The Traveling Geologist

Alpine Eclogitic Blueschist Gigapan

2013-09-25T09:23:00.000+01:00

This eclogitic blueschist was collected from Chamois, Italy in the Zermatt-Saas ophiolite. At this locality, smooshed pillow basalts are clearly seen at the top of the ophiolite section although in most areas the primary magmatic textures have been obliterated by high pressure metamorphism. This sample contains glaucophane, jadite/omphacite, lawsonite (pseudomorphs), and other secondary minerals. The term eclogitic blueschist is used because peak metamorphic conditions were eclogite facies which was followed by a strong blueschist overprint. This sample comes from near locality 4 in Angiboust and Agard (2010) and is similar to their sample ZS0716 (see here).

See other excellent geologic gigapans by Ron Schott and Callan Bentley.

Return to the Western Alps

2013-09-23T10:48:00.001+01:00

This month I was lucky enough to again participate in the "Anatomy of an Orogeny" field course in the Alps with the University of St Andrews. Last year I had four posts outlining our transect from Milan, Italy to Berne, Switerzland (clockwise from upper left: Sesia-Lanzo, Ivrea Verbano, Flysch and Molasse, Helvetic Nappes).

This year we had great weather and a great group of students. Below left: me taking magnetic suseptibility measurements of a pyroxenite; right: a student who forgot his notebook in the minibus.

Blueschist from Quincenetto (glaucophane, garnet, jadite, phengite)

Lago Blu and Cervino (Italian side of the Matterhorn) at Breuil Cervinia

Acid mine drainage from Cu-Fe Cervette mine, Saint Marcel valley, Italy

"Eve Verda" near the Chuc mine in the St Marcel valley. The left stream is coming from the Fe-Cu mine drainage in Cervette (above) and the right stream drains the Cu Chuc mine. The coloration of the left stream is due to saturation of sulfide minerals (pH = 2) whereas the right is due to copper (pH = 7.5). At the confluence, the mixing waters of different pH drives the composition into a supersaturated state causing the precipitation of an amorphous Cu hydroxide (Woodwardite).

Our fearless leaders: Drs Ed Tipper and Tony Prave at Salanfe

Dr Tipper collected an ice sample at Salanfe

S-folds at Salanfe

Reservoir at Salanfe

I am loving the versatility of the iPhone to take timelapse shots. Adding a very simple and relatively cheap rotating tripod head (here), the effect is much more striking. Thanks to Camille for finding the music from Waylon Thorton ("Eight").

Namib Sand Sea

2013-09-16T14:20:00.001+01:00

A sand sea or erg is defined as an expanse of land covered with 125 square km. The Namib Sand Sea is the second largest in the world. It covers nearly 80 thousand square kilometers (nearly the size of Austria or South Carolina). From the mountains to the dunes to the ocean, it is home of some of the most inhospitable terrane I have ever encountered.

Google Earth view of the Namib. The Orange river is in the lower left corner and the Kuiseb is the in the upper right.

The Namib begins north of the Orange River, which drains over half of the Republic of South Africa. Sand that is carried into the lower portions of the river is blown northward by the southeasterly trade winds.

The Orange River

Where the sand supply is starved, barchan dunes often with an elongated east leg are most prevalent. The elongated eastern leg is likely due to the offshore breeze blowing in from the ocean. As the sand supply increases these eastern legs increase in size and become relatively small linear dunes.

Barchan dune in the Namib

The roads are constantly being covered with sand and are cleaned continuously.

Looking southward across the Koichab Pan

Desert mirage at the southern end of the erg

Hundreds of kilometers of nothing

To put all of this in perspective the trees on the right are 10 meters tall.
The highest dunes in the Namib can reach over 300 meters above the valley floor.

Much of the dunes are stabilised by vegetation. In most large dunes only the uppermost crest is currently active. Vermeesch et al. (2010) estimated that at no point in the past million years was the Namib Desert free of dunes. This makes the Namib erg one of the oldest (if not the oldest erg) still in existence today. Furthermore, having endured arid to semi-arid condition for the past 80 million years, it is likely the oldest desert in the world as well.

Dead acacia trees in an interdunal pan

The joys of working in the desert

Herd of Oryx

Bizarre Granitoids of KwaZulu-Natal

2013-08-12T06:01:00.003+01:00

For most geologists, thoughts of South Africa are comprised of kimberlites, chromite seams, and gold. This post has nothing to do with any of those things. Along the east coast of SA near Durban is a suite of Mesoproterozoic granitoids and high-grade metamorphic rocks called the Natal Belt that were emplaced during the collision of the Kalahari Craton and Antarctica with several island arcs squooshed in between. These rocks are found in the KwaZulu-Natal State where the landscape is dominated by the inland tribal land of the Zulu tribes and a coastal paradise filled with tourists. The Zulu lands are incredibly diverse, from mountain deserts to lush river valleys.

Typical Zulu dwelling

The only place to find unweathered outcrop is in the river valleys and the coast.

The geology in KZN (kay-zed-en) extremely diverse. From the youngest Jurassic lava flows which cap the Drakensberg Mountains to the Archean Kaapvaal craton. In the middle of all of this is the Natal belt, which is a Mesoproterozoic igneous and metamorphic complex formed through the accretion of three volcanic island arc terranes. The granitoids (my current focus) have some very interesting lithologies resulting from the unique conditions during which they were emplaced. Lithologies include, charnockites, syenites, monozites, and good ole granites.

Typical riverside outcrop.

Charnockite! Plagioclase + Orthoclase + Quartz + PYROXENE

As explained very nicely by Siim Sepp (here), a charnockite is a pyroxene-bearing granitoid. It was first named by Sir Thomas Holland who first identified this rocktype from the gravestone of Job Charnock (1630–1692) in Kolkata, India. Charnockites are formed at high pressures with minor hydrous phases. There is some discussion as to the origin of charnockites and to date there seems to be some agreement that they fall into one of three classifications: magmatic, fluid, and thermal. The former being purely igneous and the latter two are metamorphic. As far as I am aware, charnockites are the only rock that can form through both igneous and metamorphic processes.

Sugar cane fields. In the evening the fields smell of caramel and molasses.

Highly weather porphyritic granite

In the Oribi River Gorge. Difficult to sample fresh surfaces in the river bed.

Garnets! (dark red minerals)

Sampling in the surf. Life doesn't get much better.

South Africa's national bird: the Blue Crane

Dinosaurs in Idaho - LJ Krumenacker

2013-08-03T19:22:00.001+01:00

The Gem State has more than gems (and potatoes). For the paleontologically inclined, Idaho also has an appreciable fossil record. Fossils range from the Precambrian to the Pleistocene, with fossils from all geological periods being reported. The only identifiable skeletal remains of dinosaurs known from Idaho are from the middle Cretaceous (latest Albian-Cenomanian stages, roughly 98 million years ago) Wayan Formation. The fossils are from a time interval that is poorly represented in the fossil record compared to the rich faunas of the Late Jurassic and Late Cretaceous of North America.

Judging from a relative abundance of partial skeletons (which may or may not be misleading about the actual paleoecology), the dominant animal at this time was the small herbivorous ornithopod dinosaur Oryctodromeus. This animal had the peculiar habit of digging burrows and spending time in them with its young, as demonstrated by the first specimens (an adult and 2 juveniles) discovered, which were found in a preserved burrow in coeval rocks in southwestern Montana. The next most abundant fossil are eggshell fragments and rare eggs of the egg Macroelongatoolithus. One complete egg measuring over 13 inches long as well as a partial egg of this type have been found, the shell microstructure suggests a large oviraptorid was laying these eggs. Frustratingly, remains of larger dinosaurs are extremely rare in the Wayan. The typical mid-Cretaceous fauna of large ankylosaurs, iguanodontids, and large theropods are mostly represented only by very rare isolated bones or teeth. This is unusual, and may reflect unique biases involved in preserving fossils in the Wayan paleoenvironments, with ongoing research hoping to address this question.

