Sequence Stratigraphy of N-Apennines Pliocene Foredeep (Italy). Inferences for the Pliocene Eustasy and Climate
Abstract
In the deformed foredeep sediments of the Northern Apennines a biostratigraphically well-calibrated succession of third-order sequences has been established. The complex morphology due to compressive tectonics allowed narrow passages from platform to deep basins, where different kinds of sediments ranging from platform carbonates to siliciclastic turbidites are variously arranged and can be physically correlated. The onset of the Pliocene is dominated by a fairly long (5.3 to 4.2 Ma) period of high stillstand of the relative sea level, giving rise to a widespread mudstone drape. A drop of the sea-level around 4.2 Ma (S.B. Pl 1) produced the progradation of a siliciclastic shelf-margin system from a nearby eroding structural high. This high was progressively drowned during the subsequent transgression, and a carbonate platform started to aggrade, colonizing the previous siliciclastic shelf. A pronounced sea-level drop at 3.75 Ma (S.B. Pl 2) shortly cut-off the carbonate factory and strongly eroded the thrust tops. At this time the most subsident (and deepest) basin was infilled by a thick interval of turbiditic sandstones fed from the Alps. The subsequent transgression (M.F. 3.3 Ma) brought mudstones over basins and swells. Organic Rich Layers (ORL) spread over most basins and slopes. Again during the highstand a small isolated carbonate platform aggraded on top of a thrust. It fed the adjacent basins via apron or small turbiditic lobes. At the top of this succession an interval rich in ORL occurs over most basins and slopes, whereas bioerosion and phosphatization at the platform margins document the abrupt increase of trophic resources. The platform then emerged, and a lobe of proximal turbidites, fed by Apenninic rivers, developed at the depocenter. The age of this unconformity is 3.0 Ma (S.B. Pl 3). After infilling of the basin, the depocenter shifted toward the foreland (Po Plain). Also carbonate platform never re-colonized the structural highs, documenting unfavourable conditions, such as the increasing influence of Apenninic rivers (turbidity, nutrients) and the cooling of climate. Despite the thrust activity, the recognized sequences appear bounded by coeval unconformities across different thrust sheets and basins. Therefore, a (glacio)eustatic cause for the observed oscillations is likely. The most important oscillation is associated to the type 1 S.B. Pl 2 (around a hundred m.), which deeply modified the palaeogeography of the area, first eroding many thrust tops and allowing progradation of Alpine turbidites, then aggrading more than 120 m of platform limestones. Glacioeustatic oscillations document climate variability since the Early Pliocene, a period usually considered constantly warm, with a first cooling around 4.2 Ma and a more vigorous at 3.75 Ma, which can be interpreted as the first signal of the onset of the Northern Hemisphere ice sheet. The middle Pliocene growth (from 3.3 to 3.1 Ma) of a carbonate platform with a well-developed photozoan assemblage points to a climatic optimum associated with a low trophic level of the surface waters (low continental runoff). At the top of this highstand the abundant ORL couple with the beginning of precession-related Mediterranean sapropels. A renewed cooling trend, since 3.0 Ma, has also been inferred by the molluscs extinction trend (Monegatti and Raffi, 2000). It is well recorded by a positive d18O shift in the Mediterranean and boreal seas, where it is coupled with ice-rafted debris (e.g. Raymo, 1994). Monegatti, P., and Raffi, S., 2001, Paleo3, v.165/3-4, p. 171-193. Raymo, M. E., 1994, Ann. Rev. Earth Pl. Sc., v. 22, p. 353-383.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFMPP42A0490C
- Keywords:
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- 4267 Paleoceanography