Carta Geologica d'Italia a scala 1:50.000Foglio n° 248 LA SPEZIA Parte marina

Many of the tectonic units recognized in the Northern Apennines crop out in La Spezia 1:50,000 Geological Sheet: from bottom to top, the Metamorphic Succession of Punta Bianca (Early Paleozoic to Late Triassic), the Tuscan Nappe (Late Triassic to Oligocene), the Marra Unit (Oligocene), the Canetolo Complex (Paleogene), the Ottone Unit (Late Cretaceous), the Mt. Gottero Unit (Jurassic to Paleocene), the Mt. Antola Unit (Late Cretaceous). The first three units developed above the continental margin of the Adria plate, the remaining four had a substratum ranging from transitional (the Subligurid Canetolo Complex) to oceanic (Ligurid Units) and are considered representative of the so-called Tethyan realm. The Metamorphic Succession of Punta Bianca crops out in the eastern pro¬montory of La Spezia gulf. It is made of by Early Paleozoic fine-grained clastic sediments metamorphosed during a Late Paleozoic (Hercynian) event and by two sedimentary (continental to shallow marine) cycles of Middle to Late Triassic age. The Triassic basins are probably related to early rifting stages con¬nected with the Tethys opening. During the Miocene the whole succession underwent “Alpine” greenschist facies metamorphism. Lenses of discontinuous thickness of cataclastic carbonate breccias (Maralunga and Lerici breccias) are interposed between the Metamorphic Succession of Punta Bianca and the overlying Tuscan Nappe. The Tuscan Nappe sedimentary formations were deposited on a carbonate platform in progressive drowning after the Early Jurassic and in deep-sea environment since the latest Jurassic. New units of different rank, from members to lithofacies, have been mapped within the Formazione di La Spezia, Scaglia Toscana and Macigno. The total thickness of the Tuscan Nappe reaches 3,500 m and more than 2,000 m are represented by the Oligocene Macigno turbidite grey¬wacke sandstones. Discontinuous lenses of Oligocene marls and silts (Marra Unit) of probable slope environment are geometrically interposed between the Tuscan Nappe and the overlying Canetolo Complex. The latter is a Paleogene highly deformed assemblage of prevailing shales with subordinate limestones and sandstones, whose original stratigraphy is obliterated. When it was possible to distinguish discrete bodies of uniform lithology, the “Groppo del Vescovo limestone” (CGV) and the “Ponte Bratica sandstones” (ARB) have been mapped. The tectonic units overlying the Canetolo Complex can be attributed to the External Ligurids, which were originally closer to the Adria margin and include the Ottone Unit, and to the Internal Ligurids, which were originally closer to the conjugate European margin and include the Mt. Gottero Unit and the Mt. Antola Unit. All the Ligurid units have a similar sedimentary succession since they con¬sist of Late Cretaceous shales topped by turbidites, that can be calcareous (Ottone and Mt. Antola) or arenaceous (Mt. Gottero). Some peculiarities can be pointed out. In the Ottone Unit the shaly portion beneath the calcareous turbidi¬tes (“Ottone flysch”, OTO) is stuffed by olistoliths derived from the ophiolitic suite. The Mt. Gottero Unit includes some chunks of serpentinites and gabbros which are regarded as the original substratum of the sedimentary succession, which at its top reaches the Paleocene. Along the northeastern flank of the Sarzana basin the highly deformed Ligurid units are sealed by a thick cover of continental deposits ranging in age from a possible Late Miocene to the Holocene. Unlike the lithostratigraphical criteria that ruled the mapping of older deposits, unconformity-bounded strati-graphic units were recognized and mapped in the Neogene sedimentary succes¬sion. The Sarzana Synthem (ASZ), the Low Magra Valley Synthem (BVM) and the Pian di Barca Synthem (PBR) have been established together with associa¬ted subsynthems and lithofacies. Fluvial and alluvial fan sediments prevail in the Low Magra Valley Synthem (BVM) and the Pian di Barca Synthem (PBR). Fluvio-lacustrine deposits with lignite seams occur in the basal portion of the Sarzana Synthem (ASZ). Along the NE slopes of the Sarzana basin the same lower units of the Sarzana Synthem (ASZ) are tilted toward SW and juxtaposed by faults to the Ligurids. Tilting decreases progressively moving basinward. The marine area of La Spezia Sheet is almost entirely occupied by a conti¬nental shelf with an extremely regular morphology. It is constituted by some Plio-Quaternary sedimentary sequences that cover an Apenninic substratum faulted and sunk. One morphological peculiarity is represented by the active head of the Levante Canyon that cuts the shelf in its southwestern part. This fea¬ture does not bear any relationships with the subaerial drainage network or tec¬tonic structures. With reference to various types of shelfs of sedimentary construction, this