Holocene sands of the Spanish Central System were exclusively derived from plutonic, middle-upper grade and low-grade metamorphic rocks. Modal composition of studied sands is mostly controlled by grain size and source area lithology. Thus, sands derived from slates and schists plot near the QR edge on the QFR diagram for all grain size fractions. Sands derived from granitic or gneissic rocks have a wide dispersion on the QFR diagram, from the R pole to the QF edge, depending on sand grain size. Percentages of quartz types in granitic-derived sands are Qnu 42 , Qu 40 , Qp2–3 14 and Qp > 3 4 . Sands of gneissic origin have Qnu 51 , Qu 15 , Qp2–3 23 and Qp > 3 11 . Sands derived from slates and schists have Qnu 20 , Qu 12 , Qp2–3 5 and Qp > 3 63 . Quartz types easily discriminate sands of low-grade origin, but distinction of sands derived from plutonic rocks from those derived from middle-upper grade metamorphic rocks is difficult because of the highly variable Qu content of plutonic rocks related to strain history and crystallization conditions. Thus, quartz types must be used with caution in source discrimination if plutonic rocks are present in the source area.
The Cameros Basin in Central Spain is an intraplate rift basin that developed from Late Jurassic to lVIiddle Albian time along NW-SE trending troughs. The sedimentary basin fill was deposited predominantly in continental environments and comprises several de positional sequences. These sequences consist of fluvial sandstones that commonly pass upward into lacustrine deposits at the top, producing considerable repetition of facies. This study focused on the western sector of the basin, where a total of seven depositional sequences (DS-
Compositions of modern first-cycle sands derived from granitic and metamorphic terrains in the Spanish Central System have been analyzed in order to evaluate the contributions of different bedrock types. The results of this work indicate that sand composition normally does not permit quantitative statements concerning sourceland composition. This is due to the fact that each rock type has a different potential to generate sand, dependent on such properties as its mineralogy, average crystal size, and microfabric. We introduce the concept of sand generation index (SGI), which is a relative measure of the capacity of one bedrock type to generate sand with respect to another in a compound source area. SGI of granitoid is 14 to 20 times greater than SGI of slate-schist when these rock types appear in a dual crystalline source. In the case of a gneiss + slate-schist source rock association, the SGI of gneiss is about five times greater than the SGI of slate-schist, whereas the SGI of gneiss is similar to that of granitoid in case of a dual granitoid + gneiss source. Finally, our results show that quantitative estimates of source land composition based on QFR diagrams are hazardous if the concept of the SGI is disregarded. Palomares, M, and Arribas, J, 1993, Modern stream sands from compound crystalline sources: Composition and sand generation index, in Johnsson, M. J., and Basu, A., eds., Processes Controlling the Composition of Clastic Sediments: Boulder, Colorado, Geological Society of America Special Paper 284. 313 on June 13, 2015 specialpapers.gsapubs.org Downloaded from
The Henares River, central Spain, flows westward from the Iberian Range (Mesozoic sedimentary rocks) under semi arid climatic conditions, In the middle and lower reaches, the Henares River receives sediment from three tributaries E-mail addresses:arribas@eucmax.sim.ucm.es(J.Arribas).critelli@irpi.cs.cnr.it(S.Critelli).lepera@irpi.cs.cnr.it (E. Le Pera) , Henares sand composition takes place by the mixing of tributary deposits with previously deposite d Henares River terraces. Abrasion during transport does not appear to cause signifi cant changes in the sand composition along the Henares River.
Plutonic and gneissic rocks of the Sila Massif in the uppermost portion of the Neto drainage basin (Calabria, Southern Italy) weather and erode under a humid Mediterranean climate. During the development of weathering profiles, a combination of chemical weathering and granular disintegration processes occurred. Chemical weathering involves a loss of both plagioclase (mainly during grus generation) and K-feldspar (mainly during soil formation). This loss is attributed to transformation of plagioclase to clay minerals and to leaching and dissolution of K-feldspar. Sand composition is quartzofeldspathic and nearly homogeneous along the main channel of the Neto River, even where the river cuts across a blanket of sedimentary cover. Thus, fluvial transport does not alter sand composition within the Neto drainage basin. Petrographic indices are effective in (1) discriminating between contributions from similar (granite and gneiss) source rocks (Qm/F); (2) relating the provenance of plutoniclastic and gneissiclastic sand found in the headwaters to grus horizons (Qm/F; Q/Rg); and (3) distinguishing between upstream first-cycle and downstream multicycle sand (Q/Rg). This last distinction is further emphasized by considering both aphanitic and phaneritic varieties of rock fragments (RgRmRs diagram). Chemical weathering is the main sand producer within the regolithic environment in northern Calabria. In addition, rapid erosion resulting from steep slopes removes weathered products, and rapid and short transport leads to minimal sediment maturation. In general, the F/Q index is climate and relief dependent; thus, it should be used in conjunction with palaeoclimatic and palaeophysiographic evidence for provenance interpretations of ancient quartzofeldspathic sandstones.
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