February 27, 2014 by Chuck Bailey
Last summer I reported on our field research in the High Plateaus of Utah. Erika Wenrich’s senior thesis project involves a gravity survey aimed at estimating the amount of sediment beneath Fish Lake, a large alpine lake developed in a high-elevation graben. In June we measured gravity at a network of stations around Fish Lake, but to complete the gravity survey, and model the sediment’s thickness in the basin, we needed gravity data on the lake itself. It’s now February and Fish Lake is covered by ice—time to return and complete the survey on the lake’s frozen surface.
Our whirlwind outbound journey included an unexpected drive to Dulles airport to catch a long flight into Las Vegas followed by an even longer drive from Nevada to Fish Lake. We arrived at the lake weary from travel, but excited to get started. The lake was crusted over with ~30 cm of ice (12”) and a layer of snow from a recent storm. The temperatures were well below freezing and accompanied by a stiff breeze from the southwest—it was brisk.
As expected measuring gravity on the lake’s icy surface during the day proved to be nearly impossible. The gravimeter is a delicate instrument that needs to be carefully leveled and works via the stretching of a spring balance with a constant mass. During sunny daylight hours the lake receives copious solar insolation that heats the ice, and as the ice expands fractures develop (not big through-going cracks, but rather small cracks here and there). When cracks propagate, seismic energy courses through the ice causing the delicate spring in the gravimeter to oscillate such that obtaining a reliable and reproducible measurement is not possible.
At night the ice is far more stable and consequentially we became nocturnal creatures wandering about on the dark icy surface making our gravity measurements. The lake was profoundly quiet during the wee hours and the veil of stars put on quite a show overhead. Working the night shift took its toll; after two consecutive evenings into the early mornings spent out on the ice we were wiped out. However, we completed three new gravity traverses across the ice and Erika is in a good position going forward with her research.
Our trip was timed to coincide with a visit by a team of collaborating geoscientists who were obtaining the first sediment core from Fish Lake. Once again the ice was critical, as the team’s coring rig was set upon the firm surface—for four days they lowered and raised the coring apparatus through 30 meters (100’) of water and into the muddy sediment at the lake’s bottom. They were rewarded with about 11 meters (35’) of core, which was safely transported to Oregon State University’s core repository to await detailed study by the team.
William & Mary alum and all-around good guy, Dr. Scott Harris from the College of Charleston used a transient electromagnetic (TEM) geophysical system to learn about the subsurface. He had quite a setup with a long (400 m) wire transmitter placed around multiple receiver loops out on the ice. The system induces an electric field and then measures the decay of that field through time, providing what is essentially a column of the conductivity in the subsurface. The lake’s fresh-water has a very low conductivity, while the infilling mud in the lake basin and underlying bedrock have much higher conductivities. His initial tests yielded subsurface information to depths of over 300 m, hopefully imaging the contact between the lake sediments and bedrock.
Our gravity data indicate that the lake is underlain by upwards of 100 meters of sediment (>300’), so the coring operation sampled just the uppermost layers of the graben fill. In the future we hope to core though the entire sediment package to fully understand the geologic history of graben development, lake formation, and glaciation.
Erika is one of 33 William & Mary geology majors in the class of 2014 and they are all working on senior research (thesis) projects. These studies range from gaging rock erodibility along the banks of the Potomac River, to understanding the complexities of agricultural runoff in the Coastal Plain, and even searching for water ice on Mercury. As college seniors, W&M geology students are contributing new knowledge about how the Earth operates (and other worlds as well). It’s cool stuff and part of what makes majoring in geology at William & Mary distinctive.
February 17, 2014 by Chuck Bailey
In 1969 Virginia embraced the travel slogan Virginia is for Lovers and at various times during the last 45 years William & Mary geology students have emblazoned departmental t-shirts with Virginia is for Lavas and turned the iconic heart into a volcano.
In that spirit, Geology Fellow Alex Johnson and I wrote a piece on the ancient lavas that once covered a large swath of what would become Virginia. What follows is an abbreviated version. Read the full version.
Stony Man is a high peak in Virginia’s Blue Ridge Mountains that tops out at just over 1200 m (4,000’). Drive south from Thornton Gap along the Skyline Drive and you’ll see the impressive cliffs of Stony Man’s northwestern face. These are the cliffs that give the mountain its name, as the cliffs and slopes have a vague resemblance to a reclining man’s forehead, eye, nose, and beard. Climb to the top and you’ll see peculiar bluish-green rocks exposed on the summit that are ancient lava flows, part of a geologic unit known as the Catoctin Formation. From the presidential retreat at Camp David to Jefferson’s Monticello, from Harpers Ferry to Humpback Rocks, the Catoctin Formation underlies much of the Blue Ridge. This distinctive geologic unit tells us much about the long geologic history of the Blue Ridge and central Appalachians.
