Tag Archives: Field work

Capitol Rock Close-Up

Close-up outcrop photograph of Capitol Rock, viewed from the north-northeast.  Image credit: Bill Mitchell (CC-BY).
Close-up outcrop photograph of Capitol Rock, viewed from the north-northeast. Image credit: Bill Mitchell (CC-BY).

Two years ago, I came tantalizingly close to Capitol Rock, an outcrop in southeastern Montana (45.572189, -104.087964) just a few miles over the border from Camp Crook, SD. Unfortunately, I did not have an opportunity at that time to explore the outcrop from any closer than about a quarter mile, but I did find the Ekalaka Quadrangle 30’x60′ (pdf) geologic map.

Recently, I was out in the area again, and this time made sure to have time to take some pictures and see some of what was to be seen. Let’s start with the quarter-mile view, which is roughly equivalent to what I saw last year.

Wide view of Capitol Rock from the east.  Image credit: Bill Mitchell (CC-BY).
Wide view of Capitol Rock from the east. Image credit: Bill Mitchell (CC-BY).

Capitol Rock has three major parts to it: an easily eroded base, a laminated sandstone middle, and a massive sandstone top. A handy turn-out from the forest service road leads right to the base of the outcrop.

The easily eroded base is made of fine, chalky, white sediment sediment, and it remains in horizontal orientation. In several places, this unit is at least superficially porous. Surprisingly, there are occasional chert clasts in the otherwise fine sediments—I’m not quite sure how those would have been deposited or formed here.

Basal unit of Capitol Rock.  Foot for scale.  Image credit: Bill Mitchell (CC-BY).
Basal unit of Capitol Rock. Foot for scale. Image credit: Bill Mitchell (CC-BY).
Cherty clast in the basal sediments at Capitol Rock.  Foot for scale.  Image credit: Bill Mitchell (CC-BY).
Chert clast embedded in the basal sediments at Capitol Rock. Foot for scale. Image credit: Bill Mitchell (CC-BY).

Above the basal unit is a somewhat more resistant, coarser-grained set of beds. These strata are finely bedded, and have a tendency toward spheroidal weathering. Occasionally interbedded with the spheroidal beds are 1–3 cm thick, well-cemented strata of a white or pink color [discoloration?].

Spheroidal weathering of finely-laminated strata.  Hand for scale.  Image credit: Bill Mitchell (CC-BY).
Spheroidal weathering of finely-laminated strata. Hand for scale. Image credit: Bill Mitchell (CC-BY).
Laminations in the unit displaying spheroidal weathering.  Hand for scale.  Image credit: Bill Mitchell (CC-BY).
Laminations in the unit displaying spheroidal weathering. Hand for scale. Image credit: Bill Mitchell (CC-BY).
Non-spheroidal bed 1–3 cm thick and slightly orange-pink in coloration, within the spheroidal beds at Capitol Rock.  Hand for scale.  Image credit: Bill Mitchell (CC-BY).
Non-spheroidal bed 1–3 cm thick and slightly orange-pink in coloration, within the spheroidal beds at Capitol Rock. Hand for scale. Image credit: Bill Mitchell (CC-BY).

The spheroidally-weathered unit also seems to have one or more channels within it.

Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below).  Possible channel cut at right.  Outcrop height in image is ~10 m.  Image credit: Bill Mitchell (CC-BY).
Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below). Possible channel cut at right. Outcrop height in image is ~10 m. Image credit: Bill Mitchell (CC-BY).
Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below).  Possible channel cut at right has been annotated.  Outcrop height in image is ~10 m.  Image credit: Bill Mitchell (CC-BY).
Contact between spheroidally-weathered strata (above) and easily-weathered basal unit (below). Possible channel cut at right has been annotated. Outcrop height in image is ~10 m. Image credit: Bill Mitchell (CC-BY).

The upper unit at Capitol Rock has more massive sandstone (see wide view above). I didn’t notice many channels in this unit, although I didn’t get very close. A butte just to the north of Capitol Rock provided a good photograph (below).

Massive unit of Capitol Rock, seen in the butte immediately to the north of Capitol Rock.  Cliff is ~30–40 m tall.  Image credit: Bill Mitchell (CC-BY).
Massive unit of Capitol Rock, seen in the butte immediately to the north of Capitol Rock. Cliff is ~30–40 m tall. Image credit: Bill Mitchell (CC-BY).

