Physical Properties of Geologic Materials

As Sedimentation Saturday was a lovely way of teaching myself more sedimentation and stratigraphy than I had previously known, for 2014, I’m taking the same route to look at the most fundamental concept in geophysics: the physical properties of geological materials. Being able to invert measurements of physical properties back to the distribution and materials most likely to produce those measurements is the key characteristic of geophysics, making it a science instead of the voodoo-magic some other geo-disciplines occasionally accuse geophysics of being.

Each month will cover a different physical physical property. The weekly updates will be on what that physical property actually is, how it relates to geological materials, and how it is measured in isolation or in situ. I’m leaving out the references; for further reading I highly recommend the ATSM International standards, and Field Geophysics by Milsom (any edition).

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Introductory Geology: setting a syllabus

Usually when I teach, I’m teaching an established class with a set curriculum. I’ve been part of team-taught or multi-term courses where we coordinate across topics, and I’ve taught APEG-mandatory courses designed to fulfill particular academic requirements for professional accreditation. I’ve even taught general-education courses that have a subtle recruitment objective, trying to lure students into geoscience degrees.

But this term, I get to do something new.
This term, I’m teaching an introductory geoscience course to students who would need to transfer to a different university to take more science courses. I’m solo-teaching with stand-alone content, with the challenge of sharing topics in geoscience that will be most relevant to their future academics and careers. I have explicit permission to design the syllabus in whatever manner I feel is most effective, covering the topics I choose instead of trudging through some preset list of “Everyone needs to memorize the Barrovian metamorphic rock sequence” (all together now, “shale, slate, phyllite, schist, gneiss, migmatite!”). It’s an absolutely heady experience, so full of potential and possibilities that it is almost overwhelming.

I’ve clearly been infected by the Carl Wieman Science Education Initiative in that my first inclination is to create a list of learning goals, the ideas that I want students to hold on to long after the course ends. I want them to use what they learn in their jobs and in their daily lives: to identify when they need to hire a geoscientist, to support fact-based public policy decisions in their communities, and to be able to access our special way of looking at the world around them. My first-draft learning goals are:

  1. Be able to tell the story of a rock or a landscape (formation, transportation, history);
  2. Identify local natural hazards, interpret forecasts, and perform appropriate personal/community mitigation;
  3. Apply geoscience knowledge to assist in their chosen careers;
  4. Participate in public policy debates related to geoscience (climate change, frakking, pipelines, mining development, hazard mitigation…); and
  5. Use fact-based reasoning in decision-making related to geoscience topics.

I suspect my academic obsession leaves me hugely biased towards disaster mitigation, although it’s hopefully balanced by my professional experience in the resource industry. However, with entire realms of geoscience I rarely touch on (I’m looking at you, geochemists and hydrologists…), I fear I’m neglecting key topics. If you had just one term with no hope of follow-up or later courses to teach geoscience to non-scientists, what would you absolutely need to teach?

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K-Pg Extinction Event: Theories

The K-Pg boundary extinction event has several distinct characteristics that can be used to differentiate between possible causes.

Supernova

Cosmic radiation from a nearby supernova could be responsible for the mass extinctions and the iridium layer. However, the boundary should also contain a plutonium isotope which has not been found during sedimentary analysis.

Gradual Extinction

The least catastrophic scenario for extinction is a gradual species loss brought about by a change in the physical environment from the Cretaceous to the Paleogene. Long term cooling, major marine regression, and acid rain associated with volcanic activity were all proposed as contributors to a gradual species loss over a million-year span. Scant fossil records of dinosaurs in Europe leading up to the K-Pg boundary could be indicative of a gradual die-off, or merely of fossil loss from a spotty stratigraphic record. In contrast, shallow marine sections in Denmark demonstrate a large, diverse population of marine life up until the K-Pg boundary, followed by an abrupt reduction in quantity and diversity of species, gradually replaced by a new large and diverse population. Other authors insist that species abundance populations were dropping for the last 100,000 years prior to the K-Pg boundary, with the trigger event killing species already on the brink of gradual extinction.

