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Bottom currents on a pelagic carbonate platform. Mounds and sediment drifts in the Jurassic succession of the Sciacca Plateau, Western Sicily
Get PDFThe stratigraphic succession in the San Vincenzo Gorge (Saccense Domain, western Sicily) documents deposition on a vast pelagic carbonate platform, the Sciacca Plateau, during the Middle and Late Jurassic. This succession caps a peritidal limestone (Inici Formation), which underwent extension during the Western Tethyan Early Jurassic rift phase, and displays a set of unique features, which have never been previously reported on a Tethyan drowned platform. The upper part of the Bositra limestone (late Bajocian-early Oxfordian p.p.) comprises elongate convex-up, mound-shaped bodies, made of thin-shelled bivalve wacke- to grainstone, a few tens of metres across and producing a topographic relief of up to 10 m. Planar beds within the mound cores are seen to thin out laterally with tangential downlaps along sections perpendicular to the mounds' longer axes, and the mounds are in lateral association with concave-up bedsets. Following halt of the Bositra-dominated deposition and demise of mound accretion, the draping units inherited an antiformal geometry. The mounds are interpreted as being part of a sediment drift, produced by bottom currents sweeping the Plateau top, the source areas being sediment-depleted sectors now documented by extremely condensed and hiatus-ridden sections, with parallel-sided beds. Following draping and partial levelling of the submarine relief by the Knobbly limestone (?middle Oxfordian/early Kimmeridgian-late Kimmeridgian), the Coquina limestone is locally a thick (>20 m) ammonite/brachiopod rudstone (Tithonian p.p.). This unit displays evidence for lateral accretion, with large-scale clinoforms dipping up to 12°, and is interpreted as a mud-poor, bioclastic-gravel drift, with the action of bottom currents being apparently linked with a bloom of cephalopods. This is an early-cemented deposit, where clotted, micropeloidal fabrics document the calcification of microbial communities and are followed by growth of early diagenetic fibrous calcite. The description and interpretation of the mounded Bositra limestone and of the clinostratified Tithonian limestone are the main focus of this paper. The San Vincenzo Gorge outcrop displays similarities with pelagic shelves, like the Upper Chalk basin of northern Europe
Evidence for extended Hercynian basement and a preserved Jurassic basin-margin tract in Northern Calabria (Southern Italy). The Longobucco Basin
No full textIn the Longobucco Basin (northeastern Calabria, Southern Italy) a Mesozoic succession covers the Hercynian basement, documenting the evolution of a continental margin during the Early Jurassic extensional tectonic phase correlated with the Western Tethys rift. The basin evolved from continental red beds around the Rhaetian/Hettangian boundary to shallow marine and eventually deep-sea siliciclastic turbidites in the late Pliensbachian/Toarcian, to dominantly pelagic deposits (Middle Jurassic onwards). Around the Sinemurian/Pliensbachian boundary, following a prominent phase of normal faulting, sedimentation took different paths in either hangingwall-block or footwall-block settings. This study, based on geological mapping, focuses on the southwestern boundary of the Longobucco Basin and in particular on the mutual relationships existing between the deeper-water basin-fill units (Fiume Trionto and Fosso Petrone Formations), the basement, and the shallow-water limestone of the Caloveto Formation which forms a narrow strip running parallel to the rift shoulder. The contacts among the above-mentioned units are generally all stratigraphic (unconformities of various types: onlap, downlap, nonconformity). These characteristics separate the study area from the depocentral parts of the basin in terms of vertical stratigraphy, nature of contacts among stratigraphic units (unconformable vs conformable), and impact of synsedimentary tectonics. The signature of Early Jurassic extension occurs in the form of (1) laterally continuous clastic bodies interpreted as products of catastrophic basin margin collapse and avalanching, (2) exhumation of the pre-rift units, which are otherwise deeply buried, and (3) neptunian dykes of at least two generations. Our new data provide evidence for the presence of a 20 km long, NW-SE trending preserved paleoescarpment, where the basinal units onlap either the basement or the Caloveto Formation which forms a fringing carbonate body with intriguing facies and bio-sedimentological features. The paleoescarpment was the submarine morphological expression of a Sinemurian/Pliensbachian fault, as testified by the age of the clastic body that seals it. Another pulse of synsedimentary extension took place in the latest Pliensbachian/Toarcian, overprinting the existing rift architecture and reviving the production of megaclastic deposits
