Elsevier

Marine Geology

Volume 213, Issues 1–4, 15 December 2004, Pages 73-89
Marine Geology

Analysis of Holocene sedimentary features on the Adriatic shelf from 3D very high resolution seismic data (Triad survey)

https://doi.org/10.1016/j.margeo.2004.10.002Get rights and content

Abstract

Very High Resolution (VHR) 3D seismic data were collected on the Adriatic shelf offshore the city of Ortona in order to define the small-scale geometric complexity of late Holocene deposits. Three groups of units are distinguished from the bottom up: (1) units U1 to U3 forming the Transgressive System Tract (TST); (2) units U4 and U5 representing the condensed interval deposited on the Maximum flooding surface (MFS) and the basal unit of the High stand System Tract (HST); and (3) U6 to U10 forming the progradational units of the HST. The muddy HST deposits show seafloor and subsurface sedimentary features over up to 40% of its extent on the Adriatic shelf. In the area offshore Ortona, the HST shows shore-parallel undulations in water depth of ca. 30–70 m and mud reliefs in water depth of ca. 70–110 m. The mud reliefs are subcircular in plan view and occur in swarms, perpendicular to the regional contour, that are connected to each other by shore-parallel junctions. The genesis of these features is interpreted as a combination of two different mechanisms: (1) sediment deformation in relation with the condensed section; and (2) depositional process related to shore-parallel currents interacting with topographic irregularities. The evolution of the HST sedimentary pattern is closely related to a “key unit” (U5) that influences both the seaward transition from linear to complex features and the stacking pattern.

Introduction

On the Adriatic shelf (Fig. 1), the late Holocene mud prism is affected by complex irregularities in the form of linear-crested seafloor undulations and near-circular mud reliefs over large extent of the seafloor (Trincardi et al., 2000, Correggiari et al., 2001) and by the presence of gas at different stratigraphic levels (Hovland and Curzi, 1989, Conti et al., 2002). The interpretation of the origin of such complex features proved difficult using conventional 2D seismic data alone as observed for other settings worldwide (Gardner et al., 1999, Faugères et al., 2002, Lee et al., 2002). For this purpose, a small area was selected for a Very High Resolution (VHR) 3D seismic survey (Marsset et al., 2003a). The survey was conducted in an area covering a surface of 800×3600 m at a water depth of 50–70 m (Fig. 1) where swath bathymetry and detailed stratigraphic data from piston cores are available (Cattaneo et al., 2003a).

The purpose of this paper is to provide a detailed description both of the geometry and the thickness distribution of the late Holocene mud wedge to tentatively explain the origin of undulations and reliefs that have previously been ascribed either to bottom-current deposition or to sediment deformation.

Section snippets

Background

The shallow stratigraphic architecture of the west Adriatic margin is composed of different sedimentological units related to late Quaternary sea level changes (Fig. 2a; Cattaneo and Trincardi, 1999, Trincardi and Correggiari, 2000):

  • The Prograding Pleistocene wedges are truncated at their top by a regional erosional surface (ES1) that records a regressive phase encompassing most of the last glacial cycle, from marine oxygen isotope stage 5e to the base of the last glacial maximum (Trincardi and

Methods

In this paper, we discuss the interpretation of the VHR 3D seismic volume and compare it to selected sections of conventional sub-bottom profiler (Chirp). The geochronological control on the identified seismic units is assumed from previous work in the area (Asioli et al., 2003, Calanchi et al., 1998, Cattaneo and Trincardi, 1999, Trincardi and Correggiari, 2000, Oldfield et al., 2003).

The 3D acquisition system developed by IFREMER (Marsset, 2001, Marsset et al., 2002) is capable of being

Results

The different seismic units are located on the inner continental shelf above the uppermost regional erosional surface (ES1) capping the Pleistocene prograding wedges (Fig. 2).

Discussion

The complex geometries observed within the late Holocene mud wedge offshore Ortona can be ascribed to two different mechanisms: (1) sediment deformation accompanied by fluid-escape (Correggiari et al., 2001); and/or (2) sediment transport and differential deposition by bottom currents.

Conclusion

The 3D VHR seismic method used during the TRIAD cruise on the Adriatic shelf allowed to study in detail the sedimentary pattern of a complex area made of two classes of seafloor and subsurface features that are elongated in directions perpendicular to each other. These results confirm that water bottom and its subsurface are by nature three-dimensional structures and should be treated as such. The use of 2D sections and extrapolation between them may be misleading in some cases. Nevertheless,

Acknowledgements

Our results were obtained as part of the Continental Slope Stability project (COSTA) funded by the European Commission under Contract EVK3-CT-1999-00006 within the 5th framework programme. This work is part of IFREMER and ISMAR CNR-Bologna (no. 1403) contributions to the EU project COSTA. Data for this paper were collected during the TRIAD cruise onboard the R/V L'Europe. The authors wish to thank F. Marillier and N. Wardell for their review and comments, which greatly improved the manuscript.

Cited by (0)

View full text