Carbonate Ramp Model Research Articles

Factors controlling sedimentation and sequence stratigraphy evolution in shallow marine (carbonates) platform: example of Middle Eocene deposits from Gafsa Basin

Based on their facies characteristics and stratal geometries, the carbonate deposits of the Middle Eocene outcropping in the Tamerza area (Western Gafsa Basin) led to the recognition of nine facies that were deposited in shallow marine environment summarized in a carbonate ramp model: homoclinal ramp under an arid climate. The depiction of facies features and the sequential analysis help to determine the depositional environments and establish the linkage between facies evolution, eustasy, climate and tectonics. The vertical evolution of facies shows a regressive trend that records a transition from infratidal to supratidal environments. The stacking pattern revealed several kinds of meter-scale, shallowing-upward cycles. Each cycle starts with relatively open-marine deposits, which are overlain by shallower and more restricted facies (tidal flat progradation). The varied facies, which are described within the Kef Eddour Formation, compose seven depositional sequences, mainly made of carbonates, marls and phosphates. Fauna associations and sedimentary structures imply a shallow subtidal to intertidal, moderately to high-energy platform. Seven sequences are interpreted as depositional sequences showing retrogradational (lowstand systems tract), aggradational (transgressive systems tract) and progradational (highstand systems tract) packages of facies associations. The seven depositional sequences which show a hierarchical organization of many cycles, as described above, suggested that eustatic sea-level oscillations caused by cyclic perturbations of the Earth’s orbit played a fundamental role in determining the formation of hierarchical cyclic rhythmicity.

Phosphate genesis and concentration a response to sea level fluctuation in shallow marine environments of the lower Eocene deposits in the southern Tethyan margin: case study of the Gafsa Basin, southern Tunisia

Detailed analysis of the Chouabine Formation exposed in the western part of the Gafsa basin, southern Tunisia shows the presence of six principal facies. Vertical and lateral evolution of facies showed a gradual transition from intertidal to circatidal environments summarized in a carbonate ramp model: homoclinal ramp under the control of synsedimentary faults and, episodically, swept by upwelling currents. Phosphogenesis indicates that early diagenetic organic matter degradation can be a source of authigenic phosphate during the first stage of transgression. The concentration of phosphate was under the control of hydrodynamic processes as testified by angular unconformities, erosive surfaces, current structures and reworked clasts. Phosphate genesis seems under the control of sea level changes. It begins with the upper LST where diagenetic processes allow the dissolution of fauna remains (bones and fish teeth) and a prelude precipitation of authigenic phosphates. During the TST stage, transgression was associated with upwelling currents, which enable faunal blooming. Phosphatization, which requires suboxic conditions, occurs in shells or in troughs, preferentially in infratidal to circatidal environments. During the HST, the shift of the sea level leads to a sea level fall and, thus, to a basinward migration of depositional environments. Accordingly, infratidal environment becomes intertidal. A detailed analysis of facies in the field shows the fossilization of current structures (cross-bedding structures capped by planar laminations and sheet deposits). Tidal currents sweep the deposits and allow phosphate concentration and deposition in troughs. The model proposed does not necessitate major rises and falls of the relative sea level to produce economic phosphorite, but emphasizes the interplay of both autocyclic and allocyclic controls to form phosphate strata during the highstand system tract.

Indication of calcareous tempestite inside the Qulqula Group in the Zagros Suture Zone, KRI

This sedimentological study focuses on carbonate storm deposits (calcareous tempestites) have been recognized for Qulqula platform in the Zagros Suture Zone outcropping at Sharbazher area in the northeastern Sulaimani City. This deposits are recorded between the oolitic limestone and calcareous turbidities in the Qulqula Group for the first time. These offer the first indication of event sedimentation in the Qulqula domain. Calcareous tempestites deposited on subsided eastern Arabian continental margin at initial stage of continental rifting, most possibly during Early Late Triassic to Early Jurassic. It is mostly encompasscalcarenite beds and indicating by some storm generated criteria such as: hummocky cross-stratification, parallel lamination, symmetrical wave-ripples, normally graded flat pebble conglomerate, and sole mark (flute caste, load cast, groove mark). Moreover, found Cruziana and Skolithos trace fossils with these calcarenite beds. Finally, for paleogeographic configuration of Qulqula carbonate platform, facies-depth connections on carbonate ramp model build up as shallowing-upward cycles from calcareous turbidities which deposited in distal part of outer ramp and carbonate distally steepened ramp to calcareous tempestites in mid ramp to oolitic limestone were formed in high-energy inner carbonate ramp.

