High-frequency detrital signals in Eocene fan-delta sandstones of mixed parentage (south-central Pyrenees, Spain): a reconstruction of chemical weathering in transit

  • Weltje G
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High-frequency sedimentation patterns in fan-delta sediments of the Eocene Montañana Group (Tremp-Ager Basin, south-central Pyrenees, Spain) are represented by a series of couplets consisting of alternating sandy conglomerate and calcareous mudstone. Each couplet is several meters thick and was deposited within a time span of the order of 20 to 30 kyr. Previous studies indicated that the couplets cannot be attributed to one of the commonly recognized mechanisms for generating high-frequency fluctuations, i.e., sea-level variations or autocyclic lateral switching of facies belts. However, the couplets can be explained as the result of intermittent variations of sediment supply. Results of detailed petrographic analysis and numericalstatistical modeling of sandstone compositions confirm this interpretation. The bases of coarse-grained units show a significantly higher quartz/feldspar ratio than the tops, which indicates that coarse-grained sediments initially supplied to the basin after a period of mudstone accumulation were subjected to prolonged chemical weathering in the source area. These systematic temporal shifts are superimposed upon compositional variations attributable to mixing of detritus from two different structural units in the lower reaches of the feeder system. Over longer time spans, tectonics and climate were equally important in determining the sediment flux from hinterland to basin in the southern Pyrenees. Which of these two factors exerted a dominant influence on the observed short-term variations cannot be determined beyond reasonable doubt. Two scenarios of climatic-physiographic variation are consistent with the observed sedimentation pattern and the information presently available: (1) high-frequency tectonic activity against a climatic background of seasonal subtropical conditions, and (2) high-frequency climate fluctuations against a tectonic background of high uplift rates and high-altitude microclimate conditions in the source area. The case history shows that provenance modeling techniques can be used to discriminate between intrabasinal and extrabasinal supply cycles. Moreover, the identification of extrabasinal supply signals is a significant step towards the reconstruction of surface processes in source areas.

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  • G.J. Weltje

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