Role of sandstone provenance in the diagenetic albitization of feldspars A 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 an extensional regime characterized by high subsidence rates. Its sedimentary infill (thicker than 6000 m) has been subdivided into eight depositional sequences (DS) mainly composed of continental sediments. DS 1 and DS 2 represent the first rifting stage (Tera Group, Tithonian), mainly formed by fluvial and lacustrine sediments. Sandstone petrofacies evolved from quartz-sedimentolithic in DS 1 to quartz-feldspathic in DS 2 due to the rifting process. In DS 2, three different types of detrital feldspars (K-feldspars, albites and polysynthetic plagioclases) with similar sodium-rich compositions (mean: Ab_(94.0) Aa_(4.5) Or_(1.5)) can be recognized. Chemically pure nonluminescent albites (Ab>99%) are common. In DS 2, diagenetic albitization of both plagioclases and K-feldspars is inferred from conventional microscopy observations, cathodoluminescence and electron microprobe analyses. DS 1 contains few plagioclase grains, which show no evidence of transformation into albite. Although the albitization is characterized as diagenetic it seems to be provenance-controlled since it affects the units showing higher original plagiodase/K-feldspar ratio (DS 2), due to the greater influence of plutonic and metamorphic source areas in DS 2. Possible Na sources are: (1) the percolation of moderate to high salinity residual brines from related alkaline lakes developed at top of DS 2 in the eastern sector of the basin, (2) clay mineral reactions (sodium smectite to illite and chlorite) indicated by mudstone composition in the interlayered mudstones, and (3) the replacement of detrital sodium plagioclases by carbonate. These three sources can be complementary.