Frans Van Buchem

Stratigraphic organization of carbonate ramps and organic-rich intrashelf basins: Natih Formation (middle Cretaceous) of northern Oman

The middle Cretaceous carbonate deposits in the Middle East are among the most productive oil-bearing stratigraphic intervals in the world, containing numerous giant fields in, for instance, the United Arab Emirates (Mauddud and Mishrif formations), Iran (Sarvak Formation), and Oman (Natih Formation). One of the main reasons for this concentration of hydrocarbons is a geological factor: the coexistence of both reservoir facies and source rocks in the same depositional sequences due to the repeated creation of organic-rich intrashelf basins. This is demonstrated in a high-resolution sequence stratigraphic study of the Natih Formation in Oman, which shows distinct and predictive patterns in the distribution and geometries of reservoir, source rock, and seal facies. The sequence stratigraphic model presented here may serve as a reference for time-equivalent deposits in the Middle East. The sedimentological analysis showed that the Natih Formation was formed by the alternation of two types of depositional systems: (1) a flat-bedded, mixed carbonate-clay ramp, dominated by benthic foraminifera, and (2) a carbonate-dominated ramp bordering an intrashelf basin, with abundant rudists in the mid-ramp environment and organic-rich basinal facies. Three fully developed third-order sequences are distinguished, showing a similar evolution of the depositional system, with a mixed carbonate-clay ramp system at the base, followed by a carbonate-dominated ramp system in the upper part. Variations occur on this pattern, however, depending on the relative influence of eustasy, environmental factors, and tectonism. The late Albiannearly Cenomanian sequence I shows an evolution from a mixed, flat ramp to a carbonate-dominated ramp and organic-rich intrashelf (Begin page 22) basin, and sedimentation is predominantly controlled by eustatic sea level. In the middle Cenomanian sequence II, the evolution from a mixed ramp to a carbonate ramp is also observed, but no intrashelf basin topography was developed in the studied area. This may be due to the high influx of clay that influenced the environment in this sequence, inhibiting the carbonate production, probably in combination with the lack of sufficient creation of accommodation space. The late Cenomaniannearly Turonian transgressive part of sequence III shows a similar evolution to that observed in sequence I, with the development of an organic-rich intrashelf basin. During highstand, however, a tectonically controlled sedimentation pattern is observed, with the development of forced regressive wedges (due to the flexural bulge of the foreland basin). Intrashelf basin formation occurred twice in the transgressive part of the third-order depositional sequences of the Natih Formation. Our study shows that this is mainly the result of differential sedimentation rates, that is, the dynamics of the carbonate sedimentary system itself in response to (rapid) rises in relative sea level, probably of eustatic origin. Tectonism was only a minor factor in the creation of the basin topography, possibly through the creation of small initial relief. The accumulation of the organic matter is not only a result of the creation of a sufficiently deep-water column to guarantee dysaerobic conditions for its preservation. The late Albian and late Cenomaniannearly Turonian were also periods of generally favorable conditions worldwide for high organic matter productivity. The time lines and stratigraphic architecture of the third-order sequences presented here have an application potential at the scale of the Arabian plate. The general sedimentation pattern is predicted by our model, but modifications due to different local conditions are likely to occur.

The Barremian-Aptian Evolution of The Eastern Arabian Carbonate Platform Margin (Northern Oman)

ABSTRACT Carbonate platform margins are sensitive recorders of changes in sea level and climate and can reveal the relative importance of global and regional controls on platform evolution. This paper focuses on the Barremian to Aptian interval (mid Cretaceous), which is known for climatic and environmental changes towards more intensified greenhouse conditions. The study area in the northern Oman mountains offers one of the very few locations where the Cretaceous carbonate margin of the Arabian Plate can be studied along continuous outcrops. Our detailed sedimentological and sequence stratigraphic model of the platform margin demonstrates how major environmental and ecological changes controlled the stratigraphic architecture. The Early Cretaceous platform margin shows high rates of progradation in Berriasian to Hauterivian times followed by lower rates and some aggradation in the Late Hauterivian to Barremian. High-energy bioclastic and oolitic sands were the dominant deposits at the margin. Turbidites were deposited at the slope and in the basin. The Early Aptian platform margin shows a marked change to purely aggradational geometries and a welldeveloped platform barrier that was formed mainly by microbial buildups. The sudden dominance in microbial activity led to cementation and stabilization of the margin and slope and, therefore, a decrease of downslope sediment transport by turbidites. In the Late Aptian, large parts of the Arabian craton were subaerially exposed and a fringing carbonate platform formed. Seven Barremian to Early Albian large-scale depositional sequences reflecting relative sea-level changes are identified on the basis of time lines constrained by physical correlation and biostratigraphy. The reconstruction of the margin geometries suggests that tectonic activity played an important role in the Early Aptian. This was most likely related to global plate reorganization that was accompanied by increased volcanic activity in many parts of the world. Along the northeastern Arabian platform the associated global changes in atmospheric and oceanic circulation are recorded with a change in platform-margin ecology from an ooid-bioclast dominated to a microbial dominated margin. Time-equivalent argillaceous deposits suggest an increase in rainfall and elevated input of nutrients onto the platform. This process contributed to the strongly diminished carbonate production by other organisms and favored microbial activity. The platform margin may thus represent a shallow-marine response to the Early Aptian global changes, commonly associated with an oceanic anoxic event in basinal environments.