Chemostratigraphy

Low strontium-calcium ratios (Sr}/Ca) in microfossil shells, and low potasium in clay mineraology leading up to the K-Pg boundary in Tunisia indicates a warm, humid climate with rising sea level. Immediately at the boundary, data suggests a maximum flooding event in an abruptly cooler and more arid climate, with decreased biotic productivity and an iridium anomaly. After the boundary event, oxygen isotope ratios suggest cooler temperatures, while low strontium-calcium ratios are indicative of high humidity and high sea level, with gradual drying over time.

Volcanic Eruption

A major eruptive sequence, like the flood basalts that formed the Deccan Traps in west-central India, could produce enough toxic outgas to destabilize climate conditions, leading to mass extinctions.

Chronostratigraphy

The age of the Deccan Traps can be determined through magnetostratigraphy and chronostratigraphy. The Deccan volcanic province locks in the magnetic reversal polarity epoch chron 29R, which encompasses the K-Pg boundary. Potassium-Argon (K-Ar) dates for whole-rock basalt samples in different sections of the traps fall in the range of 55-65 million years ago, but out of stratigraphic sequence which suggests post-crystallization loss of 40Ar* for some samples. Data from the low to intermediate temperature argon isotope (39Ar/40Ar) incremental heating studies are characteristic of fine-grained basalts exhibiting post-crystallization loss of 40Ar* and 39Ar recoil loss from internal redistribution or out of the sample entirely, producing ages greater than 70 million years old. Using the potassium data to constrain the higher-temperature argon isotope data produces ages of 64-66 million years old. Later studies confined the bulk of volcanism (~80%, 3.5 kilometers of lava flows) erupted in less than 800,000 years between 64.8-65.6 million years ago.

Biostratigraphy

Quarry outcrops in the Krishna-Godavari Basin in India contain two Deccan basalt flows (the Rajahmundry traps). The sediments directly on top of the lower flow contain Danian planktic foraminifera, the key fossil marking the K-Pg boundary, while the upper flow is deposited during chron 29N, before life bounced back from the mass extinction event.

Chemostratigraphy

The Takli interrappen sediments are a 2-meter thick shallow continental deposit from around the K-Pg boundary event in the Deccan province. The sediments are composed of lava ash, clay, and marl with Cretaceous fossils, all underlain and overlain by lava flows from chron 29R. Chemical analysis of these sediments indicate rare-earth elements concentrations similar to that of other basalts, with a few ash horizons with 2-5 times the base level concentration. The maximum observed iridium peak is two orders of magnitude smaller than the iridium peak observed in marine sediments a few hundred kilometers away in the Um Sohryngkew River section in Meghalaya, India, suggesting that the Deccan Traps are unlikely to have been the source of the iridium peak associated with the K-Pg boundary. However, fine volcanic ash from the Melbourne volcanic province recovered in Antarctica contained elevated iridium concentrations, suggesting it is plausible that some volcanic ash may be responsible for iridium anomalies.

Samples from the non-marine Raton Basin (Starkville South, Colorado, and Raton Pass, New Mexico) and marine Stevns Klint, Denmark contained two rare amino acids peaking at the K-Pg boundary, but present above and below it. The samples are otherwise typical of modern amino acid distribution. The samples specifically lack the amino acid distribution characteristic of the Murchison meteorite impact event. This suggests the exotic amino acids are a result of high-temperature hydrolysis of coal.

Computer Models

The atmospheric impact of SO2 released by volcanism over the formation of the Deccan Traps would produce an impact on climate conditions similar to a single asteroid impact. A sequence of such pulses over the period of formation would produce a runaway impact on climate that cannot be replicated by a single impact.

Extraterrestrial Impact

An asteroid impact would produce a massive crater (like the Chixulub Crater in the Yucatan Peninsula, Mexico), throwing ejecta and triggering a firestorm. The combination of direct trauma, indirect reduction in daylight, acid rain, and collapse of the food chain would result in an abrupt extinction event. A cometary impact would have similar results as an asteroid impact, but less severe due to the lower density of material involved, and spread over a longer time as the impact of the main nucleolus would be followed by the impact of much smaller cometary debris. The result would be a rapid extinction event, but less abrupt than the extinction event expected by an asteroid impact.

Chemostratigraphy

The SM-4 Sumbar river site in Turkmenistan\footnote{Formerly Turkmenia, USSR.} is an exposure of marine clay spanning the K-Pg boundary. The samples consist of detrital carbonaceous shale, and either a land plant or phytoplankton. Iridium and shocked quartz rise sharply at the boundary then quickly decline. Soot and charcoal rises sharply at the boundary, then continues to rise, suggesting fire started before the basal layer of ejecta fully settled, and continued to burn.

