Geology of Ponte di Veja area

Transcription

1 Geology of Ponte di Veja area Guido Gonzato, PhD Introduction The Lessini Mountains, north of Verona, are an outstanding area from many points of view, not least for the variety of geological features. The Ponte di Veja area is one of the most beautiful sectors in the central Lessini, where we can observe particular geological aspects. Some of them have not been thoroughly studied yet. The area offers a wonderful landscape and is rich in water courses that are usually scarce in the Lessini Mountains. This area is the result of a set of geological processes that started about 200 million years ago. Stratigraphy: rocks and fossils As in other parts of the Lessini Mountains, nearly all rocks in the Ponte di Veja are sedimentary rocks. In particular, they are carbonate rocks made of sediments (mud, lime, sand) of mainly carbonate composition. We can see different types of limestone, composed of calcium carbonate (CaCO3), and of dolostone, composed of calcium and magnesium carbonate (CaMg (CO3)2) The environment where the rocks originated can be inferred from the sedimentary structures they contain: the size of sediment grains and how they are arranged in beds (layers) and laminae. Besides, fossils contained in rocks make it possible to establish their geological age (in some cases, with great accuracy) and to further pinpoint the original environment. Sedimentary structures and fossils reveal that different marine environments existed from the early Jurassic (190 million years ago) to the late Cretaceous (90 million years ago). A set of rock layers that share similar features, corresponding to a specific sedimentary environment, can be considered as a single unit called a geological Formation. Vertical overlapping of several Formations in the same area is called the stratigraphic column. According to the principles of stratigraphy, the oldest Formations are at the bottom of the column, while more recent Formations are towards the top. Fig. 1 shows the stratigraphic column that is generally present in the area. It is composed of carbonate Formations that originate in different marine environments, plus volcanic rocks. Emerging Formations are: Upper part of Gruppo dei Calcari Grigi (Formazione di Rotzo): Pliensbachian, early Jurassic. This Formation is composed of thick-bedded, fine-grained limestones, containing thin clay and silt levels. The average thickness of Calcari Grigi is about m. These limestones originated in shallow marine conditions, lagoons and channels, in a tropical climate. The most common fossil is Lithiotis problematica, a bivalve similar to an elongated clam that lived in lagoons. Banks of Calcari Grigi are visible at the bottom of Marciora valley, near Ponte di Veja, containing thin layers of lignite (fig. 2). Formazione di Tenno and Oolite di S. Vigilio: Toarcian-Aalenian, middle Jurassic. The first Formation consists of fine-grained, slightly nodular limestones, containing fossil crinoids (sea lilies), bivalves, and echinoids (sea urchins). Oolite di S. Vigilio is mainly composed of tiny (up to 2 mm) limestone spheres, called ooids or ooliths (from the Greek òoion = egg, lithos = stone ). Ooids form by (bio)chemical precipitation of calcium carbonate clinging to tiny nuclei, in marine environments where currents continuously stir the water and the sediment. In present days, ooids form in tropical environments such as the Bahamas or the Persian Gulf. Fossil corals, crinoids, molluscs and sponges are very common, and point to a warm marine climate. This

2 Stratigraphic column in the Ponte di Veja area JURASSIC CRETACEOUS Scaglia Rossa MYA Scaglia Variegata Alpina MYA Maiolica MYA Rosso Ammonitico MYA Gruppo di S. Vigilio MYA 100 m Gruppo dei Calcari Grigi milioni di anni fa (MYA) 50 Scaglia Rossa Cenomanian-Maastrichtian KEY Rosso Ammonitico Bajocian-Tithonian Volcanites Middle Eocene Scaglia Variegata Alpina Aptian-Cenomanian Gruppo di S. Vigilio Toarcian-Aalenian Maiolica Tithonian-Aptian Gruppo dei Calcari Grigi Hettangian-Pliensbachian

3 Formation (detail in fig. 3) is at the most 100 m thick, and is very well represented; it forms the pillars of Ponte di Veja. Fig. 2: banks of Calcari Grigi. Fig. 3: details of ooids and fossil sponges in Oolite di S. Vigilio. Rosso Ammonitico: Bajocian-Titonian, late Jurassic - early Cretaceous. Commonly but incorrectly referred to as Rosso Verona marble 1, it is a reddish, very fine grained, nodular limestone 1 From a geological point of view, it is not marble. Geologists call marble only limestone that has undergone deep burial and recrystallization, while stone traders call marble all massive limestones that can polished.

4 originated by slow sedimentation in an open sea environment. This Formation is named after ammonites, which are fossils molluscs whose spiral-shaped shells are commonly found in this limestone. Belemnite rostra are not uncommon; they are the remains of the internal shell of squidlike molluscs. Similar to bivalves but completely unrelated, are brachiopoda, whose shells are fairly rare. Iron and manganese nodules, similar to those that form at the ocean bottoms, are also not uncommon. The Ponte di Veja vault is made of Rosso Ammonitico (fig. 4). Fig. 4. Rosso Ammonitico beds form the vault of Ponte di Veja; the pillars are made of Oolite di S. Vigilio. Maiolica and Scaglia Variegata: Titonian - Cenomanian, early to middle Cretaceous. These two Formations were referred to as a single Formation called Biancone in old literature. Maiolica is a white, fine-grained marly limestone, i.e. it contains a clay fraction. In the Scaglia Variegata, the amount of clay increases and forms gray-greenish layers. While macrofossils are rare, nodules and thin beds of flint (microcrystalline quartz; fig. 5) of varying colours are very common, and can be seen in the outcrop at the car park. The abundance of flint was very important in prehistoric times, as it was the most common material for tool making. Flint from the Lessini Mountains has been also found in distant areas, suggesting ancient trade. Some layers of Scaglia Variegata are fairly rich in organic substance, mainly present as tar traces, and are dark gray or black. Scaglia Rossa: Cenomanian - Maastrichtian, late Cretaceous. This Formation is well known in the area, because it is extracted in many quarries (fig. 6). it is a pink/orange, nodular marly limestone, with rare nodules and layers of flint; it formed in an open sea environment. In the lower part of the Formation, layers are separated by thin clay levels; slabs can be easily quarried and are widely employed in local architecture. Many fossils can be found, namely ammonites, the bivalves Inoceramus and Hippurites, echinoids, and marine vertebrate such as sharks, turtles, and reptiles; excellent specimens are on display at the S. Anna d'alfaedo museum. Since the rock layers are nearly horizontal, all Formations can usually be observed at the same altitude. Near Ponte di Veja, the valley bottom is incised in the Calcari Grigi. The steep valley cliffs are made of Oolite di S. Vigilio, and are topped by Rosso Ammonitico. On top of the cliffs, the level grounds are made of Maiolica, Scaglia Variegata and a few spots of Scaglia Rossa.

