A not so brief reflection on my Field Class from Fall 2011.

Below is a description of the field areas I visited for my Field Class during the Spring 2011 semester.  Before you continue reading, I should warn you that unlike at Field Camp (which I'll cover in a later post) I did not take any pictures for the majority of the field trips.  I was more concerned with learning mapping techniques and getting a good grade than documenting our field areas.  Thus most of this post will be terribly boring and entirely too text heavy.  I'm mainly writing this as a personal review of my progression as a field geologist and to serve as a record of where I've gone in southern NM.

January 15, 2011 Robledo Mountains (First Map and the Final)

This outing was mainly an excuse to get us out into the field and taking attitudes with our Brunton compasses. All of our attitudes were taken off of the Hueco Limestone in a drainage near the gravel mine or in nearby exposures. We then plotted these attitudes on a stereonet and on a field map, then wrote a short report on the lithology and similarity of attitudes in the Hueco. It was a simple outing, but you have to start your field career somewhere...


January 29, 2011 NE of San Diego Mountain

This trip required us to make independent unit descriptions of the Bliss Sandstone (burrowing suggests it is a marine sandstone, and it is of Cambrian-Ordovician age), the Precambrian granite basement (granodiorite with numerous dikes cutting it), and the El Paso Limestone (a mottled Ordovician limestone with occasional algal layers and gastropods. It is commonly silicified, probably by hydrothermal fluids from faulting). Additionally, we began mapping contacts between these units and continued to plot the attitudes of bedding when possible. We constructed a very simple cross-section from our map and attitudes and then tied everything together with a report of our interpretation of the geologic history of the area.

This field trip was the first time we had done real mapping and was the first experience I had with silicified carbonate rocks. After this report I began to feel like a bit of a field geologist, and the exposure to new rock units awakened my desire to see what else southern New Mexico's stratigraphy could offer.

February 1, 2011 SW San Diego Mountain

Seeing the other side of San Diego Mountain was like entering into another world.  The stratigraphy near the mountain is largely dominated by sedimentary rocks of volcanic origin mainly the Palm Park Fm - a maroon mudstone to fine sandstone with 2mm plagioclase clasts and the Bell Top Fm - a paler maroon, medium sandstone with common yellow staining from hydrothermal activity.

Across a listric normal fault, the map area is nearly entirely Precambrian granite with lozenges of a biotite rich schist.  A beautiful expression of a thrust fault was just a short ways from the map area (I wish I had brought my camera here), and a small travertine deposit was also spotted on one of the minor ridges of the area.  Additionally, for my cross-section, I mislabeled my units in meters instead of feet, making San Diego/Tuonuco Mountain the highest mountain in the lower 48 states!

Tonuco Mountain (AKA San Diego Mountain).  Our SW map area was
the left foothills of the main mountian.

February 12, 2011 Yoast Draw

This was the first (and only) map area in which we were provided a satellite photo of our map area as well as a topographic base to map on.  The large scale made locating oneself relatively easy, and the lithology of the area had strong contrasts on the map.

The Abo Sandstone of Permian age was a red-brown fine grained sandstone with excellently preserved ripple features which contrasted excellently with the blue gray fetid limestone of the Pennsylvanian Bar-B Formation.  This area was interesting because it was the first exposure I had to mile scale folding, which in hindsight appeared relatively well on the satellite photo.  There was also a large number of cows and a road to a ranch of sorts, which had a well.

The two non-geologic things of note for this area are that while working with a partner in this area, she managed to lose her Brunton very near the end of the day and that was a real shame.  She never found it on a return trip the next weekend, and I feel a little guilty still that I never noticed she had left it behind before it was too late.  Also, this was the hottest outing we had undertaken and I had been working without a hat of any kind.  I hate wearing hats. They feel weird, confine my vision when climbing, and are more likely to blow away than stay on my head.  This trip however I went through my three liters of water relatively quickly and had a pretty deep sunburn on my face by the end of the day, so this was my breaking point and I picked up this hat below.

Dorfman Pacific Co. 863M hat in olive green. I later added a leather strip to serve as a strap in wind.

February 26 and 28, 2011 Broad Canyon

This map area was larger than usual and so we were alotted two days to map it.  Broad Canyon lives up to its name and is a wide canyon with a large degree of alluvium (mostly loose sand and flow banded rhyolite pieces).  The Canyon also disects an ancient cinder cone which is excellently preserved.

The area also offered impressive canyon walls of ash flow tuff which had been welded and was surprisingly resistant to normal weathering and hammering.  I had never seen such thick deposits of ash flows in southern New Mexico until then.

This area was as complex as it was large, with several faults to account for although our field instructor said there were only three main faults we needed to find for the area.  We also had the disadvantage of facing high, cold winds and on the second day it even snowed a bit on us.  It was hard to believe two weeks before I was so hot I finally conceded to buying a hat!