Sedimentology, diagenesis, clay mineralogy and sequential analysis model of Upper Paleocene evaporite-carbonate ramp succession from Tamerza area (Gafsa Basin: Southern Tunisia)

Integrated sedimentological studies, diagenesis, sequential analysis and clay mineralogy on the Upper Paleocene rocks in Tamerza area provide important information on the reconstruction of the depositional basin, cyclicity, and paleoclimatic contexts. Facies analysis and petrographic studies have led to the recognition of nine facies that were deposited in three facies belts: Sebkha, inner ramp and outer ramp summarized in a carbonate ramp model: Homoclinal ramp under an arid climate.The upward and lateral changes in thickness and composition show a general regressive trend that records a transition from an outer ramp to Sebkha, creating different types of confinement. The facies stacking patterns constitute several kinds of meter-scale, shallowing-upward cycles. Nine different types of depositional cycles and several models of Sebkha sequences were defined. These different types of facies, characterized within the Thelja Formation, compose seven depositional sequences, mainly made of carbonates, marls and evaporates. Detailed multi approach analysis provides important information on evaporitic sequence stratigraphy.In carbonates beds, the diagenetic analysis provides an overview and chronology of diagenetic processes. A particular attention was paid to early stage cementation which enables us to characterize better the depositional environments. In addition to cementation, other features define the diagenetic history.X-ray diffraction reveals the presence of smectite suggesting an arid climate. Moreover, the clinoptilolite and the frequency of primary dolomite indicate different degrees of confinement.The seven depositional sequences showing a hierarchical organization of many cycles, as described above, suggested that eustatic sea level oscillations caused by cyclic perturbations of the Earth's orbit play a fundamental role in determining the formation of hierarchical cyclic rhythmicity.

Sedimentology, sequential analysis and clay mineralogy of the lower Eocene sequence at Farafra Oasis area, Western Desert of Egypt

Integrated sedimentological studies, sequential analysis and clay mineralogy on the lower Eocene rocks in the Western Desert provided important information on the reconstruction of the depositional basin, cyclicity, and paleoclimatic conditions. Two formations are recognized; the Esna and Farafra formations, with a gradational contact in-between. The studied sequence exhibits lateral facies changes as revealed from field and microfacies investigations. Eight facies were recognized and summarized in a carbonate ramp model. It represents also a general regressive trend, which records a transition from an outer ramp into a peritidal zone. The facies stacking patterns constitute several kinds of meter-scale, shallowing-upward cycles. Two different types of depositional cycles are here defined. The stratigraphic sections show a hierarchical organization of many cycles defined by five depositional sequences. It is suggested that composite eustatic sea level oscillations caused by cyclic perturbations of the Earth’s orbit played a fundamental role in determining the formation of the observed hierarchical cyclic organization. Summing up, it is believed that the paleotopography had resulted from the impact of the Syrian Arc Folding System. A confusing additional complication is introduced by syndepositional sedimentary structures, especially during the late Cretaceous/Eocene times, coupled by several tensional forces. Clay mineralogy has revealed the presence of smectite, kaolinite and illite. Their origin may be attributed to the gradual increase in the amount of erosion of the newly elevated crystalline source rocks to the south of Egypt, in areas of moderate rainfall and rapid weathering and/or to reworking processes of soils which presumably developed on basement rocks. Changes in source rocks or climatic influence during the early Eocene may account for the observed differences in clay mineral abundances.

Controls on reservoir quality in the paleogene Kalatar Formation of the southwestern region of the Tarim Basin, China

