Pelagic Sedimentation in Immature Ocean Basins

Abstract

Young ocean basins or their remains are relatively uncommon features of the earth’s surface. Whereas the world’s old ocean, the Pacific, is surrounded almost entirely by active continental margins which have ’eaten1 the early, probably pre-Mesozoic rifted margins of its primordial predecessors, passive margins characterize almost all fringes of young ocean basins which have originated since mid-Mesozoic times (Southern Ocean, Arctic Ocean, Indian Ocean, Atlantic Ocean). Such basins have evolved from rifts cutting through relatively stable continental crust. Evidence of the early history of passive continental margins, and of the invasion of the sea into the young, growing ocean basins has to be sought today along the continental rise and slope of these margins. Generally the early history of the Mesozoic basins lies deeply buried. However, Cenozoic analogues can be studied in the Gulf of Aden/Red Sea system as well as in the Gulf of California, and in the Arctic Ocean. Although a truly oceanic or pelagic environment is established only after these rifts have attained considerable depth and width, and after the initial volcanism has died out, some of the early lithofacies typify restricted depositional environments. The original distribution of such facies is controlled by the physical shape of the evolving ocean basin, which can be approximated using principles of plate rotation and subsidence of oceanic crust. The nature of these restricted lithofacies is probably entirely climate-controlled. End-members of such restricted lithofacies are halites, which have been sampled in the Red Sea and which probably occur along major segments of the Atlantic margins, or fine-grained black shales, which have been deposited in anaerobic paleoenvironments and which therefore have preserved their high contents of organic carbon (examples Red Sea, Indian and Atlantic Oceans). Such restricted depositional environments exist as long as the exchange of bottom water masses between the youthful basins and the open ocean is hampered by narrow, and/or shallow sills. The tectonic evolution of such circulation barriers is therefore one of the keys to the understanding of the depositional paleoenvironment of the “restricted” pelagic or hemipelagic sediment facies. It is these sediments which have documented the history of the early oceanic stages of old and new rifts.