Abstract:
The Indus Fan, although much smaller than its eastern Bengal counterpart, is comparable in size and discharge to the Mississippi and is one of the major geologic features of the Indian Ocean. Due to a lack of deep penetrating wells the age of the fan has been controversial, variably dated as Eocene-Mid Miocene in its onset. Multichannel seismic profiles linked to petroleum exploration wells on the Pakistani Shelf now demonstrate that much of the proximal fan is pre-Middle Miocene, although well developed channel-levee structures are limited to the Mid Miocene-Recent section. Although sedimentation since that time has been rapid, large volumes of post-rift sediment predate the onset of the channel complexes, suggesting that the fan must have started some time before then. This hypothesis is supported by Ocean Drilling Program sampling of the Owen Ridge where muddy Paleogene Indus Fan turbidites have been collected. These strata are taken to represent the fan prior to uplift of Owen Ridge in the Early Miocene. New Pb isotopic provenance data collected by ion microprobe from detrital K-feldspars at Deep Sea Drilling Project Site 224 on Owen Ridge indicate that turbidites at least as old as the Mid Eocene contain material derived from the Indus Suture and South Tibet, requiring India-Asia collision to predate that time. These data push back the start of deep-sea fan sedimentation in the western Indian Ocean, previously constrained by the Late Oligocene age for fan initiation at Site 222 at the southern toe of the fan.