The area of the Berici Hills is located on a Cenozoic structural high called the "Lessini shelf," on which, in the Oligocene, a carbonate platform was established. In particular, Lumignano is situated on the southeastern margin of this platform. From a stratigraphic point of view, the relevant succession consists of the Priabona Formation (Upper Eocene), which consists of marly limestones with macroforaminifera and red algae, including rhodoliths, capped by a paraconformity separating it from the Castelgomberto Formation. The latter is of Lower Oligocene age and contains massive limestones composed of corals with a much lower terrigenous content compared to the Priabona Formation. The Castelgomberto Formation terminates at the top with a karst surface. Both formations are intruded by volcanic chimneys and dikes. The Castelgomberto Formation forms the climbing walls for which Lumignano is famous, and it was one of the most well-known cliffs in Italy during the early days of sport climbing. There are two hypotheses about the structure of the platform itself: the first suggests it is a Caribbean-type platform with a low-energy lagoon and a scarp, while the second proposes that the coral reef is located on a gently inclined ramp, developing at a depth that does not dampen wave energy. To verify the accuracy of these theories, a systematic sampling was carried out along the line of maximum development of the mountain overlooking Lumignano village, corresponding to the core of the reef, with a thickness of about 150 meters. A total of 48 samples were collected, which were then polished and photographed at a resolution even lower than one-tenth of a millimeter, which was sufficient to observe the constituent grains. The samples from meter 0 to meter 19 come from the Priabona Formation and are predominantly composed of rudstone with coralline algae, large foraminifera (orthophramids and nummulites), bryozoans, minor elements, and marl. Subsequently, the samples belong to the Castelgomberto Formation, specifically from meter 20 to meter 110, the samples consist of systematically developed boundstone with live corals and a packstone-wackestone matrix with fragments of red algae. From meter 111 to 150, the samples are composed of rudstone with coralline algae, rhodoliths, bryozoans, and small nummulitids. The facies analysis results have revealed that the transition from the Priabona Formation to the Castelgomberto Formation corresponds to a shift from an oligophotic environment to a mesophotic one, the latter characterized by corals immersed in a muddy matrix. The coral facies was therefore deposited below the wave base. At the end of the vertical walls, a corresponding deepening of the depositional environment and a return to oligophotic conditions with the coralline algae facies are observed, resulting from a transgression. Thus, seaward of the coral facies deposited below the wave base, there is no scarp but rather a slightly deeper environment with rhodoliths. The geometry of the platform was therefore that of a gently inclined ramp.
L’area dei Colli Berici si trova su un alto strutturale Cenozoico chiamato il “Lessini shelf”, sul quale si imposta, nell’Oligocene, una piattaforma carbonatica; in particolare Lumignano si trova al margine sud est della suddetta piattaforma. Dal punto di vista stratigrafico la successione di interesse è costituita dalla Formazione di Priabona (Eocene superiore), che consiste di calcari marnosi a macroforaminiferi e alghe rosse, inclusi i rodoliti, terminata al tetto da una paraconformity che la separa dalla Formazione di Castelgomberto. Quest’ultima ha età Oligocene inferiore e contiene calcari massicci costituiti da coralli in cui il contenuto terrigeno è molto minore rispetto alla Formazione di Priabona. La Formazione di Castelgomberto termina al tetto con una superficie incarsita. Entrambe le Formazioni sono tagliate da camini e dicchi vulcanici. La Formazione di Castelgomberto dà origine alle pareti di arrampicata per cui Lumignano è famosa, le quali sono state una delle falesie più conosciute a livello italiano agli albori dell’arrampicata sportiva. Vi sono due ipotesi sulla struttura della piattaforma stessa: la prima secondo cui si tratta di una piattaforma di tipo caraibico con una laguna a bassa energia e una scarpata, mentre la seconda secondo cui il reef a coralli si trova su una rampa poco inclinata e si sviluppa a una profondità tale da non poter smorzare l’energia delle onde. Per verificare la correttezza delle teorie si è deciso di eseguire un campionamento sistematico sulla linea di maggior sviluppo del monte sovrastante l’abitato di Lumignano, corrispondente al nucleo del reef, per uno spessore di circa 150 m. Sono stati raccolti 48 campioni che sono poi stati lucidati e fotografati