The Catoctin Formation was first named by Arthur Keith in 1894 and takes its name for exposures on Catoctin Mountain, a long ridge that stretches from Maryland into northern Virginia. The word Catoctin is rooted in the old Algonquin term Kittockton. The exact meaning of the term has become a point of contention; among historians the translation “speckled mountain” is preferred, however local tradition holds that that Catoctin means “place of many deer”.
Origin of the name aside, the Catoctin Formation is a geologic unit that crops out over a large tract in the Blue Ridge region of Virginia, eastern West Virginia, Maryland, and southern Pennsylvania. Its current geographic extent does not, however, represent the original extent of the Catoctin Formation. In southern Pennsylvania and Maryland, the Catoctin Formation crops out in one contiguous area, but in Virginia there is an eastern and western outcrop belt of the formation. The Catoctin Formation is exposed on both limbs of the Blue Ridge anticlinorium, a complex regional-scale fold that has been breached by erosion thereby exposing older rocks in the center and younger rocks such as the Catoctin Formation along the flanks. Originally, the eastern and western belts were contiguous, but erosion has removed the younger Catoctin Formation to expose older rocks in the central Blue Ridge.
The Catoctin Formation is composed primarily of metabasalt, commonly referred to as greenstone due to the rock’s greenish tint. When the basalt was metamorphosed, igneous minerals such as pyroxene, plagioclase, and olivine were converted to new minerals (chlorite, actinolite, and epidote), which give the rock its distinctive color. The Catoctin Formation also contains discontinuous layers of metasedimentary rock (including phyllite, quartzite, and even marble), as well as volcanic breccia and metarhyolite.
As the Catoctin lavas cooled, columnar joints developed in many flows. Columns form as the rock volumetrically contracts during cooling. As a lava flow cools, both from its top and bottom surface, these cooling cracks propagate inward, forming hexagonal columns. Columnar joints are best developed in lava flows that extrude onto a landscape. These columns are common in the Catoctin Formation’s western outcrop belt and indicate the flows were extruded on land. In contrast, at a number of outcrops in the eastern Blue Ridge, pillow lavas are preserved in the Catoctin metabasalts. Pillow lavas are bulbous to lobate masses formed as lava rapidly cools underwater, forming a glassy shell as the surrounding water quenches the lava.
How old are the ancient lavas of the Catoctin Formation? When did a vast volcanic plain cover the terrain that would become central and northern Virginia?
Metabasalt dikes commonly intrude and cut older granitic rocks in the Blue Ridge, and in rare cases these feeder dikes can be traced upward into metabasalt flows that covered the granitic rocks. Based on these cross cutting relations, the Catoctin Formation is clearly younger than the old Blue Ridge granites that crystallized between 1.2 and 1.0 billion years ago. The Catoctin metabasalts are overlain by a sequence of sedimentary rocks that contain fossils including Skolithos, a distinctive trace fossil formed by burrowing creatures. These fossils are characteristic of sediments deposited during the early Cambrian period some 520 to 540 million years ago.
Geologists have attempted to date the Catoctin lavas with varying degrees of success. In 1988, Badger and Sinha reported a late Precambrian age of 570 ± 36 Ma for the Catoctin Formation based on the Rubidium/Strontium (Rb-Sr) dating technique, however this isotopic system can be readily disturbed by later metamorphism. Zircon is a high temperature igneous mineral that is ideal for geochronological studies. Zircon crystals invariably contain a small amount of uranium, a radioactive element that decays to lead at a constant and well-known rate. By comparing the ratio of certain uranium and lead isotopes in a given crystal, it is possible to discern how long the uranium has been decaying, and thus the age of crystal and, by association, the rock in which it is situated. However, silica-poor mafic igneous rocks, such as basalt, commonly lack zircons and thus cannot typically be dated with this technique.
Yet, all is not lost as the Catoctin Formation is composed of more than just metamorphosed basalt; in northern Virginia, western Maryland, and southern Pennsylvania, metarhyolite is interlayered with the metabasalt. Rhyolites are felsic volcanic rocks that typically contain zircon and can be dated with the U-Pb method. Based upon U-Pb ages from metarhyolites in the Catoctin Formation, the extrusion of this volcanic complex occurred around 570-550 million years ago (Aleinikoff et al., 1995; Southworth et al., 2009) during the Ediacaran Period at the end of the Neoproterozoic Era.
What is a sequence of volcanic rocks doing in the Blue Ridge?
