--> ABSTRACT: Non Marine Carbonate Precipitates: A Review Based on Recent and Ancient Case Studies

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Non Marine Carbonate Precipitates: A Review Based on Recent and Ancient Case Studies

Della Porta, Giovanna 1; Barilaro, Federica 1
(1) Earth Sciences Department, Milan University, Milan, Italy.

Non marine carbonates, in particular microbialites, have recently attracted much attention from industry and academia following the discovery of the South Atlantic carbonate reservoirs. Non marine carbonate precipitation can originate in a wide spectrum of depositional settings from freshwater, alkaline and saline lakes, to freshwater fluvial systems and hydrothermal vents. Within lakes, carbonate buildups (cms to tens of meters bioherms, mounds or pinnacles) develop either at shallow shorelines or at sublacustrine groundwater springs. In rift lakes precipitation is eventually associated with hydrothermal springs. Subaerial hot springs and rivers produce wedge-shaped and mound geometries dms to several tens of meters in thickness.

As for marine environments, non marine carbonate precipitation can be purely abiotic, driven by CO2 degassing, biologically induced involving microbes and biofilms, and biologically controlled by skeletal biota. Carbonate fabrics vary from laminated, columnar, dendritic, shrubby, oncoidal to crystalline crusts. Biota are a function of water chemistry and temperature. Diatoms, generally abundant, might play a role in the fluctuation of silica content.

Diversity of geometry, mineralogy, and fabrics is case study specific due to numerous physical, chemical and biological controlling factors influencing the modes of carbonate precipitation. Major external controls are basin morphology and hydrology, substrate lithologies, tectonic setting and climate. Water Mg/Ca ratio affects the carbonate mineralogy as in marine settings. Several factors control the Mg/Ca ratio at precipitation sites: 1) in lakes the Mg/Ca ratio is a function of variable degrees of mixing between lake, ground- or thermal water; 2) high temperature favors aragonite precipitation in hot spring settings, while in alkaline lakes ikaite forms in conditions of low temperature and high orthophosphate; 4) microbes and biofilms might also play a role due to the ability of binding Ca2+ and Mg2+ ions; 5) formation of Mg-clay minerals modifies the Mg/Ca ratio.

Primary porosity is generally high because it is associated with abundant organic matter, vegetation and degassing. Meteoric diagenesis is common in subaerial vents and shallow lakes and secondary porosity development depends on the primary mineralogy. Hot spring travertines, even if low Mg calcite, have high primary porosity and largely preserve it despite meteoric cementation and sediment infill.

 

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.