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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 6. (June)

First Page: 798

Last Page: 798

Title: Orogenic Clastic Wedges of the Alps and Apennines: ABSTRACT

Author(s): Franco Ricci Lucchi, Gian Gabriele Ori


The simple equations--flysch = turbidites, and molasse = marginal marine to continental deposits--are untenable even in Alpine collisional settings where the terms originated (e.g., the turbidite nature of most Apennine molasse). Because sedimentologic definitions of molasse and flysch are inadequate, it is necessary to rely on the tectonic setting of these deposits, although too many different usages have made the terms ambiguous in this respect. The terms are used herein only in regard to compression-related settings (including back-thrust and late-stage normal faulting), and for already completed or well-developed orogenic cycles. Time of emergence of the framework of the main chain from the mobile belt is taken as the reference for separating flysch (pre-emergence) fr m molasse (post-emergence). In Alpine belts, flysch basins have disappeared, but molasse basins are still present except for some deformed and uplifted parts. The emergence criterion requires that a significant drainage area was exposed to stream erosion, rather than just to gravity sliding and mass wasting. The time of emergence can be tested by facies analysis, as long as that analysis is made in a spreading place because chain building is incremental owing to orogeny migration (i.e., the rate of depocenter shifting is a significant parameter).

Orogenic basins of the Alps and Apennines varied much in size, lifetime, basement type, clast composition, mode and rate of filling, type of closure (tectonic or sedimentary), etc, but generally group around some of the following modes: (1) typical or Alpine flysch, (2) typical foreland molasse, (3) atypical or foreland flysch, (4) atypical foreland molasse, (5) atypical "piggy-back" molasse, and (6) intermontane and "back-arc" molasse.

(1) The typical or Alpine flysch was formerly large, turbiditic-abyssal wedges slowly deposited on a closing ocean floor and its margins, during a long "punctuated" collisional event (Late Cretaceous-Eocene). They eventually formed allochthonous slabs, detached from their basement, and obducted first on the European craton (Alpine orogeny), then on the African craton (Apennine orogeny). The main representatives of this type are the so-called Helminthoid (carbonate) Flysch.

(2) The typical foreland molasse consists of large, conformable or unconformable shallow-marine to continental wedges, that accumulated at a high rate in a major foredeep, laterally fed by the adjacent chain. These are represented by the north Alpine Molasse Group (Oligocene-upper Miocene), and the youngest undeformed "skin" (5%, upper Pliocene-Holocene) of the Po-Adriatic basin fill.

(3) The atypical or foreland flysch developed when large, conformable, deformed turbiditic-bathyal bodies were emplaced in major elongated foredeeps migrating on a shortening continental margin (African), which was activated after the Alpine collisional phase (Oligocene-Miocene). The sedimentation rate was higher than that of the typical or Alpine flysch, but did not prevail through subsidence. The basins where the foreland flysch developed were bounded mostly by subaqueous structural highs. Dispersal was longitudinal, with the main sources in the adjacent suture belt (i.e., supply of molasse type). Typical representatives of this class are the Macigno, Cervarola, and Marnoso-Arenacea Formations.

(4) Atypical foreland molasse deposits are conformable, partly deformed, large turbiditic to deep-water wedges, where the rates of subsidence and deposition were tremendous (up to 4-5 km/m.y.). The source of these deposits was sliding, giant debris flows (olistostromes), and axially deflected clastics from lateral emerging thrusts. These deposits are atypical with respect to the classic Alpine molasse, but they typify more than 90% of the Apennine foredeeps from the Po plain (upper Miocene-Pleistocene) to the Ionian Sea. They are also found in the early stage of filling of the North Alpine foredeep, where they usually have the qualities of flysch (e.g., Annot Formation).

(5) The atypical "piggy-back" molasse is mostly small to medium wedges of turbiditic-hemipelagic material filling separate basins. These basins formed at various stages on the Apennine thrusts and were carried with them to the foreland. Presently, the "piggy-back" molasse overlies autochthonous clastic wedges. Older deposits of this type (the Oligocene-Miocene Ranzano-Bismantova sequence) were close to and fed by the Alps suture zone (i.e., the source was basically Alpine molasse that was tectonically recycled in the Apennines). Younger deposits (upper Miocene-Pleistocene) reflect the emergence of the Apennines (real molasse).

(6) Intermontane and "back-arc" molasse are small to medium-size bodies unconformably filling fault-bounded basins. The highly variable subsidence and sedimentation rates resulted in a predominance of deep-water to continental deposits (i.e., partly typical and partly atypical). As the back-arc area of the Alps was the African craton (equal to the Apennine foreland), much of the "retro-Alpine" molasse was involved in Apennine thrusting. However, part of it remained in place (the Tertiary Piedmont basin). The "retro-Apennine" molasse is still accumulating in grabenlike basins around and in the Tyrrhenian Sea.

Examples of specific stratigraphic and sedimentologic attributes of the various types of wedges, with particular reference to types 3, 4, and 5 in the Apennines and Po basin, are being studied in the Pyrenees and Hellenides, in relation to their hydrocarbon potential.

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