Morphology and Quaternary sedimentation of the Mozambique Fan and environs,southwestern Indian Oceans*

Most of the Quaternary sediments of the Mozambique Fan have been derived from Africa-Madagascar and deposited by turbidity currents in Pleistocene time. Currents caused by movement of the Antarctic Bottom Water also played a significant role in reworking and redepositing sediments along the marginal areas of the fan. The inner or upper Mozambique Fan is characterized by a single, leveed valley. Due to the effects of the Coriolis force, the natural levees to the east of the valley (left, looking downstream) are higher and contain more terrigenous sediments than those to the west of the valley. The sea floor to the west of the valley returns regular hyperbolic echoes as seen on 3·5 kHz echograms, whereas to the east of the valley, the sea floor is relatively smooth. The sediments on the valley floor are coarse-grained (with median grain up to 2 mm) and poorly sorted, and occur often as massive turbidites, interbedded with hemipelagic sediments. Away from the valley, both to the east and the west, the terrigenous sediments are relatively fine-grained and have been deposited as overbank turbidite sequences. We estimate the maximum velocities of the channelized turbidity currents in the upper fan to have been 8–32 ms^?1. The middle fan has several distributary channels with no levees and has a relatively flat sea floor, characterized by lack of acoustic penetration. Thick, sheet-like, turbidite sand beds, deposited primarily by unchannelized turbidity currents, characterize the middle fan. The middle fan grades, towards the margins, into the outer (lower) fan which is relatively free of channels, has good acoustic penetration and contains hemipelagic and pelagic sediments, and thin, fine-sand turbidite and/or contourite beds. A wide zone of sediment waves, formed from the reworking of the turbidity current-fed sediments by the Antarctic Bottom Water, forms part of the outer fan.     

Distributary channels, sand lobes, and mesotopography of Navy Submarine Fan, California Borderland, with applications to ancient fan sediments

The deep-tow instrument package of Scripps Institution of Oceanography provides a unique opportunity to delineate small-scale features of a size comparable to those features usually described from ancient deep-sea fan deposits. On Navy Fan, the deep-tow side-scanning sonar readily detected steep channel walls and steps and terraces within channels. The most striking features observed in side-scan are large crescentic depressions commonly occurring in groups. These appear to be large scours or flutes carved by turbidity currents. Four distinct acoustic facies were mapped on the basis of qualitative assessment of reflectivity of 4 kHz reflection profiles. There is a distinct increase in depth of acoustic penetration, number of sub-bottom reflectors, and reflector continuity from the upper fan-valley to the lower fan. These changes are accompanied by a decrease in surface relief. Navy Fan is made up of three active sectors. The active upper fan is dominated by a single channel with prominent levees that decrease in height downstream. The active mid-fan region or suprafan is where sand is deposited. Well defined distributary channels with steps, terraces, and other mesotopography terminate in depositional lobes. Interchannel areas are rough, containing giant scours as well as other relief. The active lower fan accumulates mud and silt and is without resolvable surface morphology. The morphological features seen on Navy Fan other than levees, interchannel areas, and lobes are principally erosional. The distributary channels are up to 0.5 km wide and 5–15 m deep. Such features, because of their large size and low relief, are rarely completely exposed or easily detectable in ancient rock sequences. Some flute-shaped scours are larger than channels in cross section but many are 5-30 m across and 1-2 m deep. If observed in ancient rocks transverse to palaeo-current direction, they would perhaps be indistinguishable from channels. Surface sediment distribution combined with fan morphology can be used to relate modern sediments to facies models for ancient fan sediments. Gravel and sand occur in the upper valley, massive sand beds in the mid-fan distributary channels, classical complete Bouma sequences on depositional lobes, incomplete Bouma sequences (lacking division a) on the lower mid-fan, and Bouma sequence with lenticular shape or other limited extent on mid-fan interchannel areas and on levees.     

Magma ocean fractional crystallization and cumulate overturn in terrestrial planets: Implications for Mars

Abstract— Crystallization of a magma ocean on a large terrestrial planet that is significantly melted by the energy of accretion may lead to an unstable cumulate density stratification, which may overturn to a stable configuration. Overturn of the initially unstable stratification may produce an early basaltic crust and differentiated mantle reservoirs. Such a stable compositional stratification can have important implications for the planet's subsequent evolution by delaying or suppressing thermal convection and by influencing the distribution of radiogenic heat sources. We use simple models for fractional crystallization of a martian magma ocean, and calculate the densities of the resulting cumulates. While the simple models presented do not include all relevant physical processes, they are able to describe to first order a number of aspects of martian evolution. The models describe the creation of magma source regions that differentiated early in the history of Mars, and present the possibility of an early, brief magnetic field initiated by cold overturned cumulates falling to the coremantle boundary. In a model that includes the density inversion at about 7.5 GPa, where olivine and pyroxene float in the remaining magma ocean liquids while garnet sinks, cumulate overturn sequesters alumina in the deep martian interior. The ages and compositions of source regions are consistent with SNC meteorite data.