Recognition of a drowned delta in the northern Adriatic Sea,Italy: Stratigraphic characteristics and its significance in the frame of the early Holocene sea-level rise

Abstract

Seismic and core data off the Tagliamento river delta, northern Adriatic Sea, Italy, have allowed the recognition of a partially eroded sand ridge at 12 to 18 m water depth, which has been interpreted as a submerged wave-dominated delta with associated beach-ridge complexes. The ridge shows local channel fill features, interpreted as the result of the filling of tidal channels connecting the open sea with bays and lagoons placed adjacent to the ancient delta. This interpretation resembles the present day setting of the Tagliamento delta, located landwards. It is inferred that the recognized delta was drowned during the latest stage of the post-glacial sea-level rise, when the shoreline shifted near the present day position. New studies are needed to recognize other submerged deltas and barrier islands, which may allow the reconstruction of the post-glacial sea-level rise in the Mediterranean and of the position of ancient paleoshorelines.

Keywords

drowned delta northern Adriatic Sea post-LGM transgression sand ridge sea-level rise Trezza Grande

Introduction

The way by which continental shelves were transgressed during the high-amplitude (ca. 120 m) glacio-eustatic rise that followed the last glacial maximum (LGM, 18–20 cal. ka BP) still is not completely understood, especially for semi-enclosed basins such as the Mediterranean. On the basis of oceanic evidence, some authors highlighted that the post-LGM phase was punctuated by fast episodes of sea-level rise called melt-water pulses (MWPs), characterized by peaks that reached 60 mm/a (Carter et al., 1986; Fairbanks, 1989; Liu and Milliman, 2004). In contrast, marked steps in the rate of sea-level rise are not very apparent in reconstructed paleo sea-level curves for the Mediterranean area (Antonioli et al., 2007; Lambeck et al., 2004; Lambeck and Purcell, 2005), and their existence is therefore uncertain. As a result of the different kinds of sea-level rise, two main models for transgressive contexts were developed: the ‘continuous retreat’ (Swift et al., 1991) that refers to a relatively constant landward shift of the shoreline, and the ‘in-place drowning’ (Rampino and Sanders, 1980; Sanders and Kumar, 1975), which assumes the oversteppening of barrier islands if sea-level rise reaches a rate so high that the barrier itself is drowned and the shoreline rapidly shifts landwards.

It is therefore evident that the recognition of preserved features along shelves transgressed after the LGM, such as drowned barrier islands, deltas, and coastal cliffs, is crucial to reconstruct the post-LGM glacio-eustatic history. The aim of the present study is to assess the origin of one of such features found in the northernmost part of the Adriatic Sea, northern Italy (Figure 1), by means of CHIRP subbottom profiles (SBPs) and core data. This feature consists of a partially cemented shelf sand ridge, historically referred to as the ‘Trezza Grande’ (Gordini et al., 2002), developed off the modern Tagliamento river delta (Figures 1 and 2). Our objective is therefore twofold: (1) to define the formation and the evolution of the sand ridge following the interpretation of facies and stratigraphic architecture, and (2) to evaluate its significance in the frame of the post-LGM glacio-eustatic rise.

The present results may therefore be useful in reconstructing the most recent part of the post-LGM sea-level history of the Mediterranean Sea, integrating previous data that documented drowned barrier islands (Maselli et al., 2011; Storms et al., 2008) and terraces (Massari and Chiocci, 2006; Zecchin et al., 2011b) along shelves and shelf margins.

Geological setting

The study area is located in the northernmost part of the Adriatic Sea, that is a portion of a foreland region located between the NE-verging northern Apennines, the SSE-verging eastern South-Alpine mountain chain, and the SW-verging Dinarides (Figure 1). The northern Adriatic is an epicontinental sea having a very low south-sloping gradient and is recognizable for ca. 300 km down to the 120-m isobath (Storms et al., 2008). Much greater gradients and depths characterize the middle and southern parts of the Adriatic Sea, which is ca. 1200 m deep toward the Strait of Otranto. The northern Adriatic was completely exposed and dissected by a river network during the LGM, and a lowstand delta developed in the Meso-Adriatic Depression (MAD), which is 260 m deep (Trincardi and Correggiari, 2000).

