5 December 2017

The quarry in Contessa Gorge

Posted by Callan Bentley

After visiting Bottaccione Gorge and reuniting with the K/Pg boundary, my summer geologizing with Alan Pitts took me next to Contessa Gorge, where we saw this lovely wall of stratigraphy and structure exposed in a quarry’s cut:

Let’s zoom in a bit and see what there is to be seen:

Here it is annotated:

“Maiolica,” “Marne A Fucoidi,” and “Scaglia Bianca” are all names of different stratigraphic units, different formations in the central Apennine stratigraphic sequence. The Marne A Fucoidi is a beautiful mix of deep green and maroon. It’s alternating shale-dominated layers and calcareous-dominated layers.

The units above and below tend toward white (“bianca” in Italiano) but the upper unit, the Scaglia Bianca, has a distinctive anoxic layer within it, the Bonarelli Level, that helps distinguish it from the lower unit, the Maiolica. (Sorry, I didn’t take a photo of it.)

All this stratigraphy is well and good — but I was here for the structure!

…And annotated:

“MTD” here stands for “mass transport deposit” – a set of sedimentary layers that slumped downward and crumpled up deeper into the basin when the sediment was still soft (prior to lithification). Alan and I have enjoyed puzzling over another MTD in West Virginia‘s westernmost Valley & Ridge province. Here in Italy, the pelagic limestones seem to have cohered a bit better in layers, while the West Virginia example saw total disaggregation of the sandstone layers into segments that folded up on themselves, wrapped up in a matrix of squishy mud. The Italian example is much better behaved:

This is interpreted as syndepositional faulting and deformation. It’s not thought to be directly tectonically induced.

The quarry was a treat to see. I love outcrops like this – beautiful, with some structure and some stratigraphy, and something to compare to other places.