Detrital modes in sedimenticlastic sands from low-order streams in the Iberian Range, Spain: the potential for sand generation by different sedimentary rocks

The composition of modem stream sands derived from sedimentary source rocks in the Iberian Range has been analyzed in order to evaluate the contributions of the different bedrock types (mainly sandstones, limestones and dolostones). Temperate to subhumid climate and short transport conditions promote a weathering-limited denudation regime. As expected, sand composition proved to be essentially quartzolithic, with variable amounts of penecontemporaneous carbonates. Sand compositional data were compared with the exposure areas of the different bedrocks in the drainage sub-basins considered for semi-quantitative assessment of the sand generation potential of each bedrock type. Siliciclastic formations (sandstones) appear to be by far the most significant sand producers, with Sand Generation Indices (SGIs) in the medium sand fraction ranging from 4 to 20; i.e., 4 to 20 times greater than the SGI of carbonate rocks. Composition and texture are the main factors controlling carbonate sand generation. Sparitic limestones yield higher SGIs (2.8 to 20) when source terrains are constituted exclusively by carbonate rocks. High sparite grain content in the sands is enhanced by supplies from additional sources, such as calcitized dolostones. Dolomicrite sources are strongly under-represented in the sands analyzed (very low SGI), whereas the proportion of micritic limestone grains tends to be an accurate reflection of that bedrock at the source. Even though the results presented here refer to the first stage of sand generation with negligible transport effects, we think they may be helpful in the analysis and reconstruction of source terrains in ancient sedimenticlastic deposits. (C) 2002 Elsevier Science B.V. All rights reserved.