The Orera section (Calatayud Basin, NE Spain) - Universiteit Utrecht

The Orera section
Orera section is not so common in other parts ofthe basin except for some cycles recognised in
gypsum. Thus, the section provides an unique opportunity for undertaking studies on the
cyclostratigraphy ofMiocene lacustrine successions in the Calatayud Basin and elsewhere in the
Iberian Peninsula as these type of studies have not yet been carried out in other Spanish
continental (neither marine) Neogene basins, probably because of the lack of such clearly
developed cyclic successions. An exception, however, is the work carried out in the Armantes
area (also in Calatayud) and in southern Spain (Krijgsman, 1996). In addition, the age of the
sediments cropping out in Orera, ranging from 10.7 to 12.8 Ma (Abdul Aziz et al., 1999), makes
the succession quite suitable for extending the astronomical polarity time scale (APTS) to older
stratigraphic levels that have not yet been studied (Hilgen, 1991b). All these features make the
Orera section an exceptional site that merits particular protection and preservation.
The astronomical polarity time scale: some briefcomments about
cyclicity, orbital forcing and time
More than a century ago, G.K. Gilbert (1895) realised that sedimentary cyclicity in marine
sequences can be used to estimate the duration ofparts ofthe geological record. Gilbert linked
his sedimentary cycles to perturbations in the Earth's orbit and rotation axis caused by
gravitational interactions of our planet with the Sun, the Moon and other planets. These
interactions cause cyclic changes in the eccentricity ofthe Earth's orbit with main periods of
100,000 and 413,000 years, an in the tilt of (obliquity) and precession of the Earth's axis with
periods of41,000 and 21,000 years, respectively.
Eccentricity measures the degree of elongation of the orbit and varies between nearly zero
(circular) and 0.06 (slightly elliptical). Obliquity describes the angle between the Earth's axis of
rotation and the orbital plane varying between 22° and 25°. Precession describes the movement
of rotation axis around a circular path with one revolution of26,000 years. Due to the opposite
movement of the eccentricity orbit, the precession of the equinoxes, also called 'climatic
precession', completes one full cycle in 21,000 years.
These perturbations affect global, seasonal and latitudinal distribution of the incoming solar
radiation on our planet. Orbitally forced climatic changes are recorded in sedimentary archives
through changes in sediment properties (lithology), fossil communities and geochemistry and are
used by earth scientists to reconstruct paleoclimate. Astronomers have computed the past
variation in precession, obliquity and eccentricity. The logical next step is to date the
sedimentary cycles by matching patterns of paleoclimate variability with patterns in the
computed astronomical curves. This 'tuning' has resulted in time scales that are independent of
radioisotopic dating and are tied to recent times through this matching with the astronomical
curves. Since 1982, magnetic reversals were dated independently of readioisotope dating and
were incorporated, via the sedirr>ent:J.ry cycles. into the tilne scales producing the astronomical
polarity time scale (APTS). Radioisotope dating is also intercalibrated with the astronomical
time scale in order to eliminate uncertainties about the correct ages of the dating standards.
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