geopiedra

The Nansa river valley Bundsandstein reference outcrop (Cantabria - Spain):

Triassic Buntsandstein stratigraphic series in Cantabria has been best studied at its so considered reference outcrop, along the Nansa river, below the Cohilla dam, not far from the “Expansion of Aguayo Reversible Hydroelecrical Power Station” Project area.

 Nansa valley from near La Cohilla dam, where Cantabrian Buntsandstein reference outcrop is located.

Located in the Region of Reinosa, Cantabria (Spain) the Aguayo project would be constructed in Buntsandstein rocks. A precise knowledge on the stratigraphic and structural geological frame is a project key-stone. A familiarisation site study at the Cohilla dam outcrop allows to further correlate what the lithology at project depth is likely to be, by a precise sedimentary analysis of the lithologies encountered both in drilled boreholes and in the outcrops in Aguayo project area. Eventually Palaeozoic slates may be at reach at cavern location under the Buntsandstein.

 Geological cross section. Adapted from Geological Map of Spain, Sheet 83 “Reinosa”.

The Bundsandstein in Cantabria:

In Cantabria the Triasic, constituted of its three characteristic “germanic facies”, principally appears in the extend outcrops of the oriental edge of the Iberian Palaeozoic Asturian massif, in the Escudo de Cabuérniga Sierra and in the Besaya River area (Reinosa area). It lies discordantly over Palaeozoic carbonaceous black slates. The Buntsandstein was followed by the Muschelkalk, up to 100 m thick shallow marine calcareous facies, in turn overlied by the Keuper, constituted of lagoonal clays with evaporites, and intruded with ophites.

Triasic deposition took place in the Reinosa area in a W-E orientated tectonic graben related with the strike-slip regime involved for the rift opening of the Bay of Biscay, which at the time was determining the individualisation and separation of Iberia with respect to Europe and North America. 

Outcrop map (left) and idealized reconstruction of tectonic graben of the tectonographic evolution of the Triassic of Cantabria (right); no vertical scale implied (modified from García-Mondéjar et al (86)).

Information panel with Bunsandstein description, located on top of the valley.

Buntsandstein in Cantabria has been described as having three main fluvial origin Units:

Buntsandstein sequence in La Cohilla reference outcrop (Modified from García-Mondéjar et al. (86)). 

Higher sector of the valley, showing Unit 1 thick coarse quartzite gravel conglomerate bancs. These were originated in proximal high energy flowing alluvial, where big dimension amalgamated channels flowed with a tractive character and sudden mud waterspout charged with gravels and boulders.

Buntsanstein series Unit  2 under Cohilla dam foot. This unit consists of seven stratified coarse to medium sandstone sequences over lied by thin ending siltstone/claystone levels.

Units 2 and base of Unit 3 sandstone beds show internal cross-stratification, which was originated in a channel deposited fluvial plain environment. Coarse beds including quartz and claystone gravels were due to temporal high energy flooding leading to erosive episodes of proximal materials. 

Sandstone levels belonging to Unit 2 correspond to low winding channel filling deposits, wedged between channel edge and tractive over flooding sandstone facies. Intercalated reddish siltstone/claystone layer represents overflood and decantation facies originated in a flooding plain low energy environment. 

Internal cross-beding, (horizontal units that are internally composed of inclined layers) in sandstone bancs shows the fluvial sedimentary environment.  

Unit 3 upper levels in the lower valley sector, general view. Siltstone and claystone are predominant with fine laminated sandstone intercalations. They represent an essentially braided fluvial system which would correspond to the fluvial bodies belt near the basin edge.

Reddish siltstone/claystone layers outcrop: Upper unit 3. They represent the more distant fluvial system zones, where the prevailing environment were extended mud plains.