations and the pore structure of ten samples was quantified using microCT scanning. Morphometric parameters were calculated on 3D based surface-rendered volume models and on 2D binarized cross-section images. Eight fabric categories were identified: shrub, crystalline crust, stromatolitic, raft, wavy sheet, coated grains, bubble and reeds. Porosity types include: depositional porosity as inter-dendritic form, bubble, inter-stromatolitic laminae, shelter, and intraskeletal; and, secondary porosity as biomoldic, vuggy meteoric dissolution, and fracture. The measurements show that travertines display a wide range in porosity, from 1 to 29%, and permeability, from 0.006 to 50000mD. This wide range is a direct function of: 1) primary fabric orientation; 2) amount of cementation; 3) travertine fabric. Horizontally drilled plugs have consistently higher permeability values than the vertically drilled plugs. This reflects the intricate horizontal organization of porous and tight layers at the mm- to cm-scale. In horizontal direction the pore network is well connected. The lowest porosity and permeability values are associated with well-cemented deposits. Blocky calcite cementation partially or completely occludes the primary porosity and retards the permeability. Pore structure have a strong control on permeability: simple pore structures with large pores (from 3 to 20 mm long and from 3 to 8 high) such as those of raft and reed fabric show high permeability values. Coated bubble facies display high porosity and large pores but have low permeability as the pores are not connected. This study provides the first comprehensive evaluation of the reservoir properties of hot-spring carbonates
Pore structure, porosity and permeability of hydrothermal travertine: A case study of Pleistocene travertine (southern Tuscany, Italy)
No full textTravertines are continental carbonates precipitated from hot water (>20°C) issuing from hydrothermal vents. They display a large variety of growth fabrics and associated pore structures. This variety reflects their origin (by interplay of biotic and abiotic processes) and subsequent diagenesis. Samples from a Pleistocene–Holocene travertine bodies located in southern Tuscany (central Italy) were examined for reservoir properties. Seventy horizontally and vertically drilled plugs were measured for porosity and permeability. The results were integrated with petrographic observations and the pore structure of ten samples was quantified using microCT scanning. Morphometric parameters were calculated on 3D based surface-rendered volume models and on 2D binarized cross-section images. Eight fabric categories were identified: shrub, crystalline crust, stromatolitic, raft, wavy sheet, coated grains, bubble and reeds. Porosity types include: depositional porosity as inter-dendritic form, bubble, inter-stromatolitic laminae, shelter, and intraskeletal; and, secondary porosity as biomoldic, vuggy meteoric dissolution, and fracture. The measurements show that travertines display a wide range in porosity, from 1 to 29%, and permeability, from 0.006 to 50000mD. This wide range is a direct function of: 1) primary fabric orientation; 2) amount of cementation; 3) travertine fabric. Horizontally drilled plugs have consistently higher permeability values than the vertically drilled plugs. This reflects the intricate horizontal organization of porous and tight layers at the mm- to cm-scale. In horizontal direction the pore network is well connected. The lowest porosity and permeability values are associated with well-cemented deposits. Blocky calcite cementation partially or completely occludes the primary porosity and retards the permeability. Pore structure have a strong control on permeability: simple pore structures with large pores (from 3 to 20 mm long and from 3 to 8 high) such as those of raft and reed fabric show high permeability values. Coated bubble facies display high porosity and large pores but have low permeability as the pores are not connected. This study provides the first comprehensive evaluation of the reservoir properties of hot-spring carbonates
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