12 August 2015
As longtime readers know, late summer is when my Rockies students submit their final projects – web-based explanations of key geologic sites they examined during the trip.
Today, I offer you a guest blog post by student John Leaming. You’ll notice that I’m not *completely* absent from the post, however – I make a couple of cameos as “sense of scale.”
Glacier National Park, Belt Supergroup
I recently traveled through Glacier National Park as part of a Field Geology Course of the Northern Rocky Mountains. The distinctive and very distinguishable strata observed throughout the park’s landscape showcase the mainly sedimentary rock that comprises the Belt Supergroup.
Click here for a map of Glacier National Park, courtesy of National Park Service.
The Belt Supergroup which comprises most of the mountains in Glacier National Park, is a thick series of sedimentary rock layers that were formed from an ancient passive rifting event about 1.6 to 1.4 Ga, where the Siberian Platform separated from the North American Craton, forming a massive Mesoprotozoic age sea basin known as the Belt Basin or Belt Sea. This basin would become the depositional location for an immense amount of sediment deposited from the surrounding landscape. Due to mass wasting and erosion, the rapidly subsiding Belt Basin was able to accommodate more than 15,000 ft. of sediment deposited ~1.47 to 1.4 Ga. Sediments such as beach sands, shallow water and fine sands, mud, silt, and limestone from varying depositional environments within the basin at the time, formed the distinctive rock layers we see today in the Belt Supergroup.
Image from formontana.net
The Formations of the Belt Supergroup
The sedimentary rock formations within Glacier National Park can typically be distinguished by their color. From oldest to youngest, the formations are: Prichard and Altyn, Appekunny, Grinnell, Empire, Helena or Siyeh, Snowslip, and Shepard.
- The Prichard Formation is found in the western portions of the park and is ~1.375 to 1.4 Ga siltstone and dark-gray argillite, which is a lightly metamorphosed mudstone, and is ~4,000 ft. thick. These rocks are roughly the same age as rocks found in the Altyn Formation, but were deposited in a deeper low oxygen water environment, resulting in its darker color.
- The Altyn Formation is found in eastern portions of the park and is composed of ~1.350 to 1.450 Ga limestone and dolomite ~780 -840 ft. thick. They are light-gray or have been weathered to a light tan.
- The Appekunny Formation is composed of ~1.375 to 1.4 Ga mudstone and argillite ~2,200 ft. thick, and are gray to light green in color due to the presence of chlorite minerals, and deposition in a slightly deeper lower oxygen environment
- The Grinnell Formation is similar to the Appekunny Formation, and is composed of sandstone and argillite ~1740-2590 ft. thick, is rich in hematite, and has a deep brick-red color. This indicates that it was deposited in a shallower oxygen rich environment. The iron content in both formations is roughly equal, and it is the depth of the water and the oxygen levels that give them their distinctive color.
- The Empire Formation is a transitional formation composed of argillite and siltite ~400-500 ft. thick. It has a combination of features of the Grinnell Formation that lies below it and the Helena Formation that lies above it.
- The Helena or Siyeh Formation is composed of ~1.1 Ga limestone and dolomite ~2460-3400 ft. thick, and is medium to dark gray in color, but often has an orange buff or honey colored weathering rind.
- The Snowslip Formation is composed of ~1 Ga red and green argillite and sandstone ~1170- 1600 ft. thick, and is only exposed at high elevations within the park. Like the Shepard Formation much of the rock that involves them has been eroded away.
- The Shepard Formation is composed of argillite, siltstones, and dolomite, and quartzite ~550-1150 ft. thick. The Snowslip and Shepard Formation make up the base pf the Missoula Group and the top two layers of remaining Belt rock in Glacier National Park.
A prominent feature seen in the landscape while travelling eastward in Glacier National Park is Chief Mountain, seen at the right of the photo below. It lies in the northeastern corner of the park on the border between Glacier National Park and the Blackfeet Indian Reservation. At an elevation of 9,080 feet, its unique shape is discernable from miles away, and is regarded as a sacred location to the Blackfeet Indian Nation.
