in Jurassic and Cretaceous Stratigraphy

Earth Science Frontiers, Vol. 17, Special Issue, Aug. 2010 ISSN 1005-2321
Epioceanic Depositional Setting and Skeletals from a Cephalopod
Limestone (Betic Cordillera, SE Spain): A Geochemical Approach
R. Coimbra, F. Olóriz
Departamento de Estratigrafía y Paleontología, Facultad de Ciencias, Universidad de Granada, Campus Universiario de Fuentenueva,
Granada18002, Spain (E-mail: foloriz@ugr.es)
Geochemical data from four different types of
carbonate materials are presented: micrite matrix from
two sections (Cardador and Alamedilla), carbonate
cements, neomorphic ammonite shells and belemnite
rostra.
Analysed materials belong to Lower Tithonian
deposits, Semiforme/Verruciferum Zone, registered as
Ammonitico Rosso facies in an epioceanic high
(Cardador section), and a coeval cephalopod limestone
deposited closer to the mid-Subbetic volcanic ridge
(Alamedilla section).
The Cardador section shows more and less
calcareous, red-to-grey AR horizons with variable
nodularity. Wackstones dominate containing radiolaria,
calcisphaeres, calcareous dinoflagellates, plantik cri-
noids (Saccocoma), unidentifiable benthic forams,
ostracods, cephalopods (rare ammonitella or embrionic
shell of ammonoids; aptychi), and fragments of
juvenile and adult ammonite carcasses along with
broken mollusks, echinoderms (plates and spines),
sponge spicules and pelagic bivalves, among others;
packstone horizons mainly composed by filaments also
occur.
The Alamedilla section shows reddish-brownish-
to-pale-grey limestone and marls. Limestones are
locally rich in ammonites (cephalopod limestones)
which preserve neomorphic shells and within-bed
orientation related to shell size, although horizontal
settling is dominant. Wackstones dominate containing
rare radiolaria, common to abundant calcareous dino-
flagellates, lesser amount of plantik crinoids (Sacco-
coma), benthic forams (most probably Lenticulina),
ostracods, cephalopods (common ammonitella or em-
brionic shell of ammonoids), and fragments of juvenile
and adult ammonite carcasses and rare aptychi, along
with broken mollusks, lesser amount of small gastro-
pods, rare brachiopods and echinoderms (plates), and
variable amount of filaments (pelagic bivalves).
Hand specimens were collected under precise
stratigraphic control and two slabs were produced for
each sample: one for thin section preparation (micro-
facies and cathodoluminescence) and the other for
geochemical studies. Cathodoluminescence inspection
was carried out in order to access the degree of
diagenetic imprint. Stable isotope analysis (carbon and
oxygen) was preformed on a total of seventy six hand
drilled bulk carbonate powder samples. Aliquots
produced with the same powder sub-samples measured
for isotope composition were analysed for their Ca, Mg,
Mn, Fe and Sr elemental concentrations using indu-
ctively coupled plasma-atomic emission spectrometry
(ICP-AES).
Cathodoluminescence imaging of matrix micrite
presents distinct degrees of luminescence (Fig.1 B, F):
brighter for micrite matrix from the Alamedilla section,
attributed to an environmental enrichment in Mn 2+ , in
agreement with elemental data. A latter carbonate
cement phase is identified as secondary blocky calcite
(Fig.1 F) with alternating pattern between intrinsic and
yellow luminescence (zonation), providing evidence
for shallow marine burial (Bruckschen and Richter,
1994). Skeletal material shows overall intrinsic dark
luminescence (Fig.1 B, D), suggesting a fair presser-
vation of both neomorphic ammonite shells and belem-
nite rostra.
Carbon isotope values for matrix micrite from the
studied section at Cardador and the rock block from
Alamedilla section, neomorphic ammonite shells and
secondary blocky calcite plot within the same range of
values, from 1.5‰ to 2.1‰. The obtained values are
comparable to published data from other former
Tethyan margin locations (Jenkyns, 1996; Weissert and
Mohr, 1996; Bartolini et al., 1999) that reflect Upper
Jurassic seawater carbon isotope composition. Belem-
nite rostra carbon isotope values differ from this trend,
showing δ 13 C values depleted as much as 3.8‰ relative
to the encasing matrix micrite (Alamedilla section). A
likely lighter carbon source is metabolic carbon, that
fails to be in equilibrium with seawater carbon due to
the internal position of belemnite rostra (Price and Page,
2008; Price et al., 2009).
Oxygen isotope composition presents a higher
variability. Matrix micrite from the Cardador section
and neomorphic ammonite shells are the least depleted
materials, ranging from 0.1‰ to 0.9‰, followed by
matrix micrite from Alamedilla and encased belemnite
rostra. As most depleted, carbonate cements range from
-2.7‰ to -0.3‰. Oxygen isotope composition is here
considered to reflect marine bottom water paleo-
temperature, except for carbonate cements, that indicate
higher temperature of circulating fluids during marine
burial diagenesis, supported by the observed zonation
under cathodoluminescence.
Elemental composition revealed an exceptional
Mn enrichment on micrite matrix from Alamedilla (up
to 4000 ppm), only present at the Cardador section on
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