INTRODUCTION

The carbonate mound build-up phenomenon, driven by microbial automicrite formation, is recognized to have been a prominent process throughout the Paleozoic. This process was generally believed to be extinct since the end of the Mesozoic. Fossil mounds are important reservoirs of oil and gas in a number of hydrocarbon provinces. In recent years, industrial efforts to explore deeper water environments have yielded increasing evidence of the existence of extended modern mound provinces, rich in deepwater corals, sponges, and other colonial invertebrates, along various continental margins of the world. Ocean Drilling Program (ODP) Leg 182 shed light on environmental controls on bryozoan mounds off South Australia (Feary, Hine, Malone, et al., 1999; James et al., 2000). In the Atlantic, mound provinces have been reported off southwest Brazil, Angola, Mauritania, Rockall Trough, and Norway, but the most intensively studied site is no doubt Porcupine Basin, southwest of Ireland, which in recent years has been the focus of more than 20 cruises, mobilizing the flagships of oceanography.

An overview of Porcupine Basin and the mound provinces is shown on Figure F1. Three different types of mound provinces are identified:

Hovland Mounds

The first mound occurrences reported from industrial data on the northern slope of Porcupine Basin (Hovland et al., 1994) led to the unveiling of a complex setting with large multiphased contourite deposits and high-energy sediment fills, topped by a set of outcropping single mounds or elongated mound clusters up to 250 m high (Henriet et al., 1998; De Mol et al., 2002).

The Hovland mounds are flanked to the north and west by a crescent-shaped, well-delineated province with a very high density of buried medium-sized mounds (1 mound/km²; average height = 60–80 m). We estimate a total of more than 1500 buried mounds (Huvenne et al., 2003). Joint interpretations of high-resolution data (Henriet et al., 2001) and three-dimensional (3-D) industrial data (Huvenne et al., 2002) shed light on the presence of a past slope failure with large angular blocks, for a large part, underlying the mound cluster.

On the eastern margin of Porcupine Basin, a 45 km long range of large mounds towers from a strongly erosive surface (Fig. F2). The mounds partly root on an enigmatic, locally very thick, acoustically rather transparent horizon of unknown age and composition (Henriet et al., 1998; Van Rooij et al., 2003) and partly on a layered sequence capped by a surprising set of short-wavelength, sigmoidal depositional units (De Mol et al., 2002; Van Rooij et al., 2003). The Belgica mounds province consists of 66 conical mounds (single or in elongated clusters) in water depths ranging from 550 to 1025 m.

The mounds are partly enclosed in an impressive set of contourites (Van Rooij et al., 2003). Mounds typically trap sediment on their upslope flank, which is consequently buried, while their seaward side is well exposed and forms a steep step in the bathymetry. Average slopes amount to 10°–15°. The largest mounds have a height of ~170 m.

In the deeper part of the Belgica mound province, an extremely "lively" mound was discovered in 1998 on base of a very diffuse surface acoustic response. This mound, known as Thérèse mound, was selected as a special target site to study processes involved in mound development within the European Union Fifth Framework (EU-F5) research projects GEOMOUND (The Mound Factory: Internal Controls), ECOMOUND (Environmental Controls on Mound Formation along the European Margin), and ACES (Atlantic Coral Ecosystem Study; Foubert et al., in press; De Mol et al., unpubl. data). Video imaging revealed that Thérèse mound, jointly with its closest neighbor Galway mound, might be one of the richest deepwater coral environments in the Porcupine Seabight (PSB), remarkably in the middle of otherwise barren mounds.

Although the original proposal 573-Full "Modern Carbonate Mounds: Porcupine Drilling" identified five prime scientific objectives (see below) that optimally could be addressed by drilling in the three mound provinces of Porcupine Basin, the slim version of the proposal (573-PRL5) addresses four of these major questions by focusing on three sites in the Belgica mound province (Fig. F2, F3). What will not be addressed is the hypothesis that fluid flow may genetically link both slope failures and mound growth. Indeed, the location of Porcupine mound provinces right above spectacular buried slope failures is limited to the Magellan and Hovland provinces. Another aspect of the original proposal that will not be addressed is the reason for the apparent difference in age and development between the Belgica and Hovland mound provinces, the latter already featuring significant hiatuses and hardgrounds in the first few meters. Finally, the reason for the peculiar development of the Magellan mound province, characterized by more than a thousand mounds of medium size (some 60–80 m), which did compete with sedimentation rather than featuring an initial fast growth phase prior to burial, will not be elucidated by drilling during Expedition 307.