Click here for topographical map of Chief Mountain
Chief Mountain also represents a captivating example of overthrust faulting. This significant structural feature is known as the Lewis Overthrust.
The Lewis Overthrust began ~170 Ma, when Earth’s crustal plates were elevated in a mountain building event called the Sevier Orogeny. Towards the end of this orogeny, a huge slab of rock ~280 miles wide and ~3 miles thick was moved ~50 miles from the west to the east. The huge rock mass containing stronger rocks slid over the softer more easily deformed rock that lie below. The hanging wall moved up and over the footwall, resulting in much older rock being situated over younger rock.
In the photo above, erosional forces have stripped away much of the upper portion of the original Precambrian rock to the left of Chief Mountain, and this isolated remnant of the thrust sheet is called a klippe. Erosion has revealed much of the exposed rocks and structures seen in Glacier National Park today.
The opposite of a klippe is called a fenster, where a portion of footwall is entirely surrounded by hanging wall. It is a tectonic window that has eroded through to lower rock. It should be noted that that this geologic structure does not present itself in the area of Glacier National Park.
In the depiction created above, the fenster can be seen on the right as an eroded opening into the underlying rock.
In this photograph of Chief Mountain taken from the east, the upper plate of overlying Precambrian rocks are more than 1.4 billion years older than the underlying Cretaceous age rocks. Chief Mountain is composed of belt rock from the Altyn and Appekunny formations. The Lewis Overthrust is visible at the base of the Altyn Formation.
Most of the sedimentary rocks from the Belt Supergroup seen in Glacier National Park are Mesoproterzoic in age, and were deposited around 1.45 Ga to 800 Ma. Unlike other areas that have undergone mountain building, these rock formations in the park have preserved many of the characteristic features of sedimentation.
A photo of mud cracks from the Grinnell Formation on the way to Grinnell Glacier. These mud cracks were formed under the same conditions found today in areas exposed to wet, then dry conditions.
A photo of oscillatory ripple marks captured in the Grinnell Formation. These shallow water features are the result of gentle wave action in the fine silty clay.
Another photo of ripple marks in the Grinnell Formation along the way to Grinnell Glacier.
This photo shows mud chip rip-up clasts in the Grinnell Formation, and is indicative of an ancient storm. The high energy currents carried sand and ripped up clay particles which were deposited into the muddy surroundings.
Within the Helena Formation, a fossilized bacterial life called stromatolites can be observed in the sedimentary layers and rock outcrops. They are a cyanobacteria blue-green algae that grew in the relatively warm, shallow waters of the Belt Basin, and their existence can be traced back more than 3.5 Ga, making them one of the oldest lifeforms on this planet. They lived in mat-like colonies and would precipitate limestone on fine sediments onto successive layers. As they continued to grow higher in an effort to reach sunlight, they created their distinctive dome shaped appearance. Stromatolites generate a layer of protective slime that covers their cells, and a new layer can be generated when the old layer gets covered in too much sediment. These dome shaped layers are often referred to as a cabbage heads. Due to the abundant volume of stromatolites during this time, they are responsible for producing much of the oxygen in the Earth’s atmosphere at that time, by consuming carbon dioxide in the water, and releasing oxygen through photosynthesis. The Altyn and Snowslip Formations also contain beds of stromatolites, but the best examples seen in the park are undoubtedly seen in the Helena Formation.
A lesson on the stromatolite layer along Going-to-the-Sun Road, being taught by Professor Callan Bentley. This stromatolite layer is ~3 feet high and 25 feet long.
A closer cross-section view reveals the intricate layers within the matting, and the distinctive dome shape, also called cabbage heads. Note the buff color of the weathered Helena limestone visible above the layer of stromatolites.
This huge exposure of honey-colored stromatolites are found in the Helena formation at Grinnell Glacier. These stromatolites have been ground down over time by the glacier, providing a unique view above this particular colony.
One final picture of the stromatolites with Professor Callan Bentley, which conveys just how expansive this layer at Grinnell Glacier is.
