Lateral Lithostratigraphic Changes

I. INTRO

A. Beds laterally bounded1. erosion2. gradational

i. pinchoutii. intertonguingiii. lateral gradational

B. Pinch-out1. unit thins laterally2. small angle of convergence3. some environments, rocks thin rapidly4. good stratigraphic trap

Turbidite Pinchout

ftp.pet.hw.ac.uk/.../deep_water/research.htm

Pinchouts May Form Stratigraphic Traps.

www.elsandcompany.com/looking.htm

Pinchout of turbidites filling channels--Chile

clasticdetritus.com/.../

C. Intertonguing1. sed body splits, each w/ pinchout2. intercalation of units3. Tongues = changes

in sed rates due to tectonics or changes in sea level.

Intertonguing Relationships & Designation of formation boundaries

D. Lateral Gradations1. thinning unnecessary2. lateral lithologic change (mixed/continuous)3. changes can occur over miles4. ss goes to sh = shale out

II Causes of Changes

A Sedimentation rateB. Subsidence ratesC. Above related to:

1. Tectonics2. Climate

SEDIMENTARY FACIES

I. INTROA. Environment = lithologic/paleontologic control

1. same environment, not age2. need to differentiate:

i. vertical changes (formations, fossil zones)

ii. lateral changes in rocks)

B. Gressley (1838)1. classified rocks vertically2. unsatisfied, coined facies

i. Latin = face, figure, appearance, aspect, look, condition

3. Distinguish between facies and depositional environment

4. Facies is product of environment5. Dep environment = biological,

physical, chemical process impacting sedimentation

Rocks and Facies  From Sediment Transport to Rocks - We have been talking about sediment transport and structures. These are processes that influence sedimentary rocks. What we really need is to be able to use our understanding of the processes to interpret ancient rocks when we can no longer see the processes in action. As I mentioned in the first class, we can use the modern processes as a model for interpreting past processes, which is the principle of Uniformitarianism. However, it is often very different to see a process going on than it is to look at the ultimate deposited rock and interpret the process. For example, with bed forms, the entire shape of the structure you see as it migrates is rarely preserved. Instead, you only see a small part of it, if you get any sediment accumulation at all. Thus, we can also start the interpretation from the rock end by describing the general characteristics of the rocks and interpret flow from things like grain size, preserved cross stratification, and biogenic components. Then we can see which environments are consistent with those characteristics.   Sedimentary Facies Analysis - This approach to interpretation is called sedimentary facies analysis. A facies (Latin for aspect or appearance) is a body of rock (i.e. a sequence of beds, etc.) marked by a particular combination of compositional, physical and biological structures that distinguish it from bodies of rock above, below and adjacent to it. A sedimentary facies has a characteristic set of properties that makes it distinctive, which the geologist defines.  Facies vs Environments - By grouping characteristics of the rocks into facies, the depositional environments can be more easily compared and interpreted. It is important to remember that the sedimentary environment is the combination of physical, chemical and biological processes that influence sediment deposition, whereas sedimentary facies are the characteristics of the rocks after deposition. It is the difference between a water flow speed of 20 cm/sec and high angle cross stratification; the stratification is the result of high flow speed, but they are not the same.

C. Term not solely stratigraphic

1. metamorphic, garnet, western, trilobite2. muddled use3. descriptive vs. inferred environmenti. sandstone vs. fluvial4. distinguish petrologic/faunal characteristicsi. lithofacies vs. biofacies

III Walther’s Law (1983-1894)A. Within a given sedimentary cycle, the same succession of facies that occur laterally is also present in a vertical succession.B. Current usage:

1. The sum of the lithologic and paleontologic characteristics of a sed deposits

i. sum of all the primary characteristicsC. Applied to conformable succession

1. unconformities2. faults

Walther's Law of Facies, named after the geologist Johannes Walther, states that the vertical succession of facies reflects lateral changes in environment. Conversely, it states that when a depositional environment "migrates" laterally, sediments of one depositional environment come to lie on top of another.[3] A classic example of this law is the vertical stratigraphic succession that typifies marine trangressions and regressions. However, the law is not applicable where the contact between different lithologies is non-conformable (i.e. sedimentation was not continuous), or in instances of rapid environmental change where non-adjacent environments may replace one another.

http://faculty.weber.edu/bdattilo//fossils/notes/facies.html

Transgression

Regression

Units Do Not Correspond to Same Age

http://higheredbcs.wiley.com/legacy/college/levin/0471697435/chap_tut/chaps/chapter05-11.htm

Showing Pinchouts