Although I have those observations, I don’t have much for interpretation of them. The depositional environment seems to be relatively low-energy (give or take the chert clasts), evidenced by the flat strata, fine grain sizes, and relatively few cross-beds. Changes in the rock types would suggest changes in the sediment sources or the depositional environment (or both). There may be post-deposition alteration effects as well, such as cementation of the spheroidally-weathering strata.

View SSE from the butte just north of Capitol Rock.  Truck for scale in pull-out near Capitol Rock.  Image credit: Bill Mitchell (CC-BY).
View SSE from the butte just north of Capitol Rock. Truck for scale in pull-out near Capitol Rock. Image credit: Bill Mitchell (CC-BY).

Capitol Rock is an interesting outcrop, and if you’re in the area, I’d recommend a stop. The rocks are interesting, there are US Forest Service campgrounds nearby, and the view is quite nice. These units can probably be correlated to the Slim Buttes in South Dakota (~45 miles east).

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The Making of the Windy City Gigapan

Looking eastward at Windy City, with a person for scale. The gigapanned portion of the outcrop is at right, but two spires of similarly eroded rock outcrop further to the north of the photographed portion. The stake coming out from the outcrop is a marker for one of our temperature/light intensity sensors. Image credit: Carlos Nascimento
Looking eastward at Windy City, with a person for scale. The gigapanned portion of the outcrop is at right, but two spires of similarly eroded rock outcrop further to the north of the photographed portion. The stake coming out from the outcrop is a marker for one of our temperature/light intensity sensors.
Image credit: Carlos Nascimento

In my previous post, I discussed the gigapan of Windy City. However, the making of that gigapan was quite the adventure in field work.

After the Azorella Peninsula gigapan, the unit was packed up and taken back aboard the Braveheart for a trip to the southeast portion of the island. Rough north winds were expected, and with no protection afforded against those winds and swells from Atlas Cove, the ship had to move. Our expedition leader and two scientists not involved in the radio operations left camp and went to ride out the storm south of Stephenson Lagoon. At that time, it had become clear that I personally would not be able to go to Stephenson Lagoon—an area which was an extremely high priority for a gigapan image. I put fresh batteries into the gigapan mount, and sent it on its way. Sadly, in the almost four hours the team had on the shores of Stephenson Lagoon, they did not have an opportunity to take a gigapan. I’ll have to go back for that one!

Upon their return to camp, I knew since they had not attempted any gigapanning that there were fresh batteries in the unit. As the end of the expedition drew near, it was time to get the gigapan done at Windy City. Mid-morning, Carlos joined me for a trip to the outcrop (about 1.4 km each way). Although we didn’t have a bright sunny day, it was dry with a temperature around 5 °C. When we reached the outcrop and everything was set up, I turned on the gigapan mount. Nothing happened. With new batteries and a limited task, I hadn’t brought the whole kit with me. We headed back to camp, arriving in time for lunch.

Several of the rechargeable batteries I had for the gigapan had been sitting on the charger and were ready to go. I tossed those into the battery holder, put it under my arm to keep warm, and headed out with Carlos once again. At the outcrop I set up the rig again. When everything was set to go, I removed the batteries from inside my jacket, and put them into their slot. I powered it on. The LCD display brightened, but displayed an error message: Button-pusher disconnected or plugged in backwards. Cycling the power on and off didn’t fix it. Everything was as it had been before when it worked. Once again, this was a problem I was unable to deal with at the outcrop.

Back in camp, Carlos looked online for a solution while I tried to see if anything was likely to have come disconnected, although our team had been very gentle with the unit. Nothing stood out. Eventually we found online that the error is commonly caused not by a disconnected or backwards button-pusher, but by a low voltage. That made a bit more sense. Out came the volt-meter, and two sets of six AA alkaline batteries were verified to be fresh. One set went into the battery holder, the other went into a storage case. Now that it was late in the afternoon, Carlos had to report for radio duty, but Adam was willing to come with me—I needed this gigapan before the light died, as there was no guarantee that I would have the weather conditions or time to get it later.

Adam and I hurried over to the outcrop, the light already beginning to fade. I set up quickly, got the batteries out from my jacket, and set up the gigapan.

