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Friday Field Foto #70: Permian carbonate slope system

October 31, 2008

Earlier this week I posted about some research I did on a siliciclastic depositional slope system and mentioned how different carbonate depositional slope systems can be.

This week’s Friday Field Foto is from the Guadalupe Mts of west Texas and New Mexico. The southern rim of this mountain range is essentially an ancient carbonate shelf margin (~250 million years old). Canyons cutting back into the mountains expose fantastic cross sections of this preserved depositional system.

The photo below is taken from the fantastic Permian Reef Trail in McKittrick Canyon in the park looking across the canyon to the southwest. Click on it for a bigger version.

Permian strata in McKittrick Canyon, Guadalupe Mtns Nat'l Park (© 2008 clasticdetritus.com)

Permian strata in McKittrick Canyon, Guadalupe Mtns Nat'l Park (© 2008 clasticdetritus.com)

This next photo (below) is the same with some simple annotation highlighting the sloping stratal surfaces within the carbonate sedimentary rock. These surfaces are not structural but represent the stacking of the actual depositional surface going from shallower waters of the platform (to the right) to deeper waters (to the left).

Permian strata in McKittrick Canyon, Guadalupe Mtns Nat'l Park (© 20

Permian strata in McKittrick Canyon, Guadalupe Mtns Nat'l Park - with annotation showing reef foreslope surfaces (© 2008 clasticdetritus.com)

The steepness of these slopes are such that you can see them and map them in mountainside outcrops! This is very cool. Siliciclastic slopes, on the other hand, have at most a few degrees of relief and can be very difficult to characterize in a single outcrop.

I know Suvrat is a carbonate sedimentologist, perhaps he can add some more info and/or correct me with regards to carbonate slopes.

I’ve posted about this region several times before – see the west Texas tag.

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2 Comments leave one →
  1. October 31, 2008 8:52 pm

    Brian-

    Many thanks for the fantastic photo of the Permian reefal complexes. You have summarized quite well!

    I would just like to add that what you are seeing in the photo is actually the forereef facies i.e. the debris the reef sheds basinwards. These are likely limestone conglomerates closer to the reef grading into finer limestone sands and muds basinwards (towards the left in photo). You can also see a slight flattening of the slope basinwards. Very early marine cementation gives these forereef debris deposits some rigidity and helps maintain the steep slopes.

    The reef itself (not in the photo) does not show distinctive bedding being made up of a framework. Permian reefs of Texas are not built by corals but by a variety of encrusters and binders like red algae, byrozoans, sponges, tubiphytes and foraminifers.

    cheers

  2. November 2, 2008 1:14 pm

    Suvrat … thanks … yeah, that’s right, it’s the forereef facies. Some nice calc-turbidites as you go further down too!

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