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Chuck Bailey
Chuck Bailey

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Professor, Geology

Rolling Deep with the Penrose Conference on Orogenic Systems

April 6, 2014 by

This past week I co-convened a Geological Society of America Penrose Conference focused on Feedbacks and Linkages in Orogenic Systems.   An orogen is a geologic term for a mountain belt, and orogenesis describes the processes at work in mountain belts (derived from Greek- oros for “mountain” and genesis for “creation/origin”).  The world’s great mountain belts include massive modern ranges such as the Himalayas, Andes, and Alps as well as ancient mountain belts such as the Caledonian orogen in Greenland, Scotland, and Scandinavia, the Grenvillian orogen in Canada, and the Limpopo orogen in South Africa.

Cover image from the Penrose Coonference filed guide and technical program.

Cover image from the Penrose Conference field guide and technical program.

The Penrose Conference included structural geologists, petrologists, sedimentologists, geomorphologists, geochronologists, and geophysicists all with a common interest in orogenic processes.  Geoscientists from as far away as China and Poland traveled to Asheville, North Carolina for nearly a week’s worth of discussions, talks, posters, and field trips.  Penrose Conferences are small meetings where the participants are encouraged to present novel or controversial hypotheses and hash out those ideas with colleagues.

Penrose Conferences were first established in 1969 and over the last 45 years these meeting have helped bring forward many major advances in the realm of plate tectonics, ophiolites, and metamorphic core complexes (to name just a few topics).  For me it was a great pleasure to co-convene a Penrose conference, I reconnected with old colleagues and met many new ones.  The National Science Foundation paid the freight that enabled participation by a large contingent of graduate students, the interaction between established scientists and up-and-coming scientists was special.

The Conference honored Bob Hatcher, who first brought a plate tectonic focus to the Appalachians back in the late 1960s and early 1970s.  Today working with his large and eager group of graduate students (aka the Hatchery), Bob continues to make seminal contributions to the field.

We experienced the fickle nature of the southern Appalachian spring on our field excursions.  The first trip started under heavy overcast with a malignant wind and wet snow blanketing the outcrops.  By the last stop on the final field trip day we were broiling in Carolina sunshine.

Views from the field trip: left- snowbound in the Blue Ridge Mountains at the 2nd stop, right- broiling in the Brevard Fault Zone at the last stop.

Views from the field trip: left- snowbound in the Blue Ridge Mountains at the 2nd stop, right- broiling in the Brevard Fault Zone at the last stop.

That evening as our crew of sun-drenched and thirsty geologists pulled to the curb in downtown Asheville and headed straight towards a brewpub, a natty hipster on a skateboard took one look at the group and commented, “Ah, you’re rolling deep.

Rolling deep?  Some of the brightest geologic minds I know were utterly stumped as to just what it meant to be rolling deep.  I’ll use the phrase in a sentence:

“Me and my Penrose posse were rolling deep in the Brevard Fault Zone looking for trouble and some dextral transpression.”

The geologic lexicon is rich with colorful expressions (for instance- there are glacial erratics, faults have both throw and heave, and ocean lithosphere can be obducted).  I have no doubt we can co-opt rolling deep as geologic term with tectonic significance.

Shaded relief map of the Blue Ridge Mountains and adjacent terrain in the Inner Piedmont and Valley & Ridge provinces of western North Carolina and eastern Tennessee.  The Penrose field trip examined rocks across this region.

Shaded relief map of the Blue Ridge Mountains and adjacent terrain in the Inner Piedmont and Valley & Ridge provinces of western North Carolina and eastern Tennessee. The Penrose field trip examined rocks across this region.

I learned much about the linkages and feedbacks at work in mountain belts at this Penrose Conference–from the focused erosion in the Himalayan river systems that drive rapid exhumation to the growth dynamics of garnet porphyroblasts in metamorphic rocks from deep in the interior of thrust belts.  Heady and exciting stuff!

50 Hours in the Field: the Earth Structure & Dynamics Field Trip 2014

March 27, 2014 by

The 2014 Earth Structure & Dynamics class field trip left Williamsburg at 1 p.m. last Friday bound for the Blue Ridge Mountains and points beyond.  We would not return to campus until 3 p.m. on Sunday afternoon, some 50 hours after our departure.  The field trip is a spring tradition that’s been enjoyed by students for years.  On this year’s excursion we reveled in bountiful sunshine and mild temperatures.  We also saw an array of rocks and structures that tell the story of Virginia’s geologic history.