The Catoctin Formation is likely a continental flood basalt associated with late stage rifting that broke apart the Rodinian supercontinent and created the Iapetus Ocean. Flood basalts are large igneous provinces where low viscosity basaltic lava floods vast areas of the Earth’s surface. Due to the lava’s low viscosity, flood basalts are generally extruded quite rapidly, geologically speaking. In the case of the Catoctin Formation, more than 30,000 cubic kilometers of lava were extruded in a few million years. The origin of flood basalts is widely debated, however the most common explanation involves a combination of decompressional melting due to both continental rifting and the rise of a hot and expansive mantle plume. The origin of mantle plumes is also poorly understood, but likely involves a buoyant melt produced near the mantle-core boundary, which proceeds to rapidly rise through the mantle, melts other rocks, and drives extrusion of volcanic rocks at the surface.
Throughout geologic time, the cycle of assembly and dispersal of so-called supercontinents has been one of the most dramatic examples of plate tectonics at work. The supercontinent Rodinia is hypothesized to have been formed in the Late Mesoproterozoic and Early Neoproterozoic. At its core was Laurentia, a large landmass composed of what is now modern day North America, Greenland, and northern Scotland. As supercontinents are wont to do, Rodinia began rifting apart some 600-550 million years ago; the tectonic plates began to once again change direction and slowly drifted away from one another, forming new oceans and closing others. One of these new oceans that was created (and later destroyed during the creation of the most recent supercontinent, Pangea) was the Iapetus. The Iapetus formed between the eastern edge of the Laurentian craton and almalgam of tectonic blocks that would eventually be formed into what is referred to as Gondwana. It was during this period of rifting that the volcanic rocks of the Catoctin Formation were extruded on Laurentia’s margin.
A key method by which geologists have discerned the cycle of supercontinent formation and dissolution has been through paleomagnetism, which is the study of the magnetic properties in certain minerals as means to reconstruct the past location of tectonic plates. Although paleomagnetism has played an integral part in developing the theories of plate tectonics and continental drift, paleomagnetism in old rocks is complex. Take for instance the plight of Rodinia, different researchers have constructed multiple iterations of the supercontinent’s configuration and location. One study, focused on the Catoctin Formation in particular, place Laurentia near the South Pole at the end of the Neoproterozoic.
How did a vast plateau of volcanic rocks that were buried beneath kilometers of shallow marine sedimentary rocks become the foliated greenstones that undergird the Blue Ridge Mountains? The answer to this question involves a complex history of deformation, metamorphism, and uplift.
Recent geochronological studies indicate that the penetrative deformation and metamorphism, the tectonic event that produced the distinctive foliation in the Catoctin Formation, occurred between 320 and 350 million years ago during the Carboniferous Period. Some 20 to 30 million years later Blue Ridge rocks were thrust over sedimentary rocks of the Valley & Ridge province, during the collision that produced Pangea. The mountains produced during this collision likely rivaled the size of today’s Himalayas.
In the million of years since their uplift, the Blue Ridge has slowly been beaten down with rounded ridges replacing rugged mountains. As the processes of weathering and erosion continued their interplay, different rock types eroded at different rates resulting in the modern topography of the Blue Ridge. Compared to the overlying stratified rocks and underlying granitic basement complex, the fine-grained metavolcanic rocks of the Catoctin Formation are particularly resistant to erosion.
The great American author Nathaniel Hawthorne once noted “mountains are earth’s undecaying monuments.” Here in the central Appalachians much of that monument is shaped from the basaltic rocks of the Catoctin Formation, a unit birthed by fire during the breakup of ancient Laurentia and later changed to greenstone during the growth of the new Pangean supercontinent.
January 30, 2014 by Chuck Bailey
Oman is a sunny place and cloudy days are rather uncommon. On Friday, January 10th we awoke to cloudy skies over Muscat. Today was the day to tackle “the exposure” at Wadi Mayh about 25 km (19 mi.) south of Muscat. Wadi Mayh is a through-going drainage that offers tremendous exposures of bedrock in its channel and valley walls.
The exposure we wished to see (and photograph) is a steep north-northeast facing slope rising 170 meters (~560 ft.) above the wadi. At this time of year the face is nearly always in shadow and the bright Omani sun backlights the scene making photography tough. I thought the clouds would provide just enough cover to mellow the lighting and result in a better picture.
Alex Johnson and I climbed to a high perch across from the exposure and readied the equipment, but the sun refused to be muted behind the clouds. We waited patiently. There were moments of less sun, but we never got the lighting conditions we’d hoped for. Nevertheless, we put the GigaPan to work, taking a set of 56 images of the rocky face that we later stitched together into a seamless high-resolution image. What follows is the stitched image that spent some time getting ‘massaged’ in Photoshop to highlight this brilliant exposure and was then uploaded to the GigaPan website. Try zooming in to the image to see fine-scale details such as fractures, veins, and fold hinges.
