Abstract

The sequence stratigraphy of fluvial–deltaic strata deposited near the center of the wide, gradually subsiding Sunda Shelf differs from standard highstand examples because low accommodation and low depositional gradients restricted the thickness of fluvial–deltaic sequences and the depth of lowstand fluvial incision. In this setting sea-level fluctuations may have had less influence on fluvial depositional patterns, and fluvial stratigraphy is more likely to be defined by changes in river discharge during gradual aggradation. The upper Muda Formation in West Natuna Basin, offshore Indonesia, is a 225-m-thick, dominantly fluvial succession that prograded onto the middle part of the Sunda shelf from Pliocene to Holocene. The succession changes upward from offshore shelf deposits, through a thin interval of deltaic deposits, a succession of fluvial deposits, and finally a thin transgressive succession directly beneath the modern sea floor.

A high-resolution seismic survey (17 km by 40 km in area) of Belida Field near the middle of the Sunda shelf provides exceptional images of changing fluvial architecture within five erosionally based, fluvial sequences. Sequences, each several tens of meters thick, are characterized by low-relief basal incisions that extend laterally onto interfluves cut by a network of gullies. Although a regressive stacking pattern of successive sequences is defined to some extent by an increase in the relief of basal erosion and a decrease in average sequence thickness, it is best defined by progressive increase in the extent of gully formation onto interfluve areas (indicating longer periods of interfluve exposure).

Channel size tends to decrease upward within individual sequences, recording both a decrease in the size of the largest channels and greater preservation of smaller tributary and floodplain drainage channels. The largest channels preserved directly above the better developed sequence boundaries tend to be more sinuous and to have more extensive inner-bend accretion deposits than those that cut down from horizons higher within sequences. This suggests that incised channels had more stream power to erode their banks relative to the sediment loads they carried. Although mechanisms controlling the episodic shallow incision and then aggradation of these fluvial systems are difficult to constrain from variations observed in this small study area, the consistent thickness of the Muda Formation observed in regional 2D seismic sections is evidence against significant syndepositional tectonic deformation or differential subsidence. Low regional gradients and great distance of the study area from the paleo-shelf edge (~ 1000 km) are evidence against direct sea-level influence. Larger channel sizes directly above sequence boundaries supports temporal changes in channel discharge, perhaps related to climatic changes within the drainage basin. Recognition of episodic channel incision and bypass alternating with periods of floodplain aggradation influences predictions of channel-deposit connectivity based on channel proportion.

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