While heavy equipment and fossil preservation may not usually be congenial bedfellows, recent construction work at a site in eastern Idaho has revealed numerous fossils from these typically rare Wayan faunal elements. This construction site has turned into the most important fossil site in the Wayan Formation. What makes this site so important is that fact that it is producing remains of so many of those organisms that are not at all common anywhere else in the formation. These include teeth and bones from small dromaeosaurid theropods, medium-sized as yet unidentified theropods, ankylosaurs, larger as yet-unidentified dinosaurs, turtles, crocodylians, as well as some bones from Oryctodromeus. In over 11 years spent working in the Wayan, I could count on one hand the number of times I have found non-Oryctodromeus bones before this blessedly fossiliferous site was torn through by heavy equipment. Salvaging work is continuing at this site and will heavily contribute to a better overall picture of the Wayan Formation Fauna.

Oryctodromeus caudal (tail) vertebra in cross section at the mentioned construction site. Specimen recovered from lands administered by Caribou-Targhee National Forest.

Small dinosaur limb end in matrix block from the construction site. Specimen recovered from lands administered by Caribou-Targhee National Forest.

Outcrops of fluvial sandstones and associated redbeds of the Wayan Formation.

Oviraptorid (feathered fellow) and Oryctodromeus near a lake shore 98 million years ago in Idaho. Art by L. Krumenacker.

Poorly rendered Oryctodromeus near a Wayan river channel, with an iguanodontid in the background. Artwork by L. Krumenacker.

Uinta Mountains

2013-07-07T23:45:00.002+01:00

The Uinta Mountains are a large east-west trending mountain range (on of the few in North and South America). It is made of a 4-km thick Neoproterozoic siciliclastic succession. These strikingly monotonous sediments are hypothesized to record the development of a large E-W trending intracratonic basin during the early stages of the breakup of Rodinia. Subsequent to deposition, the Uinta Mountains were formed as the Neoproterozoic basin inverted during the Laramide orogeny. It is thought that the pre-existing weakness formed during the rifting of Rodinia allowed for basin inversion despite the long axis of the basin being parallel to the maximum stress.

from Google Earth

Within the interior of the range there is very little deformation and the sediments are essentially as flat as the day they were deposited. Photo of Joel and Kirk on Reids Peak with Baldy in the background.

Detrital zircons from the Uinta Mountain Group reveal that the bulk of these sediments were derived at least 1000 km away from the Grenville highlands that would have extended along the eastern margin of Laurentia from Texas to Labrador (Dehler et al., 2010 GSA Bull). Additionally the youngest zircons of ~760 Ma constrain the Neoproterozoic maximum depositional age.

During the last glacial maximum the Uinta Mountains were sculpted by valley glaciers that created a single narrow spine with dozens of long, thin ridges extending north and south of the main ridge. Glaciations has created the highest mountains in Utah and has 16 peaks over 13k feet.

Left foreground: Reids Peak (11,708 ft) with Dana, center distance: Hayden Peak (12,479 ft), right: Bald Mountain (11,947 ft)

Massive cirque leading out towards the southwest.

External Spanish Pyrenees - Kellen Gunderson

2013-06-10T14:48:00.000+01:00

The external Spanish Pyrenees are classic locale for geologists who want to study the interactions between sedimentation and tectonics. The Pyrenees are a fold and thrust belt resulting from the collision of Iberia and Europe. Most of the deformation of the external Pyrenees occurred during the Eocene (56-34 Million years ago). The external Pyrenees are defined as the southern-most region of the Pyrenean range. The rocks that make up the mountains there are actually the deformed foreland basin of the internal Sierras, thus they preserve a unique story of the interaction of erosion, sedimentation, and tectonics.

Growth strata, or sediments that were deposited during deformation, are well exposed on the limbs of external Pyrenean anticlines, as seen here looking east from the Pico D'Aguila anticline near Jaca, Spain. These sediments are used to determine the past rates for deformation and decipher the sequence of tectonic events. Many petroleum geologists come to this location to learn about the interactions between tectonics and sedimentation in order to better understand features seen in seismic sections.

Large alluvial fans formed at the external Pyrenean mountain front during the Eocene. These fans now exposed as large conglomerate cliffs provide information regarding the timing of deformation. In many places the Pyrenean thrust faults actually thrust over these conglomerates. Now these exposures provide popular rock-climbing routes.

Most of the deformed rocks in the external Pyrenees are marine sedimentary rocks. There are many great exposures of deep water turbidites. At one particular road-side exposure, I found evidence that previous geologists had been there before.

Evidence of marine deposition is obtained from foraminifera in the limestones. This foram is particularly large and exquisite. It is a species of Alveolina.

The external Pyrenees have many structures that trend transverse to the orogen. In many cases this is due to the influence of evaporite deposits influencing structures. This is a view of the Boltana anticline, a widely studied transverse anticline near Ainsa, Spain.

"There's some geology out there!"

The Cotiella thrust sheet marks the Northern edge of the external Pyrenees. Beyond this is the high Sierras, the suture zone, and France!

Meteorite impact and PGEs in Scotland

2013-05-25T10:00:00.000+01:00

The northwest coast of Scotland is an incredibly geologically diverse stretch of coastline. In less than 150 kilometers the rocks span over 2.8 billion years of Earth history, from the Archaean Lewisian gneiss to the Tertiary volcanic province on Skye with little missing in between. Such a wide temporal spread means that any particular age of rocks is restricted to a relatively small area. This is especially the case with one particular formation found along the Stoer Coast north of Ullapool. This formation, named after the peninsula on which it is found, is a Mesoproterozoic sedimentary sequence composed primarily of sandstones and mudstones.

The Stoer Formation

The Stoer Formation is a sedimentologist's dream land. It has just about every clastic sedimentary structure you can think of. From graded bedding, trough cross bedding, mudcracks, flame and pillow structures, ripple marks, you name it. Incredibly you can also see the facies transition from alluvial fanclomerates to silt- and mudstones all within a kilometer or so.

Kellen at Stoer

Soft sediment deformation

1.2 billion year old ripple marks

However the highlight of the Stoer Formation is the Stac Fada member. The Stac Fada Member marks the junction between dominantly fluvial silt- and sandstones below deep-water lake sediments above. It however is what initially appeared to be devitrified volcaniclastic unit 10 to 20 meter thick. It is dominated by a polymict clastic-matrix breccia with accretionary lapilli.

Accretionary lapilli of the Stac Fada member (from Anne Burgess).

Although initial investigators assumed this to be volcanic in origin it was bizzare in context of the surrounding geology. No other evidence of volcanism near this timeframe is found in the region. That was until 2008 when a team of researchers from Oxford and Aberdeen discovered shocked quartz implying the Stac Fada represented an ejecta blanket deposited by a meteorite impact. This impact was recently dated to 1177 ± 5 million years old. It was also discovered that the Stac Fada member is enriched with iridium and other platinum group elements (PGEs). The enrichment of PGEs is a very important (if not THE most important) evidence that this member was deposited following a meteorite impact. Iridium is a very rare element in the Earth's crust. hen the Earth was first formed, the dense elements (such as iron, nickel, and platinum group elements) sank into the Earth's interior to form the core. Asteroids and thus meteorites are also highly enriched in iridium because due to their small size, they did not differentiate and the dense elements are spread equally throughout the asteroid. For example, the Williamette meteorite found in Oregon and one of the largest ever recovered has 4.7 parts per million iridium. This is 4700 times more than the concentration of iridium in Earth's crust.

