Overlapping of pedogenesis and meteoric diagenesis in distal alluvial and shallow lacustrine deposits in the Madrid Miocene Basin, Spain

摘要

The Miocene distal alluvial fan and palustrine deposits of the Madrid Basin (Paracuellos de Jarama area) were examined to establish the sequence of its pedogenic-diagenetic processes and the main factors controlling them. In this area the diagenetic processes operated not only on carbonates, as commonly studied, but also in high magnesium clays and opaline cherts. This paper provides a dynamic model for saline-alkaline lake margins that complements the existing freshwater palustrine model. Three sections (BER, PEL and UPC) containing limestones, sepiolite, dolostones and opaline cherts were examined; these represent the entire transition from alluvial fan deposit to lake environment. The alluvial fan deposits (PEL section) show the most complex sequence of processes. After a weak edaphic imprint, the primary mudstone and siltstones are affected by silicification (opaline levels 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 or in interfan areas, sepiolite precipitated within free water bodies (BER section) and an intense pedogenesis led to the formation of palustrine sepiolite deposits. Later, the intense silicification of the sepiolite produced lenticular opaline levels that were locally transformed to quartz by aging.In the shallow lakes (UPC section), palustrine limestones and sepiolite were deposited. The carbonates are partially affected by silicification, the sepiolite becoming completely silicified. The opaline levels clearly preserve the structure of the limestones and sepiolite 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 sedimentary environments and facies. In the distal alluvial fan sediments of the PEL section, the initial host rock affected by silicification is not preserved due to the later dolomitization that erased both it and its textures. The silicification environment was therefore not easy to determine. In the BER and UPC section, the main silicification took place in groundwater-influenced environments but evidence was seen of cements and silicifications in vadose environments. In all study settings, the overlapping of pedogenic, vadose and groundwater processes was controlled by changes in the position of the water table. Four stages were concluded to explain the transformation from the primary deposits to the later dolomitization process and the formation of vadose cements. The majority of the 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 absorption capacity 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 silicification and ruling out the need of a biogenic source. The replacement of limestones by silica is usually driven by variations in pH and salinity, which are very common in surface environments such as shallow lakes and soils.