09 August 2022
Integrated magnetobiochronology of the Early/Middle Eocene transition at Agost (Spain): Implications for
defining the Ypresian/Lutetian boundary stratotype / J.C.Larrasoana ; C.Gonzalvo ; E.Molina ; S. Monechi;
S.Ortiz ; F.Tori ; J.Tosquella. - In: LETHAIA. - ISSN 0024-1164. - STAMPA. - 41(2008), pp. 395-415.
[10.1111/j.1502-3931.2008.00096.x]
Original Citation:
Integrated magnetobiochronology of the Early/Middle Eocene
transition at Agost (Spain): Implications for defining the
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DOI 10.1111/j.1502-3931.2008.00096.x © 2008 The Authors, Journal compilation © 2008 The Lethaia Foundation
LETHAIA
Blackwell Publishing Ltd
Integrated magnetobiochronology of the Early/Middle
Eocene transition at Agost (Spain): Implications for
defining the Ypresian/Lutetian boundary stratotype
JUAN C. LARRASOAÑA, CONCEPCIÓN GONZALVO, EUSTOQUIO MOLINA, SIMONETTA MONECHI,
SILVIA ORTIZ, FLAVIA TORI AND JOSEP TOSQUELLA
Larrasoaña, J.C., Gonzalvo, C., Molina, E., Monechi, S., Ortiz, S., Tori, F. & Tosquella, J.
2008: Integrated magnetobiochronology of the Early/Middle Eocene transition at Agost
(Spain): Implications for defining the Ypresian/Lutetian boundary stratotype.
Lethaia
,
Vol. 41, pp. 395–415
In this paper, we present an integrated study of a 115-m-thick section that spans the
Ypresian/Lutetian boundary at Agost (Betic Cordillera, SE Spain). Our study includes
magnetostratigraphic results and biostratigraphic and palaeoenvironmental data
derived from planktic foraminifera, small and larger benthic foraminifera, and calcar-
eous nannofossils. Our results demonstrate that the Agost section is continuous and
spans from Zones P9 to P12 (E7 to E10), Zones CP11 to CP14a (NP13 to NP16), Zones
SBZ11 to SBZ15, and Chrons C22n to C19r. The first occurrence (FO) of
H. nuttalli
(base of P10) and the FO of
G. nuttalli
(base of E8) are found within Chron C20r, at a
much younger age (3–5 Myr) than previously considered in standard calibration
schemes. Similarly, the boundary between SBZ12 and SBZ13 is located within Chron
C21n, also at a younger age than previously considered. On the contrary, the FO of
B. inflatus
(base of CP12b) is found within Chron C21r, which conforms to the magneto-
stratigraphically calibrated age of ca. 48 Ma (middle part of C21r) considered in standard
calibration schemes. These results corroborate earlier studies and indicate that all the
events that have been proposed to mark the Ypresian/Lutetian boundary appear at
different stratigraphic intervals and have different ages. Based on our results from
Agost and on data from other sections elsewhere, we suggest that the Ypresian/Lutetian
boundary might be approximated by the FO of
B. inflatus
(base of CP12b). The Agost
section might be considered as a potential candidate to locate the Global Stratotype
Section and Point (GSSP) of the base of the Lutetian Stage, because it includes all the
events that might be selected as marker events for the Ypresian/Lutetian boundary and
it fulfils most of the geological, biostratigraphic and infrastructure requirements
demanded for definition of a GSSP.
Agost
,
Betic Cordillera
,
biostratigraphy
,
calcareous
nannofossils
,
Global Stratotype Section and Point
,
larger benthic foraminifera
,
magnetostratigraphy
,
planktic foraminifera
,
small benthic foraminifera
,
Ypresian/
Lutetian boundary
.
