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Global patterns of continental drainage to the oceans have changed markedly over the last 200 m.y. in response to plate tectonic processes; most of the earth's major rivers now enter the sea on passive continental margins which did not exist in the early Mesozoic. This reorganization of drainage has strongly influenced the distributions of marine detrital and carbonate facies.
Analysis of changes in continental topography related to the breakup of Pangaea suggest that throughout much of the Mesozoic, drainage systems were dominated by a pole-to-pole divide directing detrital sediment away from the sites of future continental rifting. This phase was followed by rifting and formation of narrow oceans with uplifted margins. As the margins subsided by thermal relaxation, massive amounts of detrital sediment
were delivered from the continental interiors onto the young passive margins. In time, river drainage became increasingly focused, concentrating detrital sediment supply at the mouths of a few large rivers. Very large supplies of detrital sediment require large, high uplifts such as those caused by subduction of young, hot ocean crust or by continental collision.
Large sediment supplies also require drainage basins with relatively constant slope; so that sediment erosion, throughput, and delivery to the ocean margin are efficient. The result is rapid sedimentation of deltaic complexes containing an abundance of organic carbon. During most of earth history, there are no large, high uplifts, and carbonate rocks become more important in the continental margins.
In contrast to the point inputs of detrital sediments, the supply of carbonate has been from the oceanic reservoir and is diffuse. Carbonate deposition dominates the continental shelves in all warm regions where the detrital sediment input is not extremely large. Carbonate shelves become cemented, resisting erosion, so they build up until the shelf edge approximates highstands of sea level. Detrital shelves become adjusted to lowstands of sea level with the shelf breaks typically many tens of meters below the low sea level.
The clastic-carbonate shelf-slope-rise system operates to promote bypassing of detrital materials into deep water in the subtropics and tropics, with sharp facies contrasts. In higher latitudes, carbonate may be a significant proportion of the continental margin material, but facies changes are usually much more gradual.
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