Facies architecture and cyclicity in a mosaic carbonate platform: effects of fault‐block tectonics (Lower Lias, Causses platform, south‐east France)

Abstract

International audienceIn the Causses platform (south-east France), Late Hettangian to Sinemurian deposits were interpreted previously as shallow-water carbonate ramp deposits. A new look at these deposits has shown a fault-controlled mosaic carbonate platform that is different from the carbonate ramp models. Within the platform mosaic, 15 lithofacies have been recognized, which are organized in four facies associations, including peritidal, restricted shallow sub-tidal, sand dunes and sub-tidal shelf facies associations. The rapid lateral and vertical facies changes, and the lack of consistent landward or seaward direction indicated by the pattern of facies changes, question the existence of a shoreline suggested by the traditional models for this region. Instead, the facies organization and cycle stacking pattern suggest deposition in a mosaic of intertidal islands between which sub-tidal restricted or open conditions could coexist in very close proximity. Such a platform mosaic would have been defined by tectonic activities along normal faults which segmented the shallow-water Causses platform. The facies and facies associations are arranged into metre-scale, peritidal and sub-tidal cycles that are also variable. Certain cycles show the same stacking pattern in all the sections and seem to be traceable over tens of kilometres. On the contrary, other cycles cannot be correlated; they are present only in specific sections and have a maximum lateral extension of 1 or 2 km. These metre-scale cycles stack to form four medium-scale cycles bounded by surfaces that display sub-aerial exposure features. Medium-scale cycles stack into two larger-scale cycles (tens of metres thick) and are bounded by well-defined karstic surfaces. Based on their lateral continuity and their stacking pattern, the metre-scale cycles are controlled probably by high frequency eustatic variations overprinting the topographic irregularities formed by differential subsidence of fault-bounded blocks. Episodic fault activities may reorganize the topography so that, even if eustatic changes may still be the major control of cycles, the expression and number of cycles could be different. Cycles of medium and large-scale are interpreted as being allogenic, controlled by changes in eustasy and/or subsidence rates as evidenced by their lateral continuity and the correlations of the large-scale cycles with third-order depositional sequences

Peritidal cyclic sedimentation from La Manga Formation (Oxfordian), Neuquén Basin, Mendoza, Argentina

“…The remarkable cycle discontinuities are related to sea-level fluctuations (cf. Adams and Grotzinger, 1996) even though a local subsidence can not rule out (Hamon and Merzeraud, 2008). As Bádenas et al (2010) suggest, the lateral continuity of cycles can be controlled by depositional topography or by local subsi-dence, where facies distribution results from production rate and accumulation.…”

Depositional processes of the Zhushadong and Mantou formations (Early to Middle Cambrian), Shandong Province, China: roles of archipelago and mixed carbonate–siliciclastic sedimentation on cycle genesis during initial flooding of the North China Platform

“…It laterally changes to a shoal cycle in the JLS section, which reflects various types of carbonate production (cf. Hamon & Merzeraud, 2008). Cycle B3 of units 2 and 4, characterized by subtidal to intertidal stromatolites, shows limited distribution.…”

Chapter 5 Controls on sedimentation in Indian Palaeoproterozoic basins: clues from the Gwalior and Bijawar basins, central India

“…Alternation between wavy and crinkly algal microlaminite with microdigitate stromatolite (tufa) with preserved 'teepe' structure is identified as the product of stratiform, benthic microbial mat growth predominantly within shallow-water bathymetry, possibly in an intertidal to peritidal domain (Grotzinger 1989;Pratt et al 1992;Sarkar & Bose 1992;Sarkar et al 1996). In these environments, laminites commonly record a combination of precipitation of fine-grained carbonate with benthic microbial mats and trapping of detrital sediment (Grotzinger 1986;Hamon & Merzeraud 2008). In contrast, large domal/ columnar stromatolites, cabbage-headed at times and with symmetrical vertical columns of variable width and height, suggest relatively higher bathymetry without much wave or current action (Sarkar et al 1996).…”