A transgressive–regressive cyclicity, highlighted by the vertical alternation between shallow-marine and continental deposits, characterizes the late Quaternary succession in the area between the Romagna and the Friuli plains (Amorosi and Colalongo, 2005; Massari et al., 2004; Pini et al., 2009). In particular, the last glacial phase is documented in this wide region by several tens of meters of continental deposits, mostly represented by fluvial channel fills and floodplain sediments (e.g. Amorosi and Colalongo, 2005; Fontana et al., 2008; Tosi et al., 2007a, 2007b; Zecchin et al., 2011a). The top of the Pleistocene alluvial deposits is generally marked by a paleosoil developed during and slightly after the LGM, and locally by small incised valleys later filled with early Holocene estuarine sediments (Fontana et al., 2008; Tosi et al., 2007a, 2008b; Trincardi et al., 1994; Zecchin et al., 2009, 2014).

The northern Adriatic was inundated during the post-LGM transgression, and the location of ancient shorelines is marked by drowned barrier islands inferred to enclose paleo-lagoons (Storms et al., 2008), similar to those characterizing the present day coastline, like that of Venice (Zecchin et al., 2008, 2009).

Methods

This study is based on three CHIRP SBPs acquired across the northernmost Adriatic Sea, and on four cores (S19, GT2, GT3, and GT4; Figures 1 and 2). A further core (S20, Figure 1), analyzed by Marocco (1988), is considered for completeness. Seismic facies, stratal reflection terminations, and stratigraphic configurations have been used to identify and characterize seismic units. The reconstruction of depositional environments has been accomplished by integrating analysis of seismic and core data.

The CHIRP profiles have been acquired with the M/N Castorino 2 boat, using Subbottom Profiler CHIRP CAP-6600 system by Datasonic. The used tow vehicle is TTV-190. This configuration allows us to acquire on two different frequency bands, respectively 2–7 kHz and 8–23 kHz. The receiver consists of a linear array of eight hydrophones with bands ranging between 2 and 100 kHz. The tow vehicle TTV-190 has been placed at the stern of the boat, at 3 m water depth and at a distance of 12 m. The software used for the acquisition is Datasonics Dual Frequency CHIRP 2 by Benthos. The planimetric position of the boat has been obtained by using a DGPS cinematic system (receiver Ashtech G12/DGPS LandStar MkIV by Thales Tract Ltd). The precision of the position has reached 28 cm. The position of the Towfish has been defined by the same software using the layback (i.e. the GPS-Towfish antenna). The penetration varied between 10 and 20 m, as it is significantly influenced by lithology and presence of gas. The resolution ranged between 20 and 30 cm.

The cores GT2, GT3, and GT4 have been drilled with the vibrocoring AIMES-McLEAN of the oceanographic ship Bannock (C.N.R.), in the frame of two degree thesis (Frangipane, 1984; Gordini, 2001). The core S19 is continuously cored and has been drilled with a percussion system. The cores have been sectioned and opened, and then described and sampled. The lithostratigraphic features, the contacts between units and their thickness, the sedimentary structures, the color, and the macrofauna and organic contents have been described. The sediment samples have been treated with H₂O₂ at low concentration, washed through a 63 µm mesh, and then analyzed under the microscope to study the microfauna (foraminifers and ostracods). The ¹⁴C datings have been performed by the Laboratoire d’Hydrologie et de Geochimie Isotopique of the Parigi University.

Seismic stratigraphy

Two seismic units (Units 1 and 2), separated by a key stratal surface (S1), were recognized in the three CHIRP profiles (Figures 3 –5).

Unit 1

Unit 1 is characterized by high-amplitude, irregular reflectors that appear horizontal to slightly southward inclined (up to 0.1°) and show a discrete lateral continuity (Figures 3 –5). The irregularity of the reflectors is in part due to their dissection by small channel-like features up to 2 m deep and 200 m wide (Figures 3 –5). Abundant columnar opaque areas locally obscure the signal (Figures 3 –5).

