Saturday, December 16, 2006

Friday Field Foto #7: Stack of deep-water deposits

(a day late)

Here's another shot of my field area in southern Chile. The more resistant cliff-forming rocks are mostly sandstone and the gray-brown slopes are shale. These sedimentary rocks were deposited on the sea floor about 65-70 million years ago (kind of around the time the dinosaurs became extinct). Good stuff.


pirex said...

Have you figured out the depth at which the sediments were deposited? Can you make any comments about the paleoenvironment? Are any strandlines visible?

Brian said...

good questions...

we can't determine an absolute water depth, we only know that all the deposits indicate gravity flow processes are dominant. That is, there are no features to suggest shallow-water (i.e., above wave base) processes.

No shoreline deposits in this succession...the overlying formation, which is hundreds of meters above what this picture shows, does have deltaic and shallow marine deposits.

In terms of paleoenvironment, we interpret this succession as a deep-marine slope out in front of a prograding delta. We can't explicitly trace these sand bodies back to a feeder delta because they pinch out in the paleo-landward direction. But, the sedimentology and sedimentary body architecture in the succession shown here suggests a systematic increase in paleogradient ( way to actually measure, it's more of a relative thing) upwards through the stratigraphy. This could reflect the outbuilding of the slope as delta-slopes tend to have higher gradients just off the shelf edge and then decreased gradients toward the base of slope.

From a larger-scale perspective, this is consistent with the fact that we know deltas do eventually build out over the top of this formation for hundreds of kilometers...essentially filling in the basin.

i'm actually in the midst of finishing a paper that summarizes these strata.

pirex said...

Thanks for the great description. It gives me an idea of what was happening at that time. I recently looked at a book on paleosurfaces and was wondering if there may be signs of emergent marine terraces still visible? I saw your description of differential erosion to explain the stepped appearance.

When will your results be published? What are some good books and articles to read about for understanding this region? Does your study area include the Torres del Paine massif?

Brian said...

The rocks we study are very near the Torres del Paine massif. In fact they intrude into one of the formations others in my research group work on. The "photo of the month" on the front page of this blog is the view of the massif from this field area.

We aren't studying the geology of the massif, however. That is a much younger feature. The rocks we are looking at were deposited around 65 million years ago...the igneous rocks of Torres del Paine were emplaced around 8 million years ago.

One of these days I need to put together some good references for the general geology of the region...stay tuned.

As for the results of my particular study...actual publication probably won't be until a year from now, but once it is "in press" I will put a lot of it on here. Hopefully by this summer...the peer review process can take many months.