Please, light, stay with us long enough to complete this shot. Please, batteries, keep up your voltage!

It was clear from the beginning that the shot would not be truly completed. Somewhere in the middle either the light would die or the batteries would. Eventually, both did at about the same time. We quickly put everything back into the packs and headed back for camp. It was getting dark, but we arrived just in time for dinner and the start of my shift at the radio desk.

Although it was too late to be of use, I asked on Twitter what some of the other cold-weather folks had done about their gigapans. By the end of my four-hour radio shift, I had responses from @rschott and @callanbentley. Evidently this is a common problem, which is fought by insulating the gigapan unit as well as possible, and using hand/toe-warmers to add a little heat.

I think it’s time to ask Gigapan to make some design adjustments to improve the cold-weather operation of the units. It wasn’t all that cold where I was gigapanning, yet I still couldn’t get 15 minutes of operation on fresh batteries at 3–5 °C.

Windy City Gigapan

Processing the Windy City gigapan.  Image credit: Bill Mitchell (CC-BY).
Processing the Windy City gigapan. Image credit: Bill Mitchell (CC-BY).

This is the third in a series of three posts about the gigapan images taken on Heard Island (1: Big Ben, 2: Azorella Peninsula), with more information about the Windy City gigapan.

Windy City is located about 200 meters south of Atlas Cove, in the northwest portion of Heard Island. It comes from a fin of Drygalski Formation rocks, which are a mix of glacial sediments and volcanics, and is mostly surrounded by sand and gravel plains.

Looking closely at the outcrop, there are a number of interesting things to observe. First, there are the striking roughly-horizontal marks, which are particularly evident toward the base of the outcrop. Second, the outcrop is made of massive, fine-grained jointed rocks with few vesicles. Third, there are quite a few fractures within the rock, with discolorations along many of the cracks.

All of these observations combine into a remarkable tale of how Windy City has been formed. The massive, fine-grained, and jointed appearance leads to the conclusion that we are looking at a volcanic outcrop, rather than glacial sediments. Fracturing and discoloration have been brought on by weathering from the very wet, near-freezing environment. Finally, the wind has been a huge factor! Sand, gravel, snow, and graupel (ice pellets) have all been blasted against the side of this outcrop, primarily from the west (at right). On Heard Island, a 9 m/s wind is typical, with maximum recorded gusts exceeding 50 m/s on three days during the 1948-1954 period.[1] The high winds sandblast the outcrop, leading to the horizontal striations.

Here are a few wider-angle shots for context, and with better light than I ended up with for the gigapan.

Windy City outcrop, viewed from the north.  The gigapan image covers from my right arm to roughly the center of this image.  Image credit: Carlos Nascimento
Windy City outcrop, viewed from the north. The gigapan image covers from my right arm to roughly the center of this image. Image credit: Carlos Nascimento

Looking eastward at Windy City, with a person for scale.  The gigapanned portion of the outcrop is at right, but two spires of similarly eroded rock outcrop further to the north of the photographed portion.  The stake coming out from the outcrop is a marker for one of our temperature/light intensity sensors. Image credit: Carlos Nascimento
Looking eastward at Windy City, with a person for scale. The gigapanned portion of the outcrop is at right, but two spires of similarly eroded rock outcrop further to the north of the photographed portion. The stake coming out from the outcrop is a marker for one of our temperature/light intensity sensors.
Image credit: Carlos Nascimento

I also managed a close-up shot of one of the pieces of float.

Float rock at Windy City.  The 1:1000 metric scale at right is effectively a mm scale.  Some olive/green crystals are visible, mostly 1-5 mm in their longest dimension, which are likely olivine (possibly clinopyroxene). Image credit: Bill Mitchell (CC-BY).
Float rock at Windy City. The 1:1000 metric scale at right is effectively a mm scale. Some olive/green crystals are visible, mostly 1-5 mm in their longest dimension, which are likely olivine (possibly clinopyroxene).
Image credit: Bill Mitchell (CC-BY).

[1] Thost, D., Allison, I. “The climate of Heard Island” in Heard Island: Southern Ocean Sentinel, ed by K. Green and E. Woehler. Surrey Beatty & Sons, Chipping Norton 2005, p. 52-68.

Farewell, Cape Town!

