Role of sandstone provenance in the diagenetic albitization of feldsparsudA case study of the Jurassic Tera Group sandstones (Cameros Basin, NE Spain)

摘要

The Cameros Basin (Iberian Chain, NE Spain) formed during the latest Jurassic–Early Cretaceous rifting stage in anudextensional regime characterized by high subsidence rates. Its sedimentary infill (thicker than 6000 m) has beenudsubdivided into eight depositional sequences (DS) mainly composed of continental sediments. DS 1 and DS 2udrepresent the first rifting stage (Tera Group, Tithonian), mainly formed by fluvial and lacustrine sediments.udSandstone petrofacies evolved from quartz-sedimentolithic in DS 1 to quartz-feldspathic in DS 2 due to the riftingudprocess. In DS 2, three different types of detrital feldspars (K-feldspars, albites and polysynthetic plagioclases) withudsimilar sodium-rich compositions (mean: Ab94.0 An4.5 Or1.5) can be recognized. Chemically pure nonluminescentudalbites (AbN99%) are common. In DS 2, diagenetic albitization of both plagioclases and K-feldspars is inferred fromudconventional microscopy observations, cathodoluminescence and electron microprobe analyses. DS 1 contains fewudplagioclase grains, which showno evidence of transformation into albite.Although the albitization is characterized asuddiagenetic it seems to be provenance-controlled since it affects the units showing higher original plagioclase/Kfeldsparudratio (DS 2), due to the greater influence of plutonic and metamorphic source areas in DS 2. Possible Naudsources are: (1) the percolation ofmoderate to high salinity residual brines fromrelated alkaline lakes developed atudtop of DS 2 in the eastern sector of the basin, (2) clay mineral reactions (sodium smectite to illite and chlorite)udindicated by mudstone composition in the interlayered mudstones, and (3) the replacement of detrital sodiumudplagioclases by carbonate. These three sources can be complementary.