Friday Field Foto #95: Turbidites onlapping a basin margin
This week’s Friday Field Foto is from the French Alps and shows a very cool stratigraphic relationship that is commonly observed in regional-scale seismic-reflection data but not as commonly recognized in outcrops.
Note the two sandstone bodies in the photo above — they are the resistant cliff-forming rocks that are wedge-shaped and pinch out from left to right. The upper one appears to pinch out to the left but I am told by those who have climbed around on there that that is a function of it getting covered. The other thing to note is the fine-grained strata in the lower part has a slightly blueish to gray-blue color.
This next photo (above) is the same photo with some quick (and sloppy) annotation pointing out where those sandstone bodies pinch out.
The interpretation here is that the older blue marl strata was uplifted and tilted and the subsequent turbidite deposits (both the sandstone bodies and the brownish fine-grained strata they are encased in) filled in that basin — and what you are seeing here is the margin of that basin.