HDT Airbeam tent being loaded onto the Braveheart.  The tent is also blocking a nice view of Table Mountain.  Image credit: VK0EK team.

We are in the final breakfast and boarding process for departing Cape Town. Above is a picture from yesterday, where all the inspected expedition gear was loaded onto the Braveheart.

Cape Town has been very nice. Our hotel is within an easy walk of the ship, and there are many shops nearby where we have acquired food, groceries, clothing, outdoor gear, and souvenirs. Weather has been quite warm (26 °C, 79 °F) with a breeze. The local Cape Town team has been extremely helpful and have made much of the project move more smoothly. Crew from the Braveheart have also been wonderful to work with, and I’m looking forward to getting to know them more in the next six weeks. As the ships were loading, a seal was playing in the harbor, gulls were flying around, and even a few terns were spotted.

As can be seen in the photo above, there is some interesting geology around Cape Town. Most noticeable is Table Mountain, which is primarily made up of the Table Mountain Sandstone. Closer to the hotel is Signal Hill, which has slates that have been tilted nearly vertical. We were able to see these up close yesterday evening after the ship was loaded and things were under control. It’s quite a view from up there (sorry, haven’t had time to process pics). For more on the geology of Cape Town, take a look at this post by Dr. Evelyn Mervine, who writes one of the AGU blogs.

Internet connectivity on the ship is likely to be minimal, but with luck I’ll be able to get a post or two up from Heard Island! More frequent news updates can be found at vk0ek.org.

Plan for Updates from the Field

Southeastern Heard Island in true color, February 20, 2016.  Image credit: data from NASA EO-1/ALI (public domain), processed by Bill Mitchell (CC-BY).
Southeastern Heard Island in true color, February 20, 2016. Image credit: data from NASA EO-1/ALI (public domain), processed by Bill Mitchell (CC-BY).

For what I hope are obvious reasons, during the Heard Island Expedition posting around here could get infrequent or disappear entirely. I will try to get updates in when I can. Here’s the general plan for when and where to expect updates:

  • VK0EK.org will be maintained by our mission control team, and will be the best source of information. It is reasonable to expect they will be in contact with us at least daily.
  • We have a GPS tracker, which you can follow here.
  • My blog here will be updated as I am able to do so. I’ve been told that internet connectivity aboard the ship is extremely limited (few text-only emails), so don’t expect much March 10–20 and April 10–21. However, on Heard Island (est. March 21 to April 10) the situation should improve because we can aim antennas at the geostationary satellites from stable ground rather than a pitching and rolling ship.
  • This post is going to be pinned to the top of the front page, so you will need to scroll down for updates.
  • I may post things to Twitter (@i_rockhopper), but I doubt it will see much use beyond linking back here.

Here are a few reading suggestions in case you’d like some additional Heard Island flavor while I’m gone:

  • Fourteen Men by Arthur Scholes. It’s an account of the 1947-1948 Australian National Antarctic Research Expedition to Heard Island, which established many of the scientific baselines from which changes are measured on the island. I enjoyed it, and it was available at my local (large city) library. The book is written for a general audience.
  • Heard Island: Southern Ocean Sentinel, Ken Green and Eric Woehler, eds. (2006). This book has the latest research on Heard Island. It is written for a scientific audience, and is effectively a collection of research papers or review articles. The print run was small, and your local library probably doesn’t have it. However, I ordered a copy from a bookstore in Australia, and I’ve found it an invaluable resource for preparing for this expedition.
  • Heard Island 1986-1987 Scientific Expedition Report, including significant geologic and Earth science research. Open Access
  • Heard Island 1987-1988 Scientific Expedition Report, including excellent hand-drawn maps raw population counts for several species of birds, and other great early-stage science! Open Access

Geoscientist’s Toolkit: Paleomagnetic Coring

Recording rock core orientation for paleomagnetic analysis.  Image credit: Bill Mitchell.
Recording rock core orientation for paleomagnetic analysis. Image credit: Bill Mitchell.

I’ve touched on paleomagnetism a little bit before, both as a technique for tying rocks in to the geologic timescale, and as something which can be found by using a fluxgate magnetometer. It’s a pretty interesting set of techniques and uses some cool science tools, so I thought I’d explain a little bit more.

