Tectonic–palaeoenvironmental forcing of clay-mineral assemblages in nonmarine settings: the Oligocene–Miocene As Pontes Basin (Spain)

Abstract

Two small, alluvial-lacustrine subbasins developed during the early restraining overstep stages of the Oligocene -Miocene As Pontes strike-slip Basin (NW Spain). Later, the basin evolved into a restraining bend stage and an alluvial-swamp-dominated depositional framework developed. The palaeobiological record demonstrates that the Oligocene -Miocene palaeoclimate in NW Spain was subtropical, warm and humid to subhumid. The metamorphic and igneous basin catchment yielded clay assemblages made up by kaolinite, illite and Al-smectite. Illite occurred as an original mineral in the source rock area, whereas kaolinite and Al-smectite resulted mainly from weathering of feldspar and clinochlore, respectively. This detrital primary clay assemblage remained preserved in the colluvial, alluvial fan and shallow lacustrine facies, whose early diagenesis was influenced by diluted, poorly evolved pore waters with neutral to slightly alkaline pH. The original clay assemblage was mildly to strongly transformed under early diagenetic conditions in the lacustrine and swampy environments where significant hydrochemical and Eh -pH changes took place. A fibrous magnesium-rich clay mineral-dominated assemblage (palygorskite and sepiolite) formed in shallow, saline lakes and palustrine zones under the influence of magnesium-rich, alkaline waters. Moreover, kaoliniteenriched assemblages formed in deep lacustrine, swamp and swamp-related alluvial zones under the influence of slightly to highly acidic pore waters. Pore water acidic conditions, characterising environments with organic matter accumulation, led to early diagenetic transformation of Al-smectite into kaolinite. This process was relatively limited in some environments such as organic matter-rich bottoms in meromictic lacustrine zones, whereas it was pervasive in peat-forming swamp zones.The stratigraphic relationships between the diverse clay mineral assemblages in the As Pontes Basin fill demonstrate the coeval development of rather diverse clay compositions. This clearly reflects the influence on clay assemblages of palaeoenvironmental changes forced by the morphological and tectonic evolution of the catchment -basin system. The interplay between climate and tectonic processes in the source areas did not result in major variations of the clay minerals fed into the basin. Conversely, this tectonic-sedimentation interplay influenced the evolution of the drainage and the water balance in the depositional zones, causing a complex environmental -hydrochemical evolution to occur. As a consequence, drastic early diagenetic changes affected the original clay mineral assemblages and resulted in a variety of early diagenetic assemblages. The As Pontes case study emphasises the major influence of palaeogeographical and tectonosedimentary evolution on the claymineral record in nonmarine depositional systems. The depositional record in As Pontes Basin demonstrates that morphological and tectonically forced environmental and hydrochemical changes can result in variations similar to...

Large to intermediate-scale aquifer heterogeneity in fine-grain dominated alluvial fans (Cenozoic As Pontes Basin, northwestern Spain): insight based on three-dimensional geostatistical reconstruction

“…The widespread predominance of mudstone and sandy mudstone facies show that fine-grained sediments fed these systems predominantly. The rock composition of the basin catchment (major slates and schists and minor quartzites and gneisses; Figure 1b; Barsó et al 2003;Sáez et al 2003), and the warm and humid-subhumid paleoclimatic conditions (Medús 1965a and1965b;Menéndez Amor 1975;Baltuille et al 1992;Cabrera et al 1994;Cavagnetto 2002) account for this predominance.…”

“…Genetic units 2 to 5 are formed by several composite sequences, which have been defined based on sequence stratigraphic concepts applied to continental deposits, and are bounded by isochronous or near-isochronous bottom and top surfaces related to the settling and spreading of the major coal seams (Ferrús 1998;Sáez and Cabrera 2002;Sáez et al 2003; Figure 1d). …”

“…Clay minerals in the matrix are mainly kaolinite, aluminous smectite and minor illite (Sáez et al 2003). The deposits of this facies organize according to two major geometries, with changing sedimentological characteristics (Ferrús 1998): a) laterally extensive sand bodies, and b) lenticular ribbon sand bodies.…”

“…Sand grains are made up mainly of quartz and minor feldspar, whereas clay minerals (kaolinite, aluminous smectite, and illite) occur in the mud grade (Sáez et al 2003). This facies makes up laterally extensive, thick and multiepisodic bodies that were mostly deposited on the proximal to medial alluvial fan zones of the MBZ, where the core data show its close vertical and lateral relations with the sand facies.…”

“…It is composed mainly by clay minerals (kaolinite, aluminous smectite, and illite) and minor quartz and feldspars (Sáez et al 2003). Sideritebearing mudstone beds have been observed in the lower part of sequence 13, in the transition from the underlying coal seam.…”