A. Correggiari

The late-Holocene Gargano subaqueous delta, Adriatic shelf: Sediment pathways and supply fluctuations

The Gargano subaqueous delta formed on the eastern and southeastern sides of the Gargano promontory, in the western Adriatic. This subaqueous deposit represents the southernmost portion of the late-Holocene highstand systems tract (HST) growing along the western side of the Adriatic as an extensive wedge of deltaic and shallow-marine mud. The late-Holocene HST rests above a regional downlap surface that marks the time of maximum landward shift of the shoreline attained around 5.5 cal. kyr BP, at the end of the late-Pleistocene–Holocene sea-level rise. High-resolution seismic–stratigraphic and tephra correlation indicate the presence of a thin basal unit recording condensed deposition between 5.5 and 3.7 cal. kyr BP over much of the basin. Above this unit, sediment accumulation rates increased to high values (as much as 1.5 cm yr−1) reflecting the stabilisation of relative sea level and the forcing from high frequency climatic or anthropogenic changes affecting river dynamics. The late-Holocene mud wedge, of which the Gargano subaqueous delta is a significant component, reaches up to 35 m in thickness and has a volume of ca 180 km3. The shore-parallel thickness distribution of the mud wedge reflects the dominant oceanographic regime of the basin and the asymmetric location of the mostly western sediment sources (with a combined modern delivery of 51.7×106 t yr−1 of mean suspended load). In sections perpendicular to the coast the late-Holocene mud wedge appears composed of forestepping clinoforms with gently dipping foresets (typically 0.5°). The Gargano subaqueous delta is characterised by a submarine topset in water depths shallower than 25–28 m, and accounts for about 1/7th of the total volume of the late-Holocene mud wedge, despite the absence of direct river supply to the Gargano area. In the area of maximum interaction between shore-parallel currents and basin morphology, progradation occurs onto a flat and barren bedrock outcrop in about 50–80 m water depth. The rapid transition from a thickness of 30 m of late-Holocene mud to nil is a good indication of the role of southward-flowing bottom-hugging shelf currents in causing the redistribution of sediment along the Adriatic inner shelf. Additional evidence of this regime comes from: (1) the most recent sigmoid (defined at seismic–stratigraphic scale) deposited since the onset of the Little Ice Age, showing a shore-parallel thickness distribution and a main depocentre to the southeast of the Gargano promontory; (2) the maximum values of sediment accumulation rates over the last century (documented by 210Pb measurements) defining a narrow shore-parallel belt immediately seaward of the depocentre of the most recent sigmoid. The Gargano subaqueous delta grows from the outbuilding of progressively younger progradational sigmoids that tend to parallel the previous ones. The Gargano subaqueous delta differs from other documented late-Holocene subaqueous deltas because its growth reflects: (1) sediment transport dominated by bottom currents sub-parallel to the strike of the composing clinoforms; (2) a complex supply regime including the Po delta (350 km to the north) and several coalescing Apennine rivers acting as ‘line source’; (3) several alternating intervals of enhanced outbuilding and condensed deposition; and (4) an in-phase growth of the most recent sigmoid with the major progradation of the Po delta during the Little Ice Age.

Quaternary forced regression deposits in the Adriatic basin and the record of composite sea-level cycles

