Page 229 - Sedimentology and Stratigraphy

216 Shallow Sandy Seas

area lies adjacent to an uplifted continental region (Chapter 15), but can also be very abundant in sands
and there is a drainage pattern of rivers delivering and muds deposited in these seas. Whole shells and
detritus to the coast, the shallow-marine sedi- skeletons may be preserved in mudrocks because they
mentation will be dominated by terrigenous clastic are low-energy deposits. In higher energy parts of
deposits. The highest concentrations of clastic sedi- the sea, currents move sand around and a lot of bio-
ment will be near the mouths of major rivers: genic debris is broken up into bioclastic fragments
adjacent coastal regions will also be supplied with ranging from sand-sized, unidentifiable pieces up to
sediment by longshore movement of material by larger pieces of shelly material and bone. Bone is
waves, storms and tides. Shallow seas that are not also the origin for phosphates that can form as authi-
supplied by much terrigenous material may be areas genic deposits in shallow marine settings (3.4): these
of carbonate sedimentation, especially if they are in phosphates are relatively rare. However, another
lower latitudes where the climate is relatively warm. authigenic mineral, glauconite (11.5), is a common
In cooler climates where carbonate production is component of sandstones and mudrocks formed
slower, shelves and shallow seas with low terrigenous on shelves and epicontinental seas and is con-
sediment supply are considered to be starved. The sidered to be a reliable indicator of shallow marine
rate of sediment accumulation is slow and may be conditions. The characteristic dark green colour of
exceeded by the rate of subsidence of the sea floor the mineral gives sediments rich in it a distinctly
such that the environment becomes gradually deeper green tinge, although it is iron-rich and weathers to
with time. a rusty orange colour. ‘Greensands’ are shallow-
marine deposits rich in glauconite that are particu-
larly common in Cretaceous strata in the northern
14.1.2 Characteristics of shallow hemisphere.
marine sands Shallow seas are environments rich in animal
life, particularly benthic organisms that can leave
Detritus that reaches a shallow sea is likely to have traces of their activity in the sediments. Bioturbation
had a history of transport in rivers, may have passed may form features that are recognisable of the
through a delta or estuary, or could have been tem- activities of a particular type of organism (11.7),
porarily deposited along a coastline before it arrives at but also results in a general churning of the sedi-
the shelf. If there is a long history of transport thro- ment, homogenising it into apparently structureless
ugh these other environments the grain assemblage is masses. Primary sedimentary structures (wave rip-
likely to be mature (2.5.3). Texturally, the grains of ples, hummocky cross-stratification, trough cross-
sand will have suffered a degree of abrasion and the bedding, and so on) are not always preserved in
processes of turbulent flow during transport will sepa- shelf sediments because of the effects of bioturbation.
rate the material into different grain sizes. The com- Bioturbation is most intense in shallower water and is
positional maturity will probably be greater than the frequently more abundant in sandy sediment than in
equivalent continental deposits, because the more muddy deposits. This is because the currents that
labile minerals and grains (such as feldspar and lithic transport and deposit sand may also carry nutrients
fragments) are broken down during transport: shal- for benthic organisms living in the sand: many
low marine sands are commonly dominated by quartz organisms also prefer to live on and within a sandy
grains. In polar areas, the sediment supplied is much substrate.
less mature because cold weather reduces chemical The abundance of calcareous shell material in shal-
weathering of the grains and glacial transport does low-marine sandstones makes calcium carbonate
not result in much sorting or rounding of the clasts available within the strata when the beds are buried.
(7.3.4). Groundwater moving through the sediments dissolves
The detrital component is often complemented and reprecipitates the carbonate as cement (18.2.2).
by material that orginates in the shallow marine envi- Shelly fossils within sandstones are therefore some-
ronment. Shallow seas are rich in marine life, inclu- times found only as casts of the original form, as the
ding many organisms that have calcareous shells and original calcite or aragonite shells have been dissolved
skeletons. The remains of these biogenic hard parts away. Sandstone beds deposited in shallow marine
are a major component of shelf carbonate deposits settings also typically have a calcite cement.