A quantitative criterion with which to distinguish lithofacies belts from mosaics in carbonate deposystems

Abstract

The planform pattern of carbonate lithofacies in shallow tropical systems have been described as both relatively simple (i.e. the lithofacies belt model) and highly complex (i.e., the lithofacies mosaic model). The lateral continuity of the lithofacies elements differs between these two descriptions with the former containing continuous strike-oriented elements while the latter contains numerous disjointed elements with no specified orientation under two conditions. First, we consider the lithofacies elements solely by their sediment texture, and second, we do not consider ecological constraints on the benthic communities. Using variography, we develop the spatial correlation index (SCI) for distinguishing between the two types of lithofacies arrangements and deploy the index to visualize the spatial distribution of belt-like and mosaic-like patterning for 7 lithofacies in 36 carbonate platforms. The visualizations suggest that the belt versus mosaic dichotomy does not adequately capture the complexity of lithofacies arrangements in modern carbonate systems. This limitation exists because carbonates primarily consist of biogenic sediments whose distribution is controlled by physical and ecological controls. Thus, taxonomy must be considered when examining these spatial patterns. Furthermore, there appears to be a co-occurrence of arrangements within each of the systems. Four motifs in the statistical distribution of the SCI values observed within the systems are identified. The motifs appear to depend on the biotic and abiotic factors of the sedimentary system under study. Belt-like arrangements for a framestone class appeared to be strongly associated with the higher energy environments, while mosaic-like arrangement for this class often occurred in sheltered, low-energy areas. For grainstone and wackestone classes, belt-like arrangements were located along the transition from platform rim to lagoon. Finally, the results suggest that a greater variety of sediment-texture arrangements may be observed in Modern shallow-water carbonate depositional systems than may be visible in Ancient systems observed within the rock record.