Wednesday, August 01, 2007

Conglomeratic intrusions

Okay....this is another long post....maybe too long, but oh well.

A former fellow student (now professor), me, and our adviser have a paper out this month characterizing some very unique sedimentary features in Cretaceous strata of southern Chile: conglomeratic injectites.
Clastic intrusions, now termed injectites, have been recognized for a long time. In some cases, they resemble igneous intrusions (dikes, sills, etc.) with respect to their geometric relationship to the surrounding rock.

What are injectites?

They represent the violent remobilization of typically coarse sediment like sand due to over-pressurization. The best visual I can give you is imagine that you could press down on a jelly donut that was somehow sealed along the sides. That is, the jelly does not squeeze out of the side when you put pressure on it from above (overburden). Now imagine some plane of weakness in the fabric of the donut above the jelly layer. The overburden pressure will force the jelly into that plane of weakness and upward. If the movement of the jelly is of sufficient magnitude it will bust its way through the donut, creating its own path, and perhaps erupting at the surface of the donut.

Coarse sediment (usually sand) that is deposited in deep-marine settings is a mixture of sediment and water and it takes a while for the deposit to slowly de-water. If the sediment accumulation rate is high enough -- that is, if these water-laden deposits get buried relatively quickly by more sediment this layer becomes the "jelly". The increasing overburden pressure combined with some sort of trigger (seismicity? a threshold?) creates a situation for this large-scale injection to occur.

What's cool about our study (at least we think its cool) is that we have (1) a great outcrop example of this phenomenon where the features are explicitly exposed and (2) the injected material is very coarse -- cobble-sized material.

Now, we'll be the first to admit that this paper is not high science. We literally came across these features and spent a few days mapping them out and characterizing them. There's only so much time in a field season and we had other tasks to accomplish. So, this paper is very descriptive in that way...that is, we did not take it to the level of really figuring out how these things work. The mechanics of clastic injection is not well understood.

Because injectites are associated with thick piles of sediment they are being found by oil companies exploring for hydrocarbons in the subsurface. In fact, one hypothesized mechanism for contributing to over-pressurization is gas charging. Believe it or not, there is an entire book about the occurrence of clastic injectites and their relationship to petroleum systems. This paper is in that book. We were happy that the editors really wanted this stuff in there since we weren't sure where else it would ever get published. Take it when you can get it!

Okay...enough set we go.

The figure above is the location information. I've said in the past that I would put together a post about the general geology of the area i've been working in Patagonia....I still mean to do that....someday. As always, click on the image to get a higher resolution view. The study area, Cerro Benitez, is in the center of the satellite image at right. These Cretaceous strata are part of the Magallanes foreland basin and now nicely exposed in the foothills of the southernmost Andes.

The photos above show some of the close-up characteristics of these conglomeratic deposits. Notable are the near vertical groove-like features in F and G. By themselves, however, these small-scale features are not very telling.

This next set of photographs (above) takes a step back and shows the conglomerate bodies in their full glory. The one shown in A and then enlarged in B is particularly nice (note circled person [me] for scale in each). When we first came across these features we thought the juxtaposition of the conglomerate against the shale had to be a fault contact. It's not...we mapped the entire area. Then we thought these were some kind of channelized deposits, but the geometries were just not working out. When we spent a few days mapping out the multiple bodies and their relationship, an interesting picture emerged.

The last day of characterizing the injection complex, we hiked to the other side of the lake and climbed up a mountain to take the photograph above. We felt we needed a view zoomed out even took a day to get this photo....not the first time we spent a day to get a photograph down there. In the corresponding drawing below the photo, the yellow is the "regular" conglomerate bodies (referred to as stratified conglomerate) and the dark brown denotes the injected conglomerate. The cool part about this outcrop is that the old glacial lake created a flat terrace (shown in lighter gray) between two more vertical slices of the stratigraphy (shown in the dark gray). So, it's sort of a "chair" display if you can visualize it. This little bit of three-dimensionality helped immensely in mapping out the different bodies.

The schematic cartoon above (or, as the first author Steve likes to call it, the geophantasmogram) attempts to summarize the outcrop and how we've mapped it. Again, note the "chair" set up. The injectites emanate from the side of the channel-fill deposits, which is a pattern seen in injectites in the subsurface over and over again. Also, note the left complex on the diagram bifurcate and then head up through the strata until they terminate. The photos above with me for scale is the right branch of that bifurcating complex. The very ends of those injectite bodies lack conglomerate and are instead only sand reflecting a "fining-away" trend. That is, the "flow" of material had to come to a stop at some point, and the brief slowing down before stopping was enough to sort the material being carried.

The last thing I want to touch on is the scale of this injectite complex compared to other known and well-studied examples from the subsurface. The figure above (which is the bottom half of the geophantasmogram) shows the cross-sectional outline of the injectite complex we characterized with some that have been mapped using seismic-reflection data in the North Sea. Firstly, notice the "winged" appearance of nearly all of these examples. Secondly, the bottom line is -- our complex, as big as it seemed, is a tiny little sucker. For example, note the small rectangle on one of the little nubs in D. That rectangle represents the size of the complex we characterized. These North Sea injectite complexes are friggin' HUGE! It kind of blows my mind.
Anyway, i'm gonna cut myself off....this post is already ridiculously long. If you'd like to know more about the nitty gritty, please feel free to comment below or e-mail me at romansbrian AT gmail.


Sabine said...

Fascinating. I had not previously read about injectites.

Brian said...

Injectite complexes in the North Sea are, in some cases, so large that they are the entire reservoir. They are now actively targeting them.

They are also starting to be recognized more and more in the Gulf of Mexico turbidite systems as well.