The surface S1

The surface S1 truncates Unit 1, and appears as a marked, locally irregular high-amplitude reflector that in places is sub-parallel to the reflectors of Unit 1 (Figures 3 –5). The depth of S1 ranges between 23 m in the seaward part of the SBPs to 12 m below present sea level (Figures 3 –5). S1 is inclined ca. 0.05° in average toward the south, although its inclination ranges between horizontal to 0.15° (Figures 3 –5). The surface forms a step in SBP 1 just at the landward boundary of the ‘Trezza Grande’, where its inclination is maximum near its intersection with the seabed, and becomes nearly horizontal in the landward part of the profile (Figures 3 and 4).

Unit 2

Unit 2 overlies S1, and is composed of two parts, up to 7 m thick, characterized by features that vary laterally (Figures 3 –5). The landward part shows a wedge shape and is composed of both high- and low-amplitude reflectors that downlap S1 (Figures 3 –5). This is well visible in the landward part of SBPs 1 and 2, where the reflectors are up to 0.3° inclined (Figures 3 and 4). The seaward part of Unit 2 is detached from the landward component, and appears as a ridge (the so-called ‘Trezza Grande’) 20 km long and 10 km wide, parallel to the present day shoreline and elevated between ca. 18 and 12 m below present sea level (Figures 1 and 3 –5). The inner edge of the ridge is placed ca. 8 km from the shoreline (Figure 2). SBPs 1 to 3 show irregular, channel-like features up to 500 m wide within the ridge, although gently inclined reflectors downlapping S1 are evident in SBP 1 (Figures 3 –5). The seabed corresponding to the shallower part of the ridge is characterized by a highly irregular profile (Figures 3 –5). Prominent triangular-shaped features elevated ca. 2.5 m from the adjacent seabed characterize the seaward side of the ridge in SBP 3 (Figure 5). As observed in Unit 1, also in this case the signal is locally obscured by columnar opaque areas (Figures 3 –5).

Core data

The analysis of cores that intercept the recognized seismic units (cores S19, GT2, GT3, and GT4, Figures 2 and 6) has allowed us to classify three main facies associations (A, B, and C).

Facies association A

Facies association A characterizes the lower part of cores S19, GT4, and GT3 (Figures 2 and 6), and consists of gray and ochraceous laminated mudstone intervals up to 4 m thick alternated with peat beds and m-scale, fine-grained siliciclastic sandstone intervals showing sharp lower contacts. Vegetation remnants, burrow and rooth traces, and gastropods and fresh-water ostracods are common. ¹⁴C datings of the uppermost peat beds reveal a calibrated age that usually exceeds 20 cal. ka BP (Gordini et al., 2002; Figure 6).

Facies association B

Facies association B is recognizable in the middle part of core S19, in the lower part of core GT2, and in the upper part of cores GT3 and GT4 (Figures 2 and 6). This facies association, that is up to 7 m thick at the Tagliamento delta (core S19) and 3.3 m thick at ‘Trezza Grande’ (core GT2), consists of planar-laminated gray siltstones and claystones alternated with silty sandstones sharply overlying the sediments of facies association A (Figure 6). The bioturbation is locally common. The macrofauna consists of abundant marine bivalves (Abra nitida, Abra pellucida, Mysella bidentata, Corbula gibba, Tellina distorta). A 15-cm-thick bioclastic layer is found at the top of core GT3 (Figure 6). Shallow-water foraminifera and vegetation remnants are common (Figure 6). The sharp boundary that separates facies associations A and B seems to be characterized by a step just landward of the ‘Trezza Grande’ ridge, as shown by the surface S1 in SBP 1 (Figures 2 and 3).

Facies association C

Facies association C is found in the upper part of cores S19 and GT2 (Figures 2 and 6), and consists of fine- to medium-grained siliciclastic sandstone that alternates with mudstone laminae in the lower part and becomes massive toward the upper part. The thickness reaches 10 m at the Tagliamento delta (core S19) but is only ca. 2 m at ‘Trezza Grande’ (core GT2; Figure 6). The macrofauna consists of abundant shallow-marine bivalves (Chamelea gallina, Spisula subtruncata, Thracia papyracea, Loripes lacteus, Divaricella divaricata) and coastal to lagoonal bivalves (Ostrea edulis and Mitilidae). Lagoonal to fresh-water ostracods are also common (Candona neglecta). At ‘Trezza Grande’, the uppermost part is commonly characterized by seagrass remnants and associated mollusks (Figure 6), and shows abundant carbonate concretions (Gordini et al., 2002). Vegetation remnants are found in the upper part of the S19 core (Figure 6).

