Lecture 9: Stratigraphic time part II

1. Correlative surfaces
   1. Chronostratigraphy (Wheeler diagrams)
2. Time in the rock record
   1. Sadler effect
3. Cyclostratigraphy

We acknowledge and respect the lək̓ʷəŋən peoples on whose traditional territory the university stands and the Songhees, Esquimalt and W̱SÁNEĆ peoples whose historical relationships with the land continue to this day.

On/Off sediment supply with constant subsidence: time vs topography

On/Off sediment supply with constant subsidence: time vs topography

On/Off sediment supply with constant subsidence: time vs topography (erosion)

On/Off sediment supply with constant subsidence: time vs topography (erosion and hiatus)

On/Off sediment supply with constant subsidence: Wheeler diagram

On/Off sediment supply with constant subsidence: stratigraphic profile

• Now, let's take a "time" view of these stratigraphic column
• this view is sometimes called a chronostratigraphic chart, or a Wheeler diagram
• y-axis is time (top = end of model run)
• x-axis is showing presence / absense of rock at that time. Height is NOT thickness!
• white space is NON-DEPOSITION
• purple space is when EROSION was occuring, gobbling up previously deposited sediment
• can you make some observations about sedimentation occured in this basin?
• gappiest at the proximal and distal ends - FAR FROM SED SOURCE
• when sedimentaiton occurs, most complete in the middle portion
• also most erosive - LARGE GRADIENTS
• so what would a line of correlation look like on a Wheeler diagram CORRELATION = EQUAL TIME, SO FLAT

Time in the rock record

• okay, so let's end with a very big picture look at how time is distributed in sedimentary records
• we can get at this question by assessing how well can we tell time, just by looking at thickness?
• do this in a "past is the key to the present" way
  • by measuring sedimentation rates in analagous settings, and applying them

Time in the rock record

• but the true answer is very different, as told by dated ash layers at the top and bottom of this succession
• not a little wrong, but wrong by three orders of magnitude

Time in the rock record

Time in the rock record

• can show that prediction through a data compilation of accumulation rates, taken from a range of Cenozoic-aged depositional environments
• in black are the sediment trap measurements
• not linear relationship, but a power law relationship Y = kX$^{n}$
• a straight line on a log-log plot: log$_{10}$(Y) = log$_{10}$(k) + nlog$_{10}$(X)

Time in the rock record

• add some deep time examples
• why are they all at the bottom right corner?

Why is this happening?

• how could you get slow accumulation rates, and fast accumulation rates?

Time in our rock record

• we can actually analyze our model outputs, and find the Sadler effect
• a great demonstrate that our model is doing something real!

Why is this happening?

Why is this happening?

Correlating sequences: an example from our model

Correlating sequences: an example from our model

Correlating sequences: an example from our model

Why do the sea-level cycles look funny?

Why do the sea-level cycles look funny?

Correlating sequences

• an amazing key would be detecting this sea level signal
• how could this be done?

Cyclostratigraphy

• Cyclostratigraphy is a sub-discipline of stratigraphy that seeks to identify, characterize and interpret cyclic variations in the stratigraphic record
  • identify cycles $\rightarrow$ interpret timing of cycles $\rightarrow$ age models and correlations
  • fundamentally, it is about explaining the processes behind a record

Information can be described as an infinite series of sin and cos waves of differing frequencies and magnitudes.