These gray limestones lack much contrast, but the layering is readily evident. It is difficult to appreciate the scale of the image. Recall the height of the exposure exceeds 150 m (500’); the best scale markers are near the bottom of the image, they are ~7 meters tall (23’) power poles. This is a huge exposure.
In the view below (of the central part of the face), the rock almost seems to be smiling at the camera. Follow individual layers and you’ll find that they turn back on themselves and trace out a curious elliptical pattern. Clearly, the rocks are folded, but these aren’t your everyday folds. These are sheath folds, and mega-sheath folds at that.
Sheath folds are distinctive curvilinear folds in which the hinge actually wraps around on itself. In three-dimensions sheath folds look much like their name implies, a sheath that might holster a sword (or in Oman, the traditional khanjar!). When eroded, the tubular-shape of a sheath fold displays a characteristic eye-shape in cross section—that’s what we see on the slopes above Wadi Mayh.
Sheath folds were first recognized in the late 1970s and early 1980s, but, in my opinion, not properly appreciated until the 1990s. They form when layers are strongly sheared and early formed fold hinges are rotated into cone-like shapes; the long-axis of the sheath fold parallels the direction along which the rocks were most stretched.
In 2007 Mike Searle and Ian Alsop published an excellent article in the journal Geology on mega-sheath folds from the Wadi Mayh area. The sheath folds are developed in shallow marine carbonate rocks of Permian and Triassic age that are in tectonic contact with underlying high-pressure metamorphic rocks formed when the Oman ophiolite was obducted onto the Arabian margin. The folds in the photo are actually subsidiary folds of an even larger mega-sheath fold about 15 km in length!
For me, sheath folds, regardless of the scale, dramatically illustrate that solid rocks are capable of flow, often in complex, but enticingly beautiful ways.
January 14, 2014 by Chuck Bailey
A new semester awaits 11,000 kilometers away in Williamsburg. Time to depart Oman, but before heading west towards home there was one last mountain to climb. I’ve had my eye on this ridge at the north end of Jebel Akhdar for months, as the view from its crest should provide an exceptional overview of the region’s geology.
The ridge stands ~800 m (~2600 ft.) above the small villages of Murri and Ash Shakdar. We parked the saloon car in the morning shadows and set off—I headed for the ridge, and Alex bore on to the wadi that cuts dramatically through the ridge. This is an anticlinal ridge and the wadi slices neatly across the anticline providing a spectacular cross section through folded strata.
I walked up the eastern dip slope of this geologic structure to the gently dipping strata along the ridgecrest; below Alex negotiated house-sized boulders in the wadi bottom.
Rocks exposed along the ridge and in the gorge below are Cretaceous limestones deposited some 95 to 115 million years ago in reefs and shallow warm seas on the northeastern margin of Arabia. These are the strata that underlie much of the alpine scenery in northern Oman. Although these strata are folded in dramatic fashion, the rocks are essentially in the same location as where they were originally deposited. This sequence of rocks is considered autochthonous, a tough-to-spell geologic term for rocks that are still located where the formed. In contrast, allochthonous rocks are no longer where they originally formed, rather, they’ve been displaced along faults and, in many cases, are far traveled bits of wayward crust.
Look to the periphery of this photo and you’ll notice ragged brown terrain, both to the northeast and northwest of the anticlinal ridge. This is the ophiolite underlain by peridotite, a dense dark rock that originally formed in the mantle 15 to 20 kilometers (9 to 12 mi.) below the ocean floor. In some locations there are other allochthonous rocks including a complex sequence of deep-sea sedimentary rocks (known as the Hawasina sequence), exotic blocks of limestone, and mélange (which, just as the name implies is a tectonic swirly pie of many rocks) between the ophiolite and the limestones. The contact between these geologic units is a thrust fault of the first order.
While standing on the ridge taking in the scene one word came to mind—juxtaposition. I’ll use the word in a sentence:
The juxtaposition of rocks from the Earth’s mantle (highly allochthonous rocks) against the shallow marine rocks (autochthonous strata) is a profound geologic sight.
The arched nature of the sequence makes it easy to visualize that the ophiolite was thrust long distances up and over the Cretaceous limestone. Prior to erosion of the modern mountain range (the terrain we see today) the juxtaposed ophiolite from the Deep Earth would have overlain the autochthonous rocks. Later deformed folded the rocks and then erosive surface processes removed the ophiolite sequence to expose the autochthonous strata below. That is quite a story!
There are other compelling geologic stories to share about Oman. In the coming weeks I’ll post more pieces on Oman’s geology and upload our Gigapans. Alex and I are also working up a series of videos that illustrate both our travels through Oman and the geology of this wonderful country. Music Professor Anne Rasmussen and I are moving forward with plans to take a field course/study abroad program to Oman in the future. Much to do back in Williamsburg.
