Wednesday, October 8, 2008

Travertine along the Lima coast

In my article about the geology of the Lima coast I omitted to mention one prominent feature of the Lima coast. That is the occurence of Travertine or better said calcareous tuff along the cliffs of the Lima conglomerate. As a matter of fact this is a common occurence along the entire coastline of exposed conglomerate and I am sure the formation is directly linked to it.

When walking or driving along the Costa Verde (green coast) of Lima - actually it has the charm of brown-grey concrete instead of being green in most places - it is impossible to miss the sometimes 10m large and several 10s of meters long Travertine curtains that usually originate somewhere in the lower or middle of the Lima conglomerate. From my few observations while driving by or from the opposite side of the street the occurence of Travertine seems to be connected to changes in grain size within the Lima conglomerate. Either they occure at the lower contact of finer layers of sand/silt or at the lower contact of layers of very coarse conglomerate. For more on the conglomerate itself please consult the other post.

Unfortunately I only took one mediocre foto of the Travertine.

Foto showing the contact of coarse and less coarse conglomerate with travertine close to Parque de Amor/Miraflores (the sand is recent wash-out debris)

Here are several scanned images I took in late 2004 on a previous visit to Miraflores/Lima.

Travertine views from large to small scale

In the above fotos you can see different perspective on the Travertine just below the Larcomar shopping and entertainment mall. I included a few remarks into the fotos. The white thingy in the lower right foto is half my student ID, about 11cm long. Going top to bottom and left to right you can see the layer of finer material being the focal layer for travertine occurence. Travertine forms at the lower contact of the finer material and covers the sedimentary layers below it. Dead plants remains still are in place and intergrown with the travertine. On one foto you can nicely see the individual roots still in place and the empty tube-like features of the travertine. The last foto illustrates how the travertine also is cementing the conglomerate pebbeles in some places.

It seems obvious that the formation of travertine is linked to changes in grain size and sedimentary lithology of the not well consolidated sediments. I could imagine the exposed layers of sand or silt to be the former groundwater aquifer of the Lima conglomerate. Travertine would thus form at the top of less permeable layers as documented in the fotos.

From locals I know that until 4o to 50 years ago the entire coastline of Lima was green, hence the name. Only in the last decades it changed from green to concrete brown-grey. That change occured at roughly the same time as groundwater production for drinking and agriculture increased significantly when more pumps were being installed (I must apologise for not having written sources available. Those will be added as soon as available to me.). Lima today has 8 to 9 Million inhabitants that all rely in groundwater or imported water from the mountains.

Combined with the age information of the Lima conglomerate from the previous post the travertine formation is at most Plio-Pleistocene in age and continued up to recent historical times. Considering the preservation of plant remains in the fotos I assume the shown exampels to be no older than a few decades to a few hundred years. Also they illustrate nicely the results of exploiting the groundwater resources beyond their ability of regeneration.

Perhaps some of you Peruvian geobloggers can find sources on this or know more from own work? I could not find online sources regarding this issue.

To anyone working on this: These are not the results of any study or detailed work. Only the conclusions I draw from driving by in a car and a single hour of looking at the rocks from a distance.


Callan Bentley said...

A couple words in this post jump out at me:

"Travertine curtain" -- This reminds me of similar "curtains" of travertine I saw this summer in the Grand Canyon, coming down from being leached out of one the limestone layers and then being slathered all over the lower layers, colluvium, everything.

"Calcareous tuff" -- I'm confused by this one. Isn't tuff volcanic ash? And if so, how could it being calcareous? Please enlighten...

Thanks for a cool post. I love seeing these dispatches from Peru.

Lost Geologist said...

I must admit I have some problems in terms of english vocabulary. I used the term "travertine curtain" as a purely descriptive word because that is what they look like when standing in front of it - or like a frozen waterfall but that would be more confusing.

Calcereous tuff - I long pondered wether to use this term or not. The German word is "Kalktuff" which has no connection to any volcanics and is a purely descriptive term. Consulting two english (non technical) dictionaries eventually led me to use this term to describe the extremely porous texture of the travertine. It often is more holes than travertine. If that translation is wrong I will correct it within my post.

Glad you enjoyed it though.