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Pedogenetic processes coeval with Neogene faults evolution in the Barcelona's Plain, NE Spain
Journal of Geochemical Exploration 101 (2009) 21
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Pedogenetic processes coeval with Neogene faults evolution in the Barcelona's Plain, NE Spain Irene Cantarero ⁎, Anna Travé, Gemma Alías Departament de Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, Universitat de Barcelona, 08028 Barcelona, Spain
Fault rocks in basins are the result of: 1) displacement; 2) different events and mechanisms of deformation; and/or 3) later diagenetic processes related to fluids that have circulated through the faults. Therefore, the microstructural, petrographic and geochemical study of fault rocks together with the structural study of faults let us understand the geodynamic and hydrogeologic evolution of a basin because faults can act as barriers or conduits for fluids. This paper describes the fault rocks formed in minor normal NW–SE and N–S striking faults affecting a tilted block of Miocene conglomerates in Barcelona's Plain. These systems of faults are perpendicular to the main faults bordering the horsts and grabens of the Catalan margin, which have an ENE–WSW to NE–SW striking direction. All these faults are the result of the late Oligocene–middle Miocene extension that took place in the Western Mediterranean. The resulting fault rock is a cohesive, reddish mud-to-sandy material classified preliminarily as cataclasite, for which the fabric is obliterated by later diagenetic processes. The diagenetic products and features evident in the fault rock are: 1) White and orange bladed calcite 0.25–1 mm long. The orange calcites have radial fibrous textures, undulatory extinction that constitute spherulites; 2) Micrite strips that consist of bands of micrite with a symmetric distribution in their grain size, with smaller grains in the centre and bigger ones the borders; 3) Vug and channel porosity, formed later than the above products, which can remain open or cemented; and 4) Cementation of porosity, i.e., a distinct type of cement occurs in the vug porosity, consisting of layers of clear bladed calcite alternating with clays, and interpreted as having a karstic origin.
Bladed calcite and micrite strips have been interpreted as Microcodium, that is, the calcitization of root cells, because of their mineralogy, crystal habit, crystal arrangements and the destructive and matrix selective replacement of the fault rock. These domains only appear in the fault rocks, suggesting that roots take profit from these discontinuities for the plants to grow. Another interpretation could be that they are the result of calcitization of bacterial colonies. The different types of calcite show different elemental composition, highlighted by the incompatibility between Fe and Mn and between Na and Mn. Calculations using partition coefficients of these elements between calcite and groundwater suggest a meteoric origin for the fluids with temperatures between 25 and 40 °C. The presence of Microcodium, as a late diagenetic process, is important in understanding the fault and fault rock evolution: a) fault age — Microcodium abundance in the geological record decreases quickly from Miocene and, in the studied samples, it is present in fractures attributed to the Miocene extension and affected by late extensive microfractures also attributed to the Miocene. b) burial depth — Microcodium implies shallow conditions, uppermost few meters. c) diagenetic environment — Microcodium only develops in surface environments with meteoric fluids; and d) fault rock's classification — Microcodium was probably responsible for increasing the cohesiveness of a previously uncohesive fault rock, which because of the surface conditions indicated by texture and geochemistry, would have been a gouge and not the cataclasite that we interpreted first. In the studied fault rocks Microcodium is a diagenetic product responsible for impermeabilization of the fault zones.
These diagenetic products normally follow two main orientations as microfractures that affect calcite domains. One of them is subparallel to the fault orientation and the other describes a 30–45° angle with the former.
⁎ Corresponding author. Facultat de Geologia, Universitat de Barcelona, C/ Martí i Franquès s/n, 08028 Barcelona, Spain. Tel.: +34 934021416, +34 646528345. E-mail addresses: [email protected] (I. Cantarero), [email protected] (A. Travé), [email protected] (G. Alías). 0375-6742/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2008.12.003
Contents lists available at ScienceDirect
Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p
Pedogenetic processes coeval with Neogene faults evolution in the Barcelona's Plain, NE Spain Irene Cantarero ⁎, Anna Travé, Gemma Alías Departament de Geoquímica, Petrologia i Prospecció Geològica, Facultat de Geologia, Universitat de Barcelona, 08028 Barcelona, Spain
Fault rocks in basins are the result of: 1) displacement; 2) different events and mechanisms of deformation; and/or 3) later diagenetic processes related to fluids that have circulated through the faults. Therefore, the microstructural, petrographic and geochemical study of fault rocks together with the structural study of faults let us understand the geodynamic and hydrogeologic evolution of a basin because faults can act as barriers or conduits for fluids. This paper describes the fault rocks formed in minor normal NW–SE and N–S striking faults affecting a tilted block of Miocene conglomerates in Barcelona's Plain. These systems of faults are perpendicular to the main faults bordering the horsts and grabens of the Catalan margin, which have an ENE–WSW to NE–SW striking direction. All these faults are the result of the late Oligocene–middle Miocene extension that took place in the Western Mediterranean. The resulting fault rock is a cohesive, reddish mud-to-sandy material classified preliminarily as cataclasite, for which the fabric is obliterated by later diagenetic processes. The diagenetic products and features evident in the fault rock are: 1) White and orange bladed calcite 0.25–1 mm long. The orange calcites have radial fibrous textures, undulatory extinction that constitute spherulites; 2) Micrite strips that consist of bands of micrite with a symmetric distribution in their grain size, with smaller grains in the centre and bigger ones the borders; 3) Vug and channel porosity, formed later than the above products, which can remain open or cemented; and 4) Cementation of porosity, i.e., a distinct type of cement occurs in the vug porosity, consisting of layers of clear bladed calcite alternating with clays, and interpreted as having a karstic origin.
Bladed calcite and micrite strips have been interpreted as Microcodium, that is, the calcitization of root cells, because of their mineralogy, crystal habit, crystal arrangements and the destructive and matrix selective replacement of the fault rock. These domains only appear in the fault rocks, suggesting that roots take profit from these discontinuities for the plants to grow. Another interpretation could be that they are the result of calcitization of bacterial colonies. The different types of calcite show different elemental composition, highlighted by the incompatibility between Fe and Mn and between Na and Mn. Calculations using partition coefficients of these elements between calcite and groundwater suggest a meteoric origin for the fluids with temperatures between 25 and 40 °C. The presence of Microcodium, as a late diagenetic process, is important in understanding the fault and fault rock evolution: a) fault age — Microcodium abundance in the geological record decreases quickly from Miocene and, in the studied samples, it is present in fractures attributed to the Miocene extension and affected by late extensive microfractures also attributed to the Miocene. b) burial depth — Microcodium implies shallow conditions, uppermost few meters. c) diagenetic environment — Microcodium only develops in surface environments with meteoric fluids; and d) fault rock's classification — Microcodium was probably responsible for increasing the cohesiveness of a previously uncohesive fault rock, which because of the surface conditions indicated by texture and geochemistry, would have been a gouge and not the cataclasite that we interpreted first. In the studied fault rocks Microcodium is a diagenetic product responsible for impermeabilization of the fault zones.
These diagenetic products normally follow two main orientations as microfractures that affect calcite domains. One of them is subparallel to the fault orientation and the other describes a 30–45° angle with the former.
⁎ Corresponding author. Facultat de Geologia, Universitat de Barcelona, C/ Martí i Franquès s/n, 08028 Barcelona, Spain. Tel.: +34 934021416, +34 646528345. E-mail addresses: [email protected] (I. Cantarero), [email protected] (A. Travé), [email protected] (G. Alías). 0375-6742/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2008.12.003