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The Neogene sediments of east Sumba, Indonesia—products of a lost arc?

1994; Pergamon Press; Volume: 9; Issue: 1-2 Linguagem: Inglês

10.1016/0743-9547(94)90066-3

1879-1832

A.R. Fortuin, Th.B. Roep, P.A. Sumosusastro,

Geological and Geochemical Analysis

The island of Sumba is an uplifted element of the present Lombok and Savu forearc basins. An outline of the Neogene sediments of east Sumba is given, based on the study of linked individual sections along three transects. Preliminary biostratigraphic data indicate that basinal sedimentation started during early middle Miocene times, after the breakup and rapid subsidence of an Eocene-Oligocene carbonate platform developed over Paleocene-(?)Eocene volcanic eruption centres. Subsidence must have proceeded relatively fast, as can be concluded from the rapidly increasing open marine character of the sediments above the unconformity. A sequence boundary separates these basal chalky marls from widely exposed middle Miocene volcaniclastic turbidite/pelite deposits. The rapid upward disappearance of the foraminiferal fauna in the chalky marls, but frequent presence of foraminifera within overlying turbidites, is a strong indication that basinal depths soon were within the reach of the CCD. In the Kananggar area the basal volcaniclastics are developed as massive turbidite massflows, forming together a stacked channel complex of over 150 m thick. Part of this and overlying turbidite/pelite sequences have been incorporated in large, sediment slides. During the latest Miocene foraminiferal chalks start to reappear, which sediment type dominates in the youngest early Pliocene succession. The average rate of latest Miocene-Quarternary uplift of east Sumba is in the order of 0.65 mm/yr. The dominant paleocurrent direction was to NNE. With regard to the interpreted deep-sea environment and the abundant supply of pumiceous and other volcanic products of island arc composition, coming from sources south of Sumba, instead of being derived from the present volcanic arc to the north, a fundamentally different basinal setting must have existed. Because the supply of volcanic ejecta from the south decreased in the course of the latest Miocene, when volcanism started to build up the present inner arc north of Sumba and considering the southward drift of Sumba prior to the Miocene [H. Wensink, J. SE Asian Sci.9, 51–65 (1994)], it is speculated that the Sumba block may have shifted from a backarc to a forearc position.