Page 184 - Sedimentology and Stratigraphy

Chemical and Biochemical Sedimentation in Oceans 171

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Fig. 11.10 The main geostrophic current pathways (thermo-haline circulation patterns) affecting the modern oceans. Sink
points in the North Atlantic are due to input of cold glacial meltwater from the Greenland ice-cap.

are closely related (Amorosi 2003). Material made up other material, terrigenous clastic or carbonate, is
of any of these distinctive, medium to dark green slow. It therefore commonly occurs in condensed
minerals is referred to as glaucony. Glaucony miner- sections, that is, strata which have been deposited
als are authigenic, that is, they crystallise within the at anomalously low sedimentation rates. The recogni-
sedimentary environment (2.3.2): this is in contrast tion of periods of low sedimentation rate on the shelf is
to almost all other silicate minerals found within important when assessing evidence of changes in sea
sediments that are detrital (2.3.1). The process of level because outer shelf sedimentation tends to be
forming the mineral, glauconitisation, occurs at the slowest during periods of sea level rise (this is dis-
sea floor on substrates such as the hard parts of for- cussed further in Chapter 23). Thirdly, because the
aminifers, other carbonate fragments, faecal pellets mineral is authigenic and also rich in potassium, it
and lithic fragments. It appears the process requires can be dated by radiometric methods and the age
a particular microenvironment at the interface bet- obtained corresponds to the time of deposition. As
ween oxidising seawater and slightly reducing inter- will be seen in Chapter 21, direct radiometric dating
stitial waters. This typically occurs at water depths of of sedimentary material is rarely possible, but glau-
between about 50 and 500 m, on the outer parts of cony/glauconite is the exception and consequently is
continental shelves and upper parts of continental very important in relating strata to the geological
slopes. time scale (19.1.2).
Glaucony/glauconite is important in sedimentology
and stratigraphy for a number of reasons. Firstly, it is
a reliable indicator of deposition in a shallow marine 11.5.2 Phosphorites
environment, although it can be reworked into deeper
water and occasionally into shallower environments Phosphorites are sedimentary rocks that are enriched
by currents. Secondly it is most abundant within shelf in phosphorus to a level where the bulk composition
sediments under conditions where sedimentation of is over 15% P 2 O 5 . Phosphate may be present in