The Paleogene Kalatar Formation is a main target for petroleum ex125 ploration in the southwestern region of the Tarim Basin (SWRTB) and a systematical summary of controls on reservoirs of high quality has important implications for this area. According to outcrop analysis, as well as core, and laboratory experiments based on theories of sedimentology, it can be inferred that the main pore types in the Kalatar Formation are moldic, vuggy, intercrystalline and interparticle pores, fractures, and mudstone microporosity, respectively. The foreshore in the shore-shelf depositional model is the most favorable target, wherein grainstone and dolomitized grainstone are characterized by high quality in a middle ramp subfacies of the carbonate ramp model. Diagenetic factors, such as micritization, cementation, compaction, neomorphism, silicification, and pyritization, are detrimental for reservoirs. At the same time, leaching and dolomitization can improve the quality of reservoirs. Permeability of reservoirs can be greatly improved by fractures, through which quality of reservoirs can be effectively enhanced. Concentration of CO2, temperature, and humidity are affected by changes in climate, which can have an influence on depositional setting, composition and diagenesis of sediments, and eventually the properties of reservoirs. The hot and arid climate of the Paleogene was harmful to development of reservoirs, which is demonstrated in the contemporaneous diagenetic and epigenetic stages. Shallow and deep-burial diagenesis are closely related to the formation and the connate water, however these are rarely affected by paleo-climatic variation. There is possible erosion that mainly resulted from organic acid in relative well intervals according to indications of hydrocarbons in the early diagenetic stage. Fracture systems can be established when acidic fluids are emplaced during periods of uplift, by which pores can be effectively enlarged during the late diagenetic stages.

Evolution of shallow benthic communities during the Late Paleocene–earliest Eocene transition in the Northern Tethys (SW Slovenia)

A paleoecological and sedimentological study was carried out on shallow-water carbonates of the Kras Plateau (SW Slovenia) with the goal of reconstructing paleoenvironmental conditions and evolution of foraminiferal communities on the northwestern Adriatic Carbonate Platform (AdCP) during the Late Paleocene–earliest Eocene. Three facies have been recognized and summarized in a carbonate ramp model. Within these facies, six foraminiferal assemblages, representing different ramp sub-environments, have been defined: during the Late Paleocene sedimentation took place in a protected innermost ramp with (1) smaller miliolids- and (2) small benthic foraminifera-dominated assemblages thriving on partly vegetated, soft substrates. In the Uppermost Paleocene, sedimentation primarily occurred along a mid ramp. The upper mid-ramp was sporadically influenced by storms/currents and occupied by (3) Assilina-dominated assemblage occurring on a soft sandy substrate. The deeper mid-ramp was characterized by (4) ‘bioconstructors’- and (5) orthophragminids-dominated assemblages, colonizing biotopes with substrates of different nature. During the earliest Eocene, deposition occurred in an inner-ramp setting with (6) alveolinids-nummulitids assemblage thriving on muddy and sandy substrate, partly covered or close to seagrass beds. The Late Paleocene–earliest Eocene environmental conditions, coupled with the long-term evolution of larger benthic foraminifera (LBF), seem to have favored this low-light dependent group as common sediment contributors. By comparing the evolution of the shallow-water biota from the Adriatic area with data from the Pyrenees and Egypt, a general latitudinal trend can be recognized. However, on a smaller geographical scale, local conditions are likely to have played a pivotal role in promoting the evolution of biota characterized by suites of unique features.

中東地域下部白亜系炭酸塩岩油層における５点水攻法下の水付き現象とその地質学的要因

Premature water breakthrough was observed in a Lower Cretaceous carbonate reservoir under five spot pattern water injection in a Middle East oil field. Openhole log resistivity anomalies in recently drilled wells and time lapse pulsed neutron logs suggested preferential water movement in certain horizons. Reservoir heterogeneity depending on lithological variation is identified as a cause of the water breakthrough.Two high permeable lithofacies were defined by core and thin section observation. One is altered bioclast dominant coarse grainstone/packstone in the middle part of the reservoir. The other is very fine altered bioclastic peloidal grainstone/packstone in the upper part of the reservoir. These lithofacies are composed mainly of rarely micritized bioclastic grains with rigid framework preserving intergranuler porosity. Large molds and vugs associated with rudists in the upper part of the reservoir are locally connected and contribute to the fluid flow.Relative sea level change and paleo-topography are recognized as important factors to control lithofacies distribution. Areal distribution of the lithofacies shows aggradational stacking pattern and abrupt lateral lithofacies change particularly in the middle part of the reservoir. This suggests that simple carbonate ramp model currently applied is not appropriate for reservoir characterization purpose in this reservoir.