Boundary clay at geographically diverse locations including Denmark, New Zealand, the North Central Pacific Ocean, and Turkmenistan all share extremely similar chemical composition, which differs both chemically and mineralogically from proceeding and following clays in the strata. This suggests the clay is derived from a single unique source material: impact glass from a single-event impact.

Computer Models

A 10-kilometer diameter asteroid impacting the Earth would produce a crater of appropriate scale to the Chixulub Crater. The impact would release large amounts of water, dust, and climate-forcing gases — far more than would be released by large-scale volcanics. Flow models of atmospheric reentry of ejecta could cause pulses of thermal radiation, causing damage without triggering extensive fires. Detailed models of ejecta paths cannot provide the observed global distribution through purely ballistic motion; some of the ejecta must have been transported by atmospheric redistribution.

Multiple Impacts

A series of small impacts would produce local ejecta plumes instead of a global blanket of sediments; the uniformity of chemostratigraphy of marine clays thus suggest a single-impact scenario. However, the stratigraphic record is incomplete, with shallow marine sediments prone to erosion during transgression, and deep-marine stratigraphy condensed by low sedimentation rates.

Conversely, careful analysis of biostratigraphy with respect to impact glass ejecta with respect to the foraminifera that marks the K-Pg boundary indicates multiple impact events within 400,000 years. In addition to the Chixulub Crater in the Yucatan Peninsula, Mexico (120 kilometer diameter, 65.0-65.4 million years old), two other impact craters date to near the K-Pg boundary: the substantially smaller Boltysh crater in the Ukraine (24 kilometer diameter, 64.6-65.8 million years old) and the Silverpit crater in the North Sea (12 kilometer diameter, approximately 65 million years old). The Shiva structure west of India is also a potential crater of appropriate age (450-600 kilometer diameter, 66 million years old), and may have triggered the Deccan Trap eruptions. While only the Chixulub Crater and the potential Shiva structure are large enough to have created a global ejecta layer, the smaller impacts could have destabilized an already unsteady climate, enhancing extinction rates.

So, which is it?

The K-Pg boundary is a busy time, where climate change, low sea level, mass extinction, iridium anomaly, Chicxulub (and potentially Shiva) impact, and Deccan volcanism occurred within a few hundred thousand years. This is too fine of a time sequence for events to be distinguished with radiometric dates, so the law of superposition is the only way to determine the sequence of events, and untangle cause and effect. Unfortunately, the spotty stratigraphic record and potential sedimentary reworking over time make even that a fallible method. Attempts to declare the Chicxulub as the only cause of the K-Pg extinction event was immediately contradicted in letters to the same journal, which were rebutted by the authors of the original article. Thus, the debate as to how much each event contributed to mass extinctions between the Cretaceous and the Paleogene rages on to varying degrees of politeness as ever more data is analyzed and added to the puzzle.

My current backend doesn’t support in-line citation very well. Please see the bibliography for all papers used to research this topic.

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K-Pg Extinction Event: Boundary Characteristics

The end of the Cretaceous period and the start of the Paleogene period 65.5 million years ago is marked by a global extinction event at the K-Pg boundary. The Paleogene was previously part of the Tertiary before the term was deprecated; the extinction event is thus sometimes referred to as the K-T boundary event. Principles from sedimentology and stratigraphy can be used to deduce the nature and extent of event or events that triggered this extinction.

Global Stratotype Section and Point

The official stratigraphic section to mark the K-Pg boundary is at El Kef, Tunisia. This section contains a continuous sedimentary record with excellent microfossils, clear geochemical and mineralogical marker horizons of the boundary, and no discontinuities or breaks across the boundary. The unit directly before the boundary (the Upper Maastrichtian) is uniform dark grey marls, marked by the extinction of all tropical and subtropical species. The boundary is a 2 millimeter thick dark red layer high in iridium, interpreted as impact ejecta (defined by the first presence of Danian planktic foraminifera). Above this is 50 centimeters of dark, organic rich material marking a crash in plankton populations. Unfortunately, the section is currently suffering from oversampling and agricultural encroachment, so alternate sections are being examined as possible replacements.

