Open Session - SWISS GEOSCIENCE MEETINGs

1.3
The Teggiolo zone: stratigraphy and tectonics in the Val Bavona
(Lower Penninic, Ticino)
Matasci Battista, Epard Jean-Luc & Masson Henri
Institut de Géologie et Paléontologie, Université de Lausanne, 1015 Lausanne (Battista.Matasci@unil.ch)
The Teggiolo zone is the sedimentary cover of the Antigorio nappe (Lower Penninic, Central Alps). Our study is based on
detailed mapping, stratigraphy and structural analysis between Robiei and the Pizzo Castello, on the eastern side of Val
Bavona. There the Antigorio nappe is overthrusted by the Sambuco nappe (formerly northern part of the Maggia nappe, cf
Berger et al. 2007). The sedimentary cover of the latter thins out and disappears near Robiei, and SE of this locality the
Sambuco gneissic basement directly overlies the Antigorio cover. Thus the metasediments pinched between the Antigorio
and Sambuco basements entirely belong to Antigorio.
In the Teggiolo zone several sedimentary cycles are distinguished, separated by erosive surfaces that are discordant at the
map scale:
1.- Where the stratigraphic column is the most complete, it starts with Triassic layers (mainly dolomite). In the survey area
the Triassic is only preserved at Campo, elsewhere it has been eroded below the Jurassic transgression.
2.- The second sedimentary cycle is comprised of a detrital series passing upwards to a limestone formation. Its base is conglomeratic.
This conglomerate is thin in the survey area, often reduced to dispersed pebbles in a matrix of coarse-grained
sandstone, but much thicker on the opposite bank of the valley, in the Val Antabia, where it consists of several layers alternately
rich in pebbles of gneiss (eroded from the basement), dolomite (from the Triassic), and other sedimentary rocks
unknown in the normal stratigraphic column (presumably originating from the erosion of a post-Triassic and pre-conglomeratic,
completely destroyed part of the column). The sandstone shows various depositional structures (graded-bedding,
etc) and is overlaid first by a slightly micaceous and quartzic limestone (yellowish marble), then by a pure limestone (white
marble). The age of all this series (traditionally attributed to the Triassic) is clearly post-Triassic, probably Middle to Late
Jurassic. At its top there is a layer of banded marble whose presumed protolith has similarities with rocks of earliest
Cretaceous age in the Helvetic domain.
3.- The third cycle consists of the main mass of the calcschists of the Teggiolo zone. In other parts of the nappe its base truncates
the two preceding cycles and is eroding deep into the Antigorio basement. The sedimentary discontinuity with the
underlying series is supposed to be a major stratigraphic gap and the age of these calcschists is presumed to be latest
Cretaceous to Tertiary. In the Val Bavona this series is thick on the western side (Val Antabia) and on the eastern side
around Campo, but from there it disappears towards SE because it is itself truncated by the base of the overlying wildflysch.
4.- The top part of the Teggiolo zone is a more or less chaotic formation of calcschists, often rich in blocks of various rocks,
interpreted as a meta-wildflysch (Robiei Formation, Masson 2002). It seems to fill channels eroding down to various depths
the older layers of the Teggiolo zone. SW of Robiei, it can be subdivided into a lower part characterized by blocks of marble
(similar to the white marble of the underlying section), and an upper part characterized by blocks of gneiss (whose probable
source is in the Sambuco nappe, Bussien et al. 2008). In both parts a gradual vertical evolution is observed from finegrained
detritus to small blocks (of marble or gneiss, respectively), then to large blocks of dam or even hm size. SE of Robiei
the blocks of gneiss disappear and only the lower subdivision with marble blocks continues, but around the Pizzo Castello
it passes laterally to an accumulation of blocks of all sizes (up to several hm) and various lithologies. The largest ones are
often composite and present a stratigraphic succession gneiss-sandstone-marble similar to the lower part of the Teggiolo
zone in this region, with the difference that the gneiss is not identical to the usual Antigorio orthogneiss.
Conclusion: The Teggiolo zone has a complex geological history, with several sedimentary cycles and stratigraphic gaps,
presumed to cover the whole time span from Triassic to Early Tertiary. Its lower part (Triassic-Jurassic-Lower Cretaceous?)
presents similarities with some Middle Jurassic thresholds of the Helvetic realm (e.g. internal Mont Blanc), while its upper
part (Upper Cretaceous to Tertiary) typically belongs to the North-Penninic domain. Thus the Antigorio nappe occupies a
crucial position in the restoration of the main paleogeographical domains of the Alps. The highest formation of calcschists
with blocks is interpreted as a wildflysch: its sedimentation is probably contemporaneous with the beginning of the translation
of the overlying Sambuco nappe. The increasing amount and size of the blocks towards the East seems to announce a
tectonic connection of the Antigorio and Sambuco nappes..
REFERENCES
Berger, A., Mercolli, I. & Engi, M. 2007: Tectonic and petrographic map of the Central Lepontine Alps 1 : 100000, Explanatory
notes. Carta geol. spec. 127, Swisstopo.
Bussien, D., Bussy, F., Masson, H., Magna, T. & Rodionov, N. 2008: Variscan lamprophyres in the Lower Penninic domain
(Central Alps): age and tectonic significance. Bull. Soc. Géol. France 179/4, 369-381.
Masson, H. 2002: Ophiolites and other (ultra)basic rocks from the West-Central Alps: new data for a puzzle. Bull. Géol.
Lausanne 356.
Symposium 1: Structural Geology, Tectonics and Geodynamics