IX Congreso Geológico de España Geo-Temas, 16 (1) ISSN 1576-5172
387
Opaline chert nodules in maar lake sediments from Camp dels Ninots (La Selva
Basin, NE Spain)
Nodulos de chert en sedimentos lacustres del maar de Camp dels Ninots (Cuenca de La Selva,
NE España)
J. Miró
1
, J.D. Martín-Martín
1
, J. Ibáñez
2
, P. Anadón
2
, O. Oms
3
, J. Tritlla
4
and M.A. Caja
5
1 Universitat de Barcelona, Faculty of Geology. Martí i Franquès, s/n. 08028 Barcelona (Spain), jmiropad@gmail.com,juandiegomartin@ub.edu
2 Institut de Ciències de la Terra Jaume Almera, CSIC, 08028 Barcelona (Spain). jibanes@ictja.csic.es, panadon@ictja.csic.es
3 Universitat Autònoma de Barcelona. Science Faculty, Geology Department. Campus Bellaterra 08193 (Spain). joseporiol.oms@uab.cat
4 SPD, Direction of Geology, Repsol Exploración, c/ Méndez Alvaro 44, 28045 Madrid (Spain); jordi.tritlla@repsol.com
5 Geology Laboratory, Centro Tecnológico de Repsol, Crta. De Extremadura km 18, Móstoles (Spain); miguelangel.caja@repsol.com
Abstract: Chert nodule samples from three different well cores (CC, CP1 & CA) from the lacustrine infill of the Camp
dels Ninot maar-diatreme (La Selva Basin) have been studied by means of X-ray diffraction, and optical and electron
microscope technique. The chert nodules replace diatomites and carbonates layers, and varies in mineralogy between
opal-A to opal-A/CT. The microtexture of the opal-A rich nodules is characterized by smooth microspheres of ~2µm in
diameter that forms aggregates of amalgamated microspheres. Commonly, the nodules contain diatoms and their
moulds when replacing diatomites, and dolomite or ankerite crystals and their moulds when replacing carbonates. The
opal-A/CT rich nodules exhibit a microtexture consisting of microspheres of ~8µm in diameter that form aggregates
with botryoidal and finger-like morphologies. Results indicate that the early diagenetic transformation of opal-A to
opal-CT is not complete in the studied sediments.
Key words: lacustrine, maar, chert nodules, opal-A, opal-A/CT
Resumen: En el presente trabajo se estudian muestras de 3 testigos de sondeo (CC, CP1 y CA) mediante DRX,
microscopio óptico y electrónico a fin de caracterizar los nódulos de chert en el relleno sedimentario lacustre del maar
de Camp dels Ninots (La Selva Basin,). Los nódulos de chert reemplazan capas de diatomitas y carbonato, y varían
mineralógicamente entre ópalo-A y ópalo-A/CT. La microtextura de los nódulos ricos en ópalo-A se caracteriza por
microesferas lisas de ~2µm de diámetro que forman agregados de microesferas amalgamadas. Frecuentemente, los
nódulos contienen diatomeas y sus moldes cuando reemplazan diatomitas, y cristales de dolomita y ankerita y sus
moldes cuando reemplazan carbonatos. Los nódulos rico en ópalo-A/CT presentan una microtextura con microesferas
de ~8µm de diámetro que forman agregados con morfología botroidal. Los resultados indican que la transformación
diagenética de ópalo-A a ópalo-CT no es completa en los sedimentos estudiados.
Palabras clave: sedimentos lacustres, maar, nódulos de chert, ópalo-A, ópalo-A/CT
INTRODUCTION
Chert nodules are very common in the lacustrine
sediments of the Camp dels Ninots maar of Caldes de
Malavella (La Selva Basin, NE Spain) as menilitic
opal. The mineralogy and origin of such chert nodules
have been succinctly described from outcrop
specimens by Piqué (2008), who identifies the
occurrence of opal-A (i.e, amorphous opal) and opal-
CT.
Research well cores recovered at Camp dels Ninots
maar (Caldes de Malavella), during paleontological and
climatic studies, revealed that chert nodules are
common in the lacustrine infill of the basin. These
cores also allow to study the early diagenetic silica
minerals in lacustrine maar environments. Specific
objectives of this work are: (i) to characterize the
mineralogy of the nodules and (ii) to constrain the
origin and diagenetic evolution of the silica.
