Alfonso Bosellini

Subsidence and Sedimentation on Jurassic Passive Continental Margin, Southern Alps, Italy

The southern Alps of Italy preserve a tectonically intact array of Jurassic facies that record the evolution of a part of the margin of the Apulian plate from its ancestral beginnings in a complex of Permian and Triassic rifted continental basins through the initial stages of breakup and stepwise foundering of a carbonate platform. Breakup was accompanied first by the rapid accumulation of thick prisms of carbonate turbidites in newly formed fault troughs. Then, as the new Ligurian oceanic basin began to open farther west and, as subsidence gradually slowed, accumulation of a succession of slowly deposited biogenous pelagic sediments recorded not only the increasing depths of the seafloor but also fluctuations in oceanographic conditions of fertility, carbonate dissolutio levels, and the strength of bottom currents. Estimates of the history of seafloor depths, based on a simple subsidence law of the form Subsidence = K(Age)12/, provide a basis for the construction of a set of curves showing the changing depths of significant carbonate dissolution surfaces during the Jurassic in this region. The rapid 1-km deepening of the compensation depth for calcite during the Late Jurassic may be due to a change in regional oceanic vertical circulation patterns from upwelling (fertile, silica-rich, carbonate dissolving) to downwelling (less fertile, silica-poor, carbonate preserving).

Pelagic limestone and radiolarite of the Tethyan Mesozoic: A genetic model

The major lithofacies and depth of accumulation of Mesozoic pelagic carbonate and siliceous formations are expressed as a function of the history of crustal subsidence and of the depths of various dissolution surfaces. This dissolution-facies model may afford a plausible starting point for unifying and explaining the sedimentological evolution of the Mesozoic pelagic sequences.

A Jurassic “Tongue of the Ocean” infilled with oolitic sands: The Belluno Trough, Venetian Alps, Italy

Abstract The Belluno Trough is a narrow and elongate basin on the northwestern corner of the Apulian Plate continental margin. It acquired topographic identity in Early Liassic time, during the breakup of a widespread Upper Triassic carbonate shelf. At the beginning (Liassic), the Belluno Trough was a starved depression accumulating euxinic mudstone and bounded by two shallow-water carbonate banks (Trento and Friuli Platforms). This typical Bahamian physiography lasted until Late Liassic time, when the shallow-water Trento Platform collapsed tectonically and suddenly drowned, tilting northward and breaking along an old tectonic line; it became therefore a submerged pelagic plateau. In the meantime, the western edge of the Friuli Platform, now the windward margin of an oceanic platform, turned into a prolific, super-producing, oolite “factory”. Quite soon the oolite sands of the Friuli margin started to flow down the slope, reaching the basin through numerous entry points and gradually building a complex of coalescent deep-sea fans, which, prograding westward, infilled the Belluno Trough. The fans terminated westward against the faulted edge of the Trento Plateau, on which above a karst-like surface, locally encrusted with manganese, red nodular ammonitic limestone was slowly accumulating. Northward, where the topographic relief decreased, the oolite fans could spill over the escarpment onto the edge of the plateau. By the end of Middle Jurassic time, the Belluno Trough was no longer a submarine depression but a gentle slope connecting the Friuli Platform to the deep, pelagic Trento Plateau. In Callovian time, probably because of a short drop in sea level, large parts of the Friuli Platform were exposed to subaerial diagenesis. During the subsequent Oxfordian transgression, the resulting rocky bottom became the substratum for an extensive growth of reefal organisms (hydrozoans, corals, rudists), which fringed the Friuli Platform for the whole Malm and Cretaceous times.

STRATIGRAPHIC ARCHITECTURE OF THE SALENTO COAST FROM CAPO D'OTRANTO TO S.MARIA DI LEUCA (APULIA, SOUTHERN ITALY)

The Cretaceous to Quaternary succession of the Apulia Platform cropping out on the eastern coast of the Salento Peninsula shows a special stratigraphic architecture. Whereas on the platform top, i.e. on the Salento Peninsula proper, the succession is at most a few tens of metres thick and is punctuated by unconformities, on the margin and slope of the platform, along the present-day eastern coast of the peninsula, several carbonate systems are laterally disposed and grafted one upon the other. Three of these systems are clinostratified and include well developed reef tracts of Priabonian, early Chattian and early Messinian age. The geologic conclusion of our study is that, since the Late Cretaceous, the eastern coast of the Salento Peninsula grossly coincided with the margin of the Apulia Platform. This paleogeographic element acted as a foreland horst and registered important geodynamic events related to the growth of the adjacent Hellenide and Apennine thrust belts. During the last 60 m.y., the horst carapace was constantly near sea level and sediments were mainly accommodated and preserved on the deep margin and slope of the platform.

East Africa continental margins

New well data from Somalia, together with the history of sea-floor spreading in the Indian Ocean derived from magnetic anomalies, show that the East African margins from (at 15°S into the Gulf of Aden comprise four distinct segments that formed successively by the southward drift of Madagascar from Somalia during the Middle to Late Jurassic and Early Cretaceous, by the northeastward drift of India along the Owen Transform during the Late Cretaceous and Paleocene, and by the opening of the Gulf of Aden during the Neogene.

Deep-water asymmetric cycles and progradation of carbonate platforms governed by high-frequency eustatic oscillations (Triassic of the Dolomites, Italy)

The basinal San Cassiano Formation (Triassic, Dolomites, Italy) is interfingered with clinostratified megabreccia slope deposits of coeval carbonate platforms, and to a large extent is composed of metre-scale thickening, coarsening-upward cycles. These asymmetrical cycles, often representing bundles of five coarsening-upward sequences, are interpreted as platformbasin interactions governed by fourth- and fifth-order eustatic oscillations. According to this model, progradation of Triassic platforms of the Dolomites occurred mainly during fourthorder sea-level lowstands.

