Linking regional Quaternary surface evolution to the palaeoflood events recorded by cave sediments in central Portugal

<p>In karst landscape, caves are <em>loci</em> prone to trap and accumulate clastic, chemical, and organic sediments, covering a time span from several million years up to the present. Different types of cave sediments can be distinguished: (a) autochthonous (breakdown and insoluble bedrock remains) and para-autochthonous (overlaid fluvial and hydro-aeolian deposits); (b) allochthonous, with water and/or gravity-driven transport (filtrates, fluvial, lacustrine, aeolian, marine, organic); (c) chemical sediments&#8212;secondary cave minerals. The (a) and (b) are clastic cave sediments (CCS), while the most widespread and best-known chemical sediments are calcite speleothems in the form of stalactites, stalagmites, or flowstones.</p><p>Based in the knowledge of Central Portugal&#8217;s karst (Sic&#243; and Estremadura massifs), previously obtained through studies focusing geomorphology/hydrology and palaeokarst stratigraphy/evolution, an innovative and ambitious investigation is &#8220;now&#8221; necessary to overcome the current shortcomings, whose main challenge is to link surface dynamics (reasonably well studied) to the cave sedimentary record (relatively unknown), providing new insights about Quaternary landscape evolution at local/regional scale.</p><p>New data were produced through fieldwork and laboratory analyses of the sedimentary record (CCS and speleothems) preserved in the largest endokarst system of the Sic&#243; massif, which includes a set of exsurgences, and ponors linked by few caves with mainly horizontal passages that take advantage from laterally extended bedding planes to which other conducts are added that developed vertically along faults/joints. Among these caves, the Soprador do Carvalho cave stands out with an active underground river runs along its lower level, which is prone to seasonal flooding; here two main orders of ancient alluvial terraces show the evidence of fluvial dynamics during Middle Pleistocene&#8212;as confirmed by the obtained ²⁶Al/¹⁰Be data (burial ages ranging from ~0.43 to 0.18 Ma). The geometry and composition of the oldest endokarst terrace&#8212;a clast-supported and well-cemented conglomerate with imbricated shist pebbles&#8212;with a ²⁶Al/¹⁰Be burial age of 0.4 &#177; 0.03 Ma, indicate a palaeodrainage direction opposite to the present-day hydrologic flow, as well as a catchment area that extended to the Hercynian massif to the east. Instead, the sedimentary and geomorphological characteristics of younger endokarst alluvial terrace&#8212;a clayey deposit with sandy intercalated lenses&#8212;with a burial age of 0.2 &#177; 0.02 Ma, indicate a palaeodrainage direction compatible with the current one. These underground finding agree with some surface geomorphological observations that point out, for the Middle Pleistocene, to a general reorganization of the hydrographic network, with fracture adaptations, inversions, and captures, due to local tectonic deformations. Furthermore, a regional geochronological correlation between cave palaeoflood events and the oldest lower Mondego River terraces can be established, representing a sedimentary record of the period between MIS11 and 7 for the central Portugal.</p><p>In addition, detrital interlayered material (clayey layers) trapped between laminae of a stalagmite formed between ~5760 BCE and ~1920 CE (dated using U/Th and ¹⁴C methods), suggests that the studied cave was periodically affected by palaeoflood events also during Holocene. Further geochemical analysis, including speleothem&#8217;s stable isotopes and trace-element determinations, coupled by petrographic observations (microstratigraphy), could provide more valuable paleoclimate information.</p>