Accretionary lapilli cut open to show internal structure (from Anne Burgess).

Recently, my colleagues at the University of St Andrews found that much of this iridium is focused in the rims of this accretionary lapilli. Because lapilli form like snowballs rolling down a hill, this implies the iridium is not liberated into the atmosphere during the initial impact (when the core of the lapilli are formed), but rather released later on. I imagine this has much to do with the energy required to vaporize such dense elements. Silica has an enthalpy of vaporization of 300 kJ/mol, iron: 350 kJ/mol, gold: 330 kJ/mol. Iridium on the other hand takes 600 kJ/mol to vaporize. Although these ideas are still in their stages of naissance, it will be exciting to see how these ideas progress and evolve.

Stoer bay with two Munros in the background

A friendly 'heelynd coo'

The Moine Thrust

2013-05-12T22:48:00.002+01:00

In the world of geology, many firsts took place in Scotland. Scotland was the place where deep time was first demonstrated by William Hutton at Siccar Point (discussed here). It as also the birthplace of thrust tectonics. However, these tremendous discoveries were not without controversy.

Loch Eriboll

A prominent feature in the highlands is a succession of rocks known as the Moine. The Moine was composed of metamorphic gneisses and schists on top of a series of Cambro-Ordovician rocks. For many years geologists who had worked in the region claimed that these rocks had to be in stratigraphic order as at that time no once could invision any other way for layers of rocks to form. However, at the end of the 19th century, two field geologists, Benjamin Peach and John Horne led an expedition through the Scottish Highlands. They posited that the metamorphic rocks were at one time deep in the Earth and through forces, they were yet to fully understand, these rocks were thrust on top of the younger sedimentary succession. It was through the mapping and theorizing of Peach and Horne in along the Moine Thrust Belt that the theory of thrust tectonics has held true even until today.

Kellen pointing out the Arnaboll Thrust where Lewisian basement is thrust on top of Cambrian limestones and sandstones which are then imbricated.

When a geologist finds … gneiss overlying gently inclined sheets of fossiliferous quartzite, shale and limestone, he may be excused if he begins to wonder whether he himself is not really standing on his head." - Archibald Geikie (1884)

Climbing up to Knockan Crag

The Neoproterozoic Moine Schist thrust on top of the Cambrian Durness Limestone

Another fascinating aspect of the Moine Thrust Belt is the Assynt window which exposed a large series of imbricate thrusts the protrude through the Moine Thrust. Along the flanks of this imbricate stack the lithologic contrast between the hanging and footwalls caused the river to exploit this contact leaving the thrust fault beautifully exposed. Eventually the river penetrated through the fault gouge and formed a disappearing stream while flowing along the fault plane in the subsurface. While investigating the Assynt window, a famous geologist, Charles Lapworth lost sleep one particular night while pondering this new idea of thrust tectonics. While staying in Inchnadamph near the base of the Moine Thrust he recalls nightmares of being "bodily caught up in the Moine Thrust" and being crushed under "great earth engine." Despite the reality of the great Earth engine driving deformation and destruction indiscriminately, at the time the idea of deformation and fault slip rates were far from being understood.

Really excited about the thrust fault.

Disappearing stream following the fault contact

The Assynt Window and Conival

Falls at Assynt

You cannot mention the Scottish Highlands without an obligatory photo of a red stag.

Ligurian Ophiolites

2013-05-05T19:38:00.000+01:00

The Ligurian Coast is possibly the most iconic coastlines in Italy. In addition to being incredibly scenic and beautiful, the Ligurian Coast is home to some incredibly complex and exciting geology. The Ligurian Coast is composed of the Maritime Alps in the west and the Ligurian Nappes of the Apennines in the east. Along this coast the north-northwest convergence of the Alpine orogeny gives way to the active east-northeast convergence along the Apennines. Within the Ligurian Nappes are some classic examples of continental margin ophiolites. These ophiolites consist of exhumed, subcontinental lithospheric mantle lherzolite directly overlain by basaltic pillow lavas. The ophiolites along the Ligurian Coast along with those examples in the Central Alps (discussed here) were instrumental in shaping the early understanding of obducted oceanic crust.

Something that surprised me about the coastline in this region was how dramatic the relief so near the ocean. The mountains along this coast rocket out of the ocean over 1100 meters within 5 kilometers of the coast. This is greater than any of the peaks in the Scottish highlands (well known for their significant relief).

The city of Noli

Monterosso (the northernmost village of La Cinque Terre)

Gelato in Liguria - Heaven

Turbidite sea cliffs

The ophiolites along the Ligurian coast are comprised of the Piemont-Liguria oceanic crust. This ocean basin was a former piece of oceanic crust associated with the Tethys Ocean. Fragments of Piemont-Ligurian oceanic crust are also preserved in the Penninic nappes of the Alps and the Tuscan nappes of the Apennines.

Quartz slickensides in the serpentinite melange

Appalachian Ridge and Valley - Western Maryland

2013-04-24T20:31:00.000+01:00

by Kellen Gunderson
The Ridge and Valley province of the Appalachians is a fold and thrust belt that formed during the Alleghanian orogeny (260-325 Million years ago). The remnants of that ancient mountain belt consist of a series of linear ridges and valleys that can be followed for hundreds of kilometers. There are a series of particularly nice exposures or Ridge and Valley structures on a transect that follow the Potomac river in Western Maryland.

Satellite photograph of the Ridge and Valley province in Northern Maryland and and Southern Pennsylvania.

Steep forelimb of the Wills Mountain anticline near the town of Pinto, Maryland.

Many of the best exposures follow the railroad tracks.

Deformed mudcracks near Pinto, Maryland. When these mudcracks formed they were

equant but were later stretched by the deformation that formed the Ridge and Valley.

The Martin's Mountain anticline is a good place for undergraduate

structural geology students to sort out the kinematics of deformation.

You don't have to go far to find reminders that you are in Appalachia,

traditionally one of the most impoverished regions in the United States.

19th century kilns positioned on top of this fold at Round Top were used to make cement that was shipped down the Potomac River to Washington, D.C.The cement produced here helped build the famous Washington monument.

Kellen Gunderson working the outcrop at Round Top, Maryland.

LA-MC-ICP-MS @ NIGL

2013-03-31T09:08:00.001+01:00

Scientists can get often get a bit silly with acronyms. My favorite example is LA-MC-ICP-MS which stands for laser ablation multi-collector inductively coupled mass spectrometer. This instrumental method is used for in situ micro-analysis of solids. The primary application of this method is to measure the isotopic ratios of elements used in radiogenic and stable isotopic ratios. These ratios are generally applied to geochronology and using isotopic tracers wherein we can use the natural isotopic variations in radiogenic isotopes due to decay of radioactive elements to address geologic questions. In my case it is used for micro-analysis of various minerals (primarily zircon).

Let's dissect this nasty acronym:

Laser Ablation is used to sample very small volumes of material for mass spectrum analysis. In micro-analysis, lasers come in two basic flavours: solid state and excimer. Both of these systems operate using UV lasers through two different mediums. An excimer laser uses a gas source (often argon fluoride) whereas a solid state laser uses an yttrium aluminum garnet that is doped with neodymium. Excimer lasers are used at much higher power than solid state laser generally producing more efficient and consistent ablations, however solid state laser are a fraction of the cost and do not require a gas recharge when they lose power.

