Ancient shelf ridges-A potentially significant component of the transgressive systems tract: Case study from offshore northwest Java

Abstract

Detailed stratigraphic evaluation of three-dimensional (3-D) seis mic volumes calibrated with well-log and core data from the Mio cene section of the offshore northwest Java shelf reveals the exten sive presence of preserved shelf ridge deposits. These features are long linear bodies ranging from 0.3 to 2.0 km wide, more than 20 km long, and up to 17 m high. On close inspection, these features appear to be asymmetric, characteristically sharp-edged and thicker on one side and gradually thinning with an irregular edge on the other side. Possible sand waves, smaller in scale, are observed super imposed on these ridges and oriented oblique to the long axes of the ridges. The observed shelf ridge deposits tend to be sand prone and overlie ravinement surfaces. The ridges appear to be oriented parallel with the axes of broad paleoembayments associated with the structural fabric of the basin. In addition to shelf ridges, shelf ribbons, possibly less than 5 m thick and less than 100 m wide, are also imaged. Sand ridges are common on modern shelves but significantly less commonly recognized in the subsurface or in outcrop. The features observed here represent examples of preserved ancient shelf ridges. These ridges are thought to have formed as a result of erosion and subsequent reworking of sand-prone deltaic and/or coastal-plain deposits by shelf tidal currents, which became active immediately after shoreline transgression. These deposits appear to have migrated across the ancient sea floor and represent an important component of the transgressive systems tract. These transgressive systems tract deposits have significant exploration potential because they are commonly sand prone and tend to be encased in shelf mudstone seal facies. Depending on the degree to which sand is present in interridge locations, these linear sand bodies can comprise potential stratigraphic traps. (Begin page 76)