GEOLOGICAL SETTING
The Camp dels Ninots maar-diatreme (CNMD)
(41° 50′ 06″N, 2° 47′ 51″E; 95 m above sea level) is a
Pliocene maar located in Caldes de Malavella (La
Selva Basin, NE Spain), with a diameter between 650
and 400 m. La Selva Basin is a Neogene basin bounded
by NW-SE faults (Pous et al., 1990) included in
IX Congreso Geológico de España Geo-Temas, 16 (1) ISSN 1576-5172
388
FIGURE 1: A) Location of the Camp dels Ninots maar diatreme
within the Iberian Peninsula (upper left) and within the Catalan
Volcanic Field. B) Simply geological map of Camp dels Ninots maar
with the location of the wells (see the methods). Oms et al., 2015
(modified)
Catalan Volcanic Zone. This zone constitute belongs to
the Neogene European Rift System provinces (Fig.1).
The origin of the CNMD (Gómez de Soler et al., 2012;
Oms et al., 2015) is most probably associated to the
faults bounding the La Selva Basin and it is classified
as phreatomagmatic (Martí et al., 2011).
The earliest geological study in Camp dels Ninots
considered sediments as lacustrine without any
mention to volcanism (Vidal, 1882), being Vehí et al.
(1999) who identified these sediments as the infill of a
maar. During recent years, the maar sediments have
been extensively studied due to its well-preserved
Pliocene fossil record (Campeny and Gómez de Soler,
2010; Gómez de Soler et al., 2012 Jiménez-Moreno et
al., 2013). More recently, the maar structure and infill
sediments were thoroughly studied by integrating eight
cores and nine electric resistivity tomography cross
sections (Oms et al., 2015). The basement of the
CNMD is composed by Late Carboniferous to Permian
granites, schists, and Pliocene arkoses, clays, sands,
and gravels from the La Selva Basin alluvial system
(Pous et al., 1990). The lacustrine sediments fill a maar
diatreme lake rimmed by a tephra ring-shape. Three
main facies (
Jiménez-Moreno et al., 2013)
are found in
the lake sediments: (i) laminated green-to-greyish
mudstones composed mainly by clay minerals; (ii)
laminated dark mudstone rich in organic matter; and
(iii) laminated/massive whitish carbonates
.
These
lacustrine sediments contain frequent early diagenetic
silica concretions dominated by chert nodules
(menilites) that are the goal of this study. Piqué (2008)
noted that these chert nodules replaced or grew within
calcite and dolomite layers, as opal-CT lepispheres
with botryoidal textures.
METHODS
Fifty-four core samples have been selected from
three wells of the Camp dels Ninots maar (Can Cateura
– 31 m (CC), Can Pla 1 – 112 m (CP1), Can Argilera –
75.5 m (CA), Fig.1b) as well as from trench outcrops
within the area. Sampling includes: a) the main
lithological facies and b) the chert nodules and
associated host rocks. Thirty-eight thin sections and ten
polished slabs were prepared from the selected core
samples. The samples have been studied by optical
microscopy, X-Ray powder Diffraction (XRD) and
Scanning Electron Microscope (SEM).
The mineralogy of all samples was determined by
XRD using a Bruker D8-A25 powder diffractometer
equipped with a Cu x-ray source (Cu Kα radiation) and
a LynxEye position sensitive detector. The scans were
recorded between 3º and 65º (in 2θ) with a 0.05° step
size and equivalent integration times of 576 s per step.
Phase identification was performed using the Diffrac.
EVA software in combination with the Powder
Diffraction File (PDF-2) and the Crystallography Open
Database (COD).
Thirty-eight thin sections were prepared with
standard procedures and examined with a Zeiss
Axiophot petrographic microscope. Detailed textural
analyses were performed on rock chips with a
Scanning Electron Microscope (SEM) JSM-7100F
equipped with a dispersive X-ray spectrometer (EDS).
RESULTS
Core log examination indicates that the highest
concentration of diagenetic chert nodules occur within
the white carbonate facies, whereas the dark mudstones
exhibit the lowest concentrations, showing the greyish
facies intermediate concentrations. Regardless of
facies, chert nodules appear both at shallow and deep
depths, replacing both carbonates and diatomite layers.
Chert nodules are usually black (Fig. 2A) or brown in
colour, occasionally zoned, and with typical irregular
IX Congreso Geológico de España Geo-Temas, 16 (1) ISSN 1576-5172
389
shapes, often planar and semi-spherical with smoothed
edges.
XRD analyses indicate that nodules are composed
by opal-A (typical broad peak from diffuse scattering
centred at lattice spacings of ~4Å) to opal-A/CT (broad
peak centred at 4.Å with two minor individual peaks at
~4.09Å and ~4.32Å). The host rock mineralogy is
dominated by carbonates (dolomite, ankerite and/or
calcite), with subordinate of quartz, feldspars, clays
(smectite phases, kaolinite, illite), gypsum and pyrite.
