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| Titel |
Identification of flood events inside karst cavities: Fria Cave (Asturias - NW Spain) |
| VerfasserIn |
Saul Gonzalez Lemos, Heather Stoll |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250077256
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| Zusammenfassung |
| Fluvial records may be well preserved in subterranean karst drainage networks and fluvial
deposits cemented in speleothems may provide good chronology of past flood events. In
several karst systems in Asturias (NW. Spain), moments of extreme precipitation
events produce deposits from flood events in the bed and walls of caves which
we propose are also recorded in the calcium carbonate stalagmites growing in the
cave.
The final stretch of the studied cave (Fria Cave), with a development of 360 m in length,
intersects a small perennial stream which in our observation has maintained a minimum
discharge of about 0.022 m3/s but periodically overflows into the vadose cave passage.
Immediately after a flood overflow event, water marks and foam detritus are visible at various
levels on the cave walls and corresponding to heights of bottlenecks in overflow drainage
through the cave passage. Flood events deposit sand on terraces on the cave wall and move
large volumes of sand in the cave bed. These extreme events leave a long-term record in i)
wall coloration or water marks on the cave walls; and ii) detrital particles preserved as
inclusions inside the stalagmites. Throughout this cave, it is possible to recognize chromatic
changes in the walls, such as manganese oxide stains, which coincide with one of the
water marks left during a recent flood event. The most salient manganese oxide on
the walls rises up to 1.5 m measured from the thalweg and we interpreted it as
the result of a frequent process of wetting - drying related to frequent flooding
of the cave. Since 3-4 ka, drapery flowstone has been deposited over this oxide
coating in some parts of the cave and the drapery remains free of oxide coating. We
interpret this as indicating a reduction in the frequency and/or duration of flooding to
this height, coincident with a regional drying trend in late Holocene. Stalagmites
growing in the bed of the cave appear to trap fluvial sediments like sand or silts
particles, which are common weathering products in the headwaters of the rivers that
feed the cave. The abundance of detrital particles inside the coeval stalagmites
decreases as altitude and distance from the stream increase inside the cave, supporting
its fluvial affinity. During periods of high rainfall, the movement of the sand-bars
inside the cave can cover partially or completely active stalagmites, facilitating the
cementation process and trapping abundant detrital material inside the stalagmite
carbonate. 14C and U/Th dating of the stalagmites can provide a chronology, so that the
abundance of fluvial material in the stalagmites can reveal periods of enhanced
vs. reduced flooding in the cave over the past several thousand years (Holocene). |
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