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  Stop 6. The world's most famous carbonate mound

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Transcript

- [Voiceover] We're in the Sacramento Mountains looking at Muleshoe Mound, which is probably the most famous example of a microbial mound in the world. And, if you can't find the mound let me show it to you. It's that guy right there in the middle. And what I'm gonna do is I'm gonna zoom in on that thing. And when I do, you can actually see different phases of mound growth. Now this particular mound was mainly constructed by bryozoans. And on top of the bryozoan mound itself were crinoid communities. Of course it is a microbial mound where the microbes played a role as in crusters and binders where as bryzoans were bafflers. Now the rock in the mound core is basically biocementstone, very tight, low porocity, low permeability. Whereas the flank beds have much better quality. The reason for this is as the mound is aggrating sediment gravity flows come off the mound and deposit grain ridge material on to the flank beds, which you can kind of see right there all lapping against the mound, and some in the mound itself. So in this case you've got a bryzoan mound. The best place to drill is actually the flank beds and not the mound core itself. That's not just from this particular outcrop here in New Mexico. You can get the same effect in bryzoan mounds in the mid-continent, bryzoan mounds in Kansas and Illinois as well. The mound core faces tends to be tight, very cemented. Whereas the flank faces tend to be more grain-rich especially from crinoid communities that like living on top of the bryzoan mound. The next thing I want you to focus on, I'm going to zoom out, is that around the mound most of the carbonate beds are very well layered. Which means in seismic these would form parallel to set parallel reflections. Whereas if you zoom in on the mound core you can see that there's no bedding in there. If there's no bedding there's no impedance contrast. If there's no impedance contrast it's going to appear as seismically transparent or chaotic. That's one way of picking out these mounds from background sedimentation.