Thermal models and clay diagenesis in the Tertiary-Cretaceous sediments of the Alava block (Basque-Cantabrian basin, Spain)

Abstract

AB ST R ACT : Diagenesis in the Cretaceous and Tertiary sediments of the Alava Block (BasqueCantabrian basin) has been studied using the clay mineralogy (X-ray diffraction) of cuttings from three representative wells of a NÀS cross-section. More than 5500 m of various lithologies (marls, mudstones and sandstones) have been drilled in the northern part of the domain, and 2100 m in the southern zone. The illitization of smectite and the disappearance of kaolinite, due to diagenesis, are the most characteristic features in the northern well. Evolution of smectite to illite has been differentiated into four zones, from top to bottom of the series, each showing specific I-S interstratified clay assemblages. The disappearance of smectite and the distribution of kaolinite in the other two wells are explained based on source-area considerations. Burial and thermal history have been reconstructed, revealing a northward increase in thermal flow until the Oligocene (Alpine orogeny paroxysm). In the northern well, the thermal model suggests temperatures of 160 and 270ºC for the disappearance of smectite (R0) and illite-smectite (I-S) mixed-layer R1 clay minerals, respectively. The disappearance of kaolinite is related to a temperature of 230ºC, a temperature never attained in the other two wells. Retardation of these processes, in relation to temperature values in the literature, is a consequence of the poor reactivity of marly lithologies, due to the low availability of cations. In this regard, the scarcity of reactants (K-bearing phases) and the absence of pathways (low permeability) for their access and circulation imply that illitization could have taken place in a closed system, by diffusion, on a very small scale, i.e. that of the original smectite grains.

Reply to Comment by Pedreira et al. on “Reconstruction of the Exhumed Mantle Across the North Iberian Margin by Crustal‐Scale 3‐D Gravity Inversion and Geological Cross Section”

“…The mountainous region with elevations up to ~1,500 m above sea level mentioned by Pedreira et al () is enclosed within this triangular area, not in the area of mantle doming, the Biscay Synclinorium, whose average altitude is 286 m. This is a crucial point, given that Pedreira et al () also wonder whether or not flexural rigidity can account for regional support in the area of extremely reduced lithosphere. The column block over the attenuated lithosphere of the Biscay Synclinorium is made up of 7 km of syn‐rift and postrift sediments, as well as volcanics, which is in accordance with the heat‐flow history calculated in the Basque‐Cantabrian Basin (Arostegui et al, ).…”

Comment on “Reconstruction of the Exhumed Mantle Across the North Iberian Margin by Crustal‐Scale 3‐D Gravity Inversion and Geological Cross Section” by Pedrera et al.

“…Flexural rigidity of the lithosphere would hardly contribute to support the orogenic topography at a regional scale in their model, because the lithosphere is considered to be extremely thin, and therefore hot and weak (with the asthenosphere located at minimum depths of only ~15 km) from the Paleocene to the latest Oligocene (their Figure 12). This also implies very high and rather constant geothermal gradients during the whole orogenic process, something that is unrealistic and contrary to the findings of thermal studies in the basin (e.g., Arostegui et al, ; Gómez et al, ). This spatial and temporal evolution is also incompatible with a hypothetical support of the topography by hot asthenospheric upwelling (i.e., dynamic topography ) acting only since the Eocene and up to the present, a mechanism for which there is no evidence.…”

The stratigraphy of the Sima de los Huesos (Atapuerca, Spain) and implications for the origin of the fossil hominin accumulation

“…The oriented aggregates (OA) were prepared by carefully pipetting the clay suspension onto glass slides, that later were placed in a glass desiccator during 24 h for ethylene glycol solvation. Thermal treatments at 300 C and 550 C were also applied to identify the clay minerals following the procedures detailed in Arostegui et al (2006). After each treatment step, the glass slides were measured by XRD with CuKa radiation (40 kV, 40 mA), graphite monochromator, a programmable divergence slit, and a PIXcel detector.…”