January 10, 2014 by Chuck Bailey
I’ve been in Oman for over ten days and seen plenty of deformed rocks—it is what I came for. What follows are a series of images illustrating deformed Omani rocks: there are folds, faults, fractures, and veins. This stuff is eye candy for a structural geologist.
This first photo is a stitched panorama using our GigaPan apparatus of a big road cut on the main highway between Muscat and Nizwa. Notice the tilted and folded strata of the Hawasina sedimentary sequence and the lovely 4WD vehicle (unfortunately not our vehicle, we’re driving a saloon car).
Here is a small outcrop of crumpled sedimentary layers near the village of Al-Taww. It is complex in detail.
Notice the scale bar in this photo, an Omani 50 baiza piece that is about the size of a U.S. quarter. The rock in this photo is interlayered limestone (gray) and dolomite (beige) and the original sedimentary layering is tilted (lower left to upper right). The distinct white structures are tension gashes/veins, fractures that opened and immediately filled with the mineral calcite (white). Just where did the calcite in the veins originate?
This image illustrates a close-up view of slickensides on a serpentinite-coated fault in the ophiolite sequence. The linear and stepped morphology of the slickensides are useful for determining the kinematics of faulting.
The last image is of a mountain-side north of the village of Birkat al Mouz along Wadi Muaiydin, exposing a dramatic fold sequence in Mesozoic limestones. Nice stuff!
All this eye candy is wonderful to view, but also begs the question(s)
Why were these Omani rocks fodder for the tectonic cannon?
When were these rocks crumpled, broken, and faulted?
January 8, 2014 by Chuck Bailey
After four days of field work in the Western Hajar Mountains, Alex and I returned to Muscat to get clean and then joined up with William & Mary’s Middle Eastern Music Ensemble. Professor Anne Rasmussen directs this talented group of musicians who’ve been exploring and performing the music of the Middle East since 1994. Seven students and Anthropology professor Jonathan Glasser made the trip to Oman.
On a bright sunny afternoon we tagged along with Anne, Jonathan, and the crew for the first gig of their Muscat tour at the U.S. Embassy. The Ensemble commonly numbers 20+ musicians, but for my untrained ear the smaller Ensemble, with its nine performers (1 on bass, 1 on qanun, 3 on percussion, 3 on violin, and 1 on ‘ud), brought out the sound of the individual instruments.
We also accompanied the band to the U.S. Ambassador’s residence for an evening soirée. Geologists like a party, so it was great to ride the Ensemble’s collective coattails right into the festivities. Ambassador Greta Holtz and her embassy staff did an exceptional job at making the Tribe feel welcome.
While the Ensemble literally played and sang for their supper, Alex and I mingled with the assembled guests. During the course of the evening we had the pleasure of discussing our geologic work with many Omanis. The Omanis are rightly proud of their ophiolite.
As a geologist I study rocks and landscapes. For me trying to understand both the processes and history of our planet is a creative endeavor. But let’s face it; making music is a creative endeavor that provides joy in real time—it’s powerful stuff. William & Mary’s Middle Eastern Music Ensemble turned out its brand of powerful stuff here in Muscat.
January 7, 2014 by Chuck Bailey
Our travels in Oman took us north from the capital region in Muscat to Sohar, a drive of some two hours along the Batinah Coastal Plain. This coastal plain is just that, a low relief plain sloping towards the Gulf of Oman and underlain by relatively young (Tertiary to Holocene) sedimentary rocks and sediments. The ophiolite forms a distinctive and rugged terrain that rises to the west of the flatlands. As I noted in the last post, the Oman ophiolite is the largest and best exposed of its kind in the world. Wadi Jizzi is the major drainage that cuts through the ophiolite terrain to the west of Sohar and it is here we piloted our modest Toyota saloon (a British word for sedan, also used by Omanis).
One of our stops is a world classic; the pillow lavas exposed in the cliffs along the south side of the wadi (the arabic term for a valley or riverbed) are nothing less than stupendous. These exposures became famous when they graced the cover of Geotimes magazine back in 1975 and ever since then have been referred to as the “Geotimes” lava or “Geotimes” pillow lavas. I prefer the local name, Wadi Jizzi.
The external morphology of the pillows is evident, but erosion has cut cross sections through individual pillows as well. In external form the pillows are tubes or bolsters, some upwards of 3-meters (10 feet) in length and between 0.5- and 1.2-meters (1.5-4 feet) in diameter. The surface of the pillows is cracked with a series rectilinear cooling joints and green glassy material commonly occurs in the interstitial regions between the pillows. The rock itself is a brownish basalt with no visible phenocrysts or vesicles.
