научная статья по теме GEOMETRY AND SEQUENCE STRATIGRAPHY OF THE LATE EOCENE–EARLY OLIGOCENE SHELF AND BASIN FLOOR TO SLOPE TURBIDITE SYSTEMS, LEMNOS ISLAND, NE GREECE Геология

Текст научной статьи на тему «GEOMETRY AND SEQUENCE STRATIGRAPHY OF THE LATE EOCENE–EARLY OLIGOCENE SHELF AND BASIN FLOOR TO SLOPE TURBIDITE SYSTEMS, LEMNOS ISLAND, NE GREECE»

СТРАТИГРАФИЯ. ГЕОЛОГИЧЕСКАЯ КОРРЕЛЯЦИЯ, 2011, том 19, № 2, с. 87-102

GEOMETRY AND SEQUENCE STRATIGRAPHY OF THE LATE EOCENE-EARLY OLIGOCENE SHELF AND BASIN FLOOR TO SLOPE TURBIDITE SYSTEMS, LEMNOS ISLAND, NE GREECE

© 2011 A. Maravelis and A. Zelilidis

University of Patras, Department of Geology, Laboratory of Sedimentology, Patras, Greece e-mail: amaravel@upatras.gr, A.Zelilidis@upatras.gr

Abstract—The Late Eocene—Early Oligocene sedimentary fill of the Lemnos Island, NE Greece, is represented by a submarine fan and shelf deposits. Turbidites in the system occur as a laterally isolated body, with one sediment influx center present. The influx center is a proximal distributary channel that occupies a position approximately in the fan's center and displays the coarsest sediment in the study area. It also suggests in association with the main palaeocurrent direction toward NE a curved shape for the fan. The stratigraphic succession of the submarine fans indicates that their sedimentation started during the base level fall and completed shortly after the base level rise. As a consequence, the study area was filled by turbidites that correspond to forced regressive, lowstand normal regressive, and transgressive genetic units. The progradational bedsets, within the basal part of the turbidite deposits, recorded the history of the base level fall. The mixed prograda-tional/aggradational style of the upper part of the submarine fan system suggests that the regression of the shoreline is driven by sediment supply during a period of base-level rise at the shoreline, or at a time of baselevel stillstand. The overlying shelf facies consist of thick to medium bedded sandstones and mudstones, which display a general thinning upward trend. The base of the mudstone facies that overlie the thick-bedded, amalgamated sandstones corresponds to a transgressive surface. This surface separates the low-stand deposits (thick-bedded sandstones) from the high stand deposits (mudstone facies), suggesting that deposition of shelf facies occurred during a transgressive system tract.

Keywords: turbidites, sequence stratigraphy, forced regression, NE Greece.

INTRODUCTION

Interest in the architecture and evolution of deep-water sedimentary systems has grown as petroleum exploration has migrated from onshore and near-shore settings into offshore areas. Deep-water channel complexes have received special attention since exploration has shown that sand-rich turbidite-filled channel systems are major petroleum reservoirs (Weimer et al., 2000). The study of submarine fans has resulted in almost as many models to explain fan deposition, as there are fans (Bouma et al., 1985). A diverse number of fan models can be found in various literature (e.g. Mutti and Ricci Lucchi, 1972; Walker and Mutti, 1973; Mutti and Normark, 1987). Most models address fan geometries and facies relationships based almost exclusively on examination of outcrops and/or modern submarine fan morphology. Efforts have been made to integrate modern observations with ancient systems, therefore five major elements have been identified that have expressions in both systems (Mutti and Normark, 1987; Posamentier et al., 1991).

Sequence stratigraphy is considered by many as one of the most recent conceptual revolutions in the broad field of sedimentary geology (Miall, 1995), by revamping the methodology of stratigraphic analysis. Sequence stratigraphy is uniquely focused on analyz-

ing changes in facies, the geometric character of strata, and the identification of key surfaces to determine the chronological order of basin filling and erosional events (Catuneanu et al., 2009). Sequence stratigraphic concepts (Vail, 1987; Van Wagoner et al., 1988) have been applied to deep-water sediments in order to analyze temporal and spatial variations in the organization of lithofacies and architecture during a relative sea-level cycle (Coleman and Roberts, 1988; Bouma et al., 1989).

Three primary controls for fan development and deep-sea sedimentation can be identified: (1) sediment-type and supply, (2) tectonic setting and activity, and (3) sea-level variations. These controls are by no means independent; for example, tectonic factors play an important role in determining sediment supply or local sea-level changes (Stow et al., 1985). In addition the characteristics as climate, basin characteristics, and sedimentary processes should also be taken into account. Each of these primary factors is composed of a number of secondary components that are interactive within the entire set of controls (Bouma, 2004). Relative changes in sea-level are interpreted to control both spatial and temporal variations in erosion and the deposition of clastic sediments in both marine and nonmarine environments (Posamentier et al., 1988;

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17° 19° 21° 23° 25° 27° 29° 31° 33° 35° 37° 39° Fig. 1. Plate tectonic configuration of the area around the Aegean (from Papazachos et al., 1998).