Biostratigraphy

The Cretaceous ended with the extinction of the dinosaurs, along with many other species of land-based plants and animals. Marine species including calcareous plankton and tropical invertebrates also abruptly go extinct at the K-Pg boundary. The boundary is officially marked by the first arrival of Danian planktic foraminifera. The boundary is also marked by a sharp increase in disaster opportunist survivors, and freshwater species bouncing back within a decade of the K-Pg event.

Sedimentology

The K-Pg boundary is commonly marked by the presence of shocked quartz. This is commonly thought of as an impact mineral, but it could also be the result of explosive interaction between a hot mantle plume and silicic crustal material. Along with shocked quartz, altered droplets have been recovered in Haiti, and in deep marine sediments off the coast of North Carolina. These droplets are typically interpreted as impact glass, as similar sediments produced by volcanic fire-fountaining are localized to the site of formation.

By the same fluid kinematics of turbidite flows, the settling time for either volcanic ash or impact ejecta out of the atmosphere is weeks to months, with anticipated identical sedimentation patterns for a given particle size and water depth. Both impacts and eruptions could plausibly lead to global cooling, extensive wild-fires, and airborne particles blocking sunlight (leading to a reduction of photosynthesis) and triggering acid rain. Soot associated with the boundary may be from global fires, or reworked influx from other events.

Chemostratigraphy

Boundary clay at geographically diverse locations including Denmark, New Zealand, the North Central Pacific Ocean, and Turkmenistan all share extremely similar chemical composition, which differs both chemically and mineralogically from proceeding and following clays in the strata. The clays contain shocked quartz, in a higher-than-average iridium, siderophile, and platinum-group elements. All the listed chemicals are are typically depleted on the Earth’s crust. The source of a peak in these minerals could either be from the impact of an extraterrestrial body or excessive volcanic activity, as asteroids, comets, and the Earth’s mantle are all rich in these elements.

A two-million year spike in seawater strontium is also associated with the boundary. The long residence time of strontium in seawater could be a response to a quasi-instantaneous input of anomalous strontium from acid rain, acidic volcanism similar to that observed in the Seychelles Islands, or from longer-term sea level regression. However, due to the gradual increase in the strontium ratio over extremely long timescales, and the difficulty in resolving times with strontium, the evidence for this spike is somewhat hazy.

Several marine sections have been identified around the world. The K-Pg boundary is marked by a 1.5 centimetre layer of limonitic clay within a 3 metre column of shales, sandy clays, and limestones indicative of a shallow marine environment. The shales below the boundary (from the Cretaceous period) are ash grey, while those above the boundary (from the Paleocene) are brownish grey.

What does this evidence suggest for possible causes of the K-Pg boundary and extinction event?

My current backend doesn’t support in-line citation very well. Please see the bibliography for all papers used to research this topic.

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Horizontal Correlation

Why is it difficult to correlate time horizons over wide areas using lithostratigraphic units and how can fossils help resolve this problem?

The best time-markers in stratigraphy are widespread, distinct, and geologically instantaneous. Rocks that form in moments, years, or decades are effectively instantaneously, like volcanic deposits, single catastrophic slides or flows, or calm, stable lakes with a uniform rain of suspended particles. Geochemistry of stable elements, radiometric dating, magnetic reversals, isotope shifts, and iridium anomalies may all be used to pin down dates.

Horizontal correlation is difficult in stratigraphy because sediments are not uniformly laid down in homogeneous, synchronous global beds of layer-cake geology. Instead, most lithographic units have limited lateral extent, pinching out or grading into other lithographies. The combination of limited outcrop exposure, rocks lost to melting or erosion, and difficulty with determining past facies relationships limits the utility of even suitable lithographies for large-scale temporal analysis. Instead, fossils can provide cross-lithographic clues to the time of formation.

Biostratigraphy is using fossil evidence to establish time of deposition for the surrounding strata. Evolution and paleoecology result in a change over time in a particular organism, with limited range to particular facies. The result is that evolving creatures enable determining if a fossil and the surrounding material is from a particular time period, while simultaneously limiting the locations where fossils are found and preserved. The combination of evolution, extinction, immigration, and emigration provide information, but are clouded by the noise of failure to preserve (lithographic limitations), destruction (through erosion or metamorphic destruction), be inaccessible or missed during evolution. A facies change can also lead to local extinction, another source of noise.