Inductively Coupled Plasma is a plasma source that generated by electrical currents produced by electromagnetic induction. Lots of things to define here. Plasma is one of the four fundamental states of matter (along with solid, liquid, and gas). Just as heating a solid will change it to a liquid and then to a gas, heating a gas will often cause the gas to ionize by stripping off electrons. An ionized gas is by definition a plasma. Plasmas are generally maintained at very high energies and those used in mass spectrometry approach 10,000 degrees Kelvin (the surface of the sun is only 5800ºK). An ICP is used to ionize the ablated material sent from the laser system before it enter the mass spectrometer.

Mass Spectrometry is a deciphering the spectra of atomic masses of ionized material. This is done by bending a stream of ion with a magnet which splits the ions based on their mass-to-charge ratio. As the ions pass through the magnet, the lighter nuclei are diverted by the magnet more than heavier nuclei. These streams of nuclei now separated by their masses arrive at an electronic detector. This electronic detector is moved along a track at specified locations corresponding to specific atomic masses.

Multi-Collector is a type of mass spectrometer that has several detectors to capture multiple streams of ions of different masses simultaneously. This is especially important when you are analysing very small volumes of material and need to analyse a wide range of atomic masses because you don't have time to slide the detector back and forth to capture the various masses.

Neptune Plus at the University of Western Australia

I have used multi-collectors of this kind in several institutions around the world although most recently at NIGL (the National Environmental Research Counsel Isotope Geoscience Laboratory) in the UK. The facility is also associated with British Geological Survey. NIGL is a world-class analytical facility with several mass spectrometers dedicated to a wide range of analyses. See their analytical facilities here.

Here is a day in the life of a LA-MC-ICP-MS.

Thanks to Camille for putting this together.

Grand Tetons

2013-03-23T08:37:00.001Z

The Grand Tetons and the Idaho farmland

The Grand Tetons are a prominent N-S mountain range in Wyoming. I am tempted to argue that the Grand Tetons have the most complex and long lasting geologic history of any mountain range on the planet. Within the range, the Grand Teton dominates the skyline standing nearly 4200 meters high, nearly 2300 meters above the valley floor. Other dominant peaks include Mount Owen (3920 m), Mount Moran (3850 m), and Middle Teton (3900 m). The Grand Teton National Park is one of the few redeeming assets of otherwise boring Wyoming; the others being Yellowstone NP and the Wind River Mountains.

Birdseye view of the Tetons stolen from Wikipedia

The geologic history in the Tetons being in the Neoarchean with rocks as old 2673±2 Ma (the Webb Canyon gneiss of Mount Moran; Frost et al., CJES 2006). These Archean igneous rocks are thought to represent some of the earliest evidence of modern plate tectonics. The Archean magmatism and metamorphism is followed by nearly 2 billion years of erosion and tectonic quiescence (or at least nothing is preserved).

The next preserved geologic feature is the mafic dike intrusion associated with the ~780 Ma Gunbarrel Magmatic Event. These are scattered around the Tetons and seen most prominently cutting through the Middle Teton (see below) and Mount Moran. The Gunbarrel magmatic event is a large igneous province that stretches along the Neoproterozoic western margin of North America. This igneous province was emplaced during the initial rifting of the Rodinia supercontinent.

Gunbarrel dike cutting through Middle Teton

The rifting of Rodinia caused a significant amount of thermal subsidence to occur along the western margin of North America resulting in the marine transgression that deposited a thick sequence of sediments. This newly established passive margin remained the site of marine sedimentation for nearly 500 million years. This cover sequence is seen along the western flanks of the Tetons.

Along the Teton ridge with the Paleozoic sediments in the background.

During the Cretaceous Laramide Orogeny, Western Wyoming was uplifted forming a broad high plateau. This uplift caused a great deal of the overlying sediment to erode off these highlands. The Tetons as we see them today were formed as the Teton fault dropped the Jackson Valley down leaving the Teton ridge elevated. This episode of extension began during the formation of the Basin and Range that began ~30 million years ago and continues today. The Teton fault is unique in that it dips to the east whereas the majority Basin and Range faults in the area dip to the west (e.g. the Wasatch Fault). The unique dip of the Teton fault can be explained by the subsidence of the Snake River Plain. The Snake River Plain is located to the west of the Teton fault and was formed by the migration of the Yellowstone caldera across southern Idaho. As this section of crust cools it tends to subside causing the footwall of the fault to tilt down toward the west. This westward tilt accompanied with westward extension caused the Grand Tetons to emerge from the Jackson Valley.

Le Trois Tétons

The high Teton ridge was subsequently glaciated into the spectacular peaks we see today. The current landscape preserves evidence from two separate glacial advances: the Bull Lake (160-130 ka) and Pinedale (20-14 ka). The earlier Bull Lake glaciation was much more widespread than the Pinedale episode and covered the eastern slopes of the southern Rocky Mountains from Colorado to northern Montana. The second more localized glaciation formed an ice extending to just south of the Tetons and to the northern edge of Yellowstone. In the Jackson Valley, this ice sheet sheet wrapped around Signal Mountain and excavated what later became out Jackson Lake. While the main body of ice flowed from the north, dozens of alpine glaciers flowed eastward from the high peaks. As the glaciers receded, several lakes formed in the end moraines along the mountain front (seen beautifully in google maps). The ages of these glacial episodes has been greatly improved upon with the advent of cosmogenic radionuclide dating. In the Tetons, this has been done by analyzing ¹⁰Be. Cosmic rays collide with nitrogen and oxygen atoms in the atmosphere causing fragmentation into ¹⁰Be (among others). The rocks exposed at the surface absorb these radionuclides which decay to more stable forms. By analyzing the ratio between ¹⁰Be and ⁹Be will allow for calculation of how long the rock in question was exposed to the bombardment of cosmic radiation. This technique has facilitated high precision geochronology of the advance and retreat of ice sheets. See one excellent example from the Pinedale ice sheet here.

Bradley Lake (left) and Taggert Lake (right)

Even today there are a few small glaciers that remain in the Teton Range. These glaciers are not

remnants of the Pinedale event but formed during the Little Ice Age, 1400 to 1850. Today, Skillet and

Falling Ice glaciers continue to carve Mount Moran, and the Teton Glacier flows down the north flank of the Grand Teton. During the past 40 years, these glaciers have retreated 20 to 25 perscent. It is likely that in the next 40 years, these glaciers will disappear completely.

The Grand Teton

Earthquakes in South Carolina

2013-03-14T19:49:00.002Z

Charleston College

I recently returned from Charleston, South Carolina. Although the purpose of my visit was geologic in nature, it was a workshop rather than a field trip. Charleston is a charming city with many historical buildings still standing. In the major cities of the Southern United States it is rare to find any buildings older than the civil war as General Sherman laid waste to much of the region during his "March to the Sea."

In my perusing of the historic downtown I was surprised to learn that Charleston has a seismically active past. On August 31st, 1886 an estimated magnitude 7.3 earthquake struck the capitol city of Charleston. This was the most damaging earthquake to occur in the Southeast United States. This earthquake was most likely associated with the reactivation of preexisting weaknesses left over from the rifting of the Atlantic. These remnant faults can reactivate for a number of reasons; e.g. over-loading of the hanging wall or changes in far-field stresses.

Map of the Mercalli Intensity of the 1886 EQ

The Charleston Lean (from the USGS)

This earthquake damaged and destroyed many buildings in the old city of Charleston and killed 124 people. The spectacular thing about this earthquake is that structural damage was reported several hundred kilometers from Charleston (including Alabama, Ohio, Kentucky, Virginia, and Illinois), and it was felt over 1,500 kilometers away. Many of the wooden buildings in Charleston still have what is known as the "Charleston lean" where foundations sank causing the buildings to list. In some of the more dramatic cases, new floors were built on top of the original floors at an angle to restore horizontality.

