The role of internal waves in ancient carbonate systems

Résumé

Internal waves are perturbations propagating along the pycnocline, a boundary layer between two different density water masses. Although they are well known by oceanographers, their impact on the sedimentary record still poorly documented. In siliciclastic systems, breaking of internal waves on a sloping surface creates repetitive high-turbulent events and consequently erosion and transport of sediments along the shelves and continental slopes. In carbonate settings, internal waves can influence the carbonate production at the depth of pycnocliene by pumping the nutrient-rich waters to the carbonate buildups and create an ideal setting where the metazoan communities can thrive. The base of pycnocline is usually associated with the chlorophyll-maximum zone which corresponds to the lower part of the photic zone. The light penetration is one of the fundamental factor controlling loci, amount and type of carbonate productions. The light zones are named “euphotic”, “oligophotic” or “mesophotic”, and “aphotic”. Recently, the role of internal waves, as a source of water turbulence, has been considered as a useful tool in the interpretation of mesophotic carbonate communities. During the Late Jurassic and Late Oligocene, extensive carbonate reefs have been developed along the Tethys. In this PhD project, two case studies from the Upper Jurassic stromatoporoid-rich facies and Late Oligocene (Chattian) coral-rich facies have been studied in order to study the role of internal waves in development of these carbonate communities. The Upper Jurassic stromatoporoid-rich facies of Monte Sacro Limestones (MSL) crop out along the platform margin of Apulia Carbonate Platform (ACP) in Gargano area. The stromatoporoid buildups in MSL are characterized by high percentage of high-energy debris-rich facies associated with low-energy facies. The origin of these high-energy facies are still matter of debates. The MSL is characterized by three lithofacies LF1- stromatoporoid-rich facies, LF2- stromatoporoid-coral facies, and LF3- stromatoporoid-microbial facies. LF1 is the main lithofacies developed in MSL and characterized by stromatoporoids growth in low-energy mesophotic condition (LF1-S1) associated with high-energy intraclastic-bioclastic rich facies (LF1-S2). The stromatoporoid-rich buildups (LF1) in ACP can be categorized as phototrophic-heterotrophic reefs generated in a pure carbonate environment. The light penetration was confined, resulted in the high development of light-independent micro-encrusters (Tubiphytes morronensis), in a mesophotic condition, where the environment was not ideal for light-dependent microencrusters (Lithocodium- Bacinella) to grow. The origin of high-energy facies developed associated with mesophotic stromatoporoid buildups in MSL can be linked to the effect of internal waves. Firstly, internal waves can provide nutrient-rich water needed by stromatoporoid buildups to grow. Latterly, the buildups can be affected by high-energy turbulence, producing a large amount of high-energy debris rich facies (LF1-S2) in MSL. Moving on to a different age, the Late Oligocene (Chattian) coral-rich facies are well developed in Grotta San Michele Limestone (GSML) Gargano, Italy, as well as Asmari Formation, Zagros, Iran. The corals in GSML are surrounded by a mud-dominated matrix, indicating development in low-energy environments. The corals, are associated with meso-oligophotic components such as non-articulate red algae, rhodolith and Polystrata alba. However, the euphotic components such as articulated red-algae, and rare miliolids are associated with corals. Although these mesophotic corals can be mixed with euphotic components shed down from the shallower depth, the internal waves can be a factor to provide nutrient-rich water for coral colonies to develop in this low-energy settings. In the Zagros basin, Iran, the coral buildups are very well developed along the depositional profile. These coral buildups are characterized by a low-energy mud-dominated matrix and show the characteristics of cluster reefs. The corals are strongly encrusted by red algae and also associated with meso-oligophotic components such as non-articulated red algae, Neorotalia and Nephrolepidina suggesting mesophotic conditions. The internal waves can affect these mesophotic corals by two ways: provide the nutrient-rich water to buildups and produce high-energy turbulence responsible for the development of high-energy flank facies associated with coral buildups.