Timing of post-depositional events in the Burano Formation of the Secchia valley (Upper Triassic, Northern Apennines), clues from gypsum–anhydrite transitions and carbonate metasomatism

Abstract

Abstract The Burano Evaporite Formation from the Secchia River Valley is an up to 2200 m-thick sequence composed of meter-to decameter-scale interbeds of gypsum–anhydrite and dolostones with minor halite. The deposit has been affected by a complex array of post-depositional modifications, thermal events and large-scale evaporite dissolution, preventing a satisfactory reconstruction of the environment of deposition. The modifications are intense because these rocks were the main decollement horizon during the formation of the Northern Apennines chain. The carbonate rocks are massive and-or laminated dolomitic mudstone, wackestone, oolitic packstones and oolitic, peloidal, bioclastic grainstones, which commonly appear as mega-boudins within a sulfate groundmass. The dolostones (δ 18 O =−5.7 to −3.7‰; δ 13 C =+1.3 to +3.0‰; PDB) have been affected by Mg-metasomatic replacement by magnesite (δ 18 O =−14.0 to −2.6‰; δ 13 C =−2.6 to +1.4‰; PDB) induced by hydrothermal circulation. Total homogenization temperatures of fluid inclusions in hydrothermal magnesite range from 275 to 310°C. The anhydrite rocks are characterized by flow structures such as centimeter-scale pseudo-lamination composed of aligned prismatic crystals with transposed isoclinal folds outlined by dolostones fragments. Homogenization temperatures of fluid inclusions in authigenic quartz incorporated into sulfate rocks range from 260 to 305°C (Emilia) and from 230 to 315°C (Tuscany). The gypsum rocks are composed of xenotopic irregular cloudy crystals and, more rarely, by centimeter-scale idiotopic crystals showing the same structures as the anhydrite rocks. The origin of the gypsum rocks is due to late alteration of anhydrite by migration of sharp hydration fronts. The hydration is a two step process and is revealed by the presence in the gypsum rocks of corroded anhydrite micro-relics and authigenic quartz crystals which include anhydrite. The role of the Burano Evaporites during the Apennines tectogenesis can be depicted as follows: (a) prevalent deposition of gypsum in the Upper Triassic; (b) gypsum dehydration at burial conditions to form anhydrite (Cretaceous?); (c) syn-tectonic flow of anhydrite rocks, brecciation of dolostones; syn-tectonic growth stage of quartz euhedra at deep burial conditions possibly related to the development of the Oligocene–Miocene greenschist facies Apuane metamorphic complex; (d) hydrothermal deposition of sparry magnesite and partial Mg-metasomatic replacement of dolostones by magnesite; (e) sub-surface dissolution of halite to form thick matrix-supported residual caprock-like anhydrite mega-breccias; (f) complete gypsification of anhydrite at sub-surface conditions; and (g) evaporite dissolution at surface exposure producing dolostone breccias with partial calcitization and removal of most clasts (“Calcare cavernoso”).