Juan C. Larrasoaña [jclarra@ija.csic.es], Institut de Ciències de la Terra Jaume Almera,
CSIC, Solé Sabarís s/n, 08028 Barcelona, Spain; Concepción Gonzalvo [concha@unizar.es],
Eustoquio Molina [emolina@unizar.es] and Silvia Ortiz [silortiz@unizar.es], Departa-
mento de Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, 50009
Zaragoza, Spain; Simonetta Monechi [monechi@unifi.it] and Flavia Tori [flavia.tori@unifi.it],
Dipartimento di Scienze della Terra, Università di Firenze, via La Pira 4, 50121 Firenze,
Italy; Josep Tosquella [josep@uhu.es], Departamento de Geodinámica y Paleontología,
Facultad de Ciencias Experimentales, Universidad de Huelva, Avenida de las Fuerzas
Armadas s/n, 21071 Huelva, Spain; manuscript received on 16/05/07; and manuscript
accepted on 20/12/07.
Definition of global reference points and selection of
boundary stratotypes is important for delineating the
chronostratigraphic scale that frames the geological
history of the Earth (Gradstein
et al
. 2004). Still
pending definition is the base of the Lutetian Stage
(Early/Middle Eocene boundary). This boundary is
currently marked by the FO (first occurrence) of
Hantkenina nuttalli
(base of P10 of Berggren
et al
.
1995) (see Luterbacher
et al
. 2004), and has classically
been correlated to the top of Chron C22n on the
basis of magnetostratigraphic results from Gubbio,
Italy (Lowrie
et al
. 1982; Napoleone
et al
. 1983).
However, a precise correlation of this event to the
geomagnetic polarity timescale (GPTS) is still to be
established, since identification of planktic foram-
inifera in the Italian sections is made difficult by the
hard nature of the host sediments and by the poor
preservation of planktic foraminifera in some levels
(Lowrie
et al
. 1982; Napoleone
et al
. 1983; Opdyke &
Channell 1995). Further complications arise from
396
Larrasoaña
et al. LETHAIA 41 (2008)
new results retrieved from drill cores in Tanzania
(Pearson
et al
. 2004). These results suggest that the
FO of
H. nuttalli
occurs at different stratigraphic
intervals in different sections, and therefore cast
doubt on the usefulness of this event as a reliable
marker for the Ypresian/Lutetian boundary
(Berggren & Pearson 2005, 2006). Alternative
markers of the boundary include the FO of the
planktic foraminifera
Guembelitrioides nuttalli
(=
Globigerinoides higginsi
) (base of E8 of Berggren
& Pearson 2005, 2006), the FO of calcareous nanno-
fossil
Blackites inflatus
(base of CP12b of Okada &
Bukry 1980), and the boundary between shallow
benthic Zones SBZ12 and SBZ13 (Serra-Kiel
et al
.
1998). Unfortunately, assessing the suitability of these
markers has been made difficult because many sec-
tions around the world either contain sedimentary
hiatuses at the boundary (see reviews by Aubry 1995
and Opdyke & Channell 1995) or lack magnetostrati-
graphic or biostratigraphic results with the appropriate
resolution or quality (Galbrun 1992; van Fossen 1997;
Molina
et al
. 2000; Ogg & Bardot 2001; Pearson
et al
.
2004; Bowles 2006; Molina
et al
. 2006; Payros
et al
.
2006; Suganuma & Ogg 2006). A notable exception
is the Gorrondatxe section, which has recently been
studied by Bernaola
et al
. (2006) and Payros
et al
.
(2007) in the Basque–Cantabrian basin (Spain). This
section provides a new magnetobiostratigraphic
correlation scheme that bears important implications
for defining the Ypresian/Lutetian boundary. All
possible markers of the Ypresian/Lutetian boundary
appear in the section, although at different stratigraphic
levels. The FOs of
H. nuttalli
and
G. nuttalli
occur
at about 43.5 Ma and 45.5 Ma, respectively (Payros
et al
. 2007). These ages are about 3 to 5 Myr younger
compared to classic sections from Gubbio (Lowrie
et al
. 1982; Napoleone
et al
. 1983), where standard
calibration schemes of planktic foraminifera are based
on (Berggren
et al
. 1995; Luterbacher
et al
. 2004;
Berggren & Pearson 2005, 2006; Pearson
et al
. 2006a).