This was a good field area with different volcanic and sedimentary units than we had previously worked with and a lot of outcrop to cover.  I feel this area was a final preparation before we were sent to map our midterm location the next week.  I was assigned to a partner who is purely a rockhound at heart and it was difficult to keep him on task and moving with all of the interesting rocks Broad Canyon had to offer, so it was nice to look forward to working on my own.


March 12, 2011 Midterm at Selden Canyon

I hardly slept the night before our midterm field trip, because I tend to get nervous before any major event and was distressed about doing well.  We arrived at the field area and I immediately realized we were mapping mostly unconsolidated sedimentary units and a paleosol which I had minimal experience with.  I walked around the eastern portion of the map area and could only distinguish one or two of the map units in the area, which greatly worried me since I had covered over 1/3 of the map area and hadn't seen a gypsiferous sand at all.  I finally found the unit in the west and the area started to make a bit more sense, so I took attitudes on what solid rock I could find, cleaned up a few contact lines and called it a day.  We headed back to school to make the final map and I inked in my contacts and attitudes and called it a day.

When we got our maps back over half of the class had missed the faults in the area and a small fold as well.  I missed the faults entirely (thinking the sediments were unconsolidated and unlikely to be significantly faulted) and had the fold relationship mapped through my contacts but not designated by any accompanying symbol or note in the report.  Thus the main lesson I learned from the mapping is that areas can appear to be deceptively simple and fault relationships can be subtle but inferred from careful observation of bedding trends and unit contacts.


April 2, 2011 Palm Park

Palm Park was an intersting location because it contained the Palm Park Formation mentioned above, three different ash flow tuffs, two sedimentary layers between these tuffs, the Uvas Basalt, a conglomerate over that.  The most difficult part of this field area was distinguishing between the three tuffs (and thus determining what sedimentary layer could be present in succession as well) and overall I'm proud with how well I did in this regard.  The faults were easy to distinguish in this area from the landforms and the dynamic stratigraphy.  The most important things I got from this trip were an excellent sample of a tuff with slickensides on it and a bit of my confidence back after my less than impressive mapping at Selden Canyon for the midterm.



April 9, 2011 Burro Mountains

This location was by far my favorite of our field class.  We went to the Burro Mountains to map cross-cutting relationships of dikes within a gorgeous gneiss.

We also restored the regionally tilted dike orientations back to horizontal on the average attitudes of the dikes in a stereonet.  This was a difficult to do at first, due to confusing instructions but after a more clear explanation was provided the restoration was much easier than it seemed.

As you can see, the dikes are generally rhyolitic and range from granite to monzodiorite. Chilled margins are common in all of the dikes and based on the orientations and mineral compositions of the dikes I argued for a population of four distinct populations of dikes.

This field area was a wonderful location.  It was well shaded, had both intrusive and metamorphic rocks with well preserved textures, and was well concentrated within the area we were mapping.  If the Burro Mountains were closer to Las Cruces, I would go there as frequently as I could but alas the 131 miles (three hour drive) makes this difficult to justify.  Luckily, Las Cruces has enough interesting local geology to keep me down for too long!

April 22, 2011 Final: Robledo Mountains Round 2

Our first trip to the Roblado Mountains had been cursory at best and so even our investigation of the attitudes of the Hueco Limestone did not differentiate it between its four members.  We also mapped the Love Ranch, Palm Park, and Camp Rice formations, a basalt flow, and alluvium.  This made the area dynamic enough to spot even minor faults which initially caused me to over-complicate my map until I found more important faults.

I forgot to eat lunch entirely while out mapping and so at around 3 in the afternoon I was banging my head on outcrop protruding out of drainage and staggering around less coherently than usual.  In the end though I feel I determined most of the important fault relationships and contacts, but I never got the chance to check what errors I made on my map since so much of my late semester work was taking so much of my time I never made the appointment.  I feel I did much better on the final than the midterm and I don't have any regrets that I could suspect penalized me.  Making the final map at school was an anxious experience though, and I ended up making two flawed drafts I had to start over from before I finally got it right and managed to go have a celebratory beer or three with the class at a local sports bar.

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My field class experience definitely prepared me for field camp, but the shear size difference in mapping areas was difficult to get used to at field camp (even with the extra days).  My cross-section construction skills were also shown to be wanting at field camp as most of our class realized that our folding and curvature of bedding was never quite refined from a rough estimation of curve points, so long as bed thicknesses were typically preserved.  Overall though, I learned a lot about how to map in only 9 short weekends and I got to see some fascinating rocks in the process.

Photo Credits

Cool as a Cucumber "Broad Canyon: Tonuco Mountain, Dona Ana County"