Magnetism from the Earth’s magnetic field can be retained by individual layers of rocks, at least under some circumstances. If you have a bunch of layers stacked on top of each other like pancakes, the different layers (beds) can have different magnetic directions.

Stack of banana-walnut pancakes.  Although probably low on magnetic minerals and too thin individually for magnetic coring, they do illustrate the concept of layering quite nicely.  Image credit: Jack and Jason's Pancakes (CC-BY-SA).
Stack of banana-walnut pancakes. Although probably low on magnetic minerals and too thin individually for magnetic coring, they do illustrate the concept of layering quite nicely. Image credit: Jack and Jason’s Pancakes (CC-BY-SA).

As you might expect, the equipment needed to make sensitive measurements of the magnetic field are not particularly portable (and may be a topic for another post). Samples need to be collected in the field and brought back to the lab, and the sample orientation must be marked and recorded in such a way that the measured magnetic field can be related back to the magnetic field in the rock itself.*

To do that, paleomagnetists (or paleomagicians) will drill a small (1″ diameter by a few inches long) annular hole into the rock, leaving a plug of rock in the center. That will become the sample. Before it can be removed from the hole, a mark is made on the top of the plug with a brass rod. The direction of the hole is determined with a compass (or a sun compass when conditions allow), as is the angle away from vertical of the core (the hade).

When the plug is freed from the rock, the down-hole direction is marked with arrows along the mark using a permanent marker. The samples (several from each bed) are then placed into sample bags, labelled appropriately, and carefully transported back to the lab.

Are you irresistibly attracted to such a magnetic field of study? This is probably the best place to go for more information, and is freely accessible online.[1]

***
[1] Tauxe, L., Banerjee, S.K., Butler, R.F. and van der Voo R, Essentials of Paleomagnetism, 3rd Web Edition, 2014. [accessed Aug. 27, 2015]

* The field magnetic field?

Heard Island Expedition Update: T-7 Months

Visualization of a proposed Heard Island shelter setup, using two HDT Global airbeam tents.  Each shelter is 20'x21'.  Image credit: Bob Schmieder [?].
Visualization of a proposed Heard Island shelter setup, using two HDT Global airbeam tents. Each shelter is 20’x21′. Image credit: Bob Schmieder [?].

It’s only seven months until the Heard Island expedition leaves Cape Town, South Africa, heading for Heard Island. Preparations are really beginning to get going!

This morning (Minnesota time) we had a conference call with the entire on-island team (such as were able to join). Scheduling that can be tricky, because we have team members scattered around the globe, including from Australia, the US, and Ukraine.

From the conference call, it was clear that things are coming along nicely. We are gaining familiarity at least with the voices of other team members, so that when people are speaking they don’t need to identify who they are. Planning for the shelters is mostly done. Camp layouts have been presented, and are up for argument. Logistics are coming along, but there is a lot to discuss: how much testing of equipment is required, where should it take place, and how do we get the materials from that place to Cape Town in an efficient manner?

For the past few weeks, the satellite link has been worrisome. Although there are two satellites which may be “visible” from Heard Island (in the radio sense, not the optical), they were not very high above the horizon. With terrain being significant on the island (camp is in a valley), and potential for local weather—especially low-layer marine weather—to negatively affect the satellite radio link, we were concerned that there would not be reliable data/phone connection from the island. Our expedition relies on that data link for safety, to keep in touch with off-island expedition headquarters, as well as to help the VK0EK ham radio operations with real-time contact reporting.

Fortunately, while discussing the expedition with satellite service providers, our satellite team found that one of the satellites in the constellation has been repositioned over the Indian Ocean. We will now have a satellite quite high in the sky, and communications are likely to be reliable. Bandwidth may not be very high still, but it’s better than from Pluto.

I’ve been doing some things for the expedition recently, too. Our Bay Area team has acquired laptops which will be used for the radio operation, and I have been helping with software configuration specifications for that. I have also been involved in radio team discussions about how to set up these portable stations—as an apartment-dweller, I know some things about setting up and tearing down stations. Simpler is better, as are plans with fewer moving parts (and less to haul on and off the island).

Last month, I tweeted a live Q&A session, discussing some of the science that has been done (or is proposed) on Heard Island. Check out the hashtag #HeardQuestions for that, and keep an eye out for another Q&A sometime (in a few months).

