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摘要
In Provence and Languedoc, SE France, four drowning events were identified in platform carbonates of late Barremian-Bedoulian age; their timing, referred to ammonite zones or subzones, is as follows:
(1) Late Barremian (D1), at the Gerhardtia sartousiana-Imerites giraudi boundary,
(2) Middle Bedoulian (D2), at the Deshayesites weissi-Deshayesites deshayesi boundary,
(3) Mid late Bedoulian (D3) in correspondence with the "Roloboceras hambrovi subzone", and
(4) Late Bedoulian (D4) at the Deshayesites grandis-Dufrenoya furcata transition.
Notwithstanding their relatively wide regional, lateral continuity, the depositional hiatus, linkage with exposure, paleobathymetric range and offset, and geographical extent of drowning discontinuities are not uniform.
The late Barremian drowning (D1) is marked either by the development of a permanent intra-shelf basin with ammonites associated with the re-organization of platform-basin relationships, or by the wide extent of Palorbitolina-Heteraster fades, which develops to the detriment of the antecedent rudist fades. The drowning event is followed by platform recovery during the early Bedoulian, but the corresponding development of shallow water carbonates is subsequently interrupted by the emergence of an uplifted bulge trending sub-parallel to the Provence-Languedoc margin. The Middle Bedoulian drowning (D2) is concurrent with the sealing of the antecedent paleotopography and the wide development of Palorbitolina fades, then bioclastic and coral fades tend to recover. The Mid late Bedoulian drowning (D3) is characterized by an overall deepening phase, with ammonite-bearing marly facies or cherty limestones, shallow biodastics being locally present. The late Bedoulian drowning (D4) records the deposition of deep water marls.
Evidence is lacking that global sea level changes or transgressive-regressive cycles had a significant impact on drowning events, and some evidence that changes in temperature and productivity of the ocean may have contributed to these phenomena in conjunction with other factors. Though distension fault activity is regarded as a major controlling factor for the origin of the late Barremian deepening event, its associated paleogeographic re-organization, ensuing emergence of the Provence-Languedoc marginal bulge and its subsequent detumescence coeval with the Middle Bedoulian drowning. Associated changes in facies types, from rudist to Palorbitolina, reflect sea-water deepening coupled with trophic modifications.
As agents of differential subsidence, tectonic phenomena are also testified by the contrasting regional patterns of paleobathymetric offsets combined with drowning events. They are a clue for understanding thickness changes and coeval modifications in the overall orientation of the progradational polarity of the platform system, towards the adjacent basinal areas.
Environmental changes, essentially the two main OAE1a subevents, are regarded as significant agents of the Mid late and late Bedoulian drownings, whereas tectonic processes are also involved in the corresponding deepening. However, the effects of the OAE events on the functioning of the shallow carbonate factory are poorly understood and still need to be investigated.
The rise in atmospheric carbon dioxide pressure and, as a consequence, the reduction of the calcification potential of benthic organisms, both reported in the literature, are not clearly reflected in the composition of the biota: aragonite-dominated organisms such as caprinid rudists thrive throughout the Bedoulian. The role of acidification in platform demise is therefore questionable.
- 出版日期2013-12-15