Understanding the geological record of carbonate platform drowning: examples from the Lower Jurassic of the Apennines (Italy)
No full textABSTRACT - In the geological record a drowning process is documented by various types of shallow water-to-pelagic or shallow water to mixed benthic/pelagic carbonate transitions. Drowning unconformities are paraconformities, to disconformities, to angular unconformities, and their drowning surfaces range from planar to highly irregular morphologies. Drowning successions display a mix of products of both the benthic and pelagic carbonate factories. These successions can be also bounded by unconformities. Drowning unconformities and drowning successions are contrasted through a description and discussion of examples of escarpment-bounded platforms from the Jurassic of the Northern Apennines and, subordinately, from Sicily and other Tethyan sectors. The areal distribution of drowning unconformities and successions is discussed with reference to a complex depositional system whose architecture was the product of the riftinduced fragmentation of a regional Hettangian carbonate megabank. The examples in this study show drowning unconformities to be exclusive to intrabasinal highs, while drowning successions are found both on highs and in hangingwall basins. Drowning unconformities, with their long associated hiatuses, and drowning successions are often seen to merge laterally into one another over very short distances on the same intrabasinal high. The deposits of the drowning succession are sometimes missing on the top of the platform, while they are found forming clinoforms along its flanks, evidence that sediment could be permanently swept from the highs in these depositional systems at this stage
Early Cretaceous post-rift extensional faulting and inherited Jurassic structures in Central Apennines and Southern Alps
No full textThe tectonic-stratigraphic evolution of Mesozoic sedimentary successions across the Alpine Tethys
was influenced by Early Jurassic rift-related extension. Evidence tor this phase of normai faulting
are tound in the Alps and the Apennines, where vast Hettangian carbonate plattorms (Calcari Grigi
and Calcare Massiccio paleoplatform, respectively) were dismembered into fault-bounded blocks
and drowned, giving rise to pelagic carbonate platform (PCP)/basin systems (Santantonio 1994).
This is evidenced by facies and thickness variations within the syn- and post-rift Jurassic-Lower
Cretaceous pelagites. Direct and indirect evidence tor an Early Cretaceous post-rift extensional
tectonic phase has been reported across different paleogeographic domains. Typical stratigraphic
and sedimentologica! features include: i) the back-stepping of carbonate platform and PCP margins,
ii) the areai reduction or-locally- drowning of carbonate platforms, iii) the deposition of clastic bodies,
iv) the occurrence of neptunian dykes and v) the development of angular uncontormities. A field
mapping project was performed in the Narni-Amelia and Sabini mountain ranges (Centrai
Apennines), where severa! features relateci to Cretaceous normai faulting were identified. In
particular, a polygenic breccia made of clasts of lithologies not younger than the Early Cretaceous,
dispersed in a matrix of Maiolica-type facies (white Calpionellid-rich mudstone), rests through an
erosional surface on a Jurassic horst-block of Calcare Massiccio. ("Mt. Cosce Breccia" - Cipriani
2016). Comparable clastic deposits are embedded in the uppermost part of the Maiolica Fm. in
hangingwall-basin successions, associated with soft-sediment detormation. Lower Cretaceous
pelagites (Marne a Fucoidi Fm. -Aptian-Cenomanian p.p.) are locally seen onlapping the unroofed
Calcare Massiccio Fm. Neptunian dykes made of Maiolica penetrate the PCP-top, condensed
Jurassic successions as well as their pre-rift substrate (Calcare Massiccio Fm.). Impressive
similarities were recognized along the Jurassic western margin (Ballino escarpment) of the Trento
Plateau, albeit at a larger scale. The Trento Plateau is a huge morpho-structural high tormed during
the Early Jurassic extensional phase. Polyphasic extension affected the Ballino paleoescarpment
during the Mesozoic (mainly Early Jurassic, Early and Late Cretaceous), as testified by conspicuous
megaclastic deposits embedded in the basinal succession lying to the west (Lombardy Basin -
Castellarin 1972). Our attention was tocussed on the "Ballino Breccia", a Lower Cretaceous
polygenic breccia characterized by heterometric (up to >20 m across) blocks made of shallow-water
carbonates (Calcari Grigi Group), chert-rich deposits (Lombardy Succession) and horst block-top condensed facies (Venetian Succession). The clasts are associateci with pebbles of Maiolica-type facies (with and without calpionellids), and the
matrix of the ruditic deposit is a nannomicritic mudstone (Maiolica Fm.). The breccia rests
unconformably on severa I different Jurassic units of the Lombardy basin-margin succession as well
as directly on the Calcari Grigi facies, and is sealed by the uppermost part of the Maiolica Fm.