My physical training continues as well. I’ve been running, biking a little, doing core strength exercises, and stretching a lot more. Yesterday I was even convinced to take part in a 5k run. It has been several years since I last ran a 5k race, and while I’m not in the shape I was ten years ago, I definitely achieved my goals.

With seven months to go, I’m feeling really good about this expedition. Here’s hoping it comes off that well!

Walking on Lava (Flows)

A cascade along the Split Rock River, in Split Rock State Park (Minnesota).  Cascade is 2-3 m tall, and the lava is cold enough to touch.  Image credit: Bill Mitchell (CC-BY).
A cascade along the Split Rock River, in Split Rock State Park (Minnesota). Cascade is 2-3 m tall, and the lava is cold enough to touch. Image credit: Bill Mitchell (CC-BY).

On a conference call some weeks ago, Nigel Jolly, captain of the RV Braveheart which will be taking the Heard Island expedition to Heard Island in March and April, 2016, told the expedition members that they will be expected to be in good physical shape for this expedition. Specifically, he reminded us that not only will we need to be able to walk around on the uneven and slippery ground, but that we will need to do so while carrying heavy things (potentially fragile and expensive, and generally needed for a successful expedition). In order to prepare ourselves, we are to get out and try walking around with heavy stuff on uneven ground.

Naturally, my first thought was that he just told me I needed to go backpacking on the north shore of Lake Superior. Don’t twist my arm too hard!

I called my cousin, who I figured would also probably need some arm-twisting to go backpacking on the North Shore, and we figured out the logistics. We even managed to reserve a hike-in campsite in Split Rock State Park that was right along the shore. Before we left, I checked through Roadside Geology of Minnesota to see if there were any special features besides the anorthosite (rock almost exclusively made of the mineral anorthite, which is a feldspar) which makes up Split Rock itself, and I put a few places on the quick stop list for the drive home.

The geology along the Split Rock River did not disappoint. Here were lava flows, more than a billion years old (1 Ga). Along the river channel, columnar jointing was often evident (see the far bank of the cascade and the far canyon wall above). Most of the lava flows were massive. The opposite canyon wall in the photograph shows columns 5–10 m tall, which would have formed in a single flow. That’s a lot of lava! While hiking along, I was on the lookout for ropey pahoehoe flow-tops, but did not find any that I recognized.

Lava flows found along the North Shore are generally part of the North Shore Volcanic Group, and have an age of roughly 1.1 Ga. They were formed as part of the Mid-Continent Rift system, and now dip gently (~20°) toward the lake. Many of the flows are basalts (low silica, high iron), although there are rhyolites (high silica, low iron) in the area (such as Iona’s Beach).

Mid-Continent Rift system.  Volcanic rocks are in the striped regions, while the dotted regions indicate sediments derived from those volcanic rocks.  Not all of these rocks are at the surface; much of the area in central and southern Minnesota, Iowa, Nebraska, and Kansas are overlain by younger sediments (e.g. glacial till, Paleozoic carbonates).  Image source: Nicholson et al., via USGS.
Mid-Continent Rift system. Volcanic rocks are in the striped regions, while the dotted regions indicate sediments derived from those volcanic rocks. Not all of these rocks are at the surface; much of the area in central and southern Minnesota, Iowa, Nebraska, and Kansas are overlain by younger sediments (e.g. glacial till, Paleozoic carbonates). Image source: Nicholson et al., via USGS.

It was fun to get to see some igneous rocks up close in outcrop (I live on a lot of glacial sediments, and the bedrock is Paleozoic sediments). The backpacking definitely demonstrated that more such activities are needed, because my legs were quite sore by the end of the hiking and the next few days. However, we did have a gorgeous view from the campsite! In the photo below, you can see the gentle dip of the lava flows toward the lake. Obviously, the weather we had on the North Shore (quite comfortable!) was much, much better than is expected for Heard Island. I had a great trip, and hope to head back up some time for more hiking adventures.

A clear morning on Lake Superior.  The lava flows making up the points further down the shore can be seen dipping gently toward the lake.  Image credit: Bill Mitchell (CC-BY).
A clear morning on Lake Superior. The lava flows making up the points further down the shore can be seen dipping gently toward the lake. Image credit: Bill Mitchell (CC-BY).

