Page 260 - Sedimentology and Stratigraphy

16

Deep Marine Environments

The deep oceans are the largest areas of sediment accumulation on Earth but they are
also the least understood. Around the edges of ocean basins sediment shed from land
areas and the continental shelves is carried tens to hundreds of kilometres out into the
basin by gravity-driven mass flows. Turbidity currents and debris flows transport sedi-
ment down the continental slope and out on to the ocean floor to form aprons and fans of
deposits. Towards the basin centre terrigenous clastic detritus is limited to wind-blown
dust, including volcanic ash and fine particulate matter held in temporary suspension in
ocean currents. The surface waters are rich in life but below the photic zone organisms
are rarer and on the deep sea floor life is relatively sparse, apart from strange creatures
around hydrothermal vents. Organisms that live floating or swimming in the oceans
provide a source of sediment in the form of their shells and skeletons when they die.
These sources of pelagic detritus are present throughout the oceans, varying in quantity
according to the surface climate and related biogenic productivity.

16.1 OCEAN BASINS material gradually moves away from the spreading
centre and as it does so it cools, contracts and the
Altogether 71% of the area of the globe is occupied by density increases. The older, denser oceanic crust sinks
ocean basins that have formed by sea-floor spreading relative to the younger, hotter crust at the spreading
and are floored by basaltic oceanic crust. The mid- centre and a profile of increasing water depth away
ocean ridge spreading centres are typically at 2000 from the mid-ocean ridge results (Fig. 16.1) down to
to 2500 m depth in the oceans. Along them the crust around 4000 to 5000 m where the crust is more than
is actively forming by the injection of basic magmas a few tens of millions of years old.
from below to form dykes as the molten rock solidifies The ocean basins are bordered by continental
and the extrusion of basaltic lava at the surface in the margins that are important areas of terrigenous
form of pillows (17.11). This igneous activity within clastic and carbonate deposition. Sediment supplied
the crust makes it relatively hot. As further injection to the ocean basins may be reworked from the
occurs and new crust is formed, previously formed shallow marine shelf areas, or is supplied directly