Applied Stratigraphix

This one's for David Wheeler - a picture of the Cretaceous Dakota Sandstone in Fort Collins, Colorado, US.

Two interpretations here. The classic interpretation made by the folks at CSU was that this is an incised valley eroding into shoreface sandstones below.

This was made in the 1980s when anytime someone saw a scour into shoreface sandstones it was interpreted to be an "incised valley" - especially in the Book Cliffs of Utah. In 2018 John Howell presented a talk at AAPG in Salt Late City in which he reinterpreted the successions based on hundreds of measured sections, LIDAR, drones etc. All the famous incised valleys were no longer incised valleys but tidal incisions - a very common process along many modern clastic shorelines.

Brian Zaitlin defined incised valleys as at least "two parasequences deep" and in this outcrop the tidal incision has removed the foreshore, the Upper Shoreface and part of the Lower Shoreface but definitely not the entire parasequence or the one underneath.

The alternate interpretation is that this is just a tidal channel cutting into shoreface deposits with no evidence for sea-level fall. This is a simpler, less-dramatic interpretation but the outcrop is still just as cool.

WRS = Wave Ravinement Surface
TRS = Tide Ravinement Surface

Alternate interpretations are welcome!

If you're working on porphyry or epithermal deposits, this is a great opportunity to refine your understanding in some beautiful locations.