Stratigraphy as a Badly-Edited Tape

“The stratigraphic record is like a badly edited tape.” Discuss.

The rock record is incomplete. Between limited exposure, erosion, and recycling, only patches are preserved, and deposits of the same age are no longer connected. Particular lithographies can be paired by matching sedimentary properties by correlating outcrops and unit geometry, correlate geophysical and geochemical properties, and using depositional and plate tectonic frameworks to build local depositional histories. However, this leads to a layer-cake vision of the world, homogeneous layers simultaneously deposited around the world.

Modern deposits have a limited extension, with rock type diagnostic of local environment, not age. Facies are rocks with specific characteristics with limited lateral extent. Time lines cut across facies boundaries, with rock-bodies thinning out laterally and vanishing, or grading in to each other. A pinch out of a rock body is a particularly common stratigraphic trap. Rapid changes in shoreline results in a jagged stratigraphic section, leading to inter-tonguing pinch outs of particular interest to petrologists. During transgressive episodes, erosion obliterates the deposited facies from retrogressive episodes, making preservation unlikely unless subsidence is extremely rapid. The result is that the stratigraphic record is asymmetric, with more records of transgressive than regressive cycles.

Base level erosion is the point below which erosion cannot occur. The ultimate base level is ocean basins, where sediment accumulates, but local conditions can create temporary base levels at higher elevations. Similarly, base level aggregation is the point above which aggregation of sediments cannot occur. Grain size and environment vary this level greatly, but preservation of any lithography about this point is very rare, the original rocks are eroded and transported to lower elevations. These concepts lead to discontinuities in location of sedimentation, farther breaking the myth of continuous sedimentation. Oscillation in base levels can lead to sporadic sedimentation or small-scale unconformities.

Unconformities can also come from other temporal breaks. Angular unconformities, nonconformities, disconformities, and paraconformities are uncomformaties with different lithographic and geometric relationships between overlaying strata.

With all these missing sections and spotty sedimentation, geologists are left trying to patch together short, local sections into a coherent history. Outcrops are rarely extensive enough for continuous tracing, but strong lithographic similarity, position in sequence, uncomformaties (including global sequences), structural features, and evidence of deformation or metamorphism can all be used for correlation. Like an edited tape, pieces are missing; like a badly edited tape, pieces are randomly missing, potentially with key sections utterly destroyed.

Posted in Geoscience | Tagged , | Leave a comment

Charge-out Rates

For all the black magic voodoo of interpreting geophysical data, the practice of geoscience consulting can be a delightfully straightforward. When figuring out a charge-out rate, professional organizations publish annual fee guidelines depending on the responsibilities of the expected task. The trick is to pick the correct category for the job, and not get confused with the category the person who will be doing the job is capable of fulfilling.

But alas, that’s not everything. A job isn’t just the job; it’s also the overhead (office space, electricity, computers, software…) and the time spent actually finding the work to keep everybody busy. In California, this turns into a rule of thirds — each contribution gets to earn 1/3 of the total, so an employee with a $180 per hour charge-out rate working from the office of a consulting company with bosses who seek out and bid on contracts is only earning a third of the rate: $60 per hour.

The practice does get messier when working as an independent contractor, where full-time work is not guaranteed (and it’s often feast or famine) so the rate needs to amortize over all the days with no work at all. Luckily when subcontracting, it’s not all that unusual for the larger consulting company to accept a charge-out rate in line with the fee guidelines, then tack on a percentile markup (10%-15%) to their clients as their cut for arranging the project.

Of course, if you have specialized skills, or are working in unusual (or remote, or dangerous) environments, or are performing a job that doesn’t fit the fee guideline descriptions, or are otherwise unique at what you do, or are working on a contract as part of a salaried job, or countless other variables the whole system gets a lot more complicated! But hopefully by then, you’ve had enough experience to make your own judgement calls on how to charge for your time.

Posted in Geoscience, Practice of Science | Tagged , , , , | 3 Comments

Sandstone Diagenesis

During diagenesis a sandstone may undergo compaction, cementation and dissolution. Detail these processes and explain why they are important factors that must be considered by petroleum geologists.

Diagenesis is the lithification of loose sediment into solid rock through compaction, cementation, and dissolution.

Compaction is increased pressure during burial physically squeezing sedimentary grains together, compressing void pore space. Different materials compact to different extents: clays compact more, while sandstones compact less. The result is differential compaction dependent on material. Not all material can be lithified through compaction — a mix of sand and gravel without a clay matrix lacks sufficient grain-to-grain contact for even heavily compressed sediments to lithify by compaction alone.