In order for fossils to be useful for dating, they need to be abundant, distinctive, facies-independent, and rapidly changing so that a species is only present for a short-ranging period of time. The best index fossils are rapidly evolving pelagic organisms, where distribution is unaffected by sea bottom facies. Looking at the first and last appearance of a creature, overlaps of unrelated taxa, or succession of related taxa all provide additional data. Some fauna also record particular climatic information, like plankton coiling right or left depending on the temperature.

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Montreal Massacre

24 years ago, a gunman targeted female engineers, murdering them for daring to get an education.

I am a female scientist working in Canada. This year, I spent a lot of time writing or speaking about science in public, sharing my love for its mysteries. Most days, most jobs, most places, most coworkers, most audiences are good. But not all are.

This is a day of action working towards a future where violence against women daring to be human is incomprehensible. This is a day to renew a vow to speak out when witnessing something that is wrong, to intervene, and to affirm that insidious sexism is worth fighting against. This is a day to work for a future where I don’t warn the proto-scientists I mentor to brace themselves to deal with inappropriate behaviour when just trying to do their jobs.

It’s a day of vigil, and a vow. It’s foolish, and pointless, and wrong to hate half the planet. The world can be better than this.

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Beer at Conferences

Over on Twitter, Erik started a conversation about the practice of the major geoscience societies (AGU and GSA in the US) to provide free beer during the poster sessions:


It’s turned into a substantial conversation, where complex mixed-thoughts are trying to squeeze into 140 characters.

Edited for clearer context: I haven’t attended GSA, but at AGU, the alcoholic beverages are linked to a drink ticket and non-alcoholic beverages are unlimited. Each attendee receives one drink ticket per event, although it is not atypical for non-drinking or non-attending attendees to hand off their tickets to someone else (particularly to students), enabling multiple drinks. At some geoscience meetings, students or other low-cost memberships do not receive tickets. At some events (usually evening receptions), the drinks are provided via a cash bar, so it is possible to purchase additional drinks after the ticket/complementary drink.

Cian is concerned by the normalization of alcohol in geoscience culture:


As a TA at field school, I made $200 just in recycling the empties of what my students consumed in a week. And yet it was a self-correcting problem: fieldwork doesn’t have days off. If they drank too much and woke up at dawn with a hangover, all it earned them was the right to tromp around in the rain while their eyeballs pounded, and try not to puke while measuring strikes and dips.


Over-drinking at AGU doesn’t have such an instantaneous feedback mechanism of the consequences of terrible ideas (skip the morning sessions to sleep it off), but being drunk in a professional setting certainly does. The social norms of geoscience giddily embrace the Field Assistant Beer for GSA’s 125th anniversary, but who would hire a field assistant who was drunk or hungover? Ours is an active profession, where phrases like, “Must be able to hike long distances over rough terrain in all weather while carrying heavy loads” is a perfectly normal sentence in a job description. Being unaware or uncoordinated in the field damages data quality, increases likelihood of injury, and endangers field crews. Dry camps exist for a reason.

Alan commented the presence of beer as an acknowledgement that the conference attendees are (mostly) adults, and are professionals capable of consuming an intoxicant responsibly. The key aspect of that is an assumption of professionalism:


Conference-goers are adults, and adults sometimes drink, but it’s still within a professional context. Although some people will abuse the presence of free alcohol, those are people I want to learn that about now, in the safety of a gigantic poster hall where it is easy to abandon them in the crowd. I don’t want to learn it after I’ve spent time building a professional relationship and am working with them in an isolated field site. Alcohol abuse happens. I don’t know if its rate is higher or lower in exploration field camps — even some dry camps should more practically be considered damp — but the problem of what to do with someone who cannot regulate their consumption is certainly a lot more problematic when in the field. The filter works in all directions of authority: someone who is incapable of executing the good judgement and self control to drink responsibly in a professional setting is not someone I want to work for, work with, or have work for me. Anyone who manages to get unprofessionally drunk in a large, cold room with ridiculously tall ceilings and bright lights while surrounded by potential employers and employees has a lack of good judgement, and learning it before I even remember their name saves me a major problem down the line.

Finally, the poster sessions don’t just serve beer. As Eric corrects:


Although I’ve never felt any pressure to drink, the long lines, quickly emptying kegs, and alternