%0 Journal Article
%A Bustillo Revuelta, María Ángeles
%A Alonso-Zarza, Ana María
%T Overlapping of pedogenesis and meteoric diagenesis indistal alluvial and shallow lacustrine deposits in theMadrid Miocene Basin, Spain
%D 2007
%@ 0037-0738
%U https://hdl.handle.net/20.500.14352/49328
%X The Miocene distal alluvial fan and palustrine deposits of the Madrid Basin (Paracuellos de Jarama area) were examined toestablish the sequence of its pedogenic–diagenetic processes and the main factors controlling them. In this area the diageneticprocesses operated not only on carbonates, as commonly studied, but also in high magnesium clays and opaline cherts. This paperprovides a dynamic model for saline–alkaline lake margins that complements the existing freshwater palustrine model. Threesections (BER, PEL and UPC) containing limestones, sepiolite, dolostones and opaline cherts were examined; these represent theentire transition from alluvial fan deposit to lake environment. The alluvial fan deposits (PEL section) show the most complexsequence of processes. After a weak edaphic imprint, the primary mudstone and siltstones are affected by silicification (opalinelevels formed mainly by selective replacement of sepiolites) and later dolomitization. Several types of dolomite are present,rounded crystals, spherules and globular bodies being the most characteristic. In the ponds situated at the foot of the alluvial fans orin interfan areas, sepiolite precipitated within free water bodies (BER section) and an intense pedogenesis led to the formation ofpalustrine sepiolite deposits. Later, the intense silicification of the sepiolite produced lenticular opaline levels that were locallytransformed to quartz by aging.In the shallow lakes (UPC section), palustrine limestones and sepiolite were deposited. The carbonates are partially affected bysilicification, the sepiolite becoming completely silicified. The opaline levels clearly preserve the structure of the limestones andsepiolite host rocks. All the opaline levels show local aging processes.The silicification processes were different in the three settings due to the interplay of groundwater with sedimentaryenvironments and facies. In the distal alluvial fan sediments of the PEL section, the initial host rock affected by silicification is notpreserved due to the later dolomitization that erased both it and its textures. The silicification environment was therefore not easy todetermine. In the BER and UPC section, the main silicification took place in groundwater-influenced environments but evidencewas seen of cements and silicifications in vadose environments. In all study settings, the overlapping of pedogenic, vadose andgroundwater processes was controlled by changes in the position of the water table. Four stages were concluded to explain thetransformation from the primary deposits to the later dolomitization process and the formation of vadose cements. The majority ofthe silicifications seen occurred as responses to changes in the water table position linked to the aggradation of this closed basin.In all the studied environments, the sepiolite deposits were preferentially silicified; their fibrous structure and high absorptioncapacity helping to retain interstitial fluids. Moreover sepiolite and opal are stable under relatively similar geochemical conditions in arid environments. The dissolution of the sepiolite caused an increase in silica in the interstitial fluids, favouring silicificationand ruling out the need of a biogenic source. The replacement of limestones by silica is usually driven by variations in pH andsalinity, which are very common in surface environments such as shallow lakes and soils.
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