Page 228 - Sedimentology and Stratigraphy

14

Shallow Sandy Seas

Shallow marine environments are areas of accumulation of substantial amounts of
terrigenous clastic material brought in by rivers from the continental realm. Offshore
from most coastlines there is a region of shallow water, the continental shelf, which may
stretch tens to hundreds of kilometres out to sea before the water deepens down to the
abyssal depths of ocean basins. Not all land areas are separated by ocean basins, but
instead have shallow, epicontinental seas between them. Terrigenous clastic material is
distributed on shelves and epicontinental seas by tides, waves, storms and ocean
currents: these processes sort the material by grain size and deposit areas of sand and
mud, which form thick, extensive sandstone and mudstone bodies in the stratigraphic
record. Characteristic facies can be recognised as the products of transport and deposi-
tion by tides and storm/wave processes. Deposition in shallow marine environments is
sensitive to changes in sea level and the stratigraphic record of sea-level changes is
recorded within sediments formed in these settings.

14.1 SHALLOW MARINE tinental plates has forced beds deposited in shallow
ENVIRONMENTS OF TERRIGENOUS marine environments high up into mountain ranges.
CLASTIC DEPOSITION This chapter focuses on the terrigenous clastic depos-
its found in shallow seas; carbonate sedimentation,
The continental shelves and epicontinental seas which is also important in these environments, is
(11.1) are important sites of deposition of sand and covered in Chapter 15.
mud in the world’s oceans and account for over half
the volume of ocean sediments. These successions can
be very thick, over 10,000 m, because deposition may 14.1.1 Sediment supply to shallow seas
be very long-lived and can continue uninterrupted
for tens of millions of years. They occur as largely The supply of sediment to shelves is a fundamental
undeformed strata around the edges of continents control on shallow marine environments and deposi-
and also in orogenic belts, where the collision of con- tional facies of shelves and epicontinental seas. If the