Shallow burial dolomitisation of Middle–Upper Permian paleosols in an extensional tectonic context (SE Iberian Basin, Spain): Controls on temperature of precipitation and source of fluids



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Benito Moreno, María Isabel and Horra del Barco, Raúl de la and López Gómez, José and Fernández Barrenechea, José María and Luque del Villar, Francisco Javier and Arche, Alfredo (2011) Shallow burial dolomitisation of Middle–Upper Permian paleosols in an extensional tectonic context (SE Iberian Basin, Spain): Controls on temperature of precipitation and source of fluids. Sedimentary Geology, 237 . pp. 135-149. ISSN 0037-0738

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This work is focused on carbonate paleosols developed in three stratigraphic sections (Landete, Talayuelas and
Henarejos) of theMiddle–Late Permian Alcotas Formation in the SE Iberian Basin. The Alcotas Formation, of alluvial
origin, was deposited in semi-connected half-grabens developed during the early stages of the Permian–Triassic
rifting stage that affected the Iberian Basin. The studied sections were located in two of these half-grabens, the
Henarejos section being much closer to the basin boundary fault than the other two sections. The mineralogy and
texture of the carbonate precursor of paleosols in the three studied sections are not preserved because original
carbonate is replaced by coarse crystals of dolomite and/or magnesite. Dolomite crystals are typically euhedral,
displaying rhombohedral shapes and reddish luminescence, although in the Henarejos section dolomite displays
non-planar boundaries and frequently saddle habit. Micas are deformed and adapted to dolomite crystals, which, in
turn, are affected by stylolites, suggesting that dolomite precipitated before mechanical and chemical compaction.
Carbon and oxygen isotopic compositions of dolomite fromthe three sections showdifferent values (δ13CVPDB mean
values=−6.7‰,−5.5‰ and −7.5‰; δ18OVPDB mean values=−4.0‰; –5.6‰and−8.2‰, at Landete, Talayuelas
and Henarejos sections, respectively). The 87Sr/86Sr ratios are similar in the three sections yielding values between
0.71391 and 0.72213. The petrographic and geochemical features of dolomite in the three studied sections suggest
precipitation fromsimilar fluids and during shallow burial diagenesis. Assuming that theminimum temperature for
dolomite precipitation in the Henarejos sectionwas 60 °C (as suggested by the presence of non-planar saddle habit),
and that the dolomitizing fluid had similar δ18O values at the three localities, then dolomite in the Talayuelas and
Landete sections precipitated at temperatures around 16 and 25 °C cooler, respectively. In addition, the δ18OVSMOW
values of the water from which dolomite precipitated would have ranged between −0.3 and −2.9‰. Dolomite is
partially or totally replaced by non- to dark dull luminescent magnesite in the Landete and Talayuelas sections.
Magnesite crystals are affected by stylolites, indicating that it precipitated before chemical compaction. The δ13C
mean values are −6.5 and −6.0‰ and the δ18OVPDB mean values are −6.7 and −7.8‰, in the Landete and
Talayuelas sections, respectively. The 87Sr/86Sr ratios of magnesite are similar in both sections yielding values
between 0.71258 and 0.72508. This suggests that they probably precipitated from similar fluids during progressive
burial and at higher temperatures than dolomites at the same sections. Assuming thatmagnesite precipitated froma
fluid with similar δ18O values in both sections, then it had to precipitate at a temperature around 8 °C higher in
Talayuelas than in the Landete section. Dolomitisation and magnesite precipitation probably occurred via reflux of
saline to hypersaline brines from the overlying Mid-Late Triassic Muschelkalk and/or Keuper facies. The
temperatures inferred for dolomite precipitation, however, are too high for shallow burial if a normal geothermal
gradient is applied. Thus, it can be inferred that salinefluidswere heated as theyflowed through the syn-sedimentary
extensional faults that controlledMiddle Permian to Middle Triassic sedimentation; consequently fluidswould have
been at higher temperatures near the Henarejos area, which was closer to the basin boundary fault than at the
Talayuelas and Landete areas, whichwere situated further away. This contention is in agreement with recent studies
which demonstrate that an important thermal event took place during Late Triassic–Early Jurassic times in the
Iberian Peninsula.

Item Type:Article
Uncontrolled Keywords:Permian paleosols; Iberian Ranges; Dolomite; Magnesite; C, O, Sr isotopes
Subjects:Sciences > Geology > Stratigraphic geology
ID Code:17577
Deposited On:08 Jan 2013 12:44
Last Modified:11 Dec 2018 08:46

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