Tuesday, March 13, 2012

Regional Strat Column Correlations

Guest lecture by Cara Harwood on regional correlations of stratigraphic columns across the Precambrian-Cambrian boundary in the western US

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Geology 109: Sediments and Strata
March 12, 2012
Cara Harwood

Integrated Stratigraphic Analysis:  Correlating multiple columns using lithology, biostratigraphy, and chemostratigraphy


Recap:  You have been looking at interpreting stratigraphic columns based in lithology, distinctive structures, sequences of structures.  We use Walther’s law to predict how rocks will be stacked vertically.  


Today:  Looking at multiple stratigraphic columns from the same time period that correlate across a region.  Focusing on the Precambrian-Cambrian boundary and how multiple columns are used along with biostratigraphy and chemostratigraphy to understand and recognize this key time period. 
The Precambrian-Cambrian transition records one of the most important intervals in the history of life.  It encompasses appearance and diversification of metazoans, a change in style of bioturbation (organisms are moving to different types of environments within the sediment), and mineralization of skeletons.  
Sedimentologists, stratigraphers, paleontologists, have studied this interval to try to learn about: 
  • the time of the boundary, and correlating the boundary globally
  • timing of evolutionary events
  • what environments were like where these events were happening
  • how environments were distributed
They studied stratigraphic columns and looked at lithologic, paleontologic, and chemostratigraphic data.  We will talk about each of these types of data and how they are correlated in the context of the pC-C boundary in southern California. 

Stratigraphy Types
Biostratigraphy: characterization and correlation of rocks based on their fossil content, based on the principle that organisms have undergone successive changes through geologic time
Chemostratigraphy:  characterization and correlation of rocks based on their chemical composition, based on the principle that certain chemical signatures occur globally through geologic time
Why is studying the pC-C boundary in California interesting? 
  • The boundary is defined by a trace fossil that is present in siliciclastic rocks
  • It is also defined by a specific chemical signature in carbonate rocks
  • Many regions that preserve this boundary do not have both siliciclastics and carboantes together, but they are both present in this region so we can see how they correlate
Paleogeographic map of North America 
550 Ma:
http://www2.nau.edu/rcb7/namPC550.jpg
510 Ma:
http://www2.nau.edu/rcb7/namC510.jpg
Regional map showing positions of the craton margin relative to the Death Valley region and White-Inyos
From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf
 
Look at White-Inyo stratigraphic column (on left of the following figure) and talk through the whole thing - lithology, fossils including trace fossils...
From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf
Formations around the Precambrian-Cambrian boundary:
  • Wyman Formation: interbedded mudrock, siltstone, quartzite; interbedded carbonate layers that increase in number upsection --> shallow marine deposition
  • Reed Dolomite: cross bedded oolitic grainstone, stromatolites --> shallow marine, subtidal to intertidal; then hummocky cross bedded sandstone --> deposition below normal wave based, but above storm wave base. 
  • Deep Spring Formation: siliciclastic carbonate couplets, with ripple laminated quartzites with hummocky cross stratification, cross bedded oolites, intraclastic grainstone --> high energy shallow water depositional environment.  
Each of these is a formation: a mappable unit that can be defined by its lithology and stratigraphic position; has some degree of homogeneity in rock type, mineralogical composition, sedimentary structures, fossil content.  

Treptichnus pedum - pC-C boundary index fossil globally; it is one of the first (and once thought to be the first) complex trace fossils, indicating more complex organisms that made the trace.   Show examples of this trace fossil.

Once we get into the Cambrian trilobites are good index fossils that are useful for biostratigraphy; specific time zones with unique assemblages of trilobite species.

Index fossils generally - can be trace fossils or body fossils; they mark specific time periods.  Good index fossils are taxa that appear and disappear (evolve rapidly), that are widespread - global distribution, that are distinctive and abundant, and that are facies independent.   **T. pedum is only found in siliciclastic rocks, so not found everywhere at the boundary.    

Look at how the White-Inyo section is correlated with other sections in the region - Death Valley and craton margin sections.  


Note that the scale of these columns is different than others that we have been looking at - the total thickness of the White-Inyo column is 2900 m, so we’re not looking at each individual bed, but rather looking at facies and dominant lithologies.  


Look at the 510 Ma paleogeographic map again to see the broad environments where each of these columns are from.  

Marine deposits in the west correlate to the east with non-marine facies: glacial deposits, non-marine sandstone and conglomerate, and there are more unconformities.  Also notice how the overall thickness of the columns is becoming thinner to the east. 

What general observations can we make about correlating stratigraphic columns across a region? 
  • sediment packages thicken as we move from the craton (land) out into the basin
  • more unconformities (can happen when exposed, above sea level) towards land
  • facies shift from being a mix of non-marine and marine to mostly marine and deeper water facies
  • correlating long distances - can’t do lithostratigraphy (i.e. correlating sands to sands)
  • correlations based on fossil/trace fossil occurrence are robust
Look at chemostratigraphic signature (C isotopes) from ‘offshore’ (White-Inyos) to ‘onshore’ (Death Valley and craton sections):
From Corsetti and Hagadorn, 2003, Sedimentary Record: http://www.sepm.org/CM_Files/SedimentaryRecord/sedrecord1.1.pdf
Correlations across a region (and globally!) can also be made based on chemostratigraphy.   Even though the lithology is different, carbonate rocks track the concentration of ions in seawater, so the marine rocks everywhere can have the same signature. 
We’re not going to talk about what these chemical signatures mean, but notice how in the Lower Deep Spring Formation there is a positive 13 C value, and then around the pC-C boundary there is a negative value.  This is a global trend during this time period. 

Other elements also have global trends and can be used for correlating based on chemostratigraphy.  Show examples of Sr and O isotope curves.  

Using this integrated approach (chemostratigraphy and biostratigraphy) showed that the index fossil and the distinct chemical signature occur at the same time.  With this time constrained, we can apply either one of these (chemical signature or fossil) to correlation in other sections, and use this to better understand this time period.

Wrap up - this is an example of using stratigraphy.  Key points: 
  • Looking at mixed carbonate-siliciclastic facies allows us to combine data types that occur in just one
  • We get a complete picture of the time period by looking at a combination of data sets (lithostratigraphy/facies, biostratigraphy, and chemostratigraphy)...
  • ...and how they are distributed across the region - looking at multiple stratigraphic columns

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