The Parona chaotic complex: a puzzling record of the Messinian (Late Miocene) events in Monferrato (NW Italy)

Abstract

The Parona chaotic complex (PCC) from Monferrato (NW Italy) is a mélange composed of carbonate-cemented blocks floating in a matrix of weakly consolidated mud breccias. It forms a minor (1 km 2) chaotic sedimentary body enclosed in a larger Messinian mélange that rests unconformably on older deposits and is followed, again unconformably, by Pliocene sediments. Three main facies have been recognized in the blocks. (a) Bioclastic rudstones and coquinoid grainstones of early Messinian age, composed of oligotypic assemblages of gastropods and the brackish-water bivalve Cerastoderma, mixed with shallow-water, marine biota (mollusks, corallinae algae, benthic foraminifers, echinoid fragments, rare ahermatypic corals). These sediments record shallow-water carbonate sedimentation with episodic brackish incursions and show a peculiar diagenetic overprint (selective dissolution of aragonitic grains and early dolomitic cements). (b) Monogenic breccias, composed of clasts of coquinoid grainstones floating in a dolomitic microcrystalline matrix. (c) Polygenic conglomerates, composed of rounded clasts of pre-Messinian pelagic sediments, of lower Messinian bioclastic carbonates and of Messinian evaporitic carbonates. Finally, a few blocks of strongly cemented micritic limestones, interpreted as methane-derived carbonates for the geochemical signature of the carbonate phases (strongly depleted in 13C), are present. The matrix enclosing the blocks is a mud breccia containing centimeter-sized clasts of marly sediments. Micropaleontological analyses have shown that lower Messinian and upper Tortonian planktic foraminifers are mixed with lower Miocene, Oligocene and Eocene forms. The PCC results from complex processes involving: (1) deposition of the peculiar sediments preserved in the blocks and (2) their dismemberment and mixing. As for the depositional history, composition of the blocks has allowed to sketch the following evolution. (a) Deposition of bioclastic rudstones and coquinoid grainstones that are interpreted as lower Messinian carbonate platform deposits for their strong compositional and diagenetical similarity with the lower Messinian carbonates cropping out in the Mediterranean region. The Monferrato bioclastic sediments are the northernmost example of Messinian shallow-water carbonates yet known. (b) Deposition of monogenic breccias. They formed during the evaporitic phase of the Messinian that was accompanied by a relative sea-level drop and by erosion of the previously deposited coquinas. Depositional and diagenetical processes during the evaporitic phase are also recorded by clasts of evaporitic carbonates in the polygenic conglomerates. (c) Deposition of polygenic conglomerates that are interpreted as upper Messinian post-evaporitic fan delta facies resulting from an important erosional phase. After deposition, this succession was dismembered in the blocks forming the present-day mélange. Geometry, stratigraphic relationships, and internal characteristics of the PCC point to an origin related to gravity-driven phenomena, triggered by tectonics. However, the faunal mixing in the mud breccias and the occurrence of blocks of methane-derived carbonates suggest that mud diapirism also played a role for the genesis of the PCC. This latter mechanism, which has been poorly considered until now, could also have been effective in the genesis of Messinian chaotic deposits that extensively crop out in NW Italy.