Publication: Major palaeoenvironmental perturbation in an Early Aptian carbonate platform:
Prelude of the Oceanic Anoxic Event 1a?
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2011
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Elsevier
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Abstract
The Early Aptian Oceanic Anoxic Event (OAE 1a) was characterized by intensified greenhouse climate
conditions, widespread accumulation of organic deposits in open-marine settings, major perturbations in the
C cycle and a generalized increase in terrestrial runoff. Sedimentological, diagenetic and chemostratigraphic
analyses of Lower Aptian platform carbonates from the North Cantabrian basin (N Spain) illustrate the
regional impact and effects of those global conditions on shallow marine environments.
The studied interval outlines four stages of platform evolution. Stage 1 (earliest Bedoulian) is defined by an
initial rapid marine transgression that led to deposition of shallow water oligotrophic photozoan skeletal
assemblages, and by a later interval of subaerial exposure. Stage 2 (early Bedoulian) starts with a rapid
transgression followed by deposition of grainstones that yield heterozoan assemblages, more typical of
mesotrophic conditions, along with ferruginized oolites. Stage 3 (early Bedoulian) is defined by the
drowning of the carbonate platform and subsequent deposition of open-marine marls, which are thought to
represent the local expression of the OAE 1a. Finally, stage 4 shows the return of shallow water photozoan
carbonate sedimentation. The carbonate O and C stable isotope records have revealed prominent negative
excursions during deposition of the marly interval of the stage 3, which may be associated with the
important global changes that occurred at the onset of the OAE 1a. The change in skeletal assemblages that
preceded the isotopic excursions and the platform drowning documents conditions of environmental stress
caused by a combination of local and global factors. The global change, coupled with increased basin
subsidence, triggered the drowning of the platform by progressive reduction of the growth potential of the
carbonate factory.