Pillows commonly form when lava is extruded under water. As lava disgorges from its vent on the sea floor it comes in contact with the surrounding seawater that rapidly quenches the lava to a glassy solid, thereby partially clogging the conduit and forcing to lava to ooze out nearby. This repetitive process of extrusion and rapid quenching produces the tube to pillow-like morphology.
At the eastern end of the outcrop the pillow lavas are cut by two altered basaltic dikes. The cross cutting nature of the dikes indicates they are younger than the pillows and are likely the conduit by which younger lava was transported to the sea-floor, where it too would have erupted as pillow lavas.
The sequence of pillow lavas is interlayered with cherty sedimentary rocks containing radiolarian fossils; the fossil assemblage enabled geologists to date the volcanism and sea-floor sedimentation to part of the Cretaceous period (100 to 95 Ma). These were lavas erupted along a mid-ocean ridge at the bottom of an ancient ocean known as the Tethys Ocean. Tethys was a goddess from the Greek classical period, but more recently her name has been used as the moniker for the ancient ocean that once separated Eurasia from Gondwana during the Mesozoic.
Mid-ocean ridges are notoriously difficult spots to reach for field trips, primarily as a consequence of being 1) in the mid-ocean and 2) under a few kilometers of water. But ophiolites bring the mid-ocean ridge to the continents, making it possible to reach the bottom of an ancient ocean while daytripping in a saloon to an Omani wadi.
January 2, 2014 by Chuck Bailey
The New Year finds me half-a-world away from William & Mary on a research trip to Oman. I am here starting a project focused on Oman’s spectacular geology and also laying the groundwork for a W&M study abroad field program that will focus on Oman’s iconic geology, its desert environment and distinctive culture. This trip is supported in part by William & Mary’s Reves Center for International Studies. I arrived in Muscat on December 29th and was joined the following day by Alex Johnson, our Geology Department Research Fellow.
My research here is focused on quantifying the kinematics of faulting and the mechanism by which the Oman ophiolite, a vast slab of oceanic crust and deep mantle, was emplaced at the Earth’s surface (the name ophiolite is derived from the Greek: ophio for snake and lite or lithos for stone). Globally, ophiolites are rare and their origin enigmatic. When tectonic plates collide, dense ocean crust typically sinks back into the deep interior of the Earth in a process known as subduction, whereas the less dense continental crust crumples to form mountain ranges. On rare occasion however, ocean crust and the underlying mantle are tectonically shoved onto the continental crust and ophiolites are created. The Oman ophiolite is the world’s largest and best-exposed ophiolite. It is a unique setting in which to understand the dynamics of faults that place rocks from the sea floor and the deep Earth literally at our feet.
The ophiolite is well exposed because Oman is a hot and exceptionally dry country (with an average annual precipitation between 100-150 mm or 4 to 6 inches). They manage and conserve their scarce water resources with Omani’s long-ago developed aflaj, a communal system for collecting and transporting either surface or shallow ground waters via tunnels and channels to villages for domestic and agricultural use. My friend and colleague, Professor Abudullah Al-Ghafri took us on an Aflaj tour in the highlands near Nizwa. Dr. Al-Ghafri spoke on this topic while at William & Mary last spring, but to see these ancient structures conveying their life sustaining water was powerful.
Over the next two weeks we’ll be doing fieldwork in the mountains of Oman and meeting up with Omani colleagues to learn more about their country. We’ll also post an array of dispatches that we hope will vividly document our geologic adventures in Oman.
November 14, 2013 by Chuck Bailey
This post begins what I plan to be a recurring series on drainage basins and watersheds. For earth scientists interested in landscapes and surface hydrology: drainage basins are a fundamental component of these natural systems.
A drainage basin consists of all the terrain that contributes water to a particular stream or river. For instance, rain that falls on the Geology building at William & Mary runs off into College Creek, College Creek flows south into the James River, and the James River debouches into the Chesapeake Bay at Hampton Roads. Drainage basins are diverse, ranging in size from near continental portions (i.e. Mississippi/Missouri and Amazon) to modest creeks that drain a few square kilometers, and small drainage basins are nested inside progressively larger basins. In what drainage basin are you reading this post?
I grew up on a patch of rolling upland terrain near Ivy, Virginia about 10 km west of Charlottesville in Albemarle County. I spent much of my youth in the local creek that meandered across the rural landscape. This was Ivy Creek. It is a small stream, typically a meter or two or three wide, with a sandy to gravelly bottom and steep muddy banks (1 to 2 meters high).
My friends and I had many adventures in Ivy Creek and its tributaries. We traipsed through the creek on hot summer days searching for cool water and deep shade. After snowfalls we’d build a snow ramp on the edge of Ivy Creek Branch, take to our sleds, glide down the nearby hill, hit the ramp, and often land askew in the partly frozen creek – good times!
