Friday Field Foto #84: Plane- and ripple-laminated sandstones in France
This week’s Friday Field Foto is from the foothills of the French Alps (not far from Nice).
As you all know I’m a turbidite connoisseur — today’s photo is from a rather famous turbidite formation called the Annot Sandstone (Eocene-Oligocene, or ~40-30 million yrs old). It is ‘famous’ because this is the locale where Arnold Bouma did his dissertation research in the late 1950s. The story is that while trying to correlate across covered/eroded sections of outcrop he started using the sedimentary structures in the beds themselves, which led to the recognition of a general repeating pattern. This pattern of vertical stacking of types of sedimentary structures became known as the Bouma Sequence.
The photo above is from the same section of rocks and nicely shows the plane-laminated (or Tb) part of the sequence in the lower part with the wavy- to cross-laminated (or Tc) division in the middle part of the photo. The boundary between Tb and Tc is very commonly gradational and, in some cases, alternates. At the top of the photo, the structureless sandstone is very likely the basal division (or Ta) of the next turbidite bed. The Ta division typically shows normal grading — I’ll refer you to a recent post over at Hindered Settling that nicely discusses the concept of normal grading.
It is important to note that the ideal Bouma sequence is just that — ideal. In many turbiditic sequences it can be difficult to find beds with the perfect stacking of these divisions. Over the decades, sedimentologists have come to appreciate many variations on the Bouma sequence as well as deposits that represent wholly different processes but are still lumped into what we call ‘turbidites’. In many guidebooks or papers, you might see well-developed Bouma sequences referred to as ‘classical’ turbidites.
If you are doing field work and come across sandstone beds with these sedimentary structures, make sure to describe them as objectively as you can (e.g., plane-laminations, ripple-laminations, normal grading, etc.). Then, make the interpretation that they are turbidites.