STANDARDIZING TEXTURE, CLASTIC ROCK CLASSIFICATION AND GRAPHIC LOGS FOR INTERPRETING PROCESS-GENERATED STRATIGRAPHIC SEQUENCES

Process sedimentology links a hierarchy of event strata (bed by bed) with dynamic geomorphic processes. Proposed here is an integrated approach to classifying clastic sediment and rock, constructing graphic logs, and assigning facies codes, for interpreting process-generated stratigraphic sequences. This method requires a universally applicable, texturally-based classification of clastic sediment that is independent from composition and cementation, and closely allied to process sedimentology. Revisions to Folk’s (1980) texturally-based classification are proposed to accommodate this. His triangular diagram showing proportions of gravel, sand and mud is modified to include a complete range of textural classes. An assumption is made that this classification universally applies to all clastic sediment and is independent from composition.

The proposed modifications provide bases for standardizing logging practices, graphic log templates, lithofacies codes, and their derivatives – hydrofacies or permeability codes and aquifer or reservoir characteristics. A direct result is that the principles of process sedimentology can be consistantly applied to compositionally variable rock sequences, especially mixed siliciclastic and bioclastic assemblages of strata. The graphic logs produced are powerful tools that are ready for comparison with downhole logs and interpretation in a sequence stratigraphic context. The linking of genetically related lithofacies to dynamic geomorphic processes, helps define beds, bedsets, parasequences, parasequence sets, and higher orders of facies sequences, such as systems tracts in the stratigraphic hierarchy. Examples of shelf successions, Cretaceous to Holocene in age, are provided.

The proposed method is comprehensive, but flexible, and is usable in conjunction with other classification systems. It applies to a broad range of facies, depositional settings, clast compositions, and degrees of consolidation and cementation. If primary clastic fabrics are obscured or diagenetically altered, or if unit is chemically precipitated, replaced, or recrystallized, the method is limited without additional analyses.