Meter-Scale Vertical and Lateral Facies Variability in a Sequence Stratigraphic Framework: Example from Shallow-Marine Carbonates of the Middle Jurassic Izhara Formation (United Arab Emirates)
Martin Hönig, Cédric Michael John, "Meter-Scale Vertical and Lateral Facies Variability in a Sequence Stratigraphic Framework: Example from Shallow-Marine Carbonates of the Middle Jurassic Izhara Formation (United Arab Emirates)", Sequence Stratigraphy: The Future Defined, Bruce Hart, Norman C. Rosen, Dorene West, Anthony D’Agostino, Carlo Messina, Michael Hoffman, Richard Wild
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A sedimentary bed is classically defined as a distinct layer of sedimentary rock that has a relatively uniform composition. Several outcrop-based studies have shown that facies within individual beds can vary laterally on a scale of around 100 m. As facies transitions are important criterions used as a proxy for the depositional environment and often to infer sequence stratigraphic trends in subsurface studies where data is limited to one-dimensional wells, this observed facies heterogeneity has implications for both paleoenvironmental studies and sequence stratigraphy. In this study, we investigate whether sedimentary and facies heterogeneities known to occur at the hundred meters scale are also present at the meter to tens of meters scale in a well-preserved facies mosaic deposited on a carbonate ramp (Wadi Naqab, Izhara Formation, Lower Bajocian, northern UAE), where a robust, outcrop-based sequence stratigraphic framework exists.
A bed set was logged and mapped across a 120 m long curving cliff face; combined with thin section analysis, the data allowed the reconstruction and quantification of facies heterogeneity at this location. Results reveal a large amount of lateral facies transitions at the meter scale. Lithofacies types have a probability of less than 70% of being laterally continuous over 12 to 18 meters, representing the highest amount of lateral facies heterogeneity so far reported in an ancient example. The case study reveals intra-bed facies transitioning attributed to spatially heterogeneous biogenic carbonate production as well as to syndepositional homogenizing and sieving processes occurring within shallow-marine depositional environments in ancient as well as in modern analogous systems. A series of continuous hardgrounds, previously interpreted as flooding and exposure surfaces, provide an independent sequence stratigraphic framework that demonstrate that the existence of small-scale lateral facies heterogeneities complicates interpretation of the vertical stacking pattern of facies. This confirms that the best practice is to limit sequence stratigraphic interpretations based on facies trends to larger stacking patterns (>10 meters). Meter-scale vertical patterns in carbonates often do not represent a proxy for base level changes, as illustrated in our study; therefore, fine-scaled, high-resolution sequence stratigraphy or lateral correlations are not attainable in carbonate sequences based on lithostratigraphy.