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The Owen Ridge south of Oman represents oceanic crust that was uplifted by compressional tectonic forces in the early Miocene. Build-out of the Indus Fan led to deposition of a thick sequence of turbidites over the site of the Ridge during the late Oligocene and early Miocene. Early Miocene uplift of the Ridge led to a pelagic cap of nannofossil chalks. Two short sequences of turbidites from the preand syn-uplift phases were chosen for detailed grain size analysis. The upper Oligocene section at Site 731 is composed of thin (centimeter-decimeter scale) graded mud turbidites separated by relatively thick (decimeter-meter scale) intervals of homogeneous, non-bioturbated clayey siltstones. These finer intervals are unusually silt-rich (about 60%) for ungraded material and were probably deposited as undifferentiated muds from a series of turbidity current tails. By contrast, the lower Miocene section at Site 722 is comprised of a sequence of interbedded turbidites and hemipelagic carbonates. Sharp-based silt turbidites are overlain by burrow-mottled marly nannofossil chalks. The Oligocene sequence may have accumulated in an overbank setting on the middle fan—the local topographic position favoring frequent deposition from turbidity current tails and occasional deposition from the body of a turbidity flow. Uplift of the Ridge in the early Miocene led to pelagic carbonate deposition interrupted only by turbidity currents capable of overcoming a topographic barrier. Further uplift eventually led to entirely pelagic carbonate deposition. INTRODUCTION AND PALEOCEANOGRAPHIC SETTING Drilling on the Owen Ridge south of Oman during ODP Leg 117 at Sites 721, 722, and 731 (Fig. 1) penetrated hundreds of meters of turbidites underlying a pelagic cover (Prell, Nütsuma, et al., 1989). The turbidites represent portions of the Indus Fan prior to uplift of the Owen Ridge above the lysocline and level of fan deposition in the early Miocene. The Owen Ridge appears to have been initiated at a transform fault along the protoCarlsberg spreading ridge. Opening of the Gulf of Aden may have caused compression and uplift of the Ridge (Whitmarsh, 1979; Mountain and Prell, 1987). Currently the Ridge marks a minor plate boundary separating Arabia and India with exceptionally slow rates of dextral strike-slip (Gordon and DeMets, 1989). Whitmarsh, Weser, et al. (1974) established, via spot coring, the depositional history of the Ridge at DSDP Site 224 (Figs. 1 and 2). Lower Eocene lamprophyre flows are overlain by nannofossil-rich clay and volcanic glass. Upper Eocene nannofossil chalk, silty claystone and sandstone is succeeded by lower Oligocene clayey siltstones and claystones probably deposited close to the lysocline. In the late Oligocene and early Miocene almost barren graded silts and sands (turbidites) were deposited. By the earliest middle Miocene sedimentation rates had dropped, clastic supply to these sites waned, and pelagic nannofossil oozes started accumulating following the uplift of the Ridge. The present study of a few mud turbidites from the upper Oligocene and the uppermost lower Miocene may provide some insights into the changing nature of sedimentation over the nascent Owen Ridge. It was undertaken to establish the extent to which the barren Oligocene section was of turbidity current origin and to ascertain whether the thick mud turbidites described 1 Prell, W. L., Nütsuma, N., et al., 1991. Proc. ODP, Sci. Results, 117: College Station, TX (Ocean Drilling Program). 2 Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EQ, United Kingdom. by Jones (1988) and Jones et al. (in press) from the Madeira Abyssal Plain were present. Grain size investigations were based on Sedigraph X-ray settling tube analysis (Jones et al., 1988). This method allows grain-size distributions spanning fine sand to clay to be analyzed non-destructively from small samples. Size frequency distributions (as opposed to cumulative curves) are especially useful for visualization of grain size populations and permit ready identification of coarsening and fining trends (McCave, 1979). The distributions are displayed with coarser material to the right and they are usefully combined with plots of the percentage of silt (2-63 μm) and the silt/clay ratio. SAMPLES FROM HOLE 731C Thirty three samples spanning 2.6 m were collected from Core 117-731C-9W (upper Oligocene) at about 691 meters below seafloor (mbsf) (Fig. 2). An irregular sampling pattern was designed to provide a suite of specimens reflecting the full range of grain size variations. The dominant lithotypes are siltstone and clayey siltstone arranged in sharp-based fining-upward sequences a few decimeters thick. The siltstone is dark gray (10YR 4/1) with mainly plane laminations. At some levels (not sampled) rare ripple cross-lamination and low angle cross-stratification is present. The dark gray (10YR 4/1) clayey siltstone exhibits a predominantly homogeneous fabric with faint horizontal banding. The homogeneity of the clayey siltstone is not due to bioturbation (contrary to the shipboard descriptions), as layering is present and the contacts between homogeneous and laminated sediment are extremely sharp and lack burrow mottling (Fig. 6 of Site 731 chapter in Prell, Nütsuma, et al., 1989). Occasionally small slump folds and dewatering structures are present. At some levels sandstone beds of a few centimeters to decimeters occur. These have sharp bases and grade up into clayey siltstone or have sharp tops and bottoms. These coarser sequences have not been examined here. Shipboard determinations demonstrated carbonate contents of about 5%. This carbonate is detrital, the sequence being essentially barren of microfossils, probably reflecting deposition below the lysocline. The proportion of the other main components, quartz and clay minerals, is a function of bulk grain size.

Mud Turbidites from the Oligocene and Miocene Indus Fan at Sites 722 and 731 on the Owen Ridge