Within Charleston, liquifaction features were present throughout the area and sand volcanoes erupted and covered many acres of ground were overflowed with sand. Many of the liquifaction cones were nearly 6.5 meters across. In a few locations, water from the craterlets spouted to heights of about 4.5 to 6 meters.

The intraplate epicenter of this major earthquake is not unique in the Eastern and Central United States. Other major intraplate earthquakes include those at Cape Ann, Massachusetts (1755), and New Madrid, Missouri (1811-1812).

Liquifaction cone

Since the 1886 earthquake South Carolina has taken earthquake preparedness very seriously. A recent report (seen here) predicts at least 900 fatalities, 8000 serious injuries and approximately 45,000 total casualties (dead and injured). They believe over 200,000 people displaced and at least $200 billion in total economic losses from damage to buildings, interruption of businesses, and damage to transportation and utility systems. Furthermore, because of insufficient building code standards and average age of the majority of buildings, important government buildings, schools, fire stations and hospitals are especially vulnerable to damage. The catastrophic failure or partial collapse of government and school buildings during a work/school day would greatly increase casualties.

They also estimate about 800 bridges will be rendered unsafe, preventing first responders who try to reach victims. For coastal South Carolina, this will be a major problem as a number of sea island communities whose only connection to the mainland is a bridge, would be cut off from all transportation except by boats or helicopters if bridges fail. Even the new eight lane wide Arthur Ravenel Jr. Bridge is at risk. Although the bridge itself was built to sustain a magnitude 7 earthquake the elevated road leading to the bridge would likely fail leaving the bridge useless following an earthquake.

Although the chances a large earthquake will strike South Carolina in the near future are slim it would definitely be an event for which the South Carolinians should be prepared.

Friday's Fab Five #3

2013-03-01T19:28:00.004Z

With Spring nearly upon us, the geologic travels are soon to begin. I am looking forward to another season of travel and fieldwork. Heck that is why I became a geologist. Until then this Friday will be Mount Etna. I visited Mount Etna in the spring of 2007 with a group of geologists from Brigham Young University.

Etna eruption in 2008 (from swisseduc.ch)

Alvin and I walking across lava that had erupted one week earlier.
We could still feel intense heat radiating from the flows.

The Crater Rim

Snow taller than buses in May

Sulfur compounds encrusting the summit

Petroleum Exploration and Enhanced Oil Recovery

2013-01-17T09:34:00.000Z

Something that has always struck me about petroleum geology is that our petroleum resources nearly always seem to be in the most remote parts of the planet. For the most part, gone are the days of the Esso exploration geologists that first discovered much of the petroleum in these remote areas. These days petroleum companies are spend massive amounts of energy looking for ways to further exploit the oil fields discovered by these pioneering geologists in some cases over 100 years ago. Thanks to John Hoopes (of Anadarko Petroleum) for this excellent explanation of the next big thing in petroleum exploration.

Disclaimer:

The following outline is meant to cover the fundamentals of Enhanced Oil Recovery (EOR) as accepted by most of the industry and does not reflect the methods used by Anadarko Petroleum exclusively. Details regarding exact depths, pressures and other company information have been omitted and the majority of the data presented is regarded as public knowledge.

Salt Creek Oil Field Background

Salt Creek anticline as seen in surface topography -

Town of Midwest at top (2013 Google earth)

The Salt Creek oil field covers roughly 7 square miles in the north eastern Wyoming. It resides on the southwestern margin of the Powder River basin. The source rocks were principally deposited during the time of the Cretaceous seaway which bisected most of North America. Many oilfields in the Rocky Mountain region of the US and Canada are sourced from this time period and the presence of shallow to deep seas which eventually receded to the north. Laramide orogenic events (~40 Ma) created a series of westward verging anticlines along this portion of the state that are textbook examples of surface oil prospecting ‘made easy’. The field was discovered or had begun development as early as 1880. For many years the property changed ownership and struggled to see sustained or commercial development as the US was learning how to cope with westward transient expansion, steady labor and understand the complexities of oil and gas prospecting. With the discovery of several other profitable areas surrounding the city of Casper (south) and the addition of an oil refinery, Salt Creek, Teapot Dome, and other fields saw renewed interest. Initially, oil was hauled to Casper via wagon and later with small rail and pipelines.

Roughnecks in Salt Creek (1920’s) after having capped an uncontrolled oil well

Production from Salt Creek was constrained to the shallow Shannon sandstone and some porous and permeable pockets of the Niobrara shale. These intervals were so prolific that Salt Creek became the most productive field in Wyoming history. In 1915, the federal government noted the production and prolific oil reserves of the area that it set aside the Teapot Dome for the use of the Navy. Today, it stands as the sole field to have exceeded production of all others by cumulative barrels produced (approximately 692 barrels of oil by 2012).

Phases of Oil Field Development

Common practice for oil and gas development generally follows this sequence:

· Regional geologic modeling and study (basin type, modeling etc)

· Exploration (more focused surface and subsurface research and modeling-structure, source rock, reservoir rock) – Currently involves seismic imagery more than surface geology

· Test wells (either single or multiple wells drilled in an area to test for presence of hydocarbons, verify stratigraphy, porosity and permeability with a core or side wall plugs)

· Primary development (drilling an array of wells to produce productive areas, use of some water injection to maintain pressure)

· Secondary development (drilling some additional wells and beginning a water flood program [inject water to stimulate flow])

· Tertiary development (inject CO2 [or other gas] with water to stimulate flow, some additional wells drilled)

· Quaternary development (use of surfactants or other chemicals to reach zones not previously contacted by primary, secondary and tertiary methods)

Surprising to most, is the potential an oil field has since the primary phase of production usually yields 20-40% of the oil present. Secondary and Tertiary methods can be valuable assets in the right economic conditions where a single barrel is worth significantly more than it was a few or couple years earlier.

Stratigraphy and Strategy of EOR operations at Salt Creek

The salt Creek Oil Field life cycle is currently over 100 years old and is in the tertiary or EOR (Enhanced Oil Recovery) stage with some water flood processes taking place. Production of the field through the years has migrated to deeper units of the Frontier formation known as the Wall Creek 1 and Wall Creek 2. They represent a classic coarsening upward, forced regression sequences (falling stage) that extend across the eastern margin of Wyoming (left figure).

The majority of the EOR operation is designed to “sweep” or contact areas of the rock not affected or minimally affected by primary and secondary recovery efforts. In theory with steam or CO2 floods, the model is a injector-producer design. This method effectively pushes or displaces those fluids (oil) and gravitates towards a lower pressure created by the producing well (figure below).

This process can be altered further depending on well performance, fluid viscosities, and reservoir characteristics by alternating water and gas injection. This ‘pulse’ or “WAG”’ing (water alternating gas) can disrupt preferred patterns of flow and allow for new areas of the reservoir to be flooded as the gas penetrates the rock matrix.

A key component to the EOR process is the pressure and temperature of the reservoir which allows for miscibility of the CO2. Without miscibility or injecting at the appropriate pressure, CO2 would remain in a gas phase and not permeate the rock in a uniform way with the residual waters that reside at depth. Therefore, not all reservoirs and conditions allow for EOR methods with CO2. Other chemical floods or injected media in “quaternary recovery” may be possible as their chemical makeups will allow for higher or lower pressures than those of CO2.