Bypass Margins, Basin-Restricted Wedges, and Platform-to-Basin Correlation, Upper Devonian, Canadian Rocky Mountains: Implications for Sequence Stratigraphy of Carbonate Platform Systems

ABSTRACT Carbonate platforms can commonly keep up with relative sea-level rise because of high rates of sediment accumulation and platform aggradation. Surrounding basinal environments are commonly starved but can receive variable extrabasinal siliciclastic input and episodically deposited carbonate sediment. If accumulation rates in basinal settings lag behind those of the platform, a bypass or erosional margin can develop. Under these circumstances platform and basin depositional sequences become physically detached and direct correlation of basinal and platform sequences is hindered. We report here the results of high-resolution stratigraphic analyses of two Upper Devonian isolated carbonate platforms in western Alberta that provide insight into the sequence stratigraphy of bypass margins and criteria for accurate correlation of platform and basinal sequences. The slope and basin sequences surrounding the Miette and Ancient Wall platforms consist of basin-restricted, onlapping wedges of fine-grained background sediment deposited dominantly from suspension and coarse-grained platform-derived sediment redeposited by a variety of gravity-flow mechanisms. Sequence boundaries are identified within the redeposited carbonate intervals. Identification of sequence boundaries and differentiation of highstand and lowstand slope and basinal facies was based on the geometry, mineralogy, and clast content of redeposited carbonate units. Highstand carbonates contain sheet-like debris flows and turbidites with abundant slope-derived clasts and background facies with high total carbonate content. Lowstand carbonates contain sheet-like and channelized debris flows and turbidites with abundant platform-derived clasts and background facies with low carbonate content and locally high amounts of organic carbon. Transgressive facies are dominated by initially carbonate-poor and organic-rich background sediments that display a progressive increase in carbonate content and decrease in organic carbon content. These patterns are interpreted to record abundant background carbonate sedimentation during late transgression and highstand when the carbonate factory was robust. Highstand redeposited carbonates record slope erosion due to oversteepening and slope readjustment processes. Lowstand redeposited carbonates indicate platform and platform-margin erosion and low background carbonate sedimentation when the platform was either exposed or under very shallow peritidal conditions. High siliciclastic and organic contents during lowstand and early transgression may partly be the result of reciprocal sedimentation but alternatively may represent continuous siliciclastic supply during times with little dilution by fine-grained carbonate sediment. Successive stages of platform development at Miette and Ancient Wall were controlled by accommodation changes driven by relative sea-level fluctuations. Backstripping analyses of strata from both platforms confirm that significant differential subsidence was a major control on variations in platform thickness and patterns of slope development. Greater subsidence at Ancient Wall fostered the development of a steeper bypass margin and different slope evolution compared to Miette. Slope oversteepening also initiated a process of slope readjustment that eventually reduced the platform-to-basin gradient and facilitated regressive platform progradation. In conventional siliciclastic sequence stratigraphy, basin-restricted wedges are interpreted as lowstand deposits on the basis of their geometry and position relative to an updip margin. Wedge-shaped basinal units along the Miette and Ancient Wall bypass margins contain both highstand and lowstand facies that straddle sequence boundaries. The results of this study provide objective criteria for differentiating systems tracts in carbonate slope and basin environments through mineralogic and compositional analyses providing more accurate correlation of detached platform and basin sequences. Interpretation of carbonate basin-restricted wedges as purely highstand or lowstand deposits may lead to erroneous conclusions regarding sequence stratigraphy, platform-to-basin correlation, and the volumetric partitioning of sediments deposited in different systems tracts.

ABSTRACT: Reasons for The Development and Demise of An Isolated Carbonate Platform Complex: The Early Miocene (Burdigalian) of The Mut Basin, Southern Turkey

A shallow-marine carbonate platform complex developed over an abandoned delta during the Burdigalian transgression in the north of the Mut Basin (Turkey). The largest platforms (3km wide by 100m thick) grew along the edge of the underlying delta, and platform-size decreased landwards. The deltaic system eventually prograded out over the platforms terminating carbonate production: the landward platforms were directly overlain by finegrained (offshore) siliciclastics, and the seaward platforms by marls.