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ABSTRACT

Compelling evidence has been found for major fluvial incisions at the top of the Aptian Shu’aiba Formation in Block 5, offshore Qatar. Log correlation, horizontal well results and seismic mapping reveal a major east-west trending, meandering, terraced, incised valley running across Block 5, with widely distributed subsidiary valleys. Locally, the main valley reaches 8 km in width and has a depth of at least 30 m. Additional evidence for exposure comes from numerous clay-filled and, occasionally, sand-filled karst fissures that have been encountered at the top of the Shu’aiba Formation, penetrating at least 25 m below the top Shu’aiba surface.

At one location the complete channel-fill was cored and found to consists of tidally influenced estuarine sandstone deposits, rich in dispersed plant material, coal and amber at the base, passing up into shoreface deposits at the top. The overlying succession consists of a thin, sandy interval, rich in glauconite and ferruginous ooids, that blankets the entire Shu’aiba Formation. These deposits are interpreted as a transgressive, back-fill systems tract of a major valley system. The overlying oolitic ironstones and glauconitic sandstones are interpreted as highly condensed facies.

The valley-fill sandstones are of late Late Aptian age and the overlying condensed sheet sandstones are of latest Aptian to Early Albian age. This entire siliciclastic package is interpreted as an uppermost Aptian – Lower Albian depositional sequence that, in both lithological composition and age, is distinct from the overlaying Nahr Umr Formation. This is the first reported case of Upper Aptian siliciclastics in Qatar.

The importance of these observations is that they provide proof for (1) subaerial exposure of the Shu’aiba Formation in Block 5, (2) an early Late Aptian relative drop in sea-level of at least 30 m, (3) a latest Aptian sea-level rise of a similar magnitude and (4) a condensed phase, probably a sea-level stillstand, during the Early Albian. Since relative sea-level fluctuations of this magnitude are likely to have a regional expression, the here established Late Aptian sea-level curve provides a reference for the Arabian Plate. The palaeogeographic implication of these observations is that during the Late Aptian most of Qatar and adjacent offshore areas were subaerially exposed and the Bab Basin was connected with the siliciclastic sources in the northwestern part of the Arabian Plate by large river systems, which cut into the exposed carbonate platforms.

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