The profile of a river that conveys sediment without net deposition and net erosion is referred to as 'graded' with respect to vertical aggradation of the river segment. Three experimental series, designed in terms of the autostratigraphic view of alluvial grade, were conducted to clarify the diagnostic spatial behaviour of graded alluvial-deltaic rivers: an R series, which utilized a moving boundary setting with a stationary base level; an F series in a fixed boundary setting with a stationary base level to produce 'forced grade'; and an M series in a moving boundary setting with constant base level fall to produce 'autogenic grade.' The results of the three experimental series, combined with geometrical modelling of the effects of basin water depth and other experimental data, suggest the following: (1) in a graded alluvial-deltaic system, lateral shifting and avulsing of active distributary channels are suppressed regardless of whether the downstream boundary of the deltaic system is fixed; (2) in a delta with a downstream-fixed boundary, the graded streams are stabilized within a valley that is incised in the axial part of the delta plain, whereby the alluvial plain outside the valley is abandoned and terraced; (3) in moving boundary settings, the graded river simply extends basinward as a linearly elongated channel-and-lobe system without cutting a valley; and (4) a modern forced-graded alluvial river is most likely found in a valley incised into a fan delta in front of very deep water, and the stratigraphic signal of fossil autogenic-graded rivers will be 2 found in deltaic successions that accumulated in the outer to marginal areas of deltaic continental shelves during sea level falls. This renewed autostratigraphic view of alluvial grade suggests a thorough reconsideration of the conventional understanding that an alluvial river feeding a progradational delta is graded with a stationary base level.

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Three sediment cores in a north-south transect (3 • N to 13 • S) from different sediment types of the Central Indian Ocean Basin (CIOB) are studied to understand the possible relationship between magnetic susceptibility (χ) and Al, Fe, Ti and Mn concentrations. The calcareous ooze core exhibit lowest χ (12.32 × 10 −7 m 3 kg −1 ), Al (2.84%), Fe (1.63%) and Ti (0.14%), terrigenous clay core with moderate χ (29.93 × 10 −7 m 3 kg −1 ) but highest Al (6.84%), Fe (5.20%) and Ti (0.44%), and siliceous ooze core with highest χ (38.06 × 10 −7 m 3 kg −1 ) but moderate Al (4.49%), Fe (2.80%) and Ti (0.19%) contents. The distribution of χ and detrital proxy elements (Al, Fe, and Ti) are identical in both calcareous and siliceous ooze. Interestingly, in terrigenous core, the behaviour of χ is identical to only Ti content but not with Al and Fe suggesting possibility of Al and Fe having a non-detrital source.The occurrence of phillipsite in terrigenous clay is evident by the Al-K scatter plot where trend line intersects K axis at more than 50% of total K suggesting excess K in the form of phillipsite. Therefore, the presence of phillipsite might be responsible for negative correlation between χ and Al (r = −0.52). In siliceous ooze the strong positive correlations among χ, Al exc and Fe exc suggest the presence of authigenic Fe-rich smectite. High Mn content (0.5%) probably in the form of manganese micronodules is also contributing to χ in both calcareous and siliceous ooze but not in the terrigenous core where mean Mn content (0.1%) is similar to crustal abundance. Thus, χ systematically records the terrigenous variation in both the biogenic sediments but in terrigenous clay it indirectly suggests the presence of authigenic minerals.

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