In view of the implications of the results of Bernaola
et al
. (2006) and Payros
et al
. (2007), and to shed light
on the calibration of the Ypresian/Lutetian boundary,
new sections need to be studied and examined.
In this paper, we present an integrated study of a
115-m-thick section that spans the Ypresian/Lutetian
boundary at Agost (Spain). This section is located
in the vicinity of the section studied previously by
Molina
et al
. (2000), but offers better outcrop and
fossil preservation conditions. Our new study includes
magnetostratigraphic results and biostratigraphic and
palaeoenvironmental data derived from planktic
foraminifera, small and larger benthic foraminifera,
and calcareous nannofossils. Our results provide a
magnetobiostratigraphic calibration which is largely
consistent with that proposed in the Gorrondatxe
section, and therefore reinforce the suggestion by
Payros
et al
. (2007) to reconsider standard calibra-
tion schemes for the Ypresian/Lutetian boundary
(Berggren
et al
. 1995; Luterbacher
et al
. 2004;
Berggren & Pearson 2005, 2006; Pearson
et al
. 2006a).
In that case, the Agost section presented here may be
considered as a suitable candidate for defining the
Early/Middle Eocene boundary stratotype.
Geological setting
The Agost section is located ca. 1 km to the north of
the village of Agost (Alicante, southeast Spain), in the
so-called Lomas de la Beata area (Fig. 1). During
the Eocene, the studied area belonged to the passive
margin of Iberia. Carbonate sedimentation in the
platform gave way to pelagic sediments, turbidites
Fig. 1. Location of the Agost section presented in this study. The
asterisk indicates the location of the Ypresian/Lutetian section
studied previously by Molina et al. (2000). The location of the
K/T boundary section (Groot et al. 1989) is also shown.
LETHAIA 41 (2008)
Ypresian/Lutetian boundary at Agost
397
and mass flow deposits in the continental slope
located southwards (Geel 2000; Alonso-Zarza
et al
.
2002). These sediments were folded and uplifted
during the Miocene collision that led to formation
of the Betic Cordillera in southern Spain (Guerrera
et al
. 2006). Nowadays, these sediments are exposed
at the Agost section striking broadly north to south
and displaying a gentle westward dipping of about
15–20
°
(Molina
et al
. 2000). The section is composed
of 115 m of marls with intercalated limestone and
sandstone beds (Fig. 2). Marls have a distinctive
greenish-greyish colour and commonly constitute
several-metre-thick intervals. Limestones range
between white and light grey in colour, and are often
less than 50 cm thick. Marls and limestones corre-
spond to hemipelagic sediments, and predominate in
the lower and upper parts of the section. Sandstones
have beige or yellow colours, and range between
few centimetres and 2 m thick. Some sandstone beds
show evidence of slumping and, in some cases, con-
tain embedded limestone boulders. Sandstones
correspond to resedimented slope deposits (turbidites
and mass flows) accumulated in the outer part of
carbonate platforms (Geel 2000; Alonso-Zarza
et al
.
2002). Sandstone beds are clustered in several thick
packages that constitute a characteristic stratigraphic
interval located in the middle upper part of the
section. The Agost section is affected by two small
faults that show minor displacements of less than
3–4 m and, therefore, do not disrupt the stratigraphic
sequence.
