Page 185 - Sedimentology and Stratigraphy

172 The Marine Realm: Morphology and Processes

sediment as primary bioclasts such as fish teeth and The conditions for the formation of black shales
scales and vertebrate bones, but mostly it occurs as are therefore determined by the organic input, the
an authigenic precipitate, which coats grains, forms efficiency of the breakdown of that material by micro-
peloids and micronodules on the sea floor and may bial activity and the dilution effects of terrigenous
also occur as laminae encrusting the sea bed (Glenn clastic, biogenic carbonate or silica. The most favour-
& Garrison 2003). Accumulations of phosphorite are able sites are therefore deep seas where there is poor
favoured by slow sedimentation rates of other mate- circulation between the oxygenated surface water
rials and, like glaucony, are characteristic of con- and the sea floor. Basins with restricted circulation,
densed sections. Hardgrounds can be composed of such as the modern Black Sea, provide optimal con-
laminated phosphorites, while the peloids and other ditions (Wignall 1994), but not all black shales form
grains are concentrated into phosphate-rich beds in similar settings. Provided the supply of organic
by reworking of the material by seafloor currents material is greater than the rate at which it can be
(Glenn & Garrison 2003). broken down, black shales can form on shelves where
Modern phosphorite concentrations occur on con- circulation is moderately effective. They have consid-
tinental margins where there are regions of upwelling erable economic importance in sedimentology and
of nutrient-rich waters, such as off the west coast of stratigraphy as they are hydrocarbon source rocks
South America and off west Africa where Antarctic (18.7.3).
water comes to the surface. These nutrient-rich cool
waters coming up into warmer waters promote
blooms of plankton, which are at the bottom of the 11.6 MARINE FOSSILS
food chain. Ancient phosphorites are thought to
have formed in similar settings and it might also be Shelves are areas of oxygenated waters periodically
expected that concentrations would be greatest at swept by currents to bring in nutrients. As such they
times of high sea level when supply of other sediment are habitable environments for many organisms that
to the shelf is reduced. Phosphorite production is also may live swimming in water (planktonic) or on the
related to the supply of phosphate, which ultimately sea floor (benthic), either on the surface or within
comes from the weathering of continental rocks. the sediment. Plants and animals living in the marine
realm contribute detritus, modify other sediments
and create their own environments. Modern shelf
11.5.3 Organic-rich sediments: black shales environments team with life and it is rare to find an
ancient shelf deposit that does not contain some evi-
Organic material from dead plants, animals and dence for the organisms that lived in the seas at the
microbial organisms is abundant in the oceans and time.
becomes part of the material that falls to the sea floor. In shallow seas with low clastic input the calcar-
Where the sea floor is oxygenated by currents bring- eous hard parts of dead organisms make up the bulk
ing water down from the surface the organic matter is of the sediment, either as the loose detritus of mobile
oxidised or consumed by scavengers living on the sea animals or as biogenic reefs, which are whole sedi-
bed. Poor circulation reduces the oxygen in the ment bodies built up as a framework by organisms
waters at the sea floor and the conditions become such as corals and algae. Terrigenous clastic sandy
anoxic. Breakdown of the organic matter is slower and muddy shelf deposits may also contain a rich flora
or non-existent in the absence of oxygen and the and fauna, the type and diversity of which depends on
conditions are not favourable for scavenging organ- the energy on the sea bed (fragmentation can occur in
isms. The organic matter accumulates under these high-energy environments) and the post-depositional
anoxic conditions and contributes to the pelagic sedi- history (Chapter 18), which affects preservation of
ment to form black shale, a mudrock that typically material.
contains 1–15% organic carbon (Wignall 1994; Stow Many plants and animals occupy ecological niches
et al. 1996). The black or dark grey colour is partly that are defined by such factors as water depth, tem-
due to the presence of the organic matter and also perature, nutrient supply, nature of substrate and so
because of finely disseminated pyrite (iron sulphide), on. If the ecological niche of a fossil organism can be
which also forms under reducing conditions. determined this can provide an excellent indication of