Fond memories aside, Ivy Creek and its drainage basin are quite ordinary. Because the Ivy Creek drainage basin is so typical (or perhaps classic) it makes a nice embarkation point for an ongoing discussion about drainage basins.
The Ivy Creek watershed encompasses nearly 60 square kilometers (23 sq. miles). The stream begins along the northwestern slope of the Ragged Mountains and flows in a northeasterly direction for over 20 kilometers (~13 miles) where it joins the South Fork of the Rivanna River (a tributary to the James River).
The Ragged Mountains are a group of subdued and rounded hills underlain by granitic bedrock that solidified about a billion years ago. For a not so subdued and phantasmagoric story of this range read Edgar Allan Poe’s “A Tale of the Ragged Mountains” which ostensibly takes place here. All things considered, the old granitic bedrock in the Ivy Creek watershed is relatively uniform and homogeneous. As such, Ivy Creek flows across bedrock with a similar strength and erodibility throughout its drainage basin.
Consider Ivy Creek’s gradient: at its headwaters near Taylors Gap the stream has a steep gradient (~40 meters per km) that gradually lessens (~2-3 meters per km) near its confluence with the Rivanna. The longitudinal profile of Ivy Creek is steep near its headwaters and becomes progressively less steep further downstream. This concave up profile is very typical of many stream systems. Why do streams typically have such a profile?
Since the bedrock in the Ivy Creek basin is similar throughout the basin it is unlikely that the bedrock is exerting much of a control on the stream gradient. In a future post we’ll examine a stream that crosses bedrock of varying hardness and erodibility, but let’s get back to Ivy Creek.
Near Taylor’s Gap, Ivy Creek is perhaps 1 meter wide (~3’) and quite shallow; near its confluence with the Rivanna, Ivy Creek is upward of 5 meters wide (~16’) and plenty deep. As Ivy Creek flows downstream, traversing its basin, tributary streams join the main stem adding their flow to Ivy Creek, enlarging the channel and increasing the stream discharge.
These relations hold true for many stream systems:
- Stream channel slope (S) decreases downstream
- Mean stream discharge (Qm) increases downstream
Essentially, the gradient of a stream channel is proportional to an inverse function of its mean annual discharge. Perhaps a stream’s discharge plays a major role in controlling a stream’s gradient. I’ll leave it at that for the moment, but I am curious as to your thoughts.
November 4, 2013 by Chuck Bailey
My family has a tradition of going camping about once per semester. Back in the spring of 2011, as the Appalachians were beginning to green up, we headed west to Rockfish, Virginia for a weekend camping trip to my Uncle Joe’s farm. Joe’s farm is located in the foothills of the Blue Ridge Mountains and I’d call the scenery sublime.
Bedlam reigned as we unpacked and set up camp, so much bedlam that I needed quiet time alone. Off I set on a hike through the woods. A half-mile of hiking brought me to the verge of this old abandoned quarry.
It was a commanding view with spectacular rocks exposed in the walls of the quarry. The bedrock here is a metamorphosed conglomerate, a coarse-grained conglomerate chock-a-block full of clasts, many larger than basketballs. The clasts are fragments of older rocks that were eroded, transported, and deposited long ago.
The conglomerate is stratified, but the layers have been tilted and are lying on end. These layers were tilted past vertical and are overturned – that is, the older layers are structurally above the younger layers (an inverted stratigraphic sequence).
I was giddy with geologic questions. In what depositional environment was this conglomerate deposited? Why are these layers tilted and overturned?
The quarry I’d stumbled upon exposes the Rockfish Conglomerate, a geologic unit in the eastern Blue Ridge first defined by Wilbur Nelson in 1932 for outcrops along the Rockfish River about a kilometer to the southwest (geologists commonly name stratigraphic units for a particular location where the rocks are first described and typically are well exposed, it’s known as the type location). This curious conglomerate has been studied by a number of geologists, but nowhere could I find any reference to the exposure at the quarry.
Since the “discovery” I’ve taken classes to see these rocks and invited other geologists to these outcrops. Callan Bentley at Northern Virginia Community College wrote a blog post and captured a lovely Gigapan image from an early visit to the quarry. Just last year Zach Foster-Baril decided to tackle the Rockfish Conglomerate as his senior research topic. Zach just presented the results of this work as a talk last week at the Geological Society of America’s annual meeting in Denver.
We have learned much from our field and lab work over the past year. Our geologic mapping reveals that the Rockfish Conglomerate was deposited in a trough that was eroded into the underlying and older granitic basement rocks. The trough was later filled with the coarse sediment that became the Rockfish Conglomerate. Years earlier I’d posited that the contact between the basement rocks and the overlying metasedimentary cover rocks was a fault, but Zach’s detailed field data make it clear that the boundary is an unconformity.
