%0 Journal Article
%A Alonso-Zarza, Ana María
%T Palaeoenvironmental significance of palustrine carbonates andcalcretes in the geological record
%D 2003
%@ 0012-8252
%U https://hdl.handle.net/20.500.14352/51026
%X Interest in palustrine carbonates and calcretes has increased over the last 20 years since they contain significantenvironmental information. Much of the work performed in this area has focused on either of two types of terrestrialcarbonate—palustrine carbonates or calcretes (pedogenic and groundwater)—yet their simultaneous study shows there may be agradual transition from one form to the other, revealing the interplay between pedogenic, sedimentary, and diagenetic processes.Three main factors control the formation of these carbonates: the position of the water table, the host rock, and the period ofsub-aerial exposure. In pedogenic calcretes, precipitation of carbonate takes places mostly in the vadose zone above the watertable, and within a previous host rock or sediment. In groundwater calcretes, the precipitation of carbonate also occurs within aprevious host rock and around the groundwater table. In palustrine carbonates, however, the precipitation of lime mud occurs ina lacustrine water body. Palustrine carbonates necessarily form on previous lacustrine mud, whereas both types of calcretes mayform on any type of sediment or soil. The sub-aerial exposure time needed to form palustrine carbonates may by relatively short(even a season), whereas pedogenic calcretes need more time (several years to millions of years). Groundwater calcretes do notform on the topographic surfaces, so there is no need of sub-aerial exposure. However, stable surfaces favour the developmentof thick groundwater calcretes. Small fluctuations in the water table cause gradual transitions of these three types of terrestrialcarbonates and the subsequent mixture of their characteristic features, causing difficulties in the interpretation of thesecarbonates.The formation of these carbonates is controlled by palaeoenvironmental factors. Both commonly form in semi-arid climates.Arid climates are also suitable for calcretes, but sub-humid conditions are more suitable for palustrine carbonates. Moreindications of climatic conditions may be obtained through the analysis of the d18O content of both calcretes and palustrinecarbonates, and from the depth of the horizon containing carbonate nodules in pedogenic calcretes. Vegetation is also importantin the formation of these types of carbonates. Data on the prevailing vegetation can be obtained from the analysis of the microand macrofabric as well as from the d13C signal of the primary carbonates, which, in pedogenic carbonates, has also been usedto estimate atmospheric pCO2 during the Phanerozoic. These terrestrial carbonates are widely distributed on floodplains anddistal areas of alluvial basins. Their presence and characteristics can be used as indicators of aggradation, subsidence oraccommodation rates, and therefore as indicators of different tectonic regimes.Even though the study of these carbonates has notably increased in recent years, much less is known about them than aboutmarine carbonates. Presently, there is much emphasis on obtaining a general model for sequence stratigraphy in terrestrialbasins, with a need to include the carbonates analysed in this paper.
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