con una risoluzione anche inferiore al decimo di millimetro, ampiamente sufficiente per osservare i granuli costitutivi. I campioni dal metro 0 al metro 19 sono provenienti dalla Formazione di Priabona, e sono costituiti da rudstone prevalenti con alghe coralline, grandi foraminiferi (ortofragmidi e nummuliti), briozoi, elementi minori e marna. Successivamente i campioni appartengono alla Formazione di Castelgomberto, particolare: dal metro 20 al metro 110 i campioni sono boundstone costituiti sistematicamente da coralli in posizione di vita e una matrice packstone-wackestone con frammenti di alghe rosse; mentre dal metro 111 a 150 i campioni sono costituiti da rudstone ad alghe coralline, con rodoliti, briozoi e piccoli nummulitidi. I risultati dell’analisi di facies hanno permesso di stabilire che il passaggio dalla Formazione di Priabona a quella di Castelgomberto corrisponde ad una transizione da un ambiente oligofotico ad uno mesofotico, quest’ultimo con coralli immersi in una matrice fangosa. La facies a coralli si è quindi deposta al di sotto della base d’onda. Al termine delle pareti verticali si osserva un corrispondente approfondimento dell’ambiente deposizionale e un ritorno a condizioni oligofotiche con la facies ad alghe coralline, determinato da una trasgressione. Quindi, verso mare rispetto alla facies a coralli deposta sotto la base d’onda, non si riscontra una scarpata ma un ambiente di poco più profondo con rodoliti. La geometria della piattaforma era quindi di rampa poco inclinata.
La falesia di Lumignano: analisi di facies e sezione stratigrafica (Eocene - Oligocene dei Colli Berici)
STECCA, GIOVANNI
2022/2023
Abstract
The area of the Berici Hills is located on a Cenozoic structural high called the "Lessini shelf," on which, in the Oligocene, a carbonate platform was established. In particular, Lumignano is situated on the southeastern margin of this platform. From a stratigraphic point of view, the relevant succession consists of the Priabona Formation (Upper Eocene), which consists of marly limestones with macroforaminifera and red algae, including rhodoliths, capped by a paraconformity separating it from the Castelgomberto Formation. The latter is of Lower Oligocene age and contains massive limestones composed of corals with a much lower terrigenous content compared to the Priabona Formation. The Castelgomberto Formation terminates at the top with a karst surface. Both formations are intruded by volcanic chimneys and dikes. The Castelgomberto Formation forms the climbing walls for which Lumignano is famous, and it was one of the most well-known cliffs in Italy during the early days of sport climbing. There are two hypotheses about the structure of the platform itself: the first suggests it is a Caribbean-type platform with a low-energy lagoon and a scarp, while the second proposes that the coral reef is located on a gently inclined ramp, developing at a depth that does not dampen wave energy. To verify the accuracy of these theories, a systematic sampling was carried out along the line of maximum development of the mountain overlooking Lumignano village, corresponding to the core of the reef, with a thickness of about 150 meters. A total of 48 samples were collected, which were then polished and photographed at a resolution even lower than one-tenth of a millimeter, which was sufficient to observe the constituent grains. The samples from meter 0 to meter 19 come from the Priabona Formation and are predominantly composed of rudstone with coralline algae, large foraminifera (orthophramids and nummulites), bryozoans, minor elements, and marl. Subsequently, the samples belong to the Castelgomberto Formation, specifically from meter 20 to meter 110, the samples consist of systematically developed boundstone with live corals and a packstone-wackestone matrix with fragments of red algae. From meter 111 to 150, the samples are composed of rudstone with coralline algae, rhodoliths, bryozoans, and small nummulitids. The facies analysis results have revealed that the transition from the Priabona Formation to the Castelgomberto Formation corresponds to a shift from an oligophotic environment to a mesophotic one, the latter characterized by corals immersed in a muddy matrix. The coral facies was therefore deposited below the wave base. At the end of the vertical walls, a corresponding deepening of the depositional environment and a return to oligophotic conditions with the coralline algae facies are observed, resulting from a transgression. Thus, seaward of the coral facies deposited below the wave base, there is no scarp but rather a slightly deeper environment with rhodoliths. The geometry of the platform was therefore that of a gently inclined ramp.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.12608/52502