Interpretation of seismic and core data

The combined seismic and core data allow us to interpret sedimentary processes and depositional environments associated with Units 1 and 2 and with the surface S1. Unit 1 corresponds to facies association A recognized in cores, which documents a continental environment (Figures 2 and 6), in particular an alluvial plain crossed by small rivers and crevasse channels, as already known for the northern Adriatic area during the last glacial period (Ferretti et al., 1986; Tosi et al., 2007b, 2007a; Zecchin et al., 2008, 2011a). The sharp-based sandy intervals present in cores (Figure 6) are interpreted as channel fills and crevasse deposits (Amorosi, 2006; Zecchin et al., 2011a). The columnar opaque areas, locally obscuring both Units 1 and 2, are interpreted as gas seeps (Figures 3 –5), which are very abundant in the late Pleistocene continental deposits of the region (Tosi et al., 2009; Zecchin et al., 2008, 2009).

Facies associations B and C both represent marine deposits corresponding to Unit 2, and therefore the sharp boundary separating facies associations A and B is equivalent to the surface S1 (Figures 2 and 3). S1 may therefore be interpreted as a wave ravinement surface (WRS, Figure 2), truncating the underlying continental deposits by wave erosion during the post-LGM marine transgression (Demarest and Kraft, 1987; Nummedal and Swift, 1987; Swift, 1968), although a coarse-grained transgressive lag reworked from the substrate or condensed shell beds (Zecchin and Catuneanu, 2013) is not present. However, lags are discontinuous laterally, and they may locally be absent above WRSs (Zecchin et al., 2011c). The step exhibited by S1 just landward of the ‘Trezza Grande’ (Figures 2 and 3) may be the result of changes in wave erosion effectiveness shaping the local shoreface profile, possibly due to a temporary stand of the paleoshoreline during sea-level rise, or of the inherited topography (e.g. Nordfjord et al., 2009; Zecchin et al., 2011b).

The fauna contained indicates that facies association B, corresponding to the fine-grained lower part of Unit 2, accumulated in a marine environment. However, the occurrence of Corbula gibba and Tellina distorta, which colonize unstable substrates (Marocco, 1988), suggests a deltaic setting, and in particular a prodeltaic sedimentary environment (Amorosi et al., 1999, 2003; Cattaneo et al., 2007; Reading and Collinson, 1996; Figures 2 and 6). These deposits form the distal part of the Tagliamento delta and of the ‘Trezza Grande’ ridge (Figure 2). Delta progradation is also indicated by the clinoforms downlapping S1 recognized in the SBPs 1 and 2 (Figures 3 and 4).

The grain size and the fauna contained in facies association C resemble those of shoreface environments, like those found seaward of the spits and barrier islands enclosing the modern Marano and Grado lagoons, whereas the uppermost part of the S19 core corresponds to beachface and backshore deposits (e.g. Clifton, 2006). However, the occurrence of fresh-water ostracods, together with the relation between facies associations C and B, still indicates a shallowing-upward deltaic setting and in particular the accumulation in a wave-dominated delta front (Bhattacharya and Walker, 1991; Galloway, 1975; Weise, 1980; Figures 2 and 6). A wave-dominated deltaic context is also suggested by the elongation of the ‘Trezza Grande’ ridge, resembling the morphology of the Tagliamento delta, which is composed of beach-ridge complexes and spits enclosing lagoons (e.g. Psuty, 1967; Figure 1). Lagoon and bayhead delta facies are documented by the S20 core (Marocco, 1988), which is located landwards and is not considered in this study (Figures 1 and 2). A case study similar to the present one is that of the Po delta system (Amorosi et al., 1999, 2003; Stefani and Vincenzi, 2005). In contrast, a more typical shoreface environment is inferred for the deposits overlying S1 and placed near the linear coast of the S Andrea littoral (northernmost part of SBP 3, Figures 1 and 5).