During compaction, the visible fabric is deformed and distorted into new textures. Pebbles and fossils can be flattened; fine grained fragments deformed or even broken into clay particles. Grains may be subject to pressure solution: pressing concavities into each other at contact points. Textural characteristics, including an increase of fractures, can help with estimating the maximum depth reached during compaction.

Additional clues can come from minerals changing properties as the temperature increases with depth, with some fossils changing colour, organics growing increasingly shiny, and clay minerals transforming into other minerals.

Cementation is the precipitation of new minerals to hold together grains. Cementation is more common in the near surface, as groundwater flow slows with depth. Common cements are silica, calcite, and iron oxide. Magnetic cements (such as iron oxide) lock in the magnetic field at the time of formation, potentially allowing the timeframe and latitude of cementation to be determined.

Dissolution is the weathering of sandstone through the removal of minerals or grains by fluid transport.

Petroleum geologists love finding porous sandstone capped with an impermeable surface like clay that can trap natural gas. Shoreline regression and transgression forming jagged vertical switches between sandstone and offshore clays are a favourite target.

Posted in Geoscience | Tagged , , , , , , , , | Leave a comment

Day of the Doctor

I peeked at a tiny bit of planetary science, and a whole lot of astronomy and cosmology, in a piece at Physics Today for the Doctor Who 50th Anniversary.

Posted in Astronomy | Tagged , , , | Leave a comment

Accretionary Wedge #61: Geo-jobs!

For October’s Accretionary Wedge, I asked what you did in your geoscience job.

Martin Bentley works in a small geotechnical engineering company in South Africa, where he does a lot of borehole logging fieldwork, along with report creation and administration.

The Gallivanting Rockhound Ann shares her experiences with work/life balances in the petroleum industry, along with some hard-won experiences with medical issues in the field (including safety tips for new fieldworkers).

Anne Jefferson of Highly Allochthonous discusses the career profile of a hydrology professor. She also did an interview with Eureka! Lab about her job.

Continuing the watery theme, James B. of Aerial Geologist explains what it’s like to be a sourcewater protection hydrologist.

The Silver Fox on Looking for Detachment discusses exploration geology, or at least those specifics that aren’t held up by company-confidentiality.

My contributions from the archives are working as a field geophysicist, or as a science advisor in the film industry. I can’t actually talk about what I do as a government contractor, and as a science writer I spend all my time looking for interesting people to talk to, 20 minutes interviewing them, 2 weeks doing research and building a story-structure, and 8 hours actually writing each article. Fresh for the Accretionary Wedge, I’ve written up a bit more on the practical aspects of becoming a geophysicist.

For the visually-inclined, the Association for Mineral Exploration in British Columbia is currently featuring a photo contest (yes, you can vote) with a whole lot of on-the-job photography of the beautiful places we go looking for goodies.

Were you late to the party and have a link to add? Are you  proto-geoscientist with questions about what it’s like out there? Comments are open!

Martin Bently is hosting Accretionary Wedge #62: Geollowe’en edition, to take advantage of all those costumes, pumpkins, and other geonovelities you spotted around the holiday. Didn’t see anything? Pumpkins are now on sale to create a retroactive jackolantern…

Posted in Geoscience, Practice of Science | Tagged , , , , , , , , , | 1 Comment

Sequence Stratigraphy

Describe the sequences as described by Sloss (1963) and discuss the possible controls on their deposition.

Sequence stratigraphy is a system of linking unconformity-bounded sediment packages to global events to temporally correlate sedimentary units across lithographic boundaries. This is important due to the imperfect stratigraphic record, and the difficulty of correlating time across wide areas. A sequence is a conformable succession of related tracts. Sequences can be divided into parasequences — packages of strata divided by abrupt changes in sea level. By picking the peak of transgression deposition, it is possible to temporally correlate sequences across stratigraphy. It may also be possible to match unconformity cycles in a similar manner to matching magnetic reversal sequences. Sloss and others have hypothesized about repeating, global cycles that can be identified in the sediment record, to varying degrees of confidence.

1st order cycles: Supercycles (200-400 million years)
These long-term cycles are related major tectonic activity, particularly the formation and breakup of super-continents, and the continental distributions leading to icehouse and greenhouse climate conditions. Severe climate change and increased volcanic activity result in mass extinction events. The change in tectonics and climate leads to transgressive or regressive cycles, periods of high and low sea level and changes in the accommodation space for sediment accumulation.

