Friday Field Foto #92: Miocene sedimentary rocks, Tierra del Fuego

This week’s Friday Field Foto is from some Miocene sedimentary rocks exposed along the Atlantic coast of Tierra del Fuego.

The bottom part, from base of visible section to just above the top of the hammer handle, is alternating siltstone and fine-grained sandstone — most of it plane-laminated with some ripple-laminated beds. Above the hammer is alternating granule-pebble conglomerate beds (brown) and fine- to medium-grained sandstone (beige). The conglomerate beds are normally graded with some faint horizontal stratification. The sandstone beds are also normally graded with plane-lamination near base.

Miocene sedimentary rocks, Tierra del Fuego (© 2009 clasticdetritus.com)

Load structures, flame structures, and other soft-sediment deformation features are abundant, especially along bedding planes.

The light gray features sticking out of outcrop are silty concretions — they are a bit more indurated and thus more resistant to erosion.

What environment do these rocks represent? Essentially, this succession is a mix of traction (i.e., bed load) and suspension fall-out sedimentary structures. So, it’s an environment where at least silt and sand (and possibly granules) are transported in suspension. The general interpretation for these rocks is that they are in a delta-front area. I think that is a pretty good interpretation — river mouths/delta fronts can be extremely active areas with high sediment flux. When I was there looking at these, the coarse-grainedness made me think of a submarine fan delta — maybe something like this.

But, I was only there for a day … need more data … who wants to go?!

Happy Friday!