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Book Chapter

“Tuning” High-Frequency Cyclic Carbonate Platform Successions Using Omission Surfaces: Lower Jurassic of the U.A.E. and Oman

By
Gordon M. Walkden
Gordon M. Walkden
Department of Geology and Petroleum Geology, University of Aberdeen, Meston Bldg, King’s College, Old Aberdeen AB24 3UE, U.K.
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;
Jose De Matos
Jose De Matos
Petroprimo, Av. da Republica, 50–4°, 1050 Lisbon, Portugal
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Published:
January 01, 2000

Abstract

Meter-scale cyclicity is developed on a number of Lower Jurassic peri-Tethyan carbonate platforms, and comparable cyclicity is known from basinal successions in parts of Europe. Analysis of the 234-m-thick Hettangian to Early Pliensbachian section of a recently constructed reference section through inner- to mid-shelf Jurassic carbonates in Wadi Naqab, U.A.E./Oman border, shows that facies patterns alone give an inaccurate and even misleading record of this cyclicity. Omission surfaces, however, can provide far more reliable evidence for sea-level change and enable the “tuning” of cyclicity data derived from facies studies. Clear distinctions need to be made, in the field, between a number of different marine and subaerial omission surfaces, including firmgrounds, hardgrounds, karsts, corrosion surfaces, and paleosols. Some of these surfaces display similar or ambiguous characteristics in the field, and difficulties can be compounded by the re-texruring and remodeling effects of burial diagenesis. High-resolution data, incorporating systematically observed and logged omission surfaces, have potentially high value in correlation and greatly improve the reliability of interpretations. In the Wadi Naqab succession the incorporation of carefully observed cycle-boundary data more than doubles the number of cycles seen. The data show that average duration of the high-frequency cycles was ca. 90,000 years, well within the range of Milankovitch forcing factors but outside the 40,000 years obtained from basinal Milankovitch successions of the same age in Europe. The difference reflects the incompleteness of the shallow-water succession, and it is likely that both types of cyclic successions in the Lower Jurassic share the same climatic and/or eusta tic causes. Fischer-plot analysis of the 149 cycles reveals the additional operation of two strong third-order sea-level fluctuations, and possibly two further smaller ones, between late Hettangian and early Pliensbachian. Subaerial omission surfaces are best developed and most numerous on the falling limbs of these, where cycles are thin. These points would correspond to the falling stage of third-order sea-level change when there was limited creation of new accommodation and maximum subaerial exposure. Omission surface data therefore support the case for both fifth-order and third-order sea-level change in the Jurassic. A particular characteristic of this and other shallow-water Milankovitch successions is their thin-bedded nature. This is likely to impart a strong flow anisotropy in reservoir settings, enhanced by stratiform weathering and diagenetic features, but many of the definitive features are likely to be missed in seismic sections and well logs. Actual omission surfaces can be hard to detect even in core.

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SEPM Special Publication

Middle East Models of Jurassic/Cretaceous Carbonate Systems

Abdulrahman S. Alsharhan
Abdulrahman S. Alsharhan
Faculty of Science United Arab Emirates University, P.O. Box 17551, Al Ain, U.A.E.
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;
Robert W. Scott
Robert W. Scott
Precision Stratigraphy Associates, RR3 Box 103-3, Cleveland, OK 74020, U.S.A.
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SEPM Society for Sedimentary Geology
Volume
69
ISBN electronic:
9781565761919
Publication date:
January 01, 2000

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