INTERPRETING SEQUENCE SCALE ACCOMMODATION TRENDS FROM FACIES STACKING PATTERNS IN CARBONATE AND MIXED CARBONATE-CLASTIC SHELF SETTINGS: PRACTICAL CHALLENGES

The use of facies stacking patterns to define accommodation trends is a fundamental approach for defining sequence stratigraphic surfaces in carbonate shelf successions. The concept is solidly based on the observation that carbonate sediments tend to record accommodation changes well. However, this approach is fraught with difficulties when attempted with 1-D sections, as well as with some 2-D successions, in outcrops or in the subsurface. The basic reason for this is that sequence scale accommodation trends (i.e., upward-shallowing vs. upward-deepening) are not always readily definable from facies stacks because the latter are a function of a variety of variable drivers such as subsidence and sea level rise/fall rates and topography. Because continuous, dip oriented 2-D outcrop exposures can provide valid insights into this relationship, they can be used, in some cases, as models for interpreting stacking patterns. By contrast, 2-D sections that are either unoriented or along strike provide essentially no basis for defining accommodation trends. Out of context 1-D sections provide an even more problematic basis for interpretation. Some of these problems may be alleviated in 2-D sections where siliciclastic facies are interbedded with carbonate deposits especially where there is evidence that the clastics were shed from landward areas of the shelf. In these settings, the accommodation record revealed by the clastic deposits may provide a more accurate record of accommodation shifts and sequence boundary positions than the carbonate facies.