Page 92 - Sedimentology and Stratigraphy

Description of Core 79

sample is placed in. The advantage of this procedure is them individually (Mange & Maurer 1992). This pro-
that sample numbers will not be missed or duplicated cedure is called heavy mineral analysis and it can
by mistake, and they will always be legible, even in be an effective way of determining the source of the
the most unfavourable field conditions. sediment (Morton et al. 1991; Morton & Hallsworth
1994; Morton 2003).
Clay mineral analysis is also sometimes used in
5.4.1 Provenance studies provenance studies because certain clay minerals are
characteristically formed by the weathering of parti-
Information about the source of sediment, or prove- cular bedrock types (Blatt 1985): for example, weath-
nance of the material, may be obtained from an ering of basaltic rocks produces the clay minerals in
examination of the clast types present (Pettijohn the smectite group (2.4.3). Analysis of mud and
1975; Basu 2003). If a clast present in a sediment mudrocks can also be used to determine the average
can be recognised as being characteristic of a particu- chemical composition of large continental areas.
lar source area by its petrology or chemistry then its Large rivers may drain a large proportion of a con-
provenance can be established. In some circum- tinental landmass, and hence transport and deposit
stances this makes it possible to establish the palaeo- material eroded from that same area. A sample of
geographical location of a source area and provides mud from a river mouth is therefore a proxy for
information about the timing and processes of erosion sampling the continental landmass, and much sim-
in uplifted areas (6.7) (Dickinson & Suczek 1979). pler than trying to collect representative, and propor-
Provenance studies are generally relatively easy to tionate, rock samples from that same area. This is a
carry out in coarser clastic sediments because a peb- useful tool for comparing different continents and can
ble or cobble may be readily recognised as having be used on ancient mudrocks to compare potential
been eroded from a particular bedrock lithology. sources of detritus. In particular, geochemical finger-
Many rock types may have characteristic textures printing using Rare Earth Elements and isotopic dat-
and compositions that allow them to be identified ing using the neodymium–samarium system (21.2.3)
with confidence. It is more difficult to determine the can be used for this purpose.
provenance where all the clasts are sand-sized
because many of the grains may be individual miner-
als that could have come from a variety of sources. 5.5 DESCRIPTION OF CORE
Quartz grains in sandstones may have been derived
from granite bedrock, a range of different meta- Most of the world’s fossil fuels and mineral resources
morphic rocks or reworked from older sandstone are extracted from below the ground within sedimen-
lithologies, so although very common, quartz is tary rocks. There are techniques for ‘remotely’ deter-
often of little value in determining provenance. It mining the nature of subsurface strata (Chapter 22),
has been found that certain heavy minerals (2.3.1) but hard evidence of the nature of strata tens, hun-
are very good indicators of the origin of the sand dreds or thousands of metres below the surface can
(Fig. 5.10). Provenance studies in sandstones are come only from drilling boreholes. Drilling is under-
therefore often carried out by separating the heavy taken by the oil and gas industry, by companies pro-
minerals from the bulk of the grains and identifying specting mineral resources and coal, for water

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provenance indicators.