5 The stratigraphic column is often broken by faults, which are fractures in the Earth crust. Faults can rarely be seen, as they are usually hidden by vegetation or soil. Their presence can be inferred when Fig. 5: brown flint beds in Maiolica. Fig. 6: quarry of Scaglia Rossa near Ponte di Veja (Croce dello Schioppo). Scaglia Rossa is covered by volcanic tuff (upper part of the cliff).

6 Formations are somehow out of place. In addition to sedimentary rocks, the area is rich in volcanic rocks such as basalt and tuff, erupted during the Eocene (ca million years ago). Basalt is a black, very hard rock formed from the cooling of lava, similar to that of Mount Etna. Tuff is a dark brown, incoherent rock formed by sedimentation of products of submarine volcanic explosions. Unusual rocks are also present. Some of the caves around Ponte di Veja contain yellow ochre, which was excavated and exploited as pigment until the second half of the last century. Ochre is made of iron oxides and hydroxides, and fills some palaeokarst cavities. These are caves that formed millions of years ago, when the Monti Lessini first emerged from the sea between the early Oligocene and the middle Miocene. At Grotta di Fumane, a few km west of Ponte di Veja, archaeologists found a few paintings (among which the famous shaman ) made with red ochre. Fig. 7. Grotta dell'orso: cupolas in the main gallery. Morphology and karst The presence of limestone implies the presence of karst: a set of features related to the dissolution of limestone by water enriched in carbon dioxide. Karst occurs both underground, where caves (possibly very large) may develop, and on the surface, where it transforms the landscape into a typical karst morphology. The Ponte di Veja area shows both types of karst. As explained above, carbonate rocks are mainly composed of calcium carbonate, which is not easily dissolved in pure water: approximately, grams per litre. However, if the water is enriched in carbon dioxide, a chemical reaction occurs that turns calcium carbonate into bicarbonate, which is over 11,000 times more soluble: 166 grams per litre. Rain is always enriched with carbon dioxide, and other weak acids are present in the ground. As a result, rocks exposed to subaerial weathering dissolve in water relatively easily. Calcium bicarbonate dissolved in water can turn back into calcium carbonate, forming speleothems such as stalactites and stalagmites. Caves are the most spectacular karst phenomenon, and are very common in the area. The Grotta dell'orso, whose entrance is closed by a gate for archaeological protection reasons, contains

7 Quaternary deposits such as Ursus spelaeus (cave bear) bones. This species of bear became extinct approx years ago. Cave D-E contains ochre fillings, which are palaeosols probably dating back the the Oligocene regression of the area. Other karst features, such as cupolas (fig. 7), suggest that at least some of these caves originated in very different conditions than today; in particular, in hot and humid climate conditions. References BROGLIO A., DALMERI G. (eds.), Pitture paleolitiche nelle Prealpi venete. Mem. Mus. Civ. St. Nat., Verona, 2, 190 pp. MIETTO P., SAURO U., Le Grotte del Veneto: paesaggi carsici e grotte del Veneto. Regione Veneto, La Grafica Editrice, 2a ed., Verona, 480 pp. PERIN G., Scienza e poesia sui Lessini. Centro Prof. Don Calabria, Verona, 562 pp. ROGHI G., Le Formazioni Geologiche del Veronese nella nuova Cartografia Geologica Nazionale. La Lessinia - Ieri Oggi Domani, La Grafica, Verona, SAURO U., Lessinia. Cierre edizioni, Verona, 273 pp.

8 MONTI LESSINI FORMATIONS AND ENVIRONMENTS Era Period Epoch Cenozoic Mesozoic Quaternary Neogene Paleogene Cretaceous Jurassic Start (Mya) Formation Depositional environment Holocene 0,01 Fluvial sediments fluvial Pleistocene 1,81 Fluvio-glacial sediments fluvio-glacial Pliocene 2,59 Buried sediments? Miocene 23,03 Calcarenites beach Oligocene 33,9 Stratigraphic gap emersion; palaeokarst Eocene 55,8 Limestones; basalt, tuff reef, internal lagoon, beach; submarine eruptions Paleocene 65,95 Stratigraphic gap? Late 99,6 Scaglia Rossa pelagic Early 145,5 Maiolica / Scaglia Variegata abyssal plain Late 161,2 Rosso Ammonitico drowned platform Middle 175,6 Tenno / Oolite di S. Vigilio external platform Early 199,6 Gruppo dei Calcari Grigi shallow carbonate platform; coastal lagoon Triassic Late 228 Dolomia Principale tidal flat Guido Gonzato, PhD