Neptunian dykes made of Maiolica facies are also common. Field-evidence therefore suggests that
a synchronous phase of reactivation of Early Jurassic rift faults took piace in Centrai Apennines as
well as in Southern Alps during the Early Cretaceous, coeval with deposition of the upper part of the
Maiolica Fm. Cretaceous faults locally had a higher angle than Jurassic faults, so the latter were
displaced. Cretaceous extension could cause the locai exhumation of Jurassic structural highs,
which occurred through back-stepping, rejuvenation, or displacement of inherited Early Jurassic
margins
Geological mapping reveals the role of Early Jurassic rift architecture in the dispersal of calciturbidites. New insights from the Central and Northern Apennines
No full textThe role of sea-bottom topography in the dispersal of shallow water-derived calciturbidites across a submarine rift, as determined by the local extensional architecture, is under-investigated, namely with pelagic settings along ancient passive continental margins. A comparison with modern carbonate platform/basin analogues, or with siliciclastic systems, is not always feasible, as ancient carbonate systems were commonly home to anachronistic environments (e.g. the Western Tethyan Mesozoic). Our study focuses on: (a) a reconstruction of the palaeotectonic architecture of the Umbria-Marche Basin in the Jurassic and on (b) how this architecture produced a submarine topography which governed the dispersal of sediment, shed by the neighbouring Lazio-Abruzzo Carbonate Platform, for >40 million years. A geological mapping project was performed in the Apennines of Central Italy, a region which in the Jurassic displayed a pattern of intrabasinal highs (pelagic carbonate platforms) and intervening basins which was exceedingly complex due to the high density, and oddly variable trends, of faults rooted in a shallow detachment layer corresponding to thick Triassic salt. A map pairing the occurrences of these resedimented beds with an updated palaeogeography becomes the natural descriptor of the itineraries followed by sediment gravity flows. This qualitative method represents a companion, or even alternative, approach to the one strictly based on physical stratigraphy, and it greatly improves our knowledge of regional geology and rift-basin analysis. Our case history potentially represents the analogue of hydrocarbon fields both inland and in the offshore. Geological mapping shows that the marginal palaeoescarpments of pelagic carbonate platforms formed obstacles to the gravity flows as sediment load was discharged at their toes. While turbidity flows were locally vigorous enough to climb the escarpments, leaving overbank deposits on the pelagic carbonate platform-tops, a ‘shelter’ effect is evidenced by the resediment-free nature of those basins lying downflow, which were shielded by the highs
Cenozoic multiphase orogenic deformations in Northern Calabria Arc: hints from geological mapping in the Longobucco Basin
No full textConstraining the slip rate of Jurassic rift faults through the drowning history of a carbonate platform
No full text“Classic” field mapping in sedimentary successions, where lithostratigraphy, biostratigraphy and sedimentology meet, still has potential for producing answers to long-standing problems in basin analysis. In the Northern Apennines of Italy, constraints on the slip rates of Early Jurassic rift faults, related to the embryonic separation of Eurasia and Gondwana, are provided by sedimentology and ammonite biostratigraphy, coupled with the field mapping of exposed tracts of the submarine escarpments and of the clinoform slopes which bordered carbonate footwall blocks. Slip rates of 600–1,000 m/Myr are here computed for Tethyan rift faults, producing >1 km of palaeostructural relief in ~2 million years (earliest Sinemurian: Bucklandi p.p. – Semicostatum p.p. zones)
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