Together with carbonates, diatom rich layers are also
found in the host rock.
Optical petrographic observations of chert nodules
show the typical yellowish to light brown colours of
the opal under polarized light (Fig. 2B). The opal-A
rich nodules are made of smooth microspheres of
~2µm in diameter forming aggregates of amalgamated
microspheres.
The
opal-A
replacing
diatomite
frequently engulfs diatoms and their moulds that
decrease in abundance towards the centre of the nodule
(Fig. 2D). Moreover, opal-A nodules replacing
carbonate layers typically engulf crystals and their
moulds of the carbonate minerals (dolomite and
ankerite) (Fig. 2E). The opal-A/CT rich nodules show a
microstructure similar to that of opal-A rich nodules
characterized by aggregates of microspheres of ~8 µm
in diameter (Fig. 2C). Occasionally these microspheres
form finger-like aggregates (Fig. 2F). In addition to
opal-A and opal-A/CT, petrographic observations
indicate the presence of small crystalline areas, likely
microquartz crystals, appear within the opaline texture.
DISCUSSION:
The occurrence of diatom moulds in opal-A
nodules suggests that the dissolution of silica diatom
skeletons, which are abundant in the Camp dels Ninots
lacustrine sediments, contributed in part to the
formation of the nodules. Textural observations
indicate that opal-A is formed by microspheres, most
probably generated through a dissolution and re-
precipitation process (Mackay, 2007). Despite opal-
A/CT have been recognized by XRD in Camp dels
Ninots, the typical opal-CT bladed lepispheres reported
in the literature (e.g., Lynee et al., 2005) are not
recognized, indicating that the complete opal-A to
opal-CT transformation is not complete in the studied
samples.
The transformation from opal-A to opal-CT has
been extensively reported in the literature (e.g., Lynee
et al., 2005). According to these authors the complete
diagenetic transformation of opal-A includes the pass
of opal-CT to microquartz frequently with an
intermediate phase called moganite. XRD analyses of
the Camp dels Ninots nodules do not support further
transformation of opal-CT to moganite or microquartz
in the studied sediments. However, the presence of
small microquartz crystals that appear within the
opaline suggests that the transformation may have been
Figure 2: A) Hand specimen of a black and homogeneous chert nodule. B) Photomicrograph of opal-A rich nodule showing the typical yellowish
color of the amorphous mass (parallel light). C) Photomicrograph of opal-A/CT rich nodule showing the texture made of clusters of microspheres
(parallel light). D) SEM image showing diatom moulds within the opal-A amorphous mass. E) SEM image of an opal-A rich nodule showing the
amorphous mass engulfing ankerite crystals. F) SEM image of the opal-A/CT nodule showing a cluster of smooth microspheres of around 8 µm in
diameter and finger-likely morphology.
IX Congreso Geológico de España Geo-Temas, 16 (1) ISSN 1576-5172
390
initiated. The low amount of microquartz in the studied
sediments probably prevents its record in the XRD
scans, which are dominated by opal phases.
CONCLUSIONS
The study of well cores from Camp dels Ninots
reveals the presence of diagenetic chert nodules in
almost all the lacustrine sedimentary infill.
Chert nodules are composed by opal-A or opal-
A/CT,
both
showing
similar
petrographic
characteristics. The opal-A nodules are made of
microspheres of ~2µm in diameter forming aggregates.
Opal-A/CT nodules are usually made of microspheres
aggregates
showing
botryoidal
and
finger-like
morphologies. Typically, the opal-A/CT microspheres
are ~8µm in diameter. Our present data suggests that
opal-A was partially transformed to opal-A/CT during
diagenesis.
The transition from opal-A to opal-A/CT during the
diagenesis and its potential evolution to opal-CT and
microquartz needs to be further characterized by
complementary analytical techniques such as Raman
scattering or Si isotopes.
ACKNOWLEDGEMENTS
This research is funded by Repsol Exploración
through a collaborative project with the University of
Barcelona. Well drilling was financed by
El Plio-
pleistocè del Camp dels Ninots i la depressió
prelitoral:
evolució
paleoclimàtica,
dispersions
faunístiques
i
humanes
(
2014-100575)
project
(Departament de Cultura de la Generalitat de
Catalunya) and SGR2014-901 (AGAUR, Generalitat
de Catalunya). The authors would like to thank the
Town Council of Caldes de Malavella for providing
logistics and the land owners (Mr. Jaume Pla and
family). The results presented here are part of the
ongoing Msc. Thesis of J. Miró.
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