Paleo-watertable definition using cave ferromanganese stromatolites and associated cave-wall notches (Sierra de Arnero, Spain)

Abstract

The steeply-dipping-dolostone-hosted caves of the Sierra de Arnero (N Spain) contain lowgradient relict canyons with up to ten mapped levels of ferromanganese stromatolites and associated wall notches in a vertical range of 85 m, the highest occurring ~460 m above base level. Despite a plausible speleogenetic contribution related to pyrite oxidation, and irregular cave wall mesomorphologies suggestive of hypogenic speleogenesis, the Arnero relict caves are dominantly epigenic as indicated by the conduit pattern and the abundant allogenic sediments. Allogenic input declined over time due to a piracy-related decrease in the drainage area of allogenic streams, explaining the large size of the relict Arnero caves relative to the present-day outcrop of the karstified carbonates. Allogenic sediment input also explains the observed change from water-table canyons to phreatic conduits in the paleo-downstream direction. Stromatolites and associated notches arguably formed in cave-stream passages at the water table. The best-defined paleo-water tables show an overall slope of 1.7º, consistent with the present-day relief of the water table, with higher-slope segments caused by minor barriers related to sulfide mineralization. The formation of ferromanganese stromatolites at the water table favored wall notching by the combined effect of enhanced acidity by Mn-Fe oxidation and shielding of cave floors against erosion by the stromatolites themselves. The presence of abrasive clastic sediments further contributed to notch formation by preventing erosion of passage floors and promoting lateral mechanical erosion. The irregular wallrock erosional forms of Arnero caves are related partly to paragenesis and partly to the porous nature of the host dolostones, which favored intercrystalline irregular dissolution near passage walls, generating friable halos. Subsequent mechanical erosion further contributed to generate irregular forms and spongeworks. The dolostone porosity also explains the paradox that virtually all cave development in Arnero is focused on dolostone despite being much less soluble than adjacent limestone. U-series dating of carbonate speleothems and paleomagnetic data from ferromanganese stromatolites and clastic sediments indicate that the paleo-water tables recorded ~320 m above the present-day water table formed during the Matuyama Chron but prior to ~1.5 Ma, implying a long-term rate of base level lowering of ~125 to ~213 m per Ma. To our knowledge, this is the first attempt of paleomagnetic dating of cave ferromanganese deposits. Ferromanganese stromatolites are excellent geomagnetic recorders and offer a direct way to delineate and date paleo-water tables, especially in caves developed in dolostone.

Reconstructing fluvial incision rates based on palaeo‐water tables in chalk karst networks along the Seine valley (Normandy, France)

“…The presence of magnetically reversed sediments therefore indicates a minimum age for cave alluviation of 0.78 Ma. With a stacked series of normal and reversed‐polarity sediments, this method can be used to date caves back several million years (Farrant et al ., 1995; Stock et al ., 2005; Hajna et al ., 2010; Rossi et al ., 2016; Bella et al ., 2019). As a dating tool, palaeomagnetism suffers from two main limitations: first, it requires suitable fine‐grained sediments within the cave; second, it is a correlative tool that cannot yield absolute ages for stratigraphic units except when magnetic reversals are identified and reliably correlated with the global record.…”

“…Cave systems also have the benefit that they, and the deposits they contain, can be preserved over long timescales where surface landforms such as river terraces may be degraded by weathering and erosion. Therefore, mapping and dating cave systems can be used to deduce valley incision rates and continental uplift (Farrant et al ., 1995; Granger et al ., 2001; Rossi et al ., 2016). In this paper, the landscape evolution of the western Paris Basin (France) was investigated through the systematic investigation of relict caves exposed in the sides of the lower Seine River valley around Rouen (Eastern Normandy).…”

The sediment at the end of the tunnel: Geophysical research to locate the Pleistocene entrance of Gruta da Companheira (Algarve, Southern Portugal)

“…) The earlier formation stages of Gruta da Companheira probably occurred under phreatic conditions and were likely controlled by joints and bedding planes within the limestone bedrock. During our survey, we observed features characteristic of both epigenic (e.g., notches; Farrant & Smart, 2011; Slabe & Prelovšek, 2013; Rossi et al, 2016) and hypogenic caves (e.g., convection pockets and cupolas; Klimchouk, 2007; Audra et al, 2009; Dublyansky, 2013; Slabe & Prelovšek, 2013). We hypothesize that hypogenic formation was followed by a separate epigenic phase.…”

“…Further to the east, Chamber 1 shows convection ceiling pockets (Klimchouk, 2007; Audra et al, 2009; Slabe & Prelovšek, 2013; Figure 7b) and a hypogenic conical cupola (sensu Dublyansky, 2013). This feature exhibits multiple, separate notches, marking water table stands (Klimchouk, 2009; Rossi et al, 2016). After its formation, the top of the cupola was removed (probably through top‐down dissolution), opening a passage to the upper P2.…”

Jurassic Non-Carbonate Microbialites from the Betic-Rifian Cordillera (Tethys Western End): Textures, Mineralogy, and Environmental Reconstruction

“…Monty [114] and Burkhalter [115] denominated such cavity and fissure dwelling stromatolites as endostromatolites. This kind of microbial growth was also named in the literature as speleo-stromatolites [116] and cave stromatolites [8,117]. The development of the Fe crusts in protected shadow places of the sea-bottom allows them to be called cryptic crusts.…”