The Purcell Sill
Not all of the distinctive rock layers in Glacier National Park are sedimentary in origin. The dark band that stands out in stark contrast throughout most of the strata within mountains seen in the park is called the Purcell Sill. It is a 130-250 ft. layer of fine grained mafic igneous rock called diorite, which intruded between the layers of lighter colored limestone. The younger 725-775 Ma intruding sill, contact metamorphosed and baked the surrounding 1.1 Ga limestone of the Helena Formation, creating its bleached appearance.
The Purcell Sill shown here at Grinnell Glacier stands out in stark contrast as the black stripe in the middle of the strata.
Another picture of the diorite sill found within the Helena formation at Grinnell Glacier. Note the light colored limestone that has been contact metamorphosed above and below this intrusion.
Looking Glass Road
This outcrop located just outside of Glacier National Park, contains a variety of young sedimentary deposits that range from fine silt beds that crumble in your hand, to lithified beds containing siltite and larger size rocks and cobbles that have been tilted nearly 90 degrees. These layers of strata were deposited as a result of the last ice age during the Pleistocene that ended ~12 ka.
Click here to see the Google Map location
This picture shows the vertical bedding of fine glacial powder, siltstone, and glacial till within the outcrop.
This picture shows the finer grained siltstone, which results from the depositing of glacial powder. There is evidence of dropstones where fragments of rocks drop out of a glacier, and are deposited into the finer grained sedimentary layer below.
This picture shows a close-up of the beds composed of small rocks and cobbles, which are likely the result of glacial till that was transported and deposited by a glacier
Here is a synopsis of what occurred at this outcrop. After the glacier deposited its end moraine, the glacier retreated. As it melted, it deposited fine powdery silt that settled out in a glacial lake. Dropstones were transported, or floated out into the lake within pieces of the glacier, where they dropped into the silt as the glacial pieces melted. Additional gravel and small rocks were deposited in the lake by glacial outwash. The glacier then advanced, acting like a bulldozer, and scraped up all the lake sediments that were deposited previously in its path. The glacier receded a second time, and after melting away, left the vertical beds of sediment made up of the silt, gravel, cobbles, and dropstones seen here along the Looking Glass Road. It is fascinating to see, that the tilting of these beds are so unique, when compared to other structures within the park associated with mountain building.
— Justin Shinohara (@JustinShinohara) August 11, 2015
The best place for eliciting responses, however, appeared to be the AGU Facebook page.
What do you see here?http://ow.ly/QL4Ps (Mountain Beltway at #AGUblogs)
2 RepliesJuan Leonardo Vargas Fault line,also there are a number of features that resemble faces, some on the cutaway and also in the tree branches and also a person, ,possibly a boy about 10 or 11 yrs.old..Wajahat Abbasi Thurst fault smile emoticonClark Davis Not sure but thanks for asking!!Ted Asher the face of Jesus?Joe Miller A guy ready to jump.Sean Daniels anticline, syncline, with lower volcanics.. some local to regional faulting and folding as the driver of the structure.
Aradia Farmer reverse thrust fault, mud, rocks, grass, trees, humanScott Ehlert You can’t tell much for certain but probably a fault. Could be a slide or something else.
Some of those appear to be earnest efforts at interpreting this outcrop. Others are typical Facebook gobbledygook and people being smartasses.
If you’re one of those who’s interested in figuring this thing out, we’ll take the next step today. I’ll ask you to take another look, this time with annotations sketched in. I’ve traced out bedding (where visible) in the version of the image below:
Additional clues: these are Pennsylvanian aged strata exposed near the eastern edge of the Alleghany Plateau in northern West Virginia, along a newly-opened stretch of Corridor H, the east-west highway that has been under construction here for the better part of a decade.
Does that change your opinion of what’s going on here?
11 August 2015
Noodle on that for a minute. I’ll post my answer later.
No hints for now. Just examine what you see in the outcrop.
10 August 2015
I got horrible news yesterday. One of my favorite students committed suicide Saturday night.
