Geology and ground models - Practical Engineering Geology - page 58

sediments (clay rich) and reported these as ranging from 0.03m/1,000
years for deep sea conditions, about 1

2.5m/1,000 years for estuaries
and shallowmarine conditions and up to 120m/1,000 years for deltaic
situations. Sand deposited close to the shoreline tends to be quite well
sorted, bedded and often fossiliferous ( Figure 3.16). Offshore, below
the continental shelf, thick, often relatively poorly sorted sandstones
are sometimes deposited very rapidly from dense turbidity currents
arising from submarine landslides. Rocks formed in this way (once
lithi

-

ed) include greywacke. They are often bedded and cyclic with
predominantly

finer-grained horizons followed by predominantly
coarser beds ( Figure 3.17). Submarine landslides are a major hazard
to offshore engineering works, including pipelines, cables and oil rigs,
and are often triggered by earthquakes, although they may simply
occur as the sediment accumulation on the continental shelf reaches
a critically unstable geometry.

Figure 3.16
Bedded Miocene
sandstone, Capela
da Sra da Rocha,
Nr Porches,
Algarve, Portugal.

Figure 3.17
Carboniferous
schistose turbidites,
Cachopo Road,
Portugal.