High-resolution seismic and sedimentological data from the Central Adriatic basin reveals a Quaternary shelf-perched wedge that is comprised of four prograding units, the top surfaces of which are truncated by regional erosional surfaces. Internal reflector geometry indicates that each unit developed during highstand to falling sea-level conditions. Falling sea level resulted in successive downward and seaward shifts of the shoreline (forced regression). Because progradation occurs as a depositional continuum from highstand to lowstand conditions, sequence boundaries are difficult to recognize and correlate on a regional scale. In fact, continued sea-level fall produces several downshift surfaces of limited lateral extent. The regional erosional surfaces that truncate and hence bound the prograding units are composite in origin. The erosion surfaces formed during times of shelf subaerial exposure and were modified by shoreface and marine erosion during each subsequent rapid sea-level rise. All of these composite erosional surfaces become conformable seaward of the youngest depositional shoreline break formed at the end of each phase of progradation. These composite sequence-bounding erosional surfaces are draped by mud that is the distal equivalent of the overlying progradational unit deposited following rapid landward shifts of the shoreline. Facies analysis and stratigraphic data in the youngest progradational unit indicate that each of the four progradational units formed in response to fourth-order (100–120 ka) cyclicity. The four progradational units stack to form an aggradational-retrogradational sequence set that records a longer-term relative sea-level rise. Such a trend can reflect regional subsidence and/or a longer-term component of rise in the Quaternary eustatic signal. In the Adriatic basin, the key factors that favour the preservation of the forced regression deposits within the Quaternary shelf-perched wedge are: (1) the composite nature of relative sea-level cycles where a longer-term relative sea-level rise interacts with shorter-term (fourth to fifth-order) sea-level cycles; (2) the asymmetry of the eustatic signal that, reinforced by local subsidence, yields relative rises of large magnitude and short duration; (3) the high amplitude of the higher-frequency fifth-order signal that drives relative sea-level falls of short duration. This kind of composite cyclicity also controlled the formation and preservation of forced regression deposits on other Quaternary continental margins as well as in ancient sedimentary successions where these deposits may occur in backstepping or aggradational multistorey sequence sets.

Late Quaternary transgressive large dunes on the sediment-starved Adriatic shelf

Abstract The Adriatic epicontinental basin is a low-gradient shelf where the late-Quaternary transgressive systems tract (TST) is composed of thin parasequences of backbarrier, shoreface and offshore deposits. The facies and internal architecture of the late-Quaternary TST in the Adriatic epicontinental basin changed consistently from early transgression to late transgression reflecting: (1) fluctuations in the balance between sediment supply and accommodation increase, and (2) a progressive intensification of the oceanographic regime, driven by the transgressive widening of the basin to as much as seven times its lowstand extent. One of the consequences of this trend is that high-energy marine bedforms such as sand ridges and sand waves characterize only areas that were flooded close to the end of the late-Quaternary sea-level rise, when the wind fetch was maximum and bigger waves and stronger storm currents could form. We studied the morphology, sediment composition and sequence-stratigraphical setting of a field of asymmetric bedforms (typically 3 m high and 600 m in wavelength) in 20–24 m water depth offshore the Venice Lagoon in the sediment-starved North Adriatic shelf. The sand that forms these large dunes derived from a drowned transgressive coastal deposit reworked by marine processes. Early cementation took place over most of the dune crests limiting their activity and preventing their destruction. Both the formation and deactivation of this field of sand dunes occurred over a short time interval close to the turn-around point that separates the late-Quaternary sea-level rise and the following highstand and reflect rapid changes in the oceanographic regime of the basin.

Turbidite deposition from multiple sources; Quaternary Paola Basin (eastern Tyrrhenian Sea)

ABSTRACT The western margin of the Calabrian Arc is underlain by a thick (as much as 5 km) post-Miocene sedimentary succession. The uppermost part of this margin sequence is referred to as the Paola Basin System (Quaternary in age) and partially fills a present-day physiographic slope basin. Seismic and high. resolution reflection profiles reveal that a prominent unconformity marks the base of the Paola Basin System (PBS). The unconformity connects upslope to an area of recurrent shelf. edge instability and is overlain by large mass-failure deposits over much of the basin area. Basin-wide sedimentary drape deposits form stratigraphic markers that subdivide the Paola Basin System into three stages, each with unique patterns of slope erosion, including mass wasting, and lower.slope to basin-floo turbidite deposition. Three major sediment pathways elongated perpendicular to the margin developed between the unconformity and the first basin-wide drape. Upsection, sediment input for turbidite sedimentation in Paola Basin has involved many, commonly ephemeral, point sources. Most of the turbidite deposits do not form complete submarine fan sequences but consist of only a few turbidite elements. Changes in the size and type of turbidite elements through time suggest a decrease in the size and/or duration of individual sediment sources in the latest Pleistocene. The Paola Basin System is an example of a complex fill of a tectonically controlled basin where the effects of cyclic eustatic changes in sea level modify the effects of longer-term changes in sedimentation driven by tectonic ctivity in the source region and structural deformation in the depositional area.