1,721,207 research outputs found
Petrography of the quartz euhedra as a tool to provide indications on the geologic history of the Upper Triassic Burano Evaporites (Northern Apennines, Italy)
No full textTiming of post-depositional events in the Burano Formation of the Secchia valley (Upper Triassic, Northern Apennines), clues from gypsum-anhydrite transitions and carbonate metasomatism
No full textThe 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 (delta O-18 = -5.7 to -3.7 parts per thousand; delta C-13 = +1.3 to +3.0 parts per thousand; PDB) have been affected by Mg-metasomatic replacement by magnesite (delta O-18 = -14.0 to -2.6 parts per thousand; delta C-13 = -2.6 to +1.4 parts per thousand; PDB) induced by hydrothermal circulation. Total homogenization temperatures of fluid inclusions in hydrothermal magnesite range from 275 to 310 degreesC. 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 degreesC (Emilia) and from 230 to 315 degreesC (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-relies 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). (C) 2001 Elsevier Science B.V. All rights reserved
Geology of the Realmonte salt deposit, a desiccated Messinian Basin (Agrigento, Sicily)
No full textAbstractThe Realmonte salt body (Agrigento) was deposited in a syncline just ahead of a growing thrust during the Messinian “salinity crisis”. The succession can be divided into two units separated by a variable angular unconformity and an exposure surface:1) Lower unit 150 m-thick composed by cumulates of halite plate crystals with minor amount of kainite showing evidence of precipitation from a stratified water body, possibly under a significant water depth. The topmost part shows features suggesting a marked upward shallowing of the basin and carry the signature of the subaerial exposure of the salt by the occurrence of giant contraction polygons.2) Upper unit 140 m-thick characterized by cumulates of halite skeletal hoppers and chevron precipitated in a non-stratified, shallow water body.The angular unconformity existing between the salt units at the exposure surface and the deformation of the salt beds that predates the onset of the intra-Messinian truncation suggest that the desiccation could be the result of the uplift of part of the basin floor as consequence of thrust movements. The uplift of the basin floor formed a sill that temporarly cut off the basin from the sea.RiassuntoIl deposito salino di Realmonte (AG), riferibili al ciclo inferiore della Formazione Gessoso-solfifera di età messiniana, fu deposta nella sinclinale antistante un sovrascorrimento. La successione è suddivisibile in due parti separate da una discordanza angolare di pochi gradi:1) Unità inferiore di spessore attorno ai 150 m, costuita da cumuliti di cristalli tabulari di salgemma (plate). L’assenza di lamine argillose, di superfici di dissoluzione e di ulteriori accrescimenti dei cristalli una volta caduti sul fondo suggerisce che le acque del bacino fossero stratificate per densità e fossero quindi relativamente profonde. La parte alta della unità inferiore è costituita da cicli decimetrici di argilla/sale/kainite anch’essi costituiti da cristalli tabulari in giacitura cumulitica cui sono intercalati sei strati kainitici. Sono presenti anche zattere di cristalli di sale e superfici di dissoluzione che si fanno via via più frequenti verso la parte terminale dell’unità. Tali caratteristiche testimoniano l’assenza di stratificazione per densità delle salamoie ed un loro un graduale e marcato abbassamento di livello. Gli strati terminali del’unità sono stati interessati da esposizione subaerea, come suggerito dalla presenza di cavità di dissoluzione meteorica e di enormi poligoni di contrazione che si sviluppano in seguito agli effetti della escursione termica annuale sulle piane delle saline disseccate.2) Unità superiore di spessore di circa 140 m formata da cicli decimetrici argilla/salgemma costituiti da cumuliti di cristalli scheletrici che mostrano di essersi ulterioriormente accresciuti sul fondo del bacino (chevron) e sono troncati da frequenti superfici e cavità di dissoluzione. Tali caratteristiche suggeriscono una profondità estremamente ridotta delle acque nel bacino e l’assenza di una stratificazione di densità.La variabile discordanza angolare presente tra le due unità e la precoce deformazione del deposito che precede lo sviluppo della superficie di erosione intra-messiniana suggeriscono che il disseccamento possa essere stato causato da una fase tettonica. I movimenti del sovrascorrimento che limita il bacino a settentrione potrebbero aver provocato il sollevamento del fondo del bacino e a formazione di una soglia che avrebe impedito le comunicazioni con il Mediterraneo
The magnesite in the Upper Triassic Burano Evaporites of the Secchia River valley (Northern Apennines, Italy): petrographic evidence of an hydrothermal-metasomatic system
No full textIndagine petrografica delle malte, strumento per la ricostruzione dell’antico assetto del Palazzo Ducale di Sassuolo.
No full textConsiderazioni sui materiali da costruzione impiegati nella edificazione del Palazzo Ducale di Sassuolo.
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Recognition of ancient fires in archaeological sites containing gypsum rocks.
No full textThe presence in archaeological sites of architectural elements composed of gypsum rocks (selenite, gypsarenite and gypsrudite) is helpful to reveal traces of ancient fires because a characteristic white layer develop on gypsum surfaces directly exposed to temperatures in excess of 100 °C. The white surface is generally up to a few centimetres thick and is the consequence of gypsum dehydration to form a mosaic of microcrystalline bassanite, soluble anhydrite and/or anhydrite, depending on the temperature. With the exception of anhydrite, all phases rehydrate rapidly back to gypsum because of atmospheric moisture and/or groundwater capillarity rise along the walls. The shape of the original gypsum crystal outline is still recognizable, but the rock fabric is now composed of a mosaic of secondary, white microcrystalline gypsum. Anhydrite may also be present if the surface temperature during the fire was in excess of about 252 °C.
The final result of gypsum heating appears indistinguishable from natural dehydration, which may occur because of several geological processes, such as diagenesis and burial. In these cases, however, the white areas are massive or have nodular shapes and obviously formed before the rocks were quarried. Criteria to distinguish between artificial and natural dehydration are thus easily applicable. White surfaces caused by fires develop only on exposed face of blocks which were not shielded from the heat, are thin, and correlate across adjacent blocks showing different gypsum facies and crystal orientation.
The geological survey of gypsum facies in the source outcrops may reveal the presence of natural dehydration features, which may be misinterpreted as the effect of fires.
The spatial extent of correlable white surfaces in the buildings, their variable thickness and composition, may give indications on the severity of damages caused by fire events, providing significant informations to unravel the evolution of archaelogical sites
La Torre Ghirlandina, il restauro e il terremoto
No full textA meno di un anno dal completamento del restauro che l’ha restituita alla città nel pieno splendore,
la torre simbolo di Modena è stata duramente colpita dagli eventi sismici del 20 e 29 maggio 2012.
Tutto sommato il monumento ha retto bene alle forti scosse; la torre è rimasta stabile anche grazie
agli interventi antisismici di cerchiatura realizzati nel 2011. Il danno principale è stata la fratturazione
della lastra decorata di “Sansone che smascella il leone”. Questa breve nota illustra i risultati
principali del restauro e i danni prodotti alla torre dalla crisi sismica del maggio 2012
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