2nd order cycles: Sequence/Synthem (10-100 million years)
These medium-term cycles are related to change in activity in mid-ocean ridge systems, with more active volcanism increasing crust volume (young, hot rocks are more voluminous than old, cold rocks). These are also times of increased intensity of the magnetic field, possibly related to changes in mantle convection or polar wander.

3rd order cycles: Mesotherm (1-10 million years)
Evidence for these shorter term cycles is more hazy; the temporal resolution is smaller than can be determined by biostratigraphy, so these cycles may not have global distribution. If they do, they may be due to crustal flexure and changes in the geoid.

4th order cycles: Cyclotherm (0.2-0.4 million years)
These extremely short-term cycles are likely related to the growth and decay of continental ice sheets and growth and abandonment of deltas, possibly driven by the Milankovich cycle. The observational evidence to support these cycles is uncertain — on such short timeperiods, the cycle may actually be smaller than gaps in the stratigraphic record.

Posted in Geoscience | Tagged , , , | Leave a comment

Geo-Job: On how to be a Geophysicist

I’ve written before about how I love field geophysics, and find the job to be a mix of James Bond villain meets MacGyver. The post has spawned some questions, with emails to me from proto-geophysicists asking how to get from being a student to out in the field. This is a late-entry to the Geo-Jobs Accretionary Wedge #61. (If you’re also running late, I’m still taking entries until 10pm PST tonight, when I assemble the master-post of links.)

Career Trajectory
Most geophysicists start off as a field technicians . The first big promotion is to field geophysicist, crew chief for entire surveys. After a few years of field experience, many geophysicists move on to field processing, or office-work where they interpret the field data. Eventually this leads to some sort of senior geoscientist position, which I have no experience with (yet!).

Entry-Level Jobs
New geophysicists almost always start out as a field technicians. Field technicians are the skilled crew that go with a field geophysicist — the people who help transport, set up, and run the equipment. The job is effectively being an assistant to a field geophysicist. A good crew chief will treat it as an apprenticeship period, and teach their technicians how to field-repair equipment, do data quality control, what to consider when setting up the survey, and other practical aspects of all the theory learned in school. To be a technician does not require a geophysics degree, just willingness and scientific competency. A few courses in introductory geology (to identify the basic rock types), mechanical aptitude (preferably mixed with a bit of hands-on electrical tinkering or labwork), and enough outdoor experience that your crew chief doesn’t need to teach you how not to be eaten by a bear or which leaves make terrible toilet paper are all assets. In many ways, it is similar to being an assistant geologist, but lugging around heavy batteries instead of rocks, and working with electricity instead of hammers.

Geophysics is a controlled profession in Canada, that requires specific academic qualifications. Check with your local professional association or geologic survey to find out your requirements. In my region, the Association of Professional Engineers and Geoscientists – British Columbia is responsible for setting out the requirements of being a practicing geophysicist. In addition to academic requirements, the association also requires a specific period of time working under direct supervision of a fully-licensed geoscientist (experience qualifications), taking a law and ethics course, getting a good character reference, and demonstrated competence in the primary official language of the province.

The value of attending graduate school is more tricky to quantify. A lot of geoscience is fastest and easiest to learn in the field, not at school, so going to graduate school is expensive, time-consuming, and less efficient learning. Even worse, it only counts to a limited degree towards the experience qualification for APEG. For the most part, it appears only consulting companies want graduate degrees for
their employers, and then only for project-management positions. Other geoscience companies and government agencies are less inclined towards graduate degrees even in the asset qualifications of job descriptions, instead preferring more time spent with direct experience. It also appears that companies that do value graduate degrees will be willing to work with their employees to work out a part-time work/study schedule, allowing for full-time studies during the off-season in return for full-time work during the peak field and report seasons.

Geophysics vs. the other field geo-jobs
A field geophysicist typically works more with electronics and heavy equipment in the field, and processes the data through inversion mathematics and noise filtering. It requires a solid understanding of math, wave propagation (mechanical or E&M), and how the physical properties relate to the geological materials. It does not require an in-depth understanding of geology, although it is helpful during interpretation. Most other geoscientists seem to consider geophysics a bit of a black-art voodoo, not understanding how various signals can be interpreted to reveal subsurface structure. The VIEPS (a collation of universities in Victoria, Australia) offers an amazing short-course on the topic (although it’s less awesome when you’re a geophysicist trying to learn geology).

A field geologist typically works directly with the rocks (with heavy backpacks to carry them out of the field), and processes the data through more qualitative means. It requires a strong understanding of geology, formation processes, and chemistry.

A field environmental scientist surveys the current conditions: water flow, trees, wildlife. It is more of an observational science, with some fluid mechanics for hydrology & pollution.