Vail et al., 1991). Development of deep-water sediments such as turbidites, debrites, and contourites in submarine fans and in related environments have also been interpreted as being controlled by relative sea-level changes (Shanmugam and Moiola, 1982). The effect of local factors such as climate, sediment supply, and tectonics must be incorporated into models before these models can be applied to a particular basin. Once these considerations have been taken into account, the refined models can then be used predicatively to simulate local conditions in order to better predict litho-logic successions (Posamentier et al., 1988).

The detailed description of major lithofacies, the interpretation of depositional environments and sub environments on Lemnos Island, and their age determination were discussed by Maravelis et al. (2007). The aim of this study is to define the spatial distribution of submarine fans and shelf deposits, in order to reconstruct the anatomy of the depositional systems and document their complexity, and to address their

occurrence within the context of sequence stratigraphy.

GEOLOGICAL SETTING

The study area lies in the NE Aegean Sea, Greece. The plate configuration of the Aegean region consists of the Aegean plate in the south, separated by a strike-slip boundary (McKenzie 1970; Papazachos et al., 1998) from the Eurasian plate in the north, which encompasses the north Aegean, Rhodope, and adjacent areas (Fig. 1). This boundary corresponds to the North Anatolian Fault (NAF) and has been grown by westward propagation through continental lithosphere over a time range of 10 My. The Arabia/Europe collision in eastern Turkey caused Anatolia to move to the West and created the NAF in eastern Turkey in the Middle to Late Miocene. The NAF propagated along the Pontides and penetrated into the northern Aegean (Armijo et al., 1996).

The Aegean plate is overriding the African plate, accommodated by north-eastward dipping subduction in the Hellenic Trench. The strike-slip boundary between the Aegean and the Eurasian plates (the north Aegean transform zone) consists of two major strike-slip faults, which are extensions of the North Anatolian fault. Convergence between the Eurasian and African plates has played a key role in controlling mag-matism in the Balkan Peninsula since the Late Cretaceous. Since that time the collision resulted in the formation of several sub-parallel southward-migrating magmatic belts, with the youngest one being the present-day Aegean Arc (Fyticas et al., 1984).

During Late Eocene—Early Oligocene the mag-matic activity that resulted from the subduction of the African Plate beneath the Eurasian plate, occurred in the Macedonian—Rhodope—North Aegean region (Marchev and Shanov, 1991). The magmatic belt extends to the NW into Skopje and Serbia, crossing the Vardar Zone (Bonchev, 1980), and continues to the SE into the Thracian basin and Western Anatolia (Aldan-maz et al., 2000). A subduction mechanism has been proposed to explain the Late Cretaceous magmatism in the Rhodope Zone (Dabovski, 1991). High-precision U—Pb zircon and rutile age dating in Central Rhodope (Peytcheva et al., 2002) document a southward shift of this magmatism from 92 to 78 Ma. The progressive southward migration of magmatic activity in the Aegean region (Fyticas et al., 1984) that commenced in Rhodope during the Late Eocene (Yanev et al., 1998) has been confirmed by seismic tomography (Spakman et al., 1988), implying that present day North-vergent subduction in the Aegean region started by at least 40 Ma. It is generally believed that extension in the Greek portion of the Rhodope zone started no earlier than the Early Miocene (Dinter et al., 1995).

During this time interval, the studied area was characterized by the deposition of submarine fans that underlay shelf deposits, with tectonic activity responsible for the upward shallowing of the depositional environment (Maravelis et al., 2007). Turbidites were deposited in both inner and outer parts of the submarine fan system and consist of alternating sandstone and mudstone beds. Sandstones occur in both complete and incomplete Bouma sequences while shelf has been interpreted as storm-surge deposits on the deeper parts of shelves (Maravelis et al., 2007). Outer-fan facies are mostly presented at the peripheral parts, while inner facies are dominant in the central parts of the study area. Geochemical and petrographic data suggest that these sediments were deposited in a forearc basin with the outer arc ridge as a major sediment source (Maravelis and Zelilidis, 2010).

During the Miocene the Lemnos Island was the site of volcanic activity, thus magmatic rocks overlie the shelf deposits (Pe-Piper and Piper, 2001). Magmatic rocks consist of both plutonic and volcanic rocks, and cover a large part of the studied region. The end of the Miocene was characterized by accumulation of conglomerates, marls, and calcareous sandstones. Local Pleistocene porous calc

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