Mauro Caffau

Sequence stratigraphic significance of tidal channel systems in a shallow lagoon (Venice, Italy)

The Holocene succession located in the central part of the Venice Lagoon has been investigated by means of high-resolution seismic and core data, which document a full back-barrier depositional environment developed during the last 6 kyr. The studied succession, 4.5–23 m thick, consists of three seismic units bounded at the base by stratal surfaces marked by deep tidal channel incisions, recording main changes in the lagoon hydrodynamics as well as reorganizations of the tidal channel network. The recognized depositional and erosional phases in the lagoon deposits can be linked to both large-scale factors related to the Holocene sea-level rise and local factors in part due to the human impact. In particular, the lower unit is interpreted as the transgressive systems tracts of the Holocene sequence, initially characterized by the accumulation within small estuarine channels incised during the previous phase of subaerial exposure, whereas the upper units are interpreted as the highstand systems tracts, typified by the persistence of the lagoonal environment. The higher stratal surface records a hydrodynamic change related to a local transgression affecting a deltaic area placed just to the south, probably at least in part aided by human interventions. The Holocene succession accumulated in the central part of the Venice Lagoon testifies that the development of stratal surfaces in back-barrier settings, persisting during both transgressive and highstand conditions, may exhibit differences with respect to that predicted by current sequence stratigraphic models, and therefore, it is useful to improve the knowledge of these systems.

Key features of mixed carbonate-siliciclastic shallow-marine systems: the case of the Capo Colonna terrace (southern Italy)

The late Pleistocene Capo Colonna terrace (southern Italy) is composed of a mix of shallow-marine carbonate and siliciclastic sediments. Shoreface sandstones, dominated by a terrigenous component and by sedimentary structures related to waves and currents, pass seaward into shelf coralline algal frameworks and associated calcarenites. Siliciclastic sandstones and rigid bioconstructions are locally juxtaposed. Main features of the studied system include ( i ) a bipartition between mixed siliciclastic-bioclastic sedimentation in proximal settings and coralline algal growth in distal settings, ( ii ) an analogy between the stratal architecture of the clastic-dominated part of the system and that of siliciclastic shelf/ramp systems, and ( iii ) the control on the gradient of the shoreface-shelf by inherited physiography and transgressive processes rather than the carbonate productivity.

Anatomy of a submerged archipelago in the Sicilian Channel (central Mediterranean Sea)

The Sicilian Channel is a broad and shallow shelf which is geologically part of the African Plate. Its NW sector (the Adventure Plateau), where water depths rarely exceed 100 m, is punctuated by several kilometre-sized morphological highs. These elevations, formed by both sedimentary and volcanic rocks, emerged around middle Holocene time or earlier when they constituted a large archipelago. High-resolution single-channel and multichannel seismic reflection profiles, along with stratigraphic and lithological information derived from exploration wells and rock samplings, are analysed to derive the shallow and deep structural setting of these banks and identify their geological nature. The sedimentary banks (Talbot, Ante-Talbot, Panope, Nereo and Pantelleria Vecchia), presently located at water depths 8–40 m, are composed of Miocene rocks severely deformed by a late Miocene compressional phase which produced the external sector of the Sicilian–Maghrebian thrust belt. Tortonian-aged rock samples from the Pantelleria Vecchia Bank represent patch reefs that have mostly formed on structural highs. Sedimentary analogies suggest that other sedimentary banks of the Adventure Plateau may have the same origin. Galatea, Anfitrite and Tetide represent submarine volcanic edifices emplaced on major extensional faults formed during early Pliocene – Quaternary continental rifting of the Sicilian Channel. The present-day morphology of the banks is the result of repeated phases of subaerial exposure and drowning, especially since the Last Glacial Maximum.

The vertical compartmentalization of reservoirs: An example from an outcrop analog, Crotone Basin, southern Italy

The lower Pliocene shallow-marine successions deposited within half-graben subbasins exposed in the northern Crotone Basin (southern Italy) are good examples illustrating the variable development of small-scale cycles (2–15 m [6.6–49 ft] thick) within the hanging-wall blocks of normal growth faults. Shallow-marine small-scale cycles forming the successions deposited near the immediate hanging wall of half-graben subbasins show a clear alternation between well-cemented shell-rich intervals and weakly cemented siliciclastic sandstones. In contrast, the shallow-marine successions deposited in hanging-wall dip-slope locations and in the footwalls show a poorly preserved cyclicity related to lower subsidence rates and a higher degree of cycle amalgamation. The vertical alternation between well-cemented carbonate-rich intervals and less cemented sandstones creates a marked heterogeneity in the permeability of these shallow-marine half-graben fills, which are sealed by impermeable offshore and lagoonal mudstones and may be considered as an outcrop analog of potential reservoirs. As a consequence of the well-developed cyclicity, therefore, the potential reservoir is vertically compartmentalized. This feature tends to vanish toward the hanging-wall dip slopes, where the cyclicity is remarkably less appreciable. The correct prediction of geometry and effective volume of reservoirs and, in particular, the recognition of their vertical compartmentalization have an important impact on hydrocarbon exploration and production. The developed conceptual model of compartmentalized half-graben fill may be useful in several contexts to study fault-bounded basins from a reservoir point of view.

The 20,000-years-long sedimentary record of the Lesina coastal system (southern Italy): From alluvial, to tidal, to wave process regime change

The present-day Lesina area (Adriatic coast of southern Italy) preserves in the subsurface the stratigraphic signature of a recent sedimentary process regime change, which was responsible for the conversion of a former alluvial plain into a back-barrier tidal flat and, finally, into the modern barrier island. Facies-based analyses of the first 55 m of the upper Pleistocene–Holocene stratigraphic record, integrated with biostratigraphic sampling, radiocarbon data, and aerial observations of some diagnostic relict morphologies, allowed us to reconstruct the history of the last 20,000 years of this area. The succession investigated is adjacent to a salt dome, which uplifted in recent times, forming the easternmost boundary of the present-day Lesina lagoon. Three main stratigraphic intervals were detected in the subsurface: the lowermost unit is made up of conglomerates, sandstones, and mudstones of terrestrial origin, belonging to a complex system of alluvial plain filling a pre-existing Last Glacial Maximum (LGM) topography. The second interval consists of sands and muds, with subordinate conglomerates of brackish and marine origin, and lies on the previous one through a wide ravinement. Its composing lithofacies exhibit a strong tidal signature preserved in tidal rhythmites belonging to a net of tidal channels, associated with marshes, mud flat, and lagoonal deposits. These sediments record the emplacement of a back-barrier tidal flat, which developed under the strong influence of a tidal influx enhanced by the late post-LGM transgression. The third uppermost interval resulted from the deposition of coastal-marine sands and gravels accumulated during the ensuing modern normal regression, under the dominance of a wave-dominated coastal dynamics, which was responsible for the progradation of the present-day beach barrier and the closure of the Lesina lagoon. The paleogeography of the back-barrier tidal flat preceding the onset of the modern barrier island is thus reconstructed based on the results of our facies analysis, biostratigraphy, and AMS dating. Many of the elements composing this mid-Holocene tide-influenced system were also interpreted from the aerial-photograph observation of several relict morphologies, which are still preserved in many parts of the modern Lesina barrier island. We propose some new interpretation on the origin some of these elements, which possibly developed under sedimentary process regimes different from the modern ones, including some flood-tidal deltas, previously interpreted as tsunami-derived washover fans.