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

common and many of these repetitive sequences are attributed to autocyclic processes, which
result from changes within the depositional system itself These processes include, for example,
alternations of channel and overbank deposits in alluvial sequences, or of turbidites and pelagic
sediments in flysch facies (Fischer, 1986), or oceanographic processes. Cyclostratigraphy,
however, is not concerned with this type of cyclicity but rather with allocyclic changes that
reflect climate oscillations as a consequence ofastronomical forcing.
Geological time scales
Identification of orbital signals in sedimentary sequences is based on recogmsmg cycle
hierarchies, but in particular it depends on the match in frequencies between geological and
astronomical cycles in the time domain. This approach implies dating of sedimentary cycles in
geological sequences. The methods of dating geological sequences have evolved in the last
centuries from relative to absolute dating. Relative dating includes for example varve counts,
which were used to identifY precession cycles in the Permian evaporites (Anderson, 1982).
Extrapolation of varve counts was used earlier by Bradley (1928) to identifY precessioncontrolled
cyclicity in the Eocene Green River Formation (Colorado and Utah) and by Van
Houten (1964) in the Lockatong Formation of the Triassic Newark Basin (New Jersey and
Pennsylvania). Since the beginning of the 20th century, absolute dating was achieved by
determining the radioactive decay ofisotopes in minerals found in the geological record.
Since the discovery that the Earth's magnetic field has frequently reversed its polarity
throughout geological history, time scales have been constructed based on polarity reversals. The
first geomagnetic polarity time scale (GPTS), constructed by Cox et al. (1963), was based on
K/Ar datings of volcanic rocks of known polarity. Once marine magnetic anomalies were
demonstrated to represent geomagnetic polarity reversals (Vine and Matthews, 1963), Heirtzler
et al. (1988) extended the GPTS into the Mesozoic, by assuming a constant sea floor spreading
rate. The validity and accuracy of the GPTS were confirmed by magnetostratigraphy,
biostratigraphy and radiometric datings of deep-sea sediments from the Deep Sea Drilling
Project (DSDP). In 1992, Cande and Kent published their GPTS, which also included anomaly
profiles from fast spreading plates, thereby enabling the recognition ofshorter subchrons. This
GPTS, which was revised in 1995, is now widely used as a tool for absolute dating of
sedimentary sequences via magnetostratigraphic calibration ofthese sequences to the GPTS.
Meanwhile, tuning ofPleistocene cyclic variations, especially in 0 18 0 records, to astronomical
target curves led to the construction of astronomical time scales (ATS) with an unprecedented
accuracy and resolution (Hays et aI., 1976; Imbrie et aI., 1984). Subsequently, the Plio­
Pleistocene ATS was established on the basis of cyclic variations oflithology and proxies in the
marine record, which were tuned to astronomical target curves (Shackleton, 1990; Hilgen,
1991a, b). The combination ofastronomically calibrated time scales and the GPTS has resulted
in the astronomical polarity time scale (APTS). The accuracy and resolution ofthe A(P)TS was
proven beyond doubt to be a breakthrough in dating techniques used by earth scientists. Hence,
the standard GPTS and geological time scale for the time interval 0 - 5.23 Ma is entirely based
on this APTS (Kent, 1999).
The A(P)TS is now being successfully applied in paleoclimatic and oceanographic studies. For
example, Raymo et al. (1990) used Atlantic-Pacific b '3 C gradIent records ofthe past 2.5 Ma to
investigate trends between climate change (glaciations) and the formation of North Atlantic
Deep Water (NADW). Other studies are directed at understanding climate change by