***
Nicholson, S.W., Cannon, W.F., and Schulz, K.J., 1992, Metallogeny of the midcontinent rift system of North America: Precambrian Research, 58 (1-4), p. 355-386. DOI: 10.1016/0301-9268(92)90125-8

Exploring Capitol Rock, MT

Wide-angle view of Capitol Rock, MT.  Image credit: Bill Mitchell (CC-BY)
Wide-angle view of Capitol Rock, MT. Image credit: Bill Mitchell (CC-BY)

Several weeks ago, I took a road trip with some friends across the northern part of South Dakota as part of a ham radio adventure. When we reached northwestern South Dakota, we were having so much fun that we decided to continue into just across the border into Montana.

At the state line between South Dakota and Montana, we found that there was a relatively high point (Capitol Rock) which we could probably access with our vehicle. Capitol Rock is in a national forest, so no permission would be needed to go there. It would be a good place to do ham radio (primary goal), and it would get me close to some rocks (bonus)!

As we drew closer to the summit of the hills, I couldn’t help but think that the rocks looked a lot like the ones in my research area in northeastern Montana, in the Hell Creek region (Hell Creek and Tullock/Fort Union Formations).[1]

Sadly, I didn’t get quite as close to the outcrops as I would have liked (we were on a bit of a schedule), but I did get some pictures and made a few observations.

North half of Capitol Rock.  Image credit: Bill Mitchell (CC-BY).
North half of Capitol Rock. Image credit: Bill Mitchell (CC-BY).

Here we had flat-lying sedimentary strata, presumably of roughly Cretaceous-Paleogene age (somewhere around 80-50 million years ago, Ma) (introduction to geologic time). These would have been shallow marine or terrestrial sediments from along the western interior seaway, which was on its way out at the end of the Cretaceous (66 Ma, [1]). I would expect to find some fossils preserved in the sediments, and from those, a fairly accurate date on the strata could be obtained. There may even be some volcanic ash deposits which would allow for direct dating using the U-Pb system or the K-Ar system (Ar/Ar dating) .

At the top of Capitol Rock were several massive units with a slight orange color (probably from oxidized iron). Beneath those were some more finely bedded strata, with bed thicknesses probably around 3-10 cm (eyeball estimation), and displaying some rough texture (popcorn texture?). Underneath those were some fairly easily eroded strata of generally uniform grey color. The image below has these observations annotated.

Northern portion of Capitol Rock, annotated.  Image credit: Bill Mitchell (CC-BY)
Northern portion of Capitol Rock, annotated. Image credit: Bill Mitchell (CC-BY)

The ground under my feet for that previous picture was still above average terrain. Here is an additional picture, taken from the south (looking north), which shows that the light-grey sediments are underlain by more yellow-orange units.

Distant photograph of the lower portion of stratigraphy underlying Capitol Rock.  Image credit: Bill Mitchell (CC-BY).
Distant photograph of the lower portion of stratigraphy underlying Capitol Rock. Image credit: Bill Mitchell (CC-BY).

Upon returning home, I decided to see what description I could find online of Capitol Rock’s geology. It seems there are a number of different descriptions of it.

Capitol Rock, located in the Long Pines Unit in Montana, is a massive white limestone uplift that resembles the Nation’s capitol building.
Montana Office of Tourism

Capitol Rock, located in the Long Pines land unit in Montana, is a massive white sandstone remnant which originated as a volcanic ash deposit. This unique formation resembles the Nation’s Capitol Building in Washington, DC.
US Forest Service

The Bureau of Land Management (BLM) has an interesting discussion of the geology of this area from the perspective of firefighting, specifically in the avoidance of fibrous or asbestos-like minerals which are present in some of the formations in the area:

Brule Member, White River Formation [ed: Formations are a larger stratigraphic unit, and can include multiple Members] – may only be present at Capitol Rock (SE 1/4 sec. 17; T3S; R.62 E) in the Montana portion of the Sioux District. Located at the base of the Arikaree Formation. Massive pinkish gray, calcium containing, clayey siltstone: nodular claystone: and channel sandstone. Contains abundant vertebrate fossils. Thickness 0-30 ft. The member is composed of massive pink clay, exposed in the badlands just Southeast of Reva Gap, well-bedded, hard pale green sandstones alternation with very pale brownish gray clay.
Weathering causes a tread and riser affect much like a staircase. Both the sandstone and the clay are generally calcareous and Bentonitic. The lower portion of the vertical cliffs in Slim Buttes is generally Brule.

