Basin Sedimentology- Ch. 7
Outline
Background
Depositional System and Facies Models
Walther's Law and Facies Associations
Downhole tools and their graphed responses ("logs")
Continental Systems
Fluvial Channels
Meandering vs. Braided
Lacustrine Systems
Siliciclastic systems
Delta bodies
Carbonate and evaporite systems
Reefs
Rift Basin depositional styles
Collisional foreland basins depositional styles
Strike-slip basins depositional styles
(I recommend www.uga.edu/~strata/sequence/seqStrat.html for sequence
stratigraphy review for last class)
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Background
Depositional System and Facies Models
There are many depositional environments, such as coastlines, deltas and deep-sea fans
but when they all influence each other we say that they all belong to the same
depositional system. Within a depositional system, from the mountain to the deep sea,
there are internal processes that created depositional environments. For example,
overbank flooding, avlusion and crevasse splay depositional environments can be
considered internal to a fluvieal depositional environment. Tectonic rifting may be
considered a distal process (plate tectonics).
Walther's Law
A horizontal association of environmnts and their sedimentary products if preserved will
have an analogous vertical distribution in the sedimentary record.
Facies Associations
A facies is a rock unit that is distinctive by the sum of its attributes, be they color, fossil
content, grain size composition etc.
In trying to construct an environment of deposition it is best to group facies into sets of
facies that commonly occur together. Whereas an individual facies may be ambiguous a
group of facies can better identify uniquely a particular depositional environment.
For example, claystone with fossils could have been deposited in fluvial overbank
deposits, clay can be deposited in an estuarine environment and fine sand can occur in
crevasse splay deposits as well as point bars, but when we have dominantly stacked sand
channels with large trough-cross bedding, with large poorly sorted angular breccia and
small amounts of clay lenses we may be looking at a large alluvial fan/braided system or
a lahar/braided system - (gravity debris flow in a volcanic area). We use modern
examples to best guess what environments the groups of facies represent.
Downhole tools and their graphed responses ("logs")
Nowaday we have geophysical tools that can be moved up and down a well to detect
physical properties of rocks such as the neutron density (density tool) velocity of sound
(sonic tool), electrical resitivity, spontaneous potential, natural gamma ray.
A naturally radioactive source bombards the formation with neutron particles which
bounce with intensity that is a function of the density of the unit.
Natural gamma rays are indicator of terrestrially derived clay as clay contains U, Th, K
more so than silicilastic (Q-rich) sediments.
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Electrical resistance is measured by seeing how much the voltage drops over a given part
of a hole across which a known current is applied.
Spontaneous potential is the natural voltage of a unit which indicates the electrical
potentials between the unit in the borehole and the surface. So, clay has more natural
voltage than clean sands.
In summary, sands are more resistive than clays which are better conductors.
Sands emit less radioactivity than clays. So typically we would have the following, when
we plot gamma ray and resistivity together.
By exception, a lignite provides a reducing environment good for uranium to accumulate
so it has a high natural Gamma ray count and it is also highly resistive.
See attached hand-drawn figure.
Continental Systems
Fluvial Channels
Fluvial facies models have reached a high level of sophistication and were developed in
the Mississippi Valley. Here is a classical model from Fisk (1947) LSU.
When sea-level drops the valley is swept clean of its sediments. In the initial stages there
are many coarse siliciclastics that fill the valley. Then as sea-level rises, finer-grained
sediments begin to fill the valley and the system evolves from braided to meandering
river flow. This is being challenged today (thank goodness). The valley is not completely
emptied. It is more empty to closer to its mouth. It is influenced by upriver sediment
contribution and not only by sea-level fluctuations exclusively.
Meandering vs. Braided
In general meandering systems have more clay to sand ratio than braided river systems.
Braided river systems imply faster currents and coarser bedload (sediments carried along
the bottom of the river)
Figure 7.8 and 7.9 permit us to infer a vertical facies association and stacking patern of
multiple such bodies. See second hand-drawn sections.
Lacustrine Systems
Fluvial systems can have a through drainage. In the case that they have no outlet but
represent internal drainage, they cease flow within a basin. In a desert environment we
will have a gradual decrease in sediment grain size toward the center of a basin or as we
become more distal from the active fault. (Figure 7.13)
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Siliciclastic systems
Delta bodies:
Can change onsiderably in morphology and environments of deposition depending on
whether they exist on a wave or tide dominated shelf or are dominated by the river itself
(Mississippi River). Fig. 7.17 and hand-drawn handout
"Deep-sea"
Pulses of sediments down the continental slope travel as a turbidity current within a
turbulent flow. As the flow wanes the sequence fines up. Different portions of the ideal
model dominate individual packets depending on which part of a submarine fan you are
looking at. If you are looking at individual channels in the middle fan (analogous to a
submarine delta) then you may hve these coarser packages dominate. If you are looking
at more distal portions, the finer overbank deposits dominate.
Carbonate and evaporite systems
Reefs
A large poritn of the world's oil occurs in carbonate deposits. Carbonates in semi-arid
environments have a lot of variability depending on whehter the coast is wave, tidal or
sediment dominated. Here is an example of a high-energy (wave) dominated
environment. We can infer the energy of the environment analogous to the fluvial system
by interpreting the energy of deposition of the facies. The modern day depositional
model could be something like the Arabian Gulf:
Rift Basin depositional styles
Fault geometry plays a strong role in the facies distribution of fluvial systems.
Half-graben systems are the typical basin geomtery (Fig. 7.38) . Sediment input occurs
through the
Relay or accommodation zone into the valley (Fig. 7.4)
Meander belts are predominantly convex toward the shallower end of the graben (Figure
7.5)
And there is a greater stacking of sand bodies toward the deeper part of the basin (Figure
7.7)
Depositional environments are strongly controlled by climate and tectonics. For example
a passive margin sequence on the east Atlantic coast starts with rich evaporites and
limestone. (Figure 7.41) Others at higher latitudes may lack the carbonates.
In general the rift stage has volcanic composition, fluvial systems and many lakes.
Drifting is marked by a marine incursion. Generally fininf upward over the whole riftdrift transition
Collisional foreland basins depositional styles
There are three major units: deep sea sediments above shallow near-shore or continental
across an unconformity. Then, there are fining upward marine deep shelf and slope.
Then there is a terrestrial section composed of prograding coarsening upward clastic
cycles. (Hand-drawn)
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Strike-slip basins depositional styles
(Fig. 7.51) The outside basement high feeds the basin. As the basin moves by new fans
are repeatedly produced. The system incross-section looks like a rift valley, rather
narrow and deep with very steep transitions to the adjacent basement. with large lateral
facies changes over short distances. We need 3-D control to see that the source of
sediment within the basin is displaced from its deposits
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