Short Paper: A standardized model for Tethyan Tertiary carbonate ramps

Low latitude carbonates in massive and clinoform bedded sheets, termed ramps, occur throughout the Tertiary Tethyan realm, and are characterized by recurring biofacies usually including large foraminifera, rhodolithic algae, coralgal patch-reef and gastropod dominated sequences. All can be interpreted broadly within a standardized Tertiary carbonate ramp model. Biota can be used to define precise palaeobathymetric zones which have a predictive utility in palaeoenvironmental modelling, especially where data are limited. The carbonate ramp model of earlier authors, has been elaborated by Read (1982) and ramp formation occurs mostly in passive margin and extensional tectonic settings. Ramps are characterized by slope gradients of less than 1 degree and, once established, are conservative to change. Much use has been made of the ramp model for Palaeozoic and Mesozoic lithofacies modelling. In contrast, published work on Cenozoic ramps is limited, however, Tethyan ramps are particularly widespread and extend tens to hundreds of kilometres along strike and occupy tens of kilometres of slope. Many studies have dealt with Cenozoic ramp associations and sequences (e.g. Pedley 1983 ; Reiss & Hottinger 1984; Purser 1973 ) and have demonstrated the diversity of biotal and sediment detail, but without ascribing them to a ramp model. Biota were quick to adapt to the Tertiary environments left vacant after the K–T boundary extinction events. New genera of Foraminifera, Mollusca, Bryozoa and Echinoder-mata rapidly replaced the extinct forms in bursts of innovative opportunism. Codiacean algae, Rhodophyta and marine angiosperms were equally adaptive colonizers of the carbonate platforms. Novel biofacies associations were derived from

Carbonate ramp deposition: Middle to upper carboniferous microfacies of Eastern Anhui and Southern Jiangsu, China

The Huanglong limestone and the Chuanshan limestone (Middle to Upper Carboniferous) from three sections in Eastern Anhui, China, is described with regard to the microfacies types and the biota (especially the calcereous algae). 21 microfacies types have been distinguished (including 2 of the overlying Qixia limestone). The algal flora is characterized by beresellid and donezellid algae, common red algae and rare phylloid algae; the algal flora indicates the Huanglong limestone is Middle Carboniferous when compared with the Eurasian stratigraphy. Microfacies types as well as diagenetic features (vadose crystal silt, meniscus cement) point to a morphological differentiation of the depositional area into shallow marine environments bathymetrically slightly different and which in parts were affected to some extent by minor sea level fluctuations. The sedimentation pattern of the Huanglong Formation and the Chuanshan Formation can be described with the “carbonate ramp model”, i.e., sedimentation on a sloping topographic surface which is subject to open-marine and open-lagoonal conditions. A “homoclinal ramp” position can be assumed for the transgressive Huanglong carbonates as well as for the regressive Chuanshan carbonates.

Sedimentation Within Southern Oklahoma Aulacogen: Viola Limestone: ABSTRACT

The Viola Limestone (Middle Ordovician) was deposited within the southern Oklahoma aulacogen in the Arbuckle Mountains of Oklahoma. The northwest-trending aulacogen is a basement rift which opened in the Late Cambrian (535 to 525 m.y.a). During the early history of the aulacogen, the thick Cambro-Ordovician Arbuckle Group (2,050 m) was deposited as a predominantly peritidal complex. The conformably overlying Early Ordovician Simpson Group (700 m) shows more variation in water depth but still is dominated by shallow-water deposits. The Viola Limestone rests disconformably on a hardground which was developed at the top of the Simpson Group. Early Viola deposition was below wavebase probably with continental slope bathymetry; this time thus represents the deepest carbonate s dimentation within the aulacogen. Initial results show that the oldest microfacies of the Viola Limestone is a laminated calcisiltite deposited within anoxic bottom conditions by weak traction currents. Progressively increasing oxygenation and wave energy resulted in deposition of a bioturbated wackestone which then grades upward into a washed grainstone. These microfacies indicate a general upward shallowing along the axis of the aulacogen. Early workers, however, suggested that the more cratonward Viola microfacies may deepen slightly upward. As noted in earlier studies, the carbonate ramp model seems to fit best the depositional setting of the Viola. The ramp model can deal with the conflicting water depth trend by having a subsidence hinge axis upslope from the upward-deepening and u ward-shallowing sections. In addition, sedimentation rates would have been greater in the aulacogen axis than on the marginal platforms. Shallow subtidal deposits cap the Viola of both the aulacogen and platform. In both areas these shallow-water carbonates were subjected to early diagenesis by meteoric water, confirming their proximity to sea level. End_of_Article - Last_Page 632------------