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| Titel |
Meganodular anhydritization in the Tertiary Ebro basin (Spain) |
| VerfasserIn |
Federico Orti, Laura Rosell, Elisabet Playà, Josep Maria Salvany |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036692
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| Zusammenfassung |
| A number of gypsiferous units in the Tertiary Ebro Basin (Spain) are located along their
southern margins. These units, aged Paleogene to Miocene, were accumulated in small
shallow saline lakes of low ionic concentration, in which Ca-sulphates (gypsum/anhydrite)
precipitated. The lakes were nourished by groundwater from deep regional aquifers, which
had the recharge areas in the bounding chains and recycled sulphates/chlorides from the
Mesozoic (Triassic, Liassic) evaporites. Some of these units graded laterally to the thick,
highly-saline (halite, glauberite, polyhalite) evaporite units developed coevally in the basin
centre.
In the gypsiferous marginal units, meganodules and large irregular masses (from 0.5 m to >5
m in diameter/length) of secondary gypsum are present in outcrop. These particular features
originated as anhydrite, which displaced/replaced the host-gypsum rocks. Although these
features mainly display stratiform arrangements, also vertical disposals are found locally
suggesting the circulation of ascending flows. The isotopic values (δ34S and δ18O; 87Sr/86Sr)
of these features are the same than those of the gypsum host-rocks, suggesting that the
precursor anhydrite derived from the in situ replacement of the depositional sulphates.
Commonly, the host-rock of the meganodules has been preserved as primary gypsum in the
Miocene units.
The common characteristics of the meganodules/irregular masses suggest that the anhydrite
growth happened in burial conditions from shallow to moderate depths. The anhydritization
was caused mainly by the same hydraulic systems feeding the marginal saline lakes. With
progressive burial of the gypsiferous units, the gypsum-to-anhydrite conversion initiated in
few nucleation points and progressed slowly and to variable depths. At such depths (from
some metres to few hundred metres?), the regional ascending flows probably had
temperatures (>25ºC) and solute contents higher than today. Additionally, compaction brines
expelled from the thick, highly-saline central units could have mixed with the regional flows
increasing their salinity. As a result of this, these flows became anydritizing solutions and
affected irregularly the subsiding gypsiferous units before they reached deep burial
conditions.
The characteristics of this meganodular anhydritization are completely different than those of
the anhydrite formed in sabkha setting or in deep burial conditions. It is thought that this
anhydritzation mode could also have developed in the margins of other evaporitic basins in
which chemical recycling of ancient evaporites occurred. |
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