Web photogalleryThe Natural Reserve of Sasso Malascarpa is close to the Corni di Canzo group close to Lecco (Lombardy, N. Italy)). The reserve is easily accessible from Lecco and Canzo and it is a wanderful terrace on all Lombardian Prealps and Brianza. However, from a geological point of view, the late Triassic rocks are here rich in Corals and large mollusc (Conchodon).
Tectonic is also well visible with nice folds: surface karst features represent another peculiarity of the limestone beds or blocks outcropping in several points. A visitor Center is available at Primo Alpe (from Canzo) and during the summer or the week-ends a few huts are open for food support: small springs are also present. Geological and botanical plates are scattered in the area, the highest path being completed during 2008.
For guided tours contact NuovaGeoteam (martaboccaletti@alice.it)
In the succession of the Calcare di Zu (Zu Limestone) we often observe levels called coquine, containing huge quantities of shells (mostly bivalve molluscs). The origin of the piling up is mechanical, due to storms or riptides. Today we can see the same phenomenon on the sea shore or on the shallow bottoms: after a storm we find many shells, single valves of dead molluscs or still closed shells of organisms killed by the storm itself. These strata give us information about the environment where this fauna lived 215 my ago: the water was shallow, warm, teeming with organisms sometimes swept away by storms.
Corals are often colonial organisms: the small but countless polyps build their own skeleton, then making a reef, trapping sediments and other organisms in their branches. The Banco a Coralli is spread all over the lombardian Prealps, suggesting a time of tropical climate. Our area lied at lower latitudes because of plate tectonic: shallow, oxigenated waters made the rest. The bottom was literally covered by colonies (up to 1 m in diameter). Unluckily, preserservation is poor and we rarey recognize the internal structures.
The core of the synclinalic fold of Val Ravella is made of few tens of meters of Cretaceous units, the youngest marine rocks of this area. Like before, the marine bottom was still high: in the Bergamo and Como deep basins the same units became hundreds of meters thick. Microfossils like foraminifers help in dating these rock. The Marne di Bruntino formation, made of marls (limestone and shale in variable proportion), deposited during Lower Cretaceous in a deep see where shale was transported from the mouth of rivers coming down from newly formed mountains far north: the alpine orogeny was already at work.Tthe Flysch di Bergamo formation, shaly, foiled intervals alternate with thin layers of sandstone. These sequences are originated by 'torbiditic currents': sediments slide from the slope and are then redeposited on a deeper bottom starting from the coarser up to the finest grains.
The Dolomia Principale making the Monte Rai was deposited 210-220 million years ago (Late Triassic) in a hugely extended, marine carbonatic platform, with very shallow water (about 5 m): something like the Bahamas, but much wider. The calcitic particles (CaCO3) were rapidly cemented and then dolomitized (Ca, Mg (CO3)2, deleting sedimentary features and fossils. The mean thickness of the unit is about 1000 m from here to the Dolomites.Only some structures are still recognizable such as Oncoids. They are originated by algae and/or bacteria, forming an organic film which traps the calcareous muds: oncoids need 2 or 3 m of moving water that continuously rolls them so that algae can coat them.
The almost pure limestones belonging to Dolomia a Conchodon and to Calcare di Zu are often affected by superficial Karst features: along our geologic path we find Lapiez (mostly on the numerous blocks and outcropping pure limestone beds of Sasso Malascarpa) and campi solcati (Lapiez Fields). Lapiez are little, subparallel furrows caused by chemical dissolution operated by water running along the ruling gradient. Thus we can say wheater Karst features are formed prior or after the fall of the blocks. All Lapiez are along the present-day ruling gradient, allowing to say the Lapiez have been formed on the fallen blocks, after the fall.
The Sasso Malascarpa is a thick limestone layer tilted to vertical position by tectonic movements the alpine orogeny. The wall, scattered with Conchodon shells represent the sea bottom (once horizontal!) where these large bivalves lived in groups partly embedded in the sediments. The coral reef was already dead, and between the different Conchodon clusters the bottom was nearly barren (see the surface of farrowed field: no Conchodon or other fossils). The bivalves were killed by a sudden event and there thick shells remained on the sediment. The shape of the sectioned Conchodon resembles a heart or a 'devil' foot-print: the name Malascarpa means bad shoe!
From S.Tomaso we have a very nice view of the tectonic peculiarity of Sasso Malascarpa. A series of folds concern the sedimentary sequence: the most evident ones are the ´V´ shaped syncline that formed the Sasso Malascarpa itself, a broader anticlinalic fold making Monte Rai, and a syncline between the crest NE of Sasso Malascarpa and the Corni di Canzo.
The time interval of this succession is very wide, from 220 to 100 million years before present. Tthe sea bottom in fact was very high above the surrounding areas, just like a ´pillar´, so that sediments frequently slided from the steep sides to the deeper bottom. The natural reserve of Sasso Malascarpa is focussed on the calcareous outcrops of the Banco a Coralli - Calcare di Zu with its fossil Conchodon and corals and also its Karst features, lapiez and lapiez fields. Karst structures also concern the younger Dolomia a Conchodon (CH - 200 my old) which, despite the name, neither is dolomitic nor contains fossils (Conchodon are peculiar of Calcare di Zu), but the name, though originated by erroneous observations, has been maintained.
The nearly pure, massive limestones outcropping in this area give rise to cliffs. Their present aspect is the result of fracturing (due to tectonic movements) and of Karst dissolution by water. Vegetation is also peculiar and consists of the typical limestone cliffs flora and of ‘dry grassland’. On the other hand, the either absent or extremely thin soil is not able to retain water, immediately swallowed by the Karst fractures. The massive limestones of Dolomia a Conchodon make up most of the peaks here around, for example the Corni di Canzo among which the 'Corno Occidentale' resembles a tower. The cliffs we observe here are also made of the same limestone formation, which then prosecutes on the southern crest of Valle Luera. The massive limestones of Banco a Coralli (Calcare di Zu) give rise to spectacular outcrops at Sasso Malascarpa and Val Molinata: the great walls are the result of tectonic tilting which moved the thick layers to vertical position.
Along this road we find outcrops of Calcare di Zu (ZU- latest Triassic). This unit is characterized by a central calcareous sequence, named Banco a Coralli, and two similar successions where calcareous levels are intercalated to marly-shaly layers. The massive Banco a Coralli at Sasso Malascarpa is extermely rich of various fossils, among which corals, molluscs (also the large bivalve Conchodon), brachipods, microfossils like foraminifers. The other two parts of the formation are well represented in this tract of the road. The marly-shaly layers (resulting from calcareous muds mixed with shale in variable proportion) are softer and more easily erodible thus often allowing grasslands to grow. Some ‘coquina’ layers are also intercalated: they are almost exclusively made of mollusc shells, whereas fossils are normally sparsely found in the rock.
Microscopic observation of thin rock sections helps defining depositional environments as well as determinig microfossils. Above, we see two degrees of shell density; the one on the right for example suggests higher bottom energy: a storm wave wiped off the fine sediment, leaving only the coarse shells. Below, we recognize Triasina, a foraminifer whose diameter is less than 1 mm.