ALBERT

Description: :— Proposed here is a universally applicable, texturally based classification of clastic sediment that is independent from composition, cementation, and geologic environment, is closely allied to process sedimentology, and applies to all compartments in the source-to-sink system. The classification is contingent on defining the term "clastic" so that it is independent from composition or origin and includes any particles or grains that are subject to erosion, transportation, and deposition. Modifications to Folk's (1980) texturally based classification that include applying new assumptions and defining a broader array of textural fields are proposed to accommodate this. The revised ternary diagrams include additional textural fields that better define poorly sorted and coarse-grained deposits, so that all end members (gravel, sand, and mud size fractions) are included in textural codes. Revised textural fields, or classes, are based on a strict adherence to volumetric estimates of percentages of gravel, sand, and mud size grain populations, which by definition must sum to 100%. The new classification ensures that descriptors are applied consistently to all end members in the ternary diagram (gravel, sand, and mud) according to several rules, and that none of the end members are ignored. These modifications provide bases for standardizing vertical displays of texture in graphic logs, lithofacies codes, and their derivatives—hydrofacies. Hydrofacies codes are nondirectional permeability indicators that predict aquifer or reservoir potential. Folk's (1980) ternary diagram for fine-grained clastic sediments (sand, silt, and clay size fractions) is also revised to preserve consistency with the revised diagram for gravel, sand, and mud. Standardizing texture ensures that the principles of process sedimentology are consistently applied to compositionally variable rock sequences, such as mixed carbonate–siliciclastic ramp settings, and the extreme ends of depositional systems.

Description: A bstract :  This paper demonstrates a process-based method of graphic logging and stratigraphic interpretation that is based on a new, texturally based classification of clastic sediment which is independent from composition, cementation, and geologic environment. The method integrates facies analysis with sequence stratigraphy because it requires identifying surfaces, compiling vertical changes in texture and attributes between surfaces, and uses these parameters to identify facies units and interpret continuously deposited intervals. Four clastic shelf to shoreface successions from the Atlantic Coastal Plain Province (Cretaceous to Holocene in age) with established chronostratigraphic and sequence stratigraphic frameworks provide examples of the application of this method. For each, graphic logs standardized for texture show the facies and bounding surfaces that define the thickness and hierarchy of event strata (e.g., sequences, beds, bedsets, parasequences, parasequence sets); these are directly compared with gamma-ray log response patterns and the distribution of factors that impact potential permeability. Shelf to shoreface successions were chosen because these facies exhibit the greatest diversity in composition (siliciclastic, carbonate, and glauco-phosphatics), but, in spite of this, exhibit comparable and predictable graphic and gamma-ray response patterns if systematically logged using this method. For the process–response model examined, texture, i.e., principal grain size, controls the amplitude (width) of the graphic log; its shape is constant, and independent from composition. The pattern in the correlated gamma-ray log parallels the log of principal grain size, except that an inverse response is associated with coarser lags; the amplitude of the gamma log is controlled by composition. A proxy for permeability developed from standardized textural fields is applied to two of the examples, to shows how aquifer or reservoir potential is systematically tied to stratigraphic and sedimentologic parameters.