Gregor P. Eberli
Carbonate platforms are long-lived sedimentary edifices, which contain the successive record of sedimentary environments that reflect the physical processes at the time of deposition. Early diagenesis, which in carbonates is nearly simultaneous with deposition, encodes these processes in the strata and determines fluid pathways for later diagenesis. The recognition of the linkage of sedimentary facies and diagenesis provides the opportunity to relate the individual architectural elements to these processes and increases our understanding of platform evolution and enhances the predictability of the facies distribution and diagenesis.
Three examples from the modern and ancient demonstrate how both deposition and early diagenesis are related to external physical processes. The calibration of seismic images to cores and outcrops revealed the strong influence of physical processes, in particular sea level changes, on diagenesis and on the long-term platform development. Processes such as wind direction, sea level changes, and ocean currents have long been recognized as controlling factors on facies distribution on the shallow platform top. Sea level change ultimately controls the aggradation, progradation, and backstepping pattern of platforms. In addition, each high-frequency sea level fall and rise produces a characteristic succession of facies. Simultaneously with the changes in facies, the diagenetic environment c anges. For example, during a sea level rise the diagenetic environment is predominantly marine and submarine cementation stabilizes the platform margin. In contrast, when sea level drops, the platform is exposed and meteoric diagenesis may either lithify or dissolve the sediment. Thus, facies distribution and diagenesis are controlled by sea level changes and distributed in a predictable pattern within carbonate sequences.
This linkage of facies and diagenesis to sea level applies for different frequencies of sea level fluctuations. Consequently, the general platform architecture is the result of the stacking of small architectural units controlled and constructed during high-frequency sea level changes. The recognition that large platform segments consist of characteristic smaller scale architectural elements can be used to enhance predictability of margin heterogeneities and the distribution of diagenesis and sedimentary facies when only large-scale seismic images are available.
AAPG Search and Discovery Article #90948©1996-1997 AAPG Distinguished Lecturers