Chadron Member, White River Formation – only located in the southern Long Pines within Montana. Found at the base of the Arikaree formation and beneath the Brule Formation at Capitol [R]ock (SE 1/4 sec. 17 T, 3 S., R. 62 E). Basal conglomerate sandstone overlain by beds 10 to 15 ft
thick of dark gray bentonite and cream colored siltstone. Thickness 0-100 ft.
Bureau of Land Management

Well, that’s a puzzling bunch of information, isn’t it! Various sources are suggesting limestone, sandstone from volcanic ash, and a mix of sandstone and siltstone. There’s one more source to check, too: the geologic map. Specifically, we’re interested in the Ekalaka 30’x60′ quadrangle from the Montana Bureau of Mines and Geology!

In the geologic map (look along the right [eastern] edge, near the “T 19N” mark; Capitol Rock is ~1 km NE of the “Tar” label] we see the Fort Union Formation (informal Ekalaka member) at the base of the hills (i.e., under my feet), which is consistent with observations and the relatively detailed presentation from the BLM. It is also consistent with my experience that the Fort Union Formation is generally yellow-orange (in contrast to the drab, grey of the Hell Creek Formation). Then things get trickier. The rocks right at Capitol Rock are mapped as “Tar”, which is the Tertiary Arikaree Formation.

So, what is the Arikaree Formation? Well, the USGS has this to say:

Arikaree formation: gray sandstone with layers of concretions; contains volcanic ash and, locally, channels filled with conglomerate; known only in southeastern Montana.

On the other hand, the North Dakota Department of Mineral Resources breaks the Chadron, Brule, and Arikaree into distinct formations unto themselves.

I suspect this is all hitting at an important point: mapping is really hard, as is saying the rocks over here are the same as the rocks 40 km away. These difficulties are compounded when different scientists use different terminology, such as when the mapping is done by state geological surveys. The same rocks may change names when a state boundary is passed. Sometimes researchers will use the terminology from one state to apply to the rocks on both sides of the boundary, and then the literature is filled with multiple terminologies for the same rocks. It can also be very difficult to correlate rocks laterally over large distances, especially when there is poor outcrop over those distances (i.e. between buttes).

Here’s my interpretation of what’s going on at Capitol Rock: it is composed of siltstone, sandstone, and altered volcanic ash [still good for U-Pb dating!]. This volcanic ash is high in erionite, an asbestos-like mineral. Naming of the unit could include either the Arikaree Formation, or the Brule Member of the White River Formation. An age of 37–30 Ma seems reasonable.

***

[1] Renne, P. R., Deino, A. L., Hilgen, F. J., Kuiper, K. F., Mark, D. F., Mitchell, W. S., III, Morgan, L. E., Mundil, R., Smit, J. (2013) Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary. Science 339: 684-687, doi: 10.1126/science.1230492.

Geoscientist’s Toolkit: Dilute Acid

Folded outcrop of marine sediments in Berkeley, CA.  Image credit: Laikolosse (CC-BY-NC).
Folded outcrop of marine sediments in Berkeley, CA. Image credit: Laikolosse (CC-BY-NC)

When looking at sedimentary rocks in the field, one of the questions which may come up is whether or not a rock is a carbonate, such as in the outcrop pictured above. Although it is easy to determine that with an electron microprobe in the lab, there is a faster field test method: using dilute hydrochloric acid.

Sedimentary geologists will often carry a bottle of 0.1 M HCl and a watchglass with them in the field. A chip of the rock in question can be broken up and placed on the watchglass. When the acid is added, a carbonate will fizz as the acid releases carbon dioxide. This is the same process which makes a baking soda volcano erupt.

In some of my field work in the Texas Panhandle, I encountered a white layer among the redbeds. This bed was not gypsum, as many of the other white beds were. Because I was looking for volcanic ash deposits, not carbonates, an acid test was performed in the field. Unfortunately for me, the ground up sample started fizzing, so I knew it wasn’t the volcanic ash I wanted to find.