Ernie was a participant in last year’s Border to Beltway field exchange. He was a student at El Paso Community College, and was an enthusiastic participant in both phases of that program (a week in spring in West Texas, a week in May in the mid-Atlantic region). I only got to interact with him directly for those two weeks, but he made a big impression on me. He was an extraordinary young man.
I bonded with Ernie easily, as did many of his fellow students. He was impassioned and smart, friendly and vocal. He and I shared a similar worldview: excited by reality, unpopulated by supernatural beings. He had a skepticism of authority and dogma, matched (or exceeded) by an enthusiasm for participating in life, in thinking for one’s self. That’s the thing that makes his suicide so hard for me to process – he seemed so keen on being alive during our brief time together.
He had many obstacles in his path, including financial woes. After “Border to Beltway,” he dropped out of school because he didn’t have the money to pay tuition, and was working for the past year in an attempt to save up enough to return to his education as a geologist.
Without the time necessary to process his death, I went to bed last night deeply saddened. I woke up this morning missing him. It’s painful to know his gifts won’t be shared with our world in the days and decades to come. His absence is our loss.
This morning, I’m wracked with a sense of survivor’s guilt – that perhaps if I had reached out to him more, or more frequently, he might have opted for a different path. But I guess that’s the sort of thing we all think when someone important to us passes away at their own hand.
Perhaps I should flip it around into a prompt that could lead to positive action: how many of the other bright minds I’ve been so fortunate to meet in my job are facing similar struggles, are contemplating ending their lives? Who needs to be reached out to today? I’ve met so many students, struggling with a panoply of issues, and I have it in my power to help some of them.
Photo by Marcelo Arispe
Ernie was an extraordinarily talented artist. His outcrop sketches were precise, elegantly rendered, and insightful. Nobody had better field notes than him.
He was celebrated a musician, too – and his songs enlivened the campfires on our field course. His presence was positive and had an effect of building our sense of community.
He was a gifted person and would have been a talented geologist. I would have foreseen him living a full, happy life. I’m glad I got the chance to meet him.
Photo by Marcelo Arispe
Descansa en paz, Ernie.
7 August 2015
What is Matt looking at here?
Matt was one of my Rockies students this summer, a geology major at the University of Virginia. Together with another UVA student and students from Mary Washington University and George Mason University, Matt embarked on a mountain-climbing hike during our evening camping at Swift Dam, near Depuyer, Montana. The hikers were treated to an extraordinary sight when they attained the summit:
What caught my eye about this perspective was the yin-yang looking structure on the right (west). Let’s zoom in:
I found it quite readily on Google Earth:
It appears to work something like this:
Though I haven’t visited this exact outcrop, I suspect the strata here to be Mississippian-aged carbonates, and the deformation accompanied the Jurassic-to-Cretaceous Sevier Orogeny.
5 August 2015
Here are some ripple marked strata in the Silurian-aged Rose Hill Formation from West Virginia:
Tilting of these strata occurred long after the Silurian – during the late Paleozoic Alleghanian Orogeny — an application of the principle of relative dating known as “Original Horizontality.”
There are also trace fossils to be seen on these bedding planes.
This was another site that I was introduced to in April, on the GMU sedimentology and stratigraphy field trip led by Rick Diecchio.
3 August 2015
As part of my work on the GEODE project, I’m always looking for good imagery to teach key concepts in geoscience.
One important concept that I’ve been thinking about lately is the principle of relative dating on the basis of inclusions.
Just as you can’t bake a loaf of raisin bread without already having raisins in your kitchen, rock units that are included in another rock unit must be older than the rock that includes them. Chunks of sandstone included as xenoliths in a granite imply an older sandstone that had chunks broken off, and then later a granite formed, including some of those chunks among its magmatic crystals.
One local rock unit that illustrates this process well is the Sykesville Formation, a metasedimentary unit of unknown depositional age, that was metamorphosed during the late Ordovician Taconian Orogeny. It looks a lot like our other Piedmont metagraywacke units such as the Laurel Formation and the Mather Gorge Formation, but it’s distinctive because of its profusion of inclusions — so called “olistoliths.”
