--> Abstract: Cyclic Architecture of a Carbonate Sequence, Early Aptian Shuaiba Formation, Al Huwaisah Field, Oman, by J. Grotsch; #90990 (1993).

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GROTSCH, JURGEN, Shell Research, Rijswijk, Netherlands

ABSTRACT: Cyclic Architecture of a Carbonate Sequence, Early Aptian Shuaiba Formation, Al Huwaisah Field, Oman

Sequence stratigraphy of carbonates is a topic of ongoing controversy. In particular, small-scale shallowing-upward cycles can provide some key information needed for interpretation of carbonate sequences and/or third-order sea level changes.

The early Aptian Shuaiba Formation in the Al Huwaisah field consists of about 90 m of shallow-water limestones. Throughout the formation, an overall decreasing influx of fine detritus is notable toward the top. The sequence can be subdivided into a basal unit, which consists of a monotonous, restricted inner-platform facies with Lithocodium-Baccinella wackestones and an overlying unit with a highly diversified facies association of rudist-bearing pack-, float-, and rudstones (with Glossomyophorus sp., Offneria sp.). Both units are composed of meter-scale shallowing-upward cycles of different composition, which can be recognized in core and well logs. Fourier analysis of the first principle component of a set of well logs (GR, FDC, CNL, Sonic) revealed an abrupt change in spectral beha ior between the two units. Toward the top, the spectra are "cleaning upward" with an increasing pronunciation of a peak grouping of 1: 2: 5, suggesting a better preservation of orbital variations in the upper unit. Preservation of orbital forcing in shallowing-upward cycles requires rapid rates of sedimentation. In addition, increased shallow-water carbonate production on the platform is indicated by the appearance of reefal organisms. Hence, a higher rate of sedimentation and therefore a faster aggradation of the platform is inferred for the upper unit, which could have resulted from an increased rate of relative sea level rise.

The sudden facies differentiation on the broad Arabian shelf in the upper part of the early Aptian reflects the development of an intrashelf basin. Changes in rate of relative sea level rise on the Arabian shelf might explain the repeated alternation from an easily correlatable ramp-type sedimentation, with slightly higher input of fine terrigenous sediment (e.g., lower unit of Shuaiba Formation) and a differentiation into platform and intrashelf basin facies due to faster aggradation (e.g., upper unit of Shuaiba Formation).

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.