Back to the oysters

View from our work site.

Today I returned to the oyster site with just Pedro and Sam. All we had to do for the day was to measure and describe the section (which we were already quite familiar with from the day before) so Nicole and Aaron hung back in the lab to work on prep and curation stuff. I didn't exactly get a ton of sleep last night, so when I first woke I was seriously regretting not having volunteered to stay behind. This feeling only grew more powerful as I loaded up the truck with our equipment at 8:00 AM and could feel the humidity already starting to weigh down on me. Fortunately enough, my sullen attitude failed to drag the day down with me and I ended up having a really good time in the field.

The hardest part about our work for today was getting the initial general picture straight so that we'd know where to be careful and where to look out for unusual features. A lot of the outcrop was obscured by elephant grass (an invasive that grows like crazy and loves handing out scrapes and rashes to passersby) so a fair amount of digging and snooping was required to figure things out. After we had a rough idea of what was going on, Sam and I began measuring while Pedro described. Measuring is simple enough: it just involved figuring out the direction in which the plane of the rock beds extends into the ground then measuring perpendicularly up from that plane. The only challenge in it is determining what the planes are doing; what is seen on the surface is often deceiving and a true bedding plane is hard to come by (finding one that hasn't slumped is even harder). We use a tool called a Brunton (a glorified compass with some convenient bubble levels) to measure the angles of the bedding planes, and by getting measurements from multiple surfaces by multiple people, you can get an average that is pretty representative of the real situation. There ended up being about 50m of total section, and we were marking at an interval of no more than 50cm, so it took a fair bit of time to get the job done, but no real issues popped up.

Hand sample of the "cooked" sandstone.

For the most part, all there was to see along the section were alternating units of sandstone and siltstone, with the sands containing conglomeratic lenses and mollusk fossils, and the silts containing cemented burrows. The real points of interest in the area were two lengthy strips of metamorphosed sand that cut across the rest of the beds. We determined these to be evidence of alteration caused by hydrothermal injection into the already in-place beds. The metamorphosed sandstone went through a whole range of colors as you traced it from the bottom to the top of the hill and consisted of three main bands. The center band looked almost vesicular (holey), but on close inspection you could see that each hole was filled with a little ellipsoid of carbon (looked like miniature briquettes). On either side of that strip, there was a band of similar-looking sandstone that had thin carbonized plant fossils instead of the ellipsoids and was incredibly indurated - even with the pickaxe it was hard work to break it up. Finally, on the very outside, there were sandstones with large, disk-shaped grains, all aligned in more or less the same direction, still containing carbonized plants. The way we saw it, hot fluid must have entered a fossil-rich sandstone, cementing the rock hard and turning any organic matter into the ellipsoidal briquettes. The areas further away from the injection site did not get quite as hot, so groups of grains fused together rather than the entire rock body. Pretty interesting stuff!

Lots of cool mud cracks out on the flats below our section.

We had initially hoped to finish up in the early afternoon and get back to the labs early, but with the discovery of the altered areas it just didn't work out that way. We had to dig a couple of trenches in order to get good clean photos of the features and that was no easy work. Still, it was great geology so I think it's safe to speak for the group and say we all enjoyed it. Oh! And from the observations we made today, we feel pretty confident that the section represents a nearshore facies of the Culebra Formation, a ~23 million year old marine rock series that Pedro and Aaron have worked in before.