Wednesday, July 17, 2013

Notes from Spain


Structural dip on deepening water facies

IPhone dip measurement on Cretaceous deeper-water sediments near Tremp in the Spanish Pyrenees. First photo is horizontal view, second shows dip estimate of 55°. The app is Theodolite.

Alternating layers of limestone (prominent) and marl (recessive).  Note thinning of limestone layers toward the left (younger beds) indicating pulsating sea level changes with net deepening to the left.  This pattern is terminated by a massive limestone (far left) representing sea level fall (shallower water).

Dip shot with Teodolite iPhone app. I generally do 3 quick dip shots and choose the best alignment or average the 3 if they all look equally good.

Devonian chert

Chert nodule in Devonian limestone showing internal fractures of the chert with no fracturing of surrounding limestone matrix. This is a phenomenon we see abundantly in Mississippian age carbonate rocks of Northwest Arkansas, as well as subsurface rocks in Oklahoma and Kansas. The Spanish rocks contain sponge spicules that dissolved to provide enough silica to generate the chert.  This chert specimen shows very clearly that the process of replacing limestone with chert induces sufficient internal stress to fracture the chert without fracturing the surrounding limestone.  

Devonian chert in limestone.  Note fractures in chert nodule but not in surrounding limestone.


Same location. This photo demonstrates pretty clearly the chert forms in burrows. The burrowing creatures produce coarser sediments with higher permeability and thus better pathways for silica bearing fluids.  The fact that these borrowed cherts are not visibly fractured would seem to indicate a different diagentic history than the fractured chert nodules. 

Coiled burrow originally limestone then altered to chert. Surrounding rock is Devonian limestone.

Long burrow fragment converted to chert.  Note lack of visible fracturing.

This is a view of the outcrop where the chert is found. Lluis is standing on the zone containing chert, which extends up-section to the bed at Rosella's feet. Younger and older strata are chert-free.



************* original field post ********

This week I am in the Pyrenees with Lluis Ardevol on a custom Geoplay carbonate stratigraphy field trip. Also along is Dr. Rossella Sbarra of GeoScience Ltd in the UK.

We are stationed in the village of Sentarada just north of Tremp, and the Geological section is mainly Cretaceous.

We need to get back into the field so just a picture or two here to show the fantastic scenery and geology of the area.

Cretaceous carbonates

Chris in Catalonia

Fracture plane




2 comments:

Pedro Castiñeiras García said...
This comment has been removed by the author.
Gillian B said...

Oh - I wonder if that's the same mechanism that caused the black blobs in the sandstone on the NSW Central coast? It certainly fits the description. http://www.flickr.com/photos/67507070@N00/6640088175/ for a close-up pic.