Quantitative stratigraphy of rudist limestones and its bearing on spatial organisation of rudist communities: the Late Barremian, Urgonian, sequences of Provence (S.E. France)

Abstract

Rudist beds in platform carbonates of Late Barremian age (Early Cretaceous) cropping out in the Marseille region (Provence, SE France) possess a substantial lateral continuity (>40 km) whereas associated rudist bodies tend to be less continuous. Transition probabilities (Markov chain analysis) are used at a regional scale to test the lateral continuity of rudist bodies and their four associated rudist assemblages. Each rudist assemblage has a specific probability to be adjacent to itself or to any one. At a local scale, analysing the variations of transition probabilities as a function of distance, using various proxies, shows a periodic spatial trend, the style of which depends on the type of rudist assemblage, i.e., community type. The foregoing documents two groups with different properties. Group 1 includes two communities characterised by small forms restricted to very shallow and muddy settings, organised spatially into small patches (c. 1000–1500 m) with moderate spacing (c. 1500–2000 m). Group 2 includes two communities characterised by mixed size or large forms, typical of deeper water and more sandy settings, organised spatially into large patches (c. 3000 m) with a larger spacing (c. 3000–4000 m). The internal architecture and the dimensions of these rudist bodies argue against a reefal nature. Quantitative estimates of lateral continuity and spatial variations of rudist bodies have a significant potential for reservoir modelling of hydrocarbon-bearing rudist-rich platform carbonates.