Palaeobathymetric gradients across Timor during 5.7–3.3 Ma (latest Miocene–Pliocene) and implications for collision uplift

Abstract

Palaeobathymetry is interpreted for the oldest post-collision deposits present in Timor. The distributions of planktonic and benthic foraminifera in chalk, marl and mudstone successions that accumulated during 5.7–3.3Ma form the bases for age zonation and water-depth interpretation. Data from 77 samples collected from 11 stratigraphic sections in East Timor are combined with information previously published from several sections in West Timor to provide control points for palaeobathymetric contouring. The bathymetric reconstruction is based on interpreted water depths at the control points in combination with aspects of the spatial geology of Timor.Speculative bathymetric contours are illustrated for time slices represented by three planktonic foraminiferal zones: N18 (~5.5–5.7Ma), N19r (~4.3–5.5Ma), and N20r (~3.3–4.3Ma). Studied samples provide evidence for three bathymetric zones (500–1000m, 1000–1500m, and 1500–2500m) and suggestions are made for the positions of the 500m, 1000m and 1500m contours during each time slice. The contours show only very gradual variation from 5.7Ma to 3.3Ma, and suggest a deepening bathymetric gradient from the northern region (where the structurally deepest part of the orogenic pile, the Aileu Metamorphic Complex, is now exposed) to the south, east and west.The contour maps provide a suggestion for visualizing the topography of the evolving orogenic pile that forms Timor from about 5.7Ma through to 3.3Ma. The relatively undeformed friable chalk and marl deposits of Zone 18 (5.7–5.5Ma) overlie highly deformed and diagenetically more altered strata. This indicates that the initial phase of collision between the Australian continent and the Banda Arc took place prior to 5.7–5.5Ma rather than later during the Pliocene or Pleistocene as some authors have suggested. Because of lack of substantiated stratigraphic evidence from anywhere in Timor for the 9.8–5.7Ma interval (covering much of the Late Miocene) the antecedent topography cannot be determined. The bathymetric reconstructions for 5.7–3.3Ma suggest that a deep foreland basin (precursor of the present Timor Sea) had developed by 5.7Ma with uplifted areas to the north in an emerging island.