A field geological engineer is less into the great outdoors, and more confined to post-discovery monitoring. (For example, going out to camps and supervising the drilling, or the constructing of the mine.)

Taking a few classes in each discipline will help you understand which ones you like best, as would participating in summer internships or field schools. The VIEPS short-course program is open to honours and graduate students (and professionals), while many universities offer field schools (some even open to students at other universities). Geoscience is a local discipline, where you learn about the rocks where you are, so managing to take a few field adventures in locations far from home can greatly broaden your experiences.

Work-Life Balance
This is tricky. Geophysics is a field profession for the first several years at least. Some companies do regular structured shifts, but geophysics usually seems to be, “Go out until the project is done.” I’ve had jobs that were scheduled for 7 days that lasted 60, which can be a bit rough if someone is waiting at home. The only reason I can have my prickly pet or substantial container garden is because I have someone at home to care for them while I’m away. It isn’t easy to make plans with friends when you’re never certain if a job’s start-date might be moved forward, and one year I was away so often I wondered why I rented an apartment instead of a storage locker.

Sexism is alive and well in the backcountry. I have no real words of wisdom or advice on this except that pink has some practical advantages.

Fieldwork is also hard on the body. Between carrying heavy gear over rough terrain, the inevitable slips and falls, and wear-and-tear on my joints, I can feel pretty creaky. But I’ve also packed heavy backpacks over enough ground under my own power to blow away fitness recommendations and gym-monkeys. “I’m stronger than I look,” becomes a reoccurring refrain for the more slightly-built geophysicist, and scaling the office tower stairs during an elevator malfunction is no longer a daunting task.

As a geophysicist, I’ve travelled to some beautiful places, strolled on British Columbia’s glaciers and explored the countryside of Tanzania. I’ve had some amazing helicopter tours, and had lunch in front of awe-inspiring vistas.

Posted in Geoscience, Practice of Science | Tagged , , , , , , | Leave a comment

Hedgehog Heartbeats

My little hedgehog finds himself carefully quantified as I adore him each night.

Heart rate is roughly proportional to animal size: small creatures have fast heart beats, and large animals have slow ones. An often-quoted relationship with no source I can track down is:

Heart rate [bpm] = 241/⁴√(Body weight [kg])

Pet hedgehogs typically range in weight from 200-1,100 g (although some maybe much fluffier). Converting to kilograms and taking the fourth-root is 0.6-1.02; dividing 241 by the fourth-root is 236-401 beats per minute. This matches up with the occasionally-reported healthy hedgehog heart rate of 180-280 bpm. My hedgehog hovers around 450 g, with a projected heart rate of 294, just under 300 beats per minute, so aspiring speedcore drummers could try to match him.


As for my hedgehog? He’s more inclined to trying out the upright bass.

Posted in Biology | Tagged , , , | Leave a comment

Golden Spike

In a stratigraphic context, what is a “golden spike?” Explain why the choice of the Ludlow Bone Bed in the Welsh Borderland was rejected as the Golden Spike for the Silurian / Devonian boundary. What alternate section was chosen and why was it selected?

Although definitions are not strictly science, being able to communicate clearly is essential to make scientific progress. The Global Boundary Stratotype Section and Points (GSSP) are human-selected strata sections to mark boundaries between major time periods, usually defined by the appearance or disappearance of organisms. Like type sections used to define formal units in the stratigraphic code, the GSSP should be the best reference section available, a surface section with visible top and bottom contacts, and be complete and continuous with no faults and no long covered intervals. The exposure representing a period boundary needs to cover a range of facies and rich in fossils, to allow correlations between different types of fossils. It should be accessible, and protected (or at least unlikely to be destroyed) for future research.

The official boundary between the Silurian and Devonian is marked by the first occurrence of graptolite. Graptolites are pelagic creatures, so have a global distribution in marine sediments. Sections in the Czech Republic, Australia, Italy, Poland, Spain, Nevada, Quebec, Algiers, and Morocco were considered, with outcrops in the former Soviet Union, Morocco, Nevada, and the Czech Republic as the final candidates. The Golden Spike is the final selected outcrop near Klonk in the Czech Republic, a section with an uninterrupted succession of fossils, facies changes, and extensive exposure.

Posted in Geoscience | Tagged , , , , , , , | Leave a comment

Occupational Health Hazards in Mining

This is the first-ever Guest Post on GeoMika, a request that forced me to invent a Guest Post Policy! Thank you to Megan Clark, a remote researcher from University of Queensland, Brisbane, Australia, for her writeup of one of the most insidious hazards in mine-work: inhaling dust that kills you slowly.