Methods
The magnetostratigraphic study is based on 87
palaeomagnetic sites distributed along 115 m of
sedimentary succession. Between one and two
oriented cores were taken at 84 of these sites with
a portable gas-powered drill. Sampling was focused,
when possible, on hemipelagic mudstones and marly–
limestones. In the middle part of the section, samples
were also collected from sandstone layers in which
no evidence for overturned beds or slumping was
observed. This sampling scheme gives a mean resolu-
tion of 1.4 m, which allows identification of geomag-
netic polarity reversals located near the Ypresian/
Lutetian boundary according to preliminary correla-
tion of biostratigraphic data (Molina
et al
. 2000) to
the revised GPTS (Luterbacher
et al
. 2004). Three
additional samples were taken in three limestone
boulders embedded in turbidites. Palaeomagnetic
analyses were made using a 2G superconducting rock
magnetometer at the palaeomagnetic laboratory of
the Institute of Earth Sciences
Jaume Almera
(CSIC-
Universitat de Barcelona). The noise level of the
magnetometer is less than 7
×
10
–6
A/m, which is
lower than the natural remanent magnetization
(NRM) of the samples measured. Thermal demag-
netization of at least one sample per site was done
using a MMTD-80 furnace (Magnetic Measurements
Ltd, Aughton, UK). Thermal treatment involved
between eight and 14 steps at intervals of 150, 100, 50,
30 and 20
°
C to a maximum temperature of 600
°
C.
Demagnetization of a set of pilot samples represent-
ative of all the lithologies allowed optimization of the
demagnetization steps to allow accurate calculation of
the characteristic remanent magnetization (ChRM)
directions minimizing heating and formation of
new magnetic phases in the oven. ChRM directions
were calculated by fitting linear trends in orthogonal
demagnetization plots using the principal component
analysis method (Kirschvink 1980).
For the study of planktic and small benthic
foraminifera, and of calcareous nannofossils, 86
samples, most of them in marls, were collected
throughout the section. Each of these samples was
divided into two parts. The first part was disaggre-
gated in water and washed through a 100-
μ
m sieve.
Each sample was cleaned using ultrasonic agitation,
and subsequent washing and sieving were repeated
until a clean foraminiferal residue was obtained. This
residue was dried in an oven at a temperature of
50
°
C. Quantitative analyses of planktic foraminifera
from 86 samples were based on representative
random splits (using an Otto microsplitter) of
more than 300 specimens. The rest of each foram-
iniferal residue was examined for identification of
rare species. Planktic foraminiferal biozones have been
established using the scheme proposed for the Betic
Cordilleras by Gonzalvo & Molina (1998), the standard
tropical–subtropical zonation scheme of Berggren
et al
. (1995) and its latest revised versions (Berggren
& Pearson 2005, 2006). A detailed review of the dis-
tribution of relevant species is given in Pearson
et al
.
(2006a, b). Small benthic foraminifera have been
studied in 30 samples in order to examine their
potential biostratigraphic value across the Ypresian/
Lutetian boundary (Berggren & Miller 1989) and also
to detect environmental changes (Thomas 2003)
potentially associated to the boundary. The study was
carried out by picking about 300 specimens from
the same foraminiferal residue from which planktic
foraminifera were analysed.
The raw material from the second part of each
sample was used to prepare smear slides and study
calcareous nannofossils. Smear slides of 61 samples
were prepared weighing 0.05 g of sediment and
mixing it with 15 mL of distilled water. Ten to
twelve drops of the mixture were then spread on a
398
Larrasoaña
et al. LETHAIA 41 (2008)
Fig. 2. Stratigraphic log and palaeomagnetic results from the Agost section. The position of the studied samples is plotted against the
stratigraphic log of the section. Filled and open symbols indicate type 1 and type 2 samples, respectively, whereas crosses indicate type 3
samples. Circles, squares and triangles represent limestones, sandstones and marls, respectively. Examples of ortogonal demagnetization
diagrams representative of different types of palaeomagnetic behaviour are shown after tilt correction. Grey lines indicate the best fit of
the calculated ChRM directions. Reliable type 1 and type 2 ChRM directions are plotted after tilt correction in an equal-area stereonet
(filled and open symbols represent projection in the lower and upper hemispheres, respectively), together with the mean direction and
statistics of normal and reverse polarity directions.