shelf belongs to a category in which progradation and aggradation assume com¬parable importance on the whole, even if their relationship was different in the various phases of development. The seismostratigraphical analysis has confir¬med the presence of a notable reflector with regional continuity and characters of an erosive surface. This reflector divides the sedimentary prism in two parts. The upper sedimentary complex is organized in mainfold complete sequences, bounded by unconformities, as a classical expression of cycles of sea level chan¬ges, due to glacioeustatism. The remarkable thickness of the whole sedimentary prism, that increases toward SE, underlines the importance of the subsidence related to the disjunctive tectonics that has given origin to the basins of the Northern Tyrrhenian Sea. The Upper Pleistocene-Holocene succession is mainly characterized by a transgressive sequence formed by at least three bodies. The widest and thickest body corresponds to the Younger Dryas and represents the fluvial deposit of an extended coastal plain with a well-developed hydrographic network. An acoustically semitransparent layer with typical characters of an highstand deposit rests above all the described bodies. It reaches considerable, but extre¬mely varying, thickness (max 30 m). The structural framework of La Spezia Sheet is dominated by the superposi¬tion of Ligurid “Tethyan” units above Adria plate units (Punta Bianca and Tuscan Nappe). In turn, each of these major assemblages includes tectonically stacked units. The main stacking events can be dated Early to Middle Miocene. From a structural point of view, five areas can be distinguished from west to east: - continental margin. Its framework is determined by different brittle structures, mainly referable to the “Tyrrhenian” disjunctive tectonics active since the Late Miocene. As to importance and frequency, step faults of Apenninic trend, which lower the substratum towards SW, prevail. They are intersected by NE-trending transfer lines. E-W trending normal faults are more rare. A narrow belt of the substratum is deformed by some listric reverse faults, which also displace part of the sedimentary cover, producing large thrusts. Trend and continuity of these structures parallel those of the Apenninic step faults; - western promontory of La Spezia gulf: The Tuscan Nappe, the Marra Unit and the Canetolo Complex are here involved in a large, SW verging, overturned fold (“La Spezia fold”) which can be traced southward beyond the La Spezia gulf. This “Tyrrhenian” vergence, opposite to the common “Adriatic” vergence of the northern Apennine structures, has been tentatively related to a backthrust in the generally Adriatic-oriented displacement of the tectonic units. - eastern promontory of La Spezia gulf and La Spezia hinterland: in the sou¬thernmost portion of the promontory the Metamorphic Succession of Punta Bianca and the overlying Tuscan Nappe are antiformally folded. The Tuscan Nappe is involved also in low-angle normal faulting. To the North, a larg slab of “Mt. Gottero sandstone” (GOT) is locally involved in N to NE verging recum¬bent folding. A major NW-trending, high-angle normal fault (“La Spezia fault”) juxtaposes the Tuscan Nappe of the western promontory against the Mt. Gottero tectonic unit. - the Sarzana basin: like many other similar Neogene structures in the Apennines, this basin is a half graben bounded by NW trending border faults, the master fault being adjacent to the Punta Bianca promontory. It is possible that these pronounced NW/SE Apenninic lineaments were acquired during rather late stages of the basin development. - northeastern areas (Mt. Grosso/Mt.Tanna ridge): the core of this ridge is a Tuscan Nappe anticline. On both limbs of the anticline repeated thrusts of Ligurid and Subligurids units make up a belt of highly deformed rocks. A schematic structural history of the region includes the following succession of events: in connection with a subduction zone dipping beneath the Europe plate, begin since the Late Cretaceous the consumption of the Tethys realm and the stacking of the Ligurid units and their progressive translation onto the Adria plate margin. At the end of the Paleogene their superposition on the Tuscan domain is achie¬ved; end of Paleogene/beginning of Miocene intense shortening in the Adria con¬tinental crust and ensialic subduction produce greenschist metamorphism of the Punta Bianca succession and strong folding with large backthrusts; since the Late Miocene a part of the region is uplifted and emerged with development of continental intermontane basins, bounded by normal high-angle faults particularly active during the Quaternary; during the same time the continental margin is rifted and lowered as conse¬quence of Tyrrhenian Basin formation; some compressive events temporarily interrupt the disjunctive tectonics during the Late Pliocene-Early Pleistocene