The ‘Trezza Grande’ ridge is therefore interpreted as a drowned and partially truncated delta (Figures 2 and 6), which was transformed into a shelf shoal or sand ridge colonized by seagrass and locally subjected to superficial early cementation as indicated by the carbonate concretions (Gordini et al., 2002). The triangular-shaped features recognized in SBP 3 (Figure 5) indicate local reworking of the sand into dunes that migrate on the seaward side of the ridge. The channel-like features found within the ridge in the SBPs may be interpreted as old channels conveying tidal flows seaward from ancient inlets that crossed the beach-ridge complexes of the delta and connected the open sea with bays and lagoons, as observed close to the inlets dissecting the modern coast.

Discussion

Shelf shoals or sand ridges are common features along continental margins, and they usually result from the drowning of coastal wedges and barrier islands (Suter, 2006). A noticeable variability ranging from the substantial preservation of the original architecture of the precursor sedimentary body to the complete reworking by waves and currents can be exhibited by shelf sand ridges (Huthnance, 1982; Penland et al., 1988; Snedden and Dalrymple, 1999). Well known examples are those of the Atlantic margin of North America (e.g. Goff et al., 1999; Snedden et al., 2011; Stubblefield et al., 1984).

In the present case study, an ancient wave-dominated delta was drowned and partially preserved in the northernmost part of the Adriatic shelf (Figures 2 –5). The occurrence of early cementation, together with the presence of internal channelized features interpreted as tidal channels, indicate that most of the ridge is not an evolving feature controlled by present day currents, like the mid-shelf sand ridge of the New Jersey continental shelf (Snedden et al., 2011), but it preserves the architecture of the original deltaic system. Only the large dunes found on the seaward side of the ridge indicate local reworking (Figure 5).

The drowned delta, therefore, developed when sea level was lower than that of the present day, and the shoreline was placed seaward with respect to the modern one (Figure 7a and b). Since some authors highlighted that the glacio-eustatic rise that followed the LGM was typified by accelerations up to 60 mm/a (called MWPs) and by marked decelerations (Carter et al., 1986; Fairbanks, 1989; Liu and Milliman, 2004; Figure 8), it is likely that the drowned delta prograded during a phase of slow relative sea-level rise, when sediment supply temporarily outpaced the rate of creation of accommodation (Figure 7b).

The occurrence of significantly variable rates in the post-LGM glacio-eustatic rise in the Mediterranean is not unambiguous, as these steps are not so evident in some sea-level curves (e.g. Antonioli et al., 2007; Lambeck et al., 2004; Figure 8). However, clear evidence of post-LGM stepped relative sea-level changes was provided by drowned barrier islands in the northern and middle Adriatic Sea (Maselli et al., 2011; Storms et al., 2008), and by submerged paleo-coastal cliffs along the Ionian coast of Calabria, southern Italy (Zecchin et al., 2011b). The ‘in-place drowning’ mechanism (Rampino and Sanders, 1980; Sanders and Kumar, 1975) in low-gradient settings and the ‘cliff overstep’ mechanism (Zecchin et al., 2011b) in high-gradient settings both referred to stepped transgressions, and were invoked to justify the oversteppening, drowning, and preservation of late Pleistocene to early Holocene barrier islands and coastal cliffs along continental margins. Following these models, the rate of the post-LGM glacio-eustatic rise during documented relatively brief episodes was too high in order that barrier islands keep equilibrium and the deltaic systems continue to prograde, so that they underwent rapid transgression (Figure 7c).

In the present case, taking into account the depth of the ‘Trezza Grande’ ridge, the modest subsidence rate of the area (ca. 0.3 mm/a; Lambeck et al., 2004), the available sea-level curves (Figure 8), and that the progradation of the modern Tagliamento delta initiated after ca. 5–6 cal. ka BP (Cattaneo et al., 2007), the drowning of the ancient delta likely occurred between 10 and 6 cal. ka BP. Following the sea-level curve by Liu et al. (2004; Figure 8), the delta might have developed during the inferred phase of relatively slow eustatic rise between ca. 9 and 8 cal. ka BP, when sea level is thought to have risen from ca. 15 to 10 m below present sea level, and then drowned during the brief period of rapid sea-level rise called MWP 1d. However, the lack of precise datings prevents a robust correlation with these events. Following the sea-level curve by Antonioli et al. (2007; Figure 8), the delta was probably drowned before 7 cal. ka BP, when the rate of sea-level rise was higher than that characterizing the following phase. During drowning, the submerged delta was partially truncated by renewed ravinement erosion, and the shoreline shifted near the present day position (Figure 7c).