Generalized geologic map of part of central and northwestern Virginia illustrating rock units and our outbound field trip route

Generalized geologic map of part of central and northwestern Virginia illustrating rock units and our outbound field trip route.

On this trip, students do geology in the field and in the process become familiar with the tectonic history of the Appalachian Mountains.  Students work in teams of two and answer an array of questions at each outcrop (here are a few team names from this year’s trip: the Away Team, the Zesty Xenoliths, the Russian Judges, Team Stylo- Lightening, and the aptly named Despicable Fluffy Marmosets).

Hour 3- examining a lineated granodioritic gneiss  in an old quarry at Columbia, Virginia.

Hour 3- examining a lineated granodioritic gneiss in an old quarry at Columbia, Virginia.

Starting from the Coastal Plain we journeyed across the Piedmont on Friday afternoon.  We swarmed outcrops in parks and along country roads.  At an old quarry in the little town of Columbia we examined a lineated granodioritic gneiss that crystallized back in the Ordovician (~460 million years ago), and was later stretched during the continental collision that created Pangaea.

Our campsite was at the foot of the Blue Ridge Mountains and we headed straight into the range on Saturday morning to examine basement rocks, ancient lava flows, and tilted strata.  We lunched on outcrops of sheared limestone and dolostone exposed in the Shenandoah Valley.

The 2014 Earth Structure & Dynamics class atop Massanutten Mountain (note Shenandoah Valley in mid-ground and Blue Ridge Mountains in the background).

Hour 29- the 2014 Earth Structure & Dynamics class strikes a pose atop Massanutten Mountain (note the Shenandoah Valley in the mid-ground and the Blue Ridge Mountains in the background).

The afternoon hours included a mapping exercise on the flank of Massanutten Mountain.  On Massanutten’s crest, among the ever-lengthening late afternoon shadows, we marveled that the old Blue Ridge basement rocks, upon which we had stood that morning, were buried some 6 to 8 kilometers below our feet here in the Valley & Ridge province.

Geologic cross section from the Valley & Ridge to Blue Ridge in north-central Virginia. Note the basement rocks that are exposed in the Blue Ridge are deep in the subsurface in the Valley & Ridge.  Modified from bailey et al (2006).

Geologic cross section from the Valley & Ridge to Blue Ridge in north-central Virginia. Note the basement rocks that are exposed in the Blue Ridge are buried deep in the subsurface in the Valley & Ridge.  Modified from Bailey et al. (2006).

 

Saturday night fun on the Earth Structure & Dynamics field trip.

Hour 34- Saturday night fun with geologic maps and cross sections on the Earth Structure & Dynamics field trip.

Evening hours back in camp were fun and involved completing the geologic map and cross section from our afternoon foray in the Valley & Ridge province. What could be better on a Saturday night?

We also discussed the key role that time and place play in geology.  On Friday afternoon in Columbia we’d observed plutonic rocks that formed in an Ordovician volcanic arc; on Saturday afternoon in the Shenandoah Valley we examined fossiliferous strata that were deposited at the margin of an Ordovician sea whose shores lapped onto eastern North America.

In the modern world the Shenandoah Valley and Columbia, Virginia are ~80 kilometers (~50 miles) apart, in the Ordovician world they were separated by 300 to 500 kilometers (~200 to 300 miles) and in very different geologic settings.  In the late Paleozoic these rocks were deformed, metamorphosed, and transported considerable distance to the northwest forming the geologic structures that we puzzle over today.

Experience in the field is an important component in the William & Mary Geology curriculum, as going to the field and working through geologic questions on the outcrop can’t be replicated in the classroom.  Fieldwork is not always easy, but 50 hours in the field provides an opportunity for the latest crew of W&M geologists to practice their craft and revel in the wonderful world away from campus.

Working in a Winter Wonderland: The Gravity of the Situation (Part 2)

February 27, 2014 by

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.

Overview map of Fish Lake, Utah with 2014 gravity stations and core site.

