Atypical ooid diversity in the Upper Cretaceous Yacoraite Formation, Argentina

Abstract

This study reports on ooid diversity from different lithotypes of the Yacoraite Formation (Salta Group basin) in the Central Andes of north‐west Argentina. The ooids display a variety of internal and external morphologies that may be deployed as proxies for seawater chemistry and hydrodynamic processes. A short review of nomenclature problems is first discussed, followed by presentation of a two‐fold quantitative and qualitative methodology. Our proposed classification addresses internal and external ooid characters in order to understand growth in response to various environmental processes at an individual particle level. This classification allows discrimination between a variety of morphologies, but also evaluation of the complexity of the processes involved in ooid formation as seen in the fossil record. This study evaluates whether the ooids present within the Yacoraite Formation share similarities with ooids formed in marine versus marine lagoon and/or lacustrine environments. A possible lacustrine interpretation finds its origin in the diverse assemblage of ooid morphotypes present, which exceeds the variations described for marine ooids. However, growth paths and occurrence of various compound morphologies point to intense marine recycling, suggesting little accommodation. Together with other sedimentological characteristics, for example, bi‐directional cross‐bedding, tidal shoals, hummocky cross‐stratification and exposure surfaces, these features suggest that marine processes had an impact on the sediments. Therefore, the ooid assemblage of the Cretaceous Yacoraite Formation was most likely formed in a shallow coastal lagoon in the framework of an epicontinental sea that at times experienced marine flooding events. A detailed evaluation of processes involved in oolite formation is needed in order to improve the stratigraphic and palaeoenvironmental understanding of ooid formation through time. This includes examining alternating constructive and destructive stages, early binding and cementation, reworking, recycling and averaging processes.

Oolitic Limestone Induced by Photosynthetic Biofilms in the Xuzhuang Formation of Nangao Section, North China

“…The ooids of Nanga have the following additional characteristics: (1) residual calcified biofilm residues between the nuclei of the ooids and the ooids, and (2) a correlation between the ooid cortex and the nucleus of the ooid. As Coppa et al [17] stated, the formation of ooids may consist of complex sedimentary processes, and the cerebroid ooids in the Nanga section also undergo complex processes. This complexity is reflected in the diversity of the cortices and nucleus of cerebroid ooids.…”

“…The concentric laminae are dark micrite, and the radial laminae are wedge-shaped calcite (Figure 4c). The ooid cortex is not smooth and presents a petal or cerebroid pattern, distinguishing it from ordinary radial ooids or concentric radial ooids [17]. The section also shows some fully or partially dolomitized ooids (Figure 4d), with dolomite in the ooids displaying a distinct, sugar-like, "foggy nucleus and bright edge" texture, with diameters between 20 and 50 µm.…”

“…It is believed that their irregular cortex morphology may be due to the uneven transformation of calcite during the conversion process from aragonite to calcite. Another view is that the cortex of a cerebroid ooid is considered to be influenced by erosional discontinuities produced during the transformation process [17]. Recent studies have suggested that cerebroid ooids are the result of the combined action of biofilm accretion and syngenetic mineral growth.…”