Basin Sedimentology- Ch. 7OutlineBackgroundDepositional System and Facies ModelsWalther's Law and Facies AssociationsDownhole tools and their graphed responses ("logs")Continental Systems Fluvial Channels Meandering vs. BraidedLacustrine Systems Siliciclastic systemsDelta bodiesCarbonate and evaporite systemsReefsRift Basin depositional stylesCollisional foreland basins depositional stylesStrike-slip basins depositional styles(I recommend www.uga.edu/~strata/sequence/seqStrat.html for sequence stratigraphy review for last class)1BackgroundDepositional System and Facies ModelsThere 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 LawA horizontal association of environmnts and their sedimentary products if preserved will have an analogous vertical distribution in the sedimentary record.Facies AssociationsA 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.2Electrical 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, whenwe 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 thereare 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 completelyemptied. 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. BraidedIn 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)3Siliciclastic systemsDelta 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 systemsReefs 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 systemby interpreting the energy of deposition of the facies. The modern day depositional model could be something like the Arabian Gulf: Rift Basin depositional stylesFault geometry plays a strong role in the facies distribution of fluvial systems.Half-graben systems are the typical basin geomtery