In spite of being very financially rewarding, mining has never been a risk-free environment and that is a known fact. Miners expose themselves to a wide array of occupational health hazards on a constant basis, and while some hazards are related to airborne particulates such as free silica or respirable mine dust, other occupational health hazards involve ionizing radiation, oxygen deficiency or prolonged exposure to noise.

An Overview of the Dangers Posed By Airborne Particles
There are three main types of airborne hazards miners expose themselves at when descending into the mine: the naturally occurring gases (which are often the cause of fatal explosions), chemical vapors and particles resulting from natural compounds present in the earth’s crust such as coal or silica. Mining involves drilling in the earth’s crust until the miners reach the desired metal or compound: during the drilling process, the workers are exposed to silica particles and coal dust. Despite the fact that nobody can totally prevent airborne particulates, the amount of respirable compounds can be dramatically reduced, thus decreasing the risk of respiratory conditions and other diseases like Pneumoconiosis.

Silicosis – One of the Most Common Occupational Diseases

It's not dust, it's silica

It’s not dust, it’s silica. Image credit: OSHA

Also referred to as the Miner’s Phthisis, silicosis is a form of occupational lung disease that is triggered by the prolonged inhalation of silica dust. The most common signs and symptoms of silicosis are the inflammation and scarring of the lungs – this is a form of pneumoconiosis that usually takes years to develop. In addition to inflammation, some other typical signs include cyanosis, cough, fever as well as shortness of breath. This condition is very common amongst miners who use pneumatic hammer drills and sandblasting, and if the disease is left untreated it can lead to the death of the patient within years or even months.

X-ray of silicosis damage.

X-ray of silicosis damage. Image credit: OSHA

On the other hand, the prolonged exposure to silica dust can also increase the risk for lung cancer and various autoimmune diseases. Fortunately, the number of miners suffering from silicosis has decreased over the past few decades, after dust control systems have been implemented in the mining environments: a fine water mist is sprayed after sandblasting, so that the silica and coal dust particles are “trapped” inside the water molecules, therefore preventing miners from inhaling them.

The Effects of Respirable Coal Dust

Black lung.

Black Lung. Image credit: UMWA

Coal dust is just as dangerous as the silica dust, and it occurs in most coal-processing facilities (especially in the mining/coal extraction sectors). Just like it happens with silica particles, coal dust occurs after drilling or blasting techniques. Cutting machines are also known to stimulate the production of coal dust which triggers Coal Workers’ Pneumoconiosis, a condition that is strongly related to other respiratory conditions, such as emphysema or chronic bronchitis. Fortunately, the negative effects of modern drilling techniques can be reduces by avoiding the dispersion of the dust or by using regular water sprays and efficient ventilation techniques.

What is Asbestosis?
Asbestosis is a chronic inflammatory condition that affects the lungs, and it occurs after the inhalation and retention of asbestos fibers. Until the use of asbestos was discontinued back in the 1970s due to the high threats it involved, it was not uncommon for miners to develop asbestosis after long-term exposure to this compounds. As a matter of fact, this was one of the most common occupational lung diseases, and it was often accompanied by dyspnea and lung cancer. What makes asbestos so dangerous is the fact that the fibers are virtually invisible and they easily cling to the lining of the lungs, thus affecting the internal tissue of the miners’ respiratory system. As an airborne disease, asbestosis is also considered to be a severe form of pneumoconiosis, along with silicosis.

Last, but not least, the gases and vapors that are eliminated by drilling and blasting machines (which usually have diesel engines) can increase the amount of carbon monoxide and increase the risk for oxygen deficiency, one of the most dangerous problems that can occur in mines. As you may already know, diesel particulates are small and respirable and they are carcinogenic, meaning that they can increase the long-term risk for lung cancer in miners who are constantly exposed to them. The amount of dangerous and respirable diesel particulates can be significantly reduced by replacing traditional fuel with high-quality fuel that is low in sulfur, and by using modern engines that are especially designed to reduce the generation of particles.

To conclude, the occupational hazards mentioned above are only a few of the most common and dangerous risks miners have to risk. Fortunately, the modern mining environments are not as dangerous as they once used to be, in terms of both mine structure and miners’ health.

Note from Mika: Want to learn more? The 4-day Mine Safety & Environmental Engineering course at the University of Ballarat is a great high-density chunk of information with a delightful professor. It is part of the VIEPS shortcourse program open to honours undergraduate and graduate students enrolled in a university in Victoria, Australia.

Posted in Geoscience | Tagged , , , , , , , , , , | Leave a comment