Download Chapter 15

Chapter 15 Lithostratigraphy
Chapter 15 is the longest, and perhaps the most important chapter in the book.
Its key message is the reality of “facies change” – that is that even within a
relatively restricted area it is likely that several different types of sediments will be
deposited simultaneously. This is most clearly evident in near-shore
environments - (such as deltas – see Figure 9.2), where there are systematic
changes in depositional environments and deposit types as the energy level
decreases in the off-shore direction - but it applies in many other depositional
environments as well.
Transgression and regression are also extremely important in the stratigraphic
record, and can result from several different mechanisms:
• tectonic – the vertical movement of a crustal block in direct response to
the movement of plates (eg. uplift caused by the compression related to
plate convergence, or subsidence caused by the tension related to
divergence) or in response to other tectonic activity such as mountain
building
• isostatic – the vertical movement of a crustal block in response to
changes in loading (eg. by ice that comes and goes, or by rock (or
sediment) that is piled on or weathered away)
• eustatic – the global changes in sea level that are related to the formation
or degradation of very large ice sheets, or to tectonic processes such as
continental rifting or accretion, or changes in the nature of sea-floor
spreading.
When facies changes are combined with either regression or transgression the
formation of lithostratigraphic units that cut across time lines is almost inevitable
(see Figure 15.2C for example).
As described in the section entitled “Gaps in the Record” we need to forget the
concept that the stratigraphic record is like a tape recording that provides us with
a sequential record of everything that happened. Instead it is more like the Nixon
Whitehouse tape recordings, where huge amounts of information have been
erased (in this case by erosion, or by long periods of non-deposition).
Unfortunately, we rarely have a clear idea of what has been erased, nor how
much is missing.
Make sure that you understand the different types of unconformities. We are
going to see examples of angular unconformities and nonconformities and
(maybe) disconformities. As noted above, the stratigraphic record has abundant
paraconformities (ie. gaps) – but most of them are very difficult to detect.
The section entitled “The Nature of the Control” is about the many different
mechanisms – on different times scales – that can control sea and land levels.
This information can be confusing because of all of the different time scales and
Malaspina University-College – GEOL 201 – Sedimentary Geology – May 2004
mechanisms involved, and because there are some doubts as to what is actually
causing the various cycles. Just remember that all of these processes fall within
the three categories listed above – tectonic, isostatic and eustatic. (Ecstatic is
not one of these – it is the feeling that you’ll get when you understand the other
three!)
Don’t worry too much about the Stratigraphic Code, but do read the section on
measuring and describing stratigraphic sections (Box 15.1), since we will be
doing that. There are two main parts to the construction of a stratigraphic
section. The first is the description of the units and measurement of their
thicknesses (as in Fig. 15.1.1), and the second is the actual drawing of the
stratigraphic column (as in Fig. 15.1.2 B).
Chapter 16 Biostratigraphy
Biostratigraphy is the primary method, and in many cases the only method
available, to date and correlate sedimentary rocks. Sedimentary rocks cannot be
dated directly using radiometric techniques, and some of the other tools that we
have (such as magnetic dating) are only useful if we can first narrow down the
age using palaeontology.
The first part of Chapter 16 (up to p. 344 [372]) deals with some of the significant
problems of biostratigraphy – the primary one being ecological differences
between different parts of the world, and between different depositional
environments, in some cases separated by as little as a few hundred metres.
The second part of the chapter (up to p. 352 [381]) covers the use of fossils to
date rocks. The paleontological literature is huge, and there are published time
ranges for tens of thousands of different organisms. The steps in using fossils to
date a rock include: identifying the fossil, determining its age range in the
environment and region where the rock was formed; and then applying some of
the techniques described under the section entitled: “Biostratigraphic zonation”.
Don’t worry about the sections on North American mammals and quantitative
biostratigraphy.
Malaspina University-College – GEOL 201 – Sedimentary Geology – May 2004
Chapter 19 Sedimentary rocks in space and time
The most important aspect of Chapter 19 is the concept of the accumulation of
sediments in sedimentary basins. (Sediments that don’t accumulate in basins
don’t have very good prospects for survival, and thus represent only a small part
of the sedimentary rock record.) The relationships between basins and plate
tectonics is also important, and here is a quick summary of some of the types of
sedimentary basins that can exist:
Basin type
Rift basin
Passive
margin basin
Description and origin
DIVERGENT MARGINS
The down-dropped basin formed during rifting
because of stretching and thinning of oceanic
crust
Subsidence along a passive margin, mostly due
to long-term accumulation of sediments on the
continental shelf
CONVERGENT MARGINS
Modern example
& diagram in text
East Africa Rift (Fig.
19.12)
East coast of N.
America (Fig. 19.17)
Trench
Downward flexure of the subducting and nonsubducting plates (sites of accretionary wedges)
The western edge of
Vancouver Island
(Fig. 19.18B)
Forearc
basin
The area between the accretionary wedge and
the magmatic arc, largely caused by the negative
buoyancy of the subducting plate pulling down on
the overlying continental crust
Coast of Washington
& Oregon (Fig.
19.18B)
Interarc
basin
Foreland
basin
Strike-slip
basin
The area in between two island arcs, such as an
active island-arc and a remnant island arc
A depression caused by the weight of a large
mountain range pushing the adjacent crust below
sea level
TRANSCURRENT MARGINS
A pull-apart block (eg. between two transform
faults) that subsides significantly
South China Sea –
west of the
Philippines (Fig.
19.18A)
The sediment filled
plain south of the
Himalayas
Various locations on
the San Andreas
Fault (Box 19.1)
The Nanaimo Group accumulated within what is most likely a forearc basin
between the subduction zone and the magmatic arc that existed where the Coast
Range is now. One might also argue that is was partly a foreland basin, related
to depression caused by the mountain chain.
Please try to understand as much as you can about the various types of basins,
and the rocks that form within them, but don’t worry too much about the last
section of the chapter entitled “Secular changes in the sedimentary record”.
Malaspina University-College – GEOL 201 – Sedimentary Geology – May 2004