Another key component to EOR operations is the pattern design or array of wells that are utilized in the injection-production process. The “5-spot” pattern or a producer surrounded by a square of 4 injectors is the most common. Depending on isotropy, structure and depositional environment, the scale and orientation (and skewness) will need to be considered to flood and produce efficiently.

The future of oil and gas exploration, development and recovery will likely shift more and more to EOR methodology and doctrine. As these are not renewable resources, the desire of a company to extract the remaining resources in a cost effective way will be paramount and buffer the cost and risk of exploration and development elsewhere. A common adage heard among companies and other exploration geologists is: “all the easy oil has already been found”. Ironically there is a shift back to those resource plays and old oil fields where CO2 and other chemical floods are effective. These oil fields can become profitable again and become sources for economic stability for the surrounding region, be prepared for carbon sequestration and contribute to global resource supplies for several more decades.

A Work-Over rig is servicing a well on top of a mesa near the field. ‘Work over’ is a term used to generalize the operations that encompass any time an old (or new) wellbore needs to be re-entered for maintenance purposes. Considering the age of the Salt Creek field, a lot of old wellbores can be used but require this type of activity to ensure continual and safe operation.

A pump-jack in the Monell EOR field. 24 hours a day, 7 days a week and 365 days a year, pumps for injection and production are meticulously monitored for pressure and efficiency. Crews are assigned to portions of the field to care for the equipment at all times and ensure their ability to perform.

Friday's Fab Five #2

2013-01-04T18:44:00.001Z

Because there is not much traveling in the wintertime, I will post another fab five for this Friday.

This week's fab five come from India. No where in particular, just some amazing sites around the country.

The Taj Mahal is built of marble quarried from Makrana in Rajasthan. I was amazing at how little dissolution there has been over nearly 400 years.

Bengal Tiger from the Jim Corbett National Park

Dusty dusk in Rajasthan

Sacred Ghat of Har ki Pauri in Haridwar

Asian Elephants in Uttarpradesh

Friday's Fab Five #1

2012-12-28T12:20:00.002Z

This weeks fab five comes from my old stomping grounds: the Colorado Plateau.

Prof. Tom Morris (in the aquamarine shirt) standing atop Glass Mountain in Capitol Reef National Park, Utah.
Glass Mountain is a large diapir of selenite. Selenite is a variety of gypsum (CaSO4•2H2O) in the form of glassy crystals. Glass Mountain formed as a result of groundwater flowing through the Entrada Sandstone. Some of the selenite collected from the region stand over a meter tall and 30 cm across.

Side on view of Delicate Arch in Arches National Park, Utah. These arches were formed as fractures in the Entrada sandstone formed during the Laramide orogeny in the Cretaceous. Groundwater subsequent flowed through these fractures dissolving the cement holding the sand grains together. Once these fractures were exposed at the surface, the loose uncemented sand nearest the fractures was blown and washed away leaving only the arches we see today.

Boulder, Utah: quite possibly the greatest inside joke ever.

Navajo Sandstone in Little Wildhorse Canyon, San Rafael Swell, Utah

The Cockscomb, Teasdale, Utah

If you have five fabulous pictures of a particularly spectacular place shoot me an email to contribute for the next Friday's Fab Five.

Labrador

2012-12-13T15:21:00.002Z

Labrador has a fabulous diversity of geology. It has rocks as old as 3.8 billion years old to the present. In addition to rocks, Labrador also is known for:

Bears...

Wide expanses of pine trees...

Blueberries!

Most of the best exposures in Labrador are to be found in quarries. These quarries are large scars in the landscape nearly fifty feet deep and hundreds of meters wide. Because these quarries are less than 50 years old there has been little to no weathering resulting is excellent outcrop.

Left: garnet-bearing, K-feldspar megacrystic biotite granitoid rock with ovoid megacrysts. Right: mafic dike associated with the 615 Ma Long Range dike swarm with small quenched plagioclase phenocrysts.

Wouter Bleeker testing the magnetic susceptibility of the Long Range dike.

Top to the right shear fabric in a K-feldspar megacrystic granitoid from the Lake Melville terrane.

Biotite and K-feldspar pegmatite with 1 meter wide biotite crystal.

graphite+quartz+K-spar+biotite+titanite gneiss
Think of it graphite in a granitic gneiss!

More graphite.

This excellent specimen of fergusonite [(Y, rare earths)NbO4] found on HighREE Island. That's right High Rare Earth Element Island. The island was named by the mining company Search Minerals because of the bizarre occurrence of rare earth elements. Rare earth mineralization is found primarily in pegmatite and aplite intrusions. The rare earth elements were discovered by prospectors walking around with geiger counters. When the geigers click the prospectors dig.

More fergusonite in a pegmatitic dike.

Allanite [(Ce,Ca,Y,La)2(Al,Fe+3)3(SiO4)3(OH)] crystal in pegmatite intruding the Alexis River Anorthosite/Leucogabbonorite. The radial fractures around the allanite are formed from radiation damage.

On top of all the crystalline rocks is the Cambro-Ordovician rocks of the Sauk transgression (the Labrador group). From Estonia to Scotland, Labrador, Argentina, Texas, California, Yukon, and the Northwest Territories all display this same sequence of rocks.

New York City

2012-11-18T16:47:00.001Z

Although New York City is not thought of as a mecca for geologists you would be surprised at the hidden gems it has to offer. My first stop in New York City was the top of the Rockefeller Center. A terribly touristy place offers an unparalleled view of the city. I will never cease to be amazed how the human race has modified the landscape to our liking. With the Hudson River to the West and the East River to the ... well to the East the isle of Manhattan has been transformed into a sea of concrete and lights. It is hard for me to understand what draws so many people into such a small space. Current estimates of Manhattan's population density of nearly one person inhabiting every 20 square feet. This is over two and three times larger than Chicago and Los Angeles, respectively.

Top: George Washington Bridge over the Hudson River; bottom-left: Radio City Music Hall; bottom-right: looking south to the Empire State Building and the East River

No trip to New York would be complete without a trip on the Staten Island Ferry and a quick visit to the 9/11 Memorial. Currently the new World Trade Center building is nearly complete and dominates the Manhattan skyline. Staten Island still looks like a war zone after the devastation of hurricane Sandy. Many residents still do not have clean drinking water and electricity.

Clockwise from top left: new World Trade Center, Staten Island Ferry and Statue of Liberty, downtown Manhattan from the air, the South Tower Memorial Pool at the 9/11 monument.

My favorite site in New York is the American Museum of Natural History. I spent five hours in the museum and still felt like I was rushed.

Clockwise from top left: true to life scale model of a Blue Whale, main entrance with bronze statue of Theodore Roosevelt, herd of African elephants, diplodocus and allosaurus(?) skeletons, eight-foot high Aztec calendar, and the actual remains of Lucy.

Although the Anthropocene sites are neat, the real treat is found throughout Central Park where the bedrock is exposed. Central Park exposes allochthonous bedrock of the Taconic Sequence (Hartland and Manhattan formations) and minor exposures of the Tippecanoe Sequence (Walloomsac formation). The Hartland and Manhattan formations are deformed in complex fold structures with three generations deformation features present. These pelitic migmatites have been intensively mapped and studied by Charles Merguerian of Hofstra University. He has published a fantastic overview of Central Park geology here.

Hartland Schist near 77th St.