Alternatively, the drowning of the delta might have been unrelated to sea-level rise but to autocyclic delta abandonment during constant rate of eustatic rise. However, the lacking evidence of adjacent deltaic systems related to a paleo-Tagliamento river and which developed after delta switching, together with the observed depth of the sand ridge and its position in front of the modern delta (Figure 1), indicates that a drowning mechanism related to eustatic sea-level rise is likely (e.g. Boyd et al., 1989).

The step shown by S1 just landward of the ‘Trezza Grande’ ridge in SBP 1 (Figure 3) highlights a change in the dip of the basal WRS, that is expected to be higher close to the paleoshoreline adjacent to the older delta, whereas the surface becomes flatter further landwards, reflecting the inferred deepening that led to delta drowning (Figure 7c). The drowned delta possibly correlates with a field of sand ridges found in 20 to 24 m water depth off the Venice lagoon, which were interpreted as the result of reworking of a coastal wedge (Trincardi et al., 1994).

In addition to the Adriatic area (see Maselli et al., 2011; Storms et al., 2008), evidence of coastal features drowned during the post-LGM glacio-eustatic rise was also found worldwide, such as in Florida and Gulf of Mexico (Gardner et al., 2007), along the Rhone delta (Berné et al., 2007), the Indian shelf (Wagle et al., 1994), the Great Barrier Reef and the South Island, New Zealand (Carter et al., 1986), the Red Sea (Bailey et al., 2007), the Japan Sea (Korotkii, 1985), and the South Africa shelf (Green et al., 2014; Salzmann et al., 2013). These examples have documented paleoshorelines at similar water depths close to −100 and −60 m, which have been correlated to conditions immediately preceding MWPs 1a and 1b, respectively between 14.6 and 13.5 cal. ka BP, and between 11.6 and 11 cal. ka BP (Green et al., 2014). In contrast, the more recent MWPs 1c and 1d (Liu et al., 2004; Figure 8) are much less documented and need further studies.

In the study area, the most recent phase was characterized by the progradation of the present day Tagliamento delta (i.e. the landward part of Unit 2, Figures 3, 4, and 7d), initiated after ca. 5–6 cal. ka BP, the prodelta sediments of which were swept and resuspended by currents and waves (Cattaneo et al., 2007).

Conclusion

Integrated CHIRP and core data have allowed the recognition of the architecture and origin of a sand ridge, referred to as the ‘Trezza Grande’, located in the northern Adriatic Sea, off the modern Tagliamento delta. This deposit was originated by deltaic progradation, and then was transgressed and transformed into a shelf sand ridge that preserves most of its original architecture. The most recent phase of the post-LGM glacio-eustatic rise, either stepped or more gradual, was probably responsible for delta drowning. Current data prevent to conclusively discriminate between a stepped or gradual mechanism; however previous studies on several continental margins have highlighted that brief phases of rapid sea-level rise (i.e. MWPs) may justify the ‘in-place drowning’ of coastal features.

Further research is needed to recognize the characteristics of the post-LGM sea-level rise and the occurrence of marked variations in its rate, which are thought to be associated with diagnostic features found along the continental margins, such as drowned deltas, barrier islands, and coastal cliffs.

Footnotes

Acknowledgements

The authors thank Professor Ruggero Marocco for making available the core data and for his suggestions in the sedimentological study, Professor Nevio Pugliese and Dr Romana Melis for the biostratigraphic analysis and for the environmental interpretation, Dr Luigi Tosi for his helpful comments, and the colleagues of the National Institute of Oceanography and Experimental Geophysics (OGS) for having participated in the acquisition of the CHIRP data.

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

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