Friday Field Foto #79: Shazam!
When stratigraphers construct correlation diagrams of sedimentary rocks across many 10s to 100s of km the depiction of small-scale relationships has to be simplified. It’s impossible to show meter-scale detail on a diagram meant to depict regional relationships*.
For example, here is a correlation diagram for the siliciclastic depositional systems in Utah related to the Cretaceous Western Interior Seaway (from the Univ of South Carolina sequence strat website, a great resource).
Note how boundary between the yellow (Ferron Member, Emery Member, Blackhawk Fm, etc.) with the gray (Mancos Shale) is not a smooth line — it is drawn as a jagged line. Stratigraphers informally refer to these as ‘shazam’ lines and they are meant to represent facies changes among different rock types that are the same age.
But when you zoom in to the scale of an outcrop, the ‘shazams’ are typically larger than what you can see in any one spot.
This week’s Friday Field Foto is an example of this type of relationship at the outcrop scale. The below photo is from Triassic (?) strata in Dinosaur National Monument in northeastern Utah (note: the below photo is not part of geology shown on diagram above, that was just an example).
The next image is zoomed in a bit more.
Note how the finer-grained layers (dark red is siltstone) are pinching out towards the right. Just below that, a sandstone bed is thinning to the left. So there you go — shazam! Over several kilometers most of this sandstone cliff is replaced by finer-grained strata by this style of small-scale interfingering of facies.
* That said, I can envision a visualization tool similar to the Gigapan viewer in which one can zoom in and out of a correlation diagram and observe multiple scales of relationships. That would be a fun and challenging diagram to construct.