Record of climatic change in neritic carbonates: turnover in biogenic associations and depositional modes (Late Miocene,southern Spain) |
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Authors: | T C Brachert C Betzler J C Braga J M Martin |
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Institution: | 1. Institut für Geowissenschaften, Pal?ontologie, Saarstr. 21, D-55099, Mainz, Germany 2. Geologisch-Pal?ontologisches Institut, Senckenberganlage 23-34, D-60054, Frankfurt, Germany 3. Departamento de Estratigrafia y Paleontologia, Campus de Fuentenueva, E-18002, Granada, Spain
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Abstract: | In order to evaluate the geological record of climatic change in neritic carbonates, we studied Late Miocene rock outcrops
in southern Spain. Six episodes of reef growth are documented (Burdigalian to Messinian) in Neogene basins of the Betic Cordillera,
which were located close to the margin of the global reef belt. The reefs are characterized by various zooxanthellate corals
which decrease in diversity with time, andHalimeda; the youngest reefs of the latest Messinian are characterized by the dominance of the genusPorites. Late Miocene coral reefs and reef-rimmed platforms alternate over time with non-reefal carbonate ramps characterized by
skeletal calcirudites or with gypsum such as that formed during the Messinian salinity crisis. The calcirudites lack reef
corals, calcified green algae and extensive marine cement, but exhibit skeletal components described from both modern and
fossil nontropical carbonates. These include bryozoans, mollusks, foraminifers, echinoderms and minor balanids, as well as
coralline algae of a bryomol association. The presence of some larger foraminifers indicates high temperatures, close to the
lower temperature threshold of the reef assemblage. Sea level lowstands and highstands are documented by wedges of bryomol
carbonate and chlorozoan patch reefs or prograding platforms. Thus, temperate climate depositional modes correspond to relatively
low sea levels, and warm-water modes to high sea levels. The Neogene infill of the Agua Amarga and Sorbas basins documents
two of these cycles. Other climate/sea-level cycles (including Messinian gypsum in the cool water depositional mode) are well
established in adjacent Neogene basins in southern Spain. This type of composite sequence seems to occur only along the margin
of the global reef belt and indicates an oscillatory latitudinal movement of the margin, which is associated with global climatic
change. The analysis of turnover in neritic depositional carbonate systems may therefore be considered a sensitive tool for
reconstructing climatic change from the fossil record. However, warm-water modes and temperate-water modes of carbonate sedimentation
and diagenesis differ significantly. For this reason the interpretation of composite system sequences by sequence stratigraphy
requires an extended concept. The particular type of mixed bryomolchlorozoan depositional sequence also bears some potential
for drowning, because sea level rise may be faster than the net production rate of temperate carbonate systems. |
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Keywords: | Miocene Palaeoceanography Climatic changes Sea-level fluctuations Chlorozoan reefs Bryomol ramps |
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