As the rocks in the Rockfish area are tilted into an almost vertical orientation we can turn the geologic map on its side to approximate the original geometry of this sequence.
Instead of north, south, east, and west we’ve got stratigraphic up (top) and down (bottom) with the oldest rocks at the bottom. From this perspective it is clear that the granitic basement rocks (green) originally were beneath the Rockfish conglomerate (orange), which occupies an irregular trough a few hundred meters deep. An intrusive gabbro body (lavender), as sills and dikes, cuts both the granitic rocks and overlying metasedimentary rocks.
In what environment were these sediments (now thoroughly lithified rocks) deposited?
Cobbles and boulders are not easily transported: high-energy processes are needed to move bowling ball-sized rocks. Suitable conditions occur in mountain streams where fast-moving turbulent waters rage during floods. As the torrent reaches the mountain front, the current slackens and the stream’s load of cobbles and boulders is deposited at the mountain front creating a distinctive wedge of alluvial deposits out into the valley.
But fast moving water effectively sorts sediment based on its grain size. Particles of similar size are deposited along with each other; so as pebbles are deposited, the sand and mud (the smaller particles) continue to be transported by the flowing water. The Rockfish Conglomerate is a very poorly sorted rock and lacks many of the sedimentary structures we’d expect in alluvial fan deposits.
In the 1980’s Frederick Wehr, at the time a graduate student at Virginia Tech, studied the exposures along the Rockfish River and concluded that the sequence was not a terrestrial alluvial fan deposit, but rather a subaqueous marine deposit formed from glacial outwash and mass flow turbidites. Our observations at the quarry are consistent with Wehr’s interpretations. A glaciomarine origin for these rocks is compatible with 1) the wide range of grain sizes (boulder to silt/sand), 2) the prominent parallel stratification present throughout the Rockfish Conglomerate, and 3) the conformable nature of the overlying strata in the Lynchburg Group.
After the talk my colleague Michelle Markley from Mt. Holyoke College plaintively asked Zach to show the audience some dropstones.
Dropstones are literally stones dropped from above into sediment layers at the bottom of a lake or ocean. They are inferred to form when rock laden lake or sea ice rafted off glaciers melts, thereby “dropping” the sediment, from boulders to mud, that was entrained in the ice. Dropstones commonly deform or deflect the underlying strata and are draped by younger layers of sediment. These sedimentary features are not only visually stunning, but typically taken as key evidence for glaciogenic sedimentation into a basin.
Michelle’s question was right on point, we’d not offered up any dropstones for viewing.
Individual dropstones (called by some geologists – lonestones) are not common in the Rockfish Conglomerate. The picture below highlights a candidate for a Rockfish dropstone, but it won’t win any dropstone beauty contest.
A few plausible reasons for the lack of readily identifiable dropstones in the Rockfish Conglomerate include: 1) the coarse conglomerate was deposited in a proximal position relative to the glacial ice and as such the flux of coarse-grained sediment was huge, 2) abundant ice-rafted debris falling into a coarse mixture of sand is less likely to stand out as a dropstone when there are lots of clasts (no lonestones here), and 3) these rocks experienced later deformation and metamorphism such that the matrix is a recrystallized mixture of quartz and mica which does not preserve fine-scale details of the depositional environment.
Zach’s research provides new data on the Rockfish Conglomerate, but many questions remain. A few include:
- When was the Rockfish Conglomerate deposited? Stratigraphic relations indicate that the Rockfish Conglomerate was deposited during the Neoproterozoic Era between 570 and 1,000 million years ago. The possible age range for when these rocks were deposited is >400 million years, that’s lousy age control.
- What created the trough in the basement complex into which the Rockfish Conglomerate was deposited? Possibilities include either glacial or fluvial erosion during a low stand of sea level.
- Is the Rockfish Conglomerate associated with widespread glacial episodes that occurred during the Neoproterozoic? The Neoproterozoic was a time of tremendous climate oscillation, some researchers argue for a Snowball Earth in which glacial ice covered much of the planet.
- How much strain/deformation has the Rockfish Conglomerate experienced? Outcrop-scale deformation structures are common. To better understand original sedimentary geology we need to quantify the amount of shortening, stretching, and rotation these rocks enjoyed.
- When was the Rockfish Conglomerate deformed, metamorphosed, and tilted? Regional data suggest these events occurred during the Paleozoic Era, but as with the Neoproterozoic: the Paleozoic encompasses an expansive amount of Earth history.
That is the nature of research – some questions get answered and other new ones arise. The good news is that Gussie Maguire, a Geology/English double major and mystery tweeter, has taken up the charge. Her thesis research is aimed at answering some of the outstanding questions regarding the Rockfish Conglomerate. On we go.