Overview map of Fish Lake, Utah with 2014 gravity stations and core site.

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.

View to the east of Fish Lake's frozen surface and Mytoge Mountain which rises steeply from the southeastern edge of the lake.

View to the east of Fish Lake’s frozen surface and Mytoge Mountain which rises steeply from the southeastern shore of the lake.

Erika Wenrich makes a gravity measurement.

Erika Wenrich makes a gravity measurement.

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.

After a Hard Day's Night. W&M geologists Erika Wenrich and Peter Steele in the early morning after their nighttime gravity survey.

After a hard day’s night. W&M geologists Erika Wenrich and Peter Steele in the early morning light after their nighttime gravity survey.

 

The coring team at work on Fish Lake's icy surface.

The coring team hard at work on Fish Lake’s icy surface.

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.

FFLfig6

Dr. Scott Harris (W&M class of ’88), kneeling on the ice, runs the TEM geophysical system on a breezy day at Fish Lake.

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.

Glimpses of the Past: the Catoctin Formation – Virginia is for Lavas

February 17, 2014 by

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.

VAlavas

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.

Stony Man’s summit and northwestern slope, Shenandoah National Park, Virginia. Cliffs exposed metabasaltic greenstone of the Neoproterozoic Catoctin Formation.

Stony Man’s summit and northwestern slope, Shenandoah National Park, Virginia. Cliffs expose metabasaltic greenstone of the Neoproterozoic Catoctin Formation.

 

Geologic cross section of Stony Man summit area (modified from Badger, 1999).

Geologic cross section of Stony Man summit area (modified from Badger, 1999).

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.

Map illustrating the distribution of the Catoctin Formation in the central Appalachians.

Map illustrating the distribution of the Catoctin Formation in the central Appalachians.

Column joints in the Catoctin Formation exposed along the Skyline Drive in Shenandoah National Park.

Column joints in the Catoctin Formation exposed along the Skyline Drive in Shenandoah National Park.

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.

Pillow structures in the Catoctin Formation exposed along the south bank of the Hardware River in southern Albemarle County, VA.

Pillow structures in the Catoctin Formation exposed along the south bank of the Hardware River in southern Albemarle County, VA.

 

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.

Graph illustrating isotopic ages and their associated uncertainty for the Catoctin Formation.

Graph illustrating isotopic ages and their associated uncertainty for the Catoctin Formation.

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.

Schematic diagram of a rising mantle plume 1) moving through the mesosphere 2) spreading in the asthenosphere 3) piercing thelithosphere and extruding onto the surface.

Schematic diagram of a rising mantle plume 1) moving through the mesosphere 2) spreading in the asthenosphere 3) piercing the
lithosphere and extruding onto 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.

Paleogeographic reconstruction of Laurentia and surrounding continents at ~550 Ma. Note Laurentia was in the southern hemisphere (data from numerous sources).

Paleogeographic reconstruction of Laurentia and surrounding continents at ~550 Ma. Note Laurentia was in the southern hemisphere (data from numerous sources).

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.

Oman’s Mega-Sheath Folds

January 30, 2014 by

Shaded relief map of the Muscat area, Oman with Wadi Mayh highlighted.

Shaded relief map of the Muscat area, Oman with Wadi Mayh highlighted. 30-m data from the Shuttle Radar Topography Mission.

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.

 

Google Earth image of the Wadi Mayh exposure, note its north-facing aspect.

Google Earth image of the Wadi Mayh exposure, note its north-facing aspect.

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.

(View the Oman Sheath Folds GigaPan.)

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.

Close up of the central part of the exposure at Wadi Mayh, Oman.  Lower image is a tracing of the layers wrapping around the sheath fold.

Close up of the central part of the exposure at Wadi Mayh, Oman, note bushes for scale.  Lower image is a tracing of the layers wrapping around the sheath fold.  Sheath fold axis trends into the cliff face, approximately normal to the photograph.

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.

Schematic model illustrating the development of a sheath fold.  Note distinctive eye-shape in cross section normal to sheath axis.

Schematic model illustrating the development of a sheath fold. Note distinctive eye-shape in cross section normal to sheath axis.

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.

Annotated and traced image of the sheath folds at Wadi Mayh, Oman

Annotated and traced image of the sheath folds at Wadi Mayh, Oman.