Here are six new GigaPans I shot last Friday morning to highlight some of the features of the Sykesville, as exposed near the Potomac River at “Chain Bridge Flats,” the very un-flat section of bedrock terrace / floodplain immediately upstream of Chain Bridge, in Washington, DC’s westernmost corner.
One possibility is that these rocks represent an accretionary wedge complex, and so they have been “tectonically mixed” and jumbled up different sized particles (from different original depositional settings) into what you see here, prior to being metamorphosed. Another possibility is that these rocks’ textures reflect their original depositional character (matrix-supported diamictite), and therefore imply perhaps submarine landslides or glacial dropstones. Some of the clasts are quite angular whereas others appear to be well-rounded. Of particular note is the fact that some of the inclusions are amphibolites and gneisses with internal foliations that differ from the Taconian foliation that is so prominent in the matrix. This implies a previous generation of tectonism in the clasts’ source area, and suggests to me that that source area was continental crust (as opposed to, say, the Taconian volcanic island arc).
This is a great spot to introduce students to key concepts in relative dating, explore the differential weathering of clasts of different compositions, and delve into some fascinating questions of the interpretation of enigmatic rocks.
31 July 2015
Some time ago, I featured as Friday fold the extraordinarily complex duplex structure to be seen in the Cretaceous “gastropod limestone” member of the Kootenai Formation at Sandy Hollow, Montana. Today, let’s take a deeper look through a couple of hand-shot GigaPan images:
Here’s the bigger of the two:
Here’s one with students for scale:
30 July 2015
My favorite place to have lunch in Montana is at the Grinnell Glacier cirque in Glacier National Park. This is the dining room table:
You’re looking at a bedding-plane-parallel exposure of Mesoproterozoic stromatolites here. Every few years, I’m lucky enough to hike up there with motivated students and share food atop this unparalleled view into the shallow seas of more than a billion years ago.
Stromatolites are sedimentary structures that preserve the bulbous forms of ancient microbial mats.
The biofilm was probably photosynthetic, as the stromatolites domed upward toward the sun. You’re seeing them sectioned here like a head of cabbage sliced laterally through its ‘equator’… hence the middle of each of these concentric circles represents the oldest part of each stromatolite, and the outermost layers represent the most recent laminations to be tacked on before burial.
On the hike down from lunch, my students and I also saw some stromatolites sectioned vertically – so this is a different view, a side view, of the same dome-like structure:
These structures formed at a time when stromatolites were apparently the most advanced life on Earth. This planet was “Slimeworld” in the Mesoproterozoic, to use Gabrielle Walker’s memorable phrase.
Here, I positioned the students so each had stromatolitic laminations draped over the tops of their heads:
A stromatolite of a different color, seen in float below the cirque:
One more beast: a big erratic (note the glacial striations) full of stromatolites. Jake gives it a hug:
…And two close-ups of this boulder:
…Delicious repast for those who like nibbling on our planet’s Slimeworld past!
Glad you could join me for lunch.
29 July 2015
My friend Barbara am Ende sent along this lovely image of a dike in Colorado:
Here’s the site. You can see the dike in Google Earth.
Dikes are fractures, filled with molten rock, which then cools and solidifies, sealing the crack shut. In this case, once it got uplifted to Earth’s surface and exposed, the dike rock is tougher (more resistant to weathering) that the older rock it cut across. As a result is weathers “proud” of the landscape. It is being eroded at a rate slower than the rock on either side of it. This isn’t necessarily the case – not all dikes weather out like the Great Wall of China. Along Mather Gorge (Potomac River, downstream of Great Falls), a quartet of lamprophyre dikes weather away negatively, resulting in tabular recessed features on the landscape, which is otherwise dominated by metagraywacke:
And here’s another negatively-weathering example: basalt (later metamorphosed to lower greenschist facies) intruding granite on Old Rag Mountain, Shenandoah National Park, Virginia:
Thanks for sharing, Barbara!