Tuesday, September 23, 2008

Small Geology of Lima coast

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As you know I am currently in Lima, Peru. Lima is the capital of Peru and a crowded melting pot directly at the Pacific seaside. The seaside itself is pretty steep and vertical cliffs dominate the entire coastline here which of course provides for a magnificent view.


The coastline at Miraflores

Lima is situated on the alluvial fans of the Rimac and Chillon rivers. The palaofans are well exposed along the coastside in the cliffs that can be up to 80 m high. The Cordillera is the supplier of the pebbles and gravels that can be encountered here. Conglomerate clasts can be up to 80cm large at the coast. The Rimac river has a steep gradient of 1:25 and, according to my sources, thus is largly providing bedload transport. Clasts are mostly granites, diorites, gabbros and mesozoic to cenozoic volcanic rocks. The Lima Conglomerate has a thickness of up to 86m. It is interrupted in parts by lenticular sand- and siltstone lenses that likely represent estuarine incursions caused by sea-level variations.


One of the cliffs with erosional evidence and gullies forming. Note the lenses of finer material.


A sand lense in close-up. Note the deviation from the horizontal in the "layering".

According to what I read they can be subdivided into several layers which, by all honestly, I cannot see. The only difference is the average clast size which perhaps is not well exposed in Miraflores where I am. Neither could I observe the imbrication of the clasts and pebbles indicating flow directions and such alike. Well maybe I did but I have to stretch my imagination a lot to see it. There are no fossils

In terms of age we are talking Plio-Pleistocene here when sedimentation was affected by the movement of the Nazca Ridge towards the south proving for impulses of uplift and subsidence and the Lima anticline that blocks direct access for the Rimac river to the sea.


Movement of the Nazca Ridge. Taken from the source cited below.


The pebbles of the Lima Conglomerate get washed out and form the beach. The beach is only pebbles but it makes a great and amazing sound when the waves are washing over them and the water flows back. Klack-klack-klack...


Beach pebbles

If you like to read more take a look at the cited article.

References:

  • Roux, J.P. et al (2000): Sedimentology of the Rimac-Chillon alluvial fan at Lima, Peru, as related to Plio-Pleistocene sea-level changes, glacial cycles and tectonics, Journal of South American Earth Sciences 13, 499 - 510

9 comments :

Silver Fox said...

Nice post about clastic-type geology! Hope the meeting is going well.

abrashid ahmad said...

Walking along the beach is like having a foot massage.

Lost Geologist said...

The course will start september 28th. Two days where I will meet whom we talked about in Email SX. September 30th to October 3rd will be literally hundreds of presentations. I already made a list of where to go.

I just hope I also have the courage to go talk to the people I find interesting.

abrashid: Indeed but currently temperatures are not high enough for that yet. Also it sounds quite dangerous for the toes...

Sabine said...

"I just hope I also have the courage to go talk to the people I find interesting."

I have a tough time with that myself.

enmorrenas said...

What a post man! Welcome to Peru, I'm glad that you are enjoying Lima.
I hope we can meet us in the Congress.

Best regards and your blog is on my favorites now!

Linda D said...

Thank you so much for posting that explanation. I went to that very beach years ago, and I have always wondered how that magnificent cliff of round stones was formed. I gathered up about 30 lbs of those rocks, put them in a canvas bag and carried them in my lap all the way back on the plane to the US. I still treasure them. I call them my Lima Beach Front Property.

Nicola said...

Hi, my name is Nicola and I work with tourists in Latin America. one tourist asked me today about the pebbles and rounded boulders in the cliffs and how the walls are cemented and why as they seem to be composed of gravel and rocks, they do not crumble and fall down. Your post seems to have the answers to these questions, but as I am not a geologist I don't entirely understand the jargon! Please can you email me on nicolagude@gmail.com with a Dummies Guide to the question asked!
I'd really very much appreciate it!

Linda D said...

Nicola, to put it in simple terms, the glaciers high up in the Andes are slowly pulled by gravity down the slopes. As they travel, they pick up rocks and boulders, dragging them against the ground, grinding down their rough edges, turning them into smooth, round rocks while the glacier picks up more rocks and boulders. As the glaciers descend into warmer elevations, the ice melts, releasing the worn, rounded rocks and grit within. As the melting causes the glacier to disappear, what it left is a huge population of rounded rocks encased in the grit left from their grinding.

Lost Geologist said...

I can't leave it like that Linda. :-)

The Lima Conglomerate is not primarily of glacial origin. Though deposited during or before the last glacial period. The rocks were transported by the Rimac river (or it's ancient counter-parts) who deposited the pebbles. I don't remember seeing any clear glacial marks on the pebbles like striations or such. Neither are there any surface features present like drumlins (long stretched small "hills"). Glaciers rarely make smooth and round pebbles.

The pebbles are largely rock that formed below or close to the earth surface from magma and/or lava. The transport smoothed and rounded them.

The bad sorting, meaning having all kinds of sizes present in the conglomerate (how we call such gatherings of pebbles) already may provide some support. Also the rocks are being cemented by freshwater limestone that formed on the cliff-face from groundwater. The cliff is actually much less stable than it seems. Small rock avalanches are very common and localy it must be artifically supported.

Hope to have answered all questions.