Dispatches from Oman: Juxtaposition

January 14, 2014 by

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.

Shaded relief map of a part of northern Oman.  30-m data from the Shuttle Radar Topography Mission.

Shaded relief map of a part of northern Oman. 30-m data from the Shuttle Radar Topography Mission.

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.

View from the top of the Murri anticline, northern Oman (view to north-northwest). Annotated geology in lower image.  This will be posted as a Gigapan in the coming weeks.

View from the top of the Murri anticline, northern Oman (view to north-northwest). Annotated geology in lower image, note the ophiolite on either side of the Cretaceous strata. The wadi bottom is ~700 m below.  This image will be posted as a Gigapan in the coming weeks.

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.

Geologic map and cross section of the Murri anticline and Oman ophiolite.  Ophiolite is juxtaposed against mélange and autochthonous limestones.

Geologic map and cross section of the Murri anticline and Oman ophiolite. Ophiolite is juxtaposed against mélange and autochthonous limestones.

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.

Dispatches from Oman: Fodder for the Tectonic Cannon

January 10, 2014 by

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.

Big road cut near Nizwa, Oman exposed folded and tilted strata.

Big road cut near Nizwa, Oman exposed folded and tilted strata.

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).

crumpled strata in Oman

Crumpled strata in Oman.

Here is a small outcrop of crumpled sedimentary layers near the village of Al-Taww.  It is complex in detail.

Tension gashes/veins cutting layered limestone and dolomite, Oman.

Tension gashes/veins cutting layered limestone and dolomite, Oman.

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?

Slickensides on serpentinite coated fault, Oman ophiolite.

Slickensides on serpentinite-coated fault, Oman ophiolite.

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.

A mountain-side of folded strata, Wadi Muaiydin, Oman.

A mountain-side of folded strata, Wadi Muaiydin, Oman.

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?

     and

When were these rocks crumpled, broken, and faulted?

 

Dispatches from Oman: We’re with the Band

January 8, 2014 by

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 the road with the band! W&M's Middle Eastern Music Ensemble heads to their first gig in Muscat.

On the road with the band! W&M’s Middle Eastern Music Ensemble heads to their first gig in Muscat.

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.

The Ensemble making their music at the U.S. Ambassador's residence.  (Photo courtesy of Megan Porter)

The Ensemble making their music at the U.S. Ambassador’s residence in Muscat, Oman. (Photo courtesy of Megan Porter)

Dispatches from Oman: Wadi Jizzi – standing at the bottom of the Tethys Ocean

January 7, 2014 by

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).

Shaded relief map of a part of northern Oman.  Note the different topography between the Batinah Coastal Plain and the Western ajar Mountains.

Shaded relief map of a part of northern Oman. Note the different topography between the Batinah Coastal Plain and the Western Hajar Mountains. 30-m data from the Shuttle Radar Topography Mission.

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.

Alex Johnson, William & Mary Geology Fellow, standing tall and proud in front of the pillow lavas exposed at Wadi Jizzi, northern Oman

Alex Johnson, William & Mary Geology Fellow, standing tall and proud in front of the pillow lavas exposed at Wadi Jizzi, northern Oman.

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.

Close up of Wadi Jizzi pillows (both external morphology and in cross section).  Note abundant fractures.  Hammer is ~42 cm (~17") in length.

Close up of Wadi Jizzi pillows (both external morphology and in cross section). Note abundant fractures. Hammer is ~42 cm (~17″) in length.

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.

Basaltic dikes cutting pillow lavas at Wadi Jizzi.  Alex Johnson for scale.

Basaltic dikes cutting pillow lavas at Wadi Jizzi. Once again Alex Johnson for scale.

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.

Dispatches from Oman: Ophiolite to Aflaj

January 2, 2014 by

northern Oman map

Shaded relief map of northern Oman.

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.

Ophiolitic mountains rising above the Oman desert.

Ophiolite terrain rising above the Oman desert.

 

The flowing waters of Falaj al Khatmeen in the village of Birkat al Mawz.  This ...

The flowing waters of Falaj al Khatmeen in the village of Birkat al Mawz (literally “Pool of Bananas” from the crop grown by these waters).

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.