Dolomitization Processes and Their Relationships with the Evolution of an Orogenic Belt (Central Apennines and Peri-Adriatic Foreland, Italy)

Abstract

In the Jurassic–lower Cretaceous sequence of central Apennines and their foreland (onshore and offshore Marche–Abruzzi regions), the dolomitization processes enhanced the petrophysical properties of the carbonate platform and slope series. The area experienced a first phase of passive-margin regime, from the Lower Jurassic to the Miocene: the Liassic carbonate platform underwent extensional tectonics that established the southern Apulian–Apennines persisting platforms and the northern Umbria-Marche basin; within the basin, differential subsidence rates created faulted horsts characterized by condensed series. From upper Miocene, the area entered the collision-margin phase when its eastern part was involved in the Apennines orogeny. The aim of this study was to analyze the dolomitization process in relation with the fluid-flow changes caused by the evolution of the geological framework. Dolomitized bodies are mainly located at the Jurassic–Early Cretaceous platform edges and in the paleohigh areas, particularly in the platform Calcare Massiccio and basinal Corniola formations, with minor extension to younger slope successions (up to Maiolica Formation). The petrographic observations evidenced a multiphase dolomitization of alternated dolomite replacement, dissolution, and recrystallization. The carbon and oxygen isotopic analyses suggest a seawater-derived diagenesis in a wide temperature range; this is confirmed by the fluid-inclusion analyses that detected a few stages of dolomitization events during a progressive heating of the carbonate series. The reconstructed paragenetic sequence is almost the same in all the studied successions, both in outcrop and subsurface. The data collected show that the dolomitization processes changed during the evolution of the area from a passive-margin domain to the collision-margin regime. In both phases, the interaction between the increasing burial temperature and the fluid migration paths, which is driven by the approaching orogenic wave, is suggested. The first dolomitization event (dolomite 1) is characterized by low homogenization temperature (Th) and is interpreted as a replacement of calcite precursor; its formation occurred during the passive-margin domain and is strongly dependent on the rate of subsidence of the different areas. The dolomite 1 time of generation varies from Cretaceous, in the highly subsiding areas of the thrust zone, to the Miocene in the paleohighs of the foreland. Dolomite overgrowths (dolomite 2) precipitated at higher temperatures. In the depocenters of thrust zone, those temperatures (as much as 130C) were reached during maximum overburden during the upper Miocene, whereas in the foredeep and foreland areas, the high-temperature dolomitizing fluids seem to flow through faults during the orogenic phases (Pliocene). In the whole studied area, pore-filling dolomite cements (dolomite 4 and saddle dolomite) are supposed to be precipitated from heated fluids coming from deep strata along fault planes. Regional considerations and salinity data of the fluid inclusions support the hypothesis that the dolomitizing fluids of the last phases could come from Triassic evaporites that are present in the area and represent the detachment surface of the thrusts.