Stairs cut into the Hartland Schist

Black Hills and Badlands - Kellen Gunderson

2012-10-29T21:47:00.004Z

from Kellen Gunderson:

Western South Dakota contains two gems that are often forgotten in discussions of great landscapes of the American Rocky Mountains. Just off I-80 near the Wyoming border lies the famous South Dakota Black Hills Mountains and Badlands National Park. Before the Black Hills became the holy mountains of the Lakota Sioux Tribe, it was formed during one of the last major pulses of uplift during the Laramide Orogeny; the mountain building event responsible for the creation of many of the iconic ranges we know in the Rockies today.

Just east of the Black Hills lies Badlands national park. Most of the rocks in the Badlands were deposited during the time the Black Hills were being uplifted as sediments shed off the uplifted mountains were deposited in an adjacent basin. Erosion and escarpement retreat has exposed these rocks in deep canyons and steep cliffs that are flanked by South Dakota shortgrass prairies.

The Brule formation dominates the Badlands landscape and contains numerous vertebrate fossils.

This normal fault in Badlands National Park shows about 10 m of offset in the Chadron Fm

Clastic dike in Brule Formation in Badlands National Park

A goldmine in the Black Hills exposes spectacular dikes and sills.

No visit to the Black Hills is complete without seeing the famous Mt. Rushmore.

The Black Hills bleed into Wyoming where you can see Devil's Tower, the volcanic plug made famous in "Close Encounters of the 3rd Kind." Devil's Tower is the remanent of a volcanic intrusion into surround sedimentary country rock. Because the sedimentary rocks are more easily eroded, they have since been removed leaving the igneous intrusion standing above the surrounding country side.

Sunset and moonrise over the prairies and badlands in South Dakota.

Central Alps - Flysch and Molasse

2012-10-22T20:37:00.002+01:00

Flysch and Molasse are terms that describe different sets of sedimentary facies associated with orogenic belts. Molasse is defined as terrestrial and shallow marine deposits formed nearest the rising mountain front. It consists of various interbedded rock types including conglomerates, sandstones, and shales. The molasse on the other hand is made up of deep marine sediments deposited in the distal part of the orogenic sedimentary wedge.

In the Alps (where these terms were originally defined), the flysch and molasse was deposited north of the Alps throughout the development of the Alps from the Cretaceous to the present. Prior to this time, the Alpine area was covered with what is known as the Tethys Ocean. The Tethys Ocean extended from Spain all the way to Australia and Indonesia. As rifting in the Tethys Ocean stopped and southward subduction began, sediment began accumulating in front of the growing mountain front. As the Apulian plate continued to override the European plate the weight of the Apulian plate and the sediment accumulating on the European plate began to flex the subducting plate adding to the accumulation space of the basin. The interplay of accretionary loading and sedimentation continues as the fold and thrust belt of the advancing orogeny. Eventually the fold and thrust belt overthrusts the oldest part of the flysch and molasse basin.

The Helvetic nappes discussed in the last post overthrust this basin over 10 kilometers. This overthrusting of the Helvetic nappes raised the orogenic wedge to a critical taper. This means the strength of the rocks do not allow for the angle of the orogenic wedge to exceed this critical taper forcing the convergence to be accommodated further into the foreland. As thrust faults cut through the more distal portions of the foreland basin, a large part of the basin is caught up in the fold and thrust belt (the Jura Mountains) and forms a piggy-back basin (the Geneva basin) and extends the active portion of the flysch and molasse basin north of the Jura Mountains.

Sub-Helvetic Molasse

Flute and Cast structures

Zoophycos trace fossil

More flutes

Ooids in the foreland basin sediments

The piggy back basin with Zurich in the middle and the high peaks of the Alps above the clouds.

Sauropod tracks in the molasse

Marbach, Switzerland

Central Alps - Helvetic Nappes

2012-10-04T19:44:00.000+01:00

First off we should define what is a nappe. A nappes is a package of rock that has been folded and deformed to the point that it is completely allocthonous, that is disconnected from its origin (generally over 5km from the original position). They are characterized by rootless, recumbent, and isoclinal folds. The word nappe is derived from the French word for tablecloth. One of the most recognizable nappes and actually where the word originates is in the Helvetic Nappes of the Suisse and French Alps. Just about every textbook about structural geology features a classic picture of the Helvetic Nappes and more specifically the Morgels Nappe in western Switzerland.

The Morcles Nappe

Closeup of the Morcles Nappe

In the upper Rhone river valley near the cities of Sion and Martigny, the Helvetic Nappes consist of three nappes stacked on top of each other. These nappes allow for a spectacular reconstruction of the paleoenvironment prior to the formation of the Alps and the emplacement of the Helvetic Nappes. The basal nappe (Morcels) is primarily composed of near shore sedimentary rocks. Above the Morcles is the Diablerets nappe which is made up of rocks deposited on the shallow to deep water slope face. At the top of the stack, the Wildhorn (pronounced "vild-horn") represents the deepest water pelagic sediments. When we restore these three nappes back to their original position we have a fully restored ocean margin from the shallow water (Morcles nappe), to the continental slope (Diablerets nappe), and the deep ocean basin (Wildhorn nappe).

Morcles nappe at sunset

Lake Salanfe and the Morcles Nappe

Looking north towards the Pré-Alpes and the Jura Mountains

Les Dents Blanche (the white teeth) above Lake Salanfe

Can you see the fold closures in the center of the photo? (click to enlarge)

Perfect S-fold at the base of the Wildhorn Nappe

Wildhorn Nappe at Saneche

The Helvetic Nappes were the location of some of the most influential studies in structural geology when geology was still in its youth. In the shales of the Helvetic Nappes are thousands upon thousands of little marine fossils known as Belemnites. These small marine cephalopods were similar to modern day Cuttlefish although these creatures had a calcite shell over their shuttle. When these animals died, their calcite shuttles were deposited on the ocean floor and eventually fossilized. Going way back to 1876, Daubrée noted that in the Helvetic Nappes, these fossils were variably stretched and broken depending their orientation. The voids between the shell fragments were filled with calcite. If the belemnite was oriented in the direction of maximum stress, the belemnite exhibited the maximum degree of strain. Nearly 100 years later, John Ramsay (perhaps the father of modern structural geology) deciphered a method through which the amount of strain within these belemnites could be used to quantify the amount of strain and the direction of maximum stress within the Helvetic Nappes. Near the town of Sion, I visited the outcrop where Ramsay and others first described this phenomena. A group of students calculated the amount of strain in nearly 20 belemnites. The strain measurements were normal distributed varying between 120 and 250% (150% average) of the original length depending on the orientation of the belemnites.

Next week we will end this tour of the Alps with the flysche and molasse basin north of the Alps.

Central Alps - Sesia-Lanzo

2012-09-22T12:14:00.000+01:00

Last week we transected the Apulian crust through the Ivrea-Verbano Zone to the Insubric Line. The Insubric Line is the suture between the Apulian crust that makes up Italy and the European continent. Within this suture zone is a sliver of oceanic crust zone known as the Sesia-Lanzo Zone. The Sesia Lanzo and other units of a similar affinity (Dent Blanche, Zermatt-Saas, and Adula Nappe) are tectonically interleaved above the Hercynian Basement of the Apulian crust within the ophiolitic fragment of the subducted Tethys ocean. These units lie adjacent to the Insubric line to the south and continue over 100 km north where the Penninic Nappes form large klippe of the Sesia-Lanzo ophiolite.