The Mesozoic succession of Dire Dawa, Harar Province, Ethiopia

Abstract The Mesozoic succession of Dire Dawa, Harar Province, Ethiopia, consists of a lower fluviatile sandstone (Adigrat Sandstone); an intermediate carbonate-marly unit, formerly called Antalo Limestone; and an upper fluviatile sandstone (Amba Aradam Formation). This study has shown that the intermediate unit consists of four different formations grouped into two depositional sequences. These sequences and their boundaries, Middle-Late Jurassic in age, are well correlated with sequences recognised throughout East Africa and a large part of Yemen. The base of the lower sequence (Antalo Supersequence) is time-transgressive (Pliensbachian to Oxfordian) and is the result of the first flooding of this sector of the Gondwana continent during the Mesozoic. The second major sequence boundary is also time-transgressive and corresponds to an abrupt deepening of East Africa and southern Arabia shallow water ramps and carbonate platforms, a collapse most probably related to the separation of Madagascar from Africa. A major tectonic event occurred in Early Cretaceous from northern Ethiopia to Yemen, and southern Ethiopia and Somalia. This vast uplift, testified by faults and angular uncomformities, was followed by deposition of fluviatile sediments over the entire region.

CARBONATE FACIES ZONATION OF THE UPPER JURASSIC-LOWER CRETACEOUS APULIA PLATFORM MARGIN (GARGANO PROMONTORY, SOUTHERN ITALY)

The Late Jurassic-Early Cretaceous Apulia platform margin and the transition to adjacent basinal deposits (inner platform to basin) are well exposed in the Gargano Promontory. Detailed field work has allowed to recognize eight main facies associations which reflect various depositional environments, and which document a differentiated zonation, from the inner platform to the basin. A shallow lagoon existed in the internal part of the Gargano Promontory with a transition to tidal flat areas (F1). Oolitic shoals (F2) bordered this internal peritidal area passing seaward to a reef-flat with abundant corals (F3). A reef-front, associated with a coral rubble zone, has been found in some areas (F4). In the external margin zone, massive wackestones with Ellipsactinia occur (F5) and pass gradually to a rudstone facies on the proximal slope (F6). The base-of-slope facies association consists of pelagic sediments interbedded with gravity-displaced deposits (F7 and F8). The depositional profile of the Apulia Platform is typical of the Tethyan Jurassic-Early Cretaceous platforms, with slope declivities in the order of 25°-28°. The remarkable progradation of the platform in the northern tract of the Gargano (Lesina and Varano lakes area) and its substantial stability east- and southwards (Mattinata area) suggest a possible windward position of the margin in this latter portion and, in contrast, a leeward position of the northern portion.

Inherited structures in the hangingwall of the Valsugana Overthrust (Southern Alps, Northern Italy)

Abstract In the hangingwall of the Valsugana Line (north side), the most important Neogene overthrust of the Venetian Alps, two major palaeostructural pre-thrusting elements are recognizable, the Atesina Platform to the west and the Carnico-Bellunese Basin to the east. These palaeostructures, which are oblique to the overthrust, are demonstrated by variations in thickness of the sedimentary cover. The present structural framework of the hangingwall of the Valsugana Line, with the crystalline basement more uplifted toward the west, is genetically related to these pre-thrusting Permo-Mesozoic structural elements. The apparent variation in throw along the Valsugana Line is mainly a result of these palaeostructural geometries, and was not, for example, produced by a lateral hangingwall ramp.

Il Calcare di Antalo nella regione di Macallè (Tigrai, Etiopia settentrionale)

The study of the Antalo Limestone in its type-area has shown that the formation is part of a classic depositional sequence, here termed Mekele Sequence, which consists of a number of parasequences (4° or 5° order cycles). These parasequences display systematic vertical changes in thickness and facies through the entire sequence. At the base, during the relative sea-level rise (transgressive systems tract), they form a thickening-upward retrogradational parasequence set, in the middle, during the highstand, they are of similar thickness with the subtidal facies dominating (aggradational parasequence set), and finally in the upper part they present a thinning-upward trend (progradational parasequence set). The Mekele Sequence (Antalo Limestone and Agulà Shale) was deposited on a gently sloping ramp during a second-order sea-level cycle (Oxfordian-Kimmeridgian). The lower boundary, with the Adigrat Sandstone, is diachronous due to ramp geometry, and occurs via a migrating transitional interval (lagoonal, estuarine); the upper boundary, with the Amba Aradam Formation, is the result of a forced regression and coincides with an angular unconformity.RiassuntoNella sua area-tipo il Calcare di Antalo (Oxfordiano-Kimmeridgiano) risulta costituito da gruppi di cicli o parasequenze, retrogradanti nella parte inferiore, aggradanti in quella centrale e progradanti in quella superiore. Questa organizzazione stratigrafico-sequenziale è il risultato di un ciclo trasgressi-vo-regressivo che ha portato alla deposizione di una sequenza deposizionale di secondo ordine, costituita dal Calcare di Antalo e dal sovrastante Agulà Shale. Tale sequenza, qui denominata Sequenza di Macallè, si depositò su di una rampa a debolissima pendenza e può essere considerata l’equivalente cratonico della Sequenza di Uarandab, presente in prossimità del margine continentale est-africano.