The Great Matterhorn

These ophiolitic fragments display blueschist to eclogite facies metamorphism of Mississippian and Eocene age. The overprinting (ultra-)high pressure metamorphic textures are indicative of double-dip tectonics. When continents collide part of the previously subducted oceanic slab remains too buoyant to subduct and squeezes its way up through the suture zone (see Beaumont et al. 2009 - EPSL for the classic Himalayan example). After tens and sometimes hundreds of millions of years, the suture zone of old collision systems become the weakest link when the continents are rifted apart (e.g. Caledonide Orogeny and the North Atlantic Rift, East Africa Orogeny and Rift). The Carboniferous Hercynian Orogeny was no different. Nearly 300 million years ago the Rheic Ocean closed via subduction and collision during the Hercynian orogeny where ophiolitic fragments were subducted to nearly 80 km into the mantle only to be exhumed back up the subduction channel to the surface. About 50 million years after the closure of the Rheic Ocean, the Hercynian orogeny was torn in two along its ancient suture zone forming the Teyths Ocean. This wide ocean was subsequently closed by the Alpine orogeny in the west and Himalayan orogeny in the east. In similar fashion to the Hercynian orogeny, the Alpine orogeny subducted the Tethys ocean to depth over 100 km where a portion returned to the surface incorporating a portion of the Hercynian suture zone material within it. Herwartz et al. (2011), were able to date single garnets using Lu-Hf chronometry within the Adula Nappe that had 336 million year old cores with 37 million year old rims with compositions both indicative of high pressure metamorphism.

For my journey through this blueschist and ecolgitic ophiolite, I visited the Valtournenche off the Aosta Valley in northern Italy. Starting at the base of the crust we see serpentinized peridotite. Peridotite is an ultramafic, olivine- and pyroxene-bearing rock found in the mantle. When peridotite is exhumed to the surface it is most often serpentinized by water-rock interactions. As the oceanic crust is subducted, hydrous mineral such as amphibole (e.g. Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂) begin to dehydrate. The water that is driven off the downgoing slab interacts with the overlying mantle (peridotite) causing these reactions:

Forsterite (Mg-Olivine) + water → serpentine + brucite
2Mg₂SiO₄ + 3H₂O → Mg₃Si₂O₅(OH)₄ + Mg(OH)₂

Serpentine is found in three basic forms and each form generally corresponds with the varying temperatures of formation. The lowest temperature form is chrysotile (<300ºC), with antigorite next (300-600ºC), and lizardite as the highest temperature form (>600ºC).

Lizardite, brusite, and chromite in a serpentinite

Chevron folds in a serpentinite

Above the peridotite are the layered gabbros and sheeted dikes which are poorly exposed (if at all) in the Sesia Lanzo, which brings us to the pillow basalts. In the Valtournenche, the pillow basalts have all been to blueschist facies metamorphic conditions. When basalt is subducted to >50 km the basaltic mineralogy of plagioclase, orthopyroxene, and olivine is transformed into glaucophane, lawsonite, and often garnet and titanite.

Lawsonite (peach-colored, orthorhombic), Garnet (dark-red, round), and Glaucophane (blue groundmass)

Squashed pillow basalt

Deeper still in the subduction zone are the eclogite-facies rocks. To see eclogites I jumped across the Aosta Valley to the south up the Saint-Marcel river to the now abandoned Servette mine. The Servette mine is a Fe-Cu mine that had been exploited since Roman times. The eclogitic blocks here are heavily altered in some places where hydrothermal fluids altered the composition of the host rock.

Look up the Saint-Marcel valley from the Servette Mine

Eclogite with garnet (red) and omphacite (green Na-pyroxene)

Garnets weathering out of the rock

These eclogites are thought to represent Jurassic black smokers on the sea-floor and are responsible for the Fe-Cu mineralization. Black smokers are underwater geysers that erupt high levels of sulfur-bearing minerals and form larges fields hundreds of meters wide where hot water comes to the ocean floor through fractures near the mid-ocean ridge. When this superheated water comes in contact with cold ocean water, many minerals precipitate, forming black chimneys around the vents. Sulfides metals such as pyrite and chalcopyrite are deposited often forming massive sulfide ore deposits after millions of years of hydrothermal activity.

Pyrite and Chalcopyrite in chert

Stibnite in chert (Sb₂S₃)

Old mining establishment

Next week, we will cross the Penninic divide to the Helvetic Nappes.

Central Alps - Ivrea Verbano Zone

2012-09-16T08:52:00.001+01:00

I recently returned from a trip to the Central Alps in Italy and Switzerland. All things considered, the Alps might be my favorite mountain range to visit. It has the incredible relief of the Andes and Himalaya (almost), accessibility of the Rockies, and food of ... Italy! The Alpine mountain chain extends basically along the entire northern Italian border. The Alps were formed from the continental collision of the Apulian plate with the European plate from the late-Cretaceous to Tertiary time. The next four post will take you on a transect through the central Alps from Milan, Italy to Bern, Switzerland.

During the Tertiary convergence of the Apulian microplate and Europe, the Tethys ocean subducted beneath Apulia to the south consuming over 500 km of oceanic crust. As ocean crust became the European continent, the entire thickness of Apulian crust began to turn in on top of itself and rotated over 90º overturning the continental crust. The resulting impact has exposed a spectacular cross section through the Apulian crust. Traveling north from Milan, we crossed the Po foreland basin and deformed Mesozoic strata of the Southern Alps into the Hercynian basement of granulites and layered gabbros. The Hercynian basement was formed during a ~300 Ma orogenic event time equivalent to the Alleghanian orogeny in the US.

Lithotectonic map from ETHZ

Further north we crossed the Insubric line which demarcates the boundary between Apulia and Europe and is in essence the original suture potentially dating back to the Hercynian orogeny. Across this line we entered a mysterious lozenge shaped crustal block cored with granulite facies rocks and rimmed by retrogressed greenschist. This is the Ivrea-Verbano Zone. The Ivrea-Verbano zone is dominated by variably migmatized metapelitic schists at the base grading to a massively banded migmatitic gneiss, known locally as a Stronalite (after the Val Strona).

Traveling northwards and up-metamorphic grade there are a large number of thick Permian-age mafic intrusions eventually dominating the Ivrea Verbano zone. The most spectacular of these is found in the southernmost part of the IV zone where a 10 km wide layered mafic intrusion is sandwiched between the high grade pelitic migmatites and the European crust.
This layered mafic intrusion is predominantly a plag-pyx gabbro with crosscutting pegmatitic pyroxenite dikes and olivine cumulates at its base. Although the compositional layering could be argued to be gneissic banding and mineral segregation, primary magmatic cross-bedding is observed affirming the original layered nature of the intrusion.

In the northeast portion of the IV zone, the base of the layered gabbro is exposed at the petrological Moho with a thin sliver of mantle lherzolite above the Insubric line. The Mohorovičić discontinuity (or Moho for short), is the boundary between the crust and the mantle. For a petrologist the Moho is the contact between the layered gabbros or granulites of the lower crust and the peridotite of the mantle. However, for a seismologist the Moho is the seismic discontinuity after whom the boundary gets its name (after Croatian seismologist Andrija Mohorovičić). Although the Moho is imaged all over the Earth by seismologist, there are few places where the petrological Moho is exposed at the surface. The Ivrea Verbano zone gives a unique view through the Apulian crust all the way to the mantle.

Garnet-bearing migmatite in a large boulder in the Lys river at Fontainmore presumably derived from the metapelites of the Ivrea-Verbano zone.

Another "IV zone" pelitic boulder.

Spectacular chloritized garnet rims. Coin is 2.3 cm across.

Layered gabbro. Faint cross-bedding can be seen right of the hammer.

Olivine cumulate altered to iddingsite.

Pyroxenite pegmatite

Progressive close-up of the Moho. Left - layered gabbro, Right - serpentinized lherzolite

Lizardite and Antigorite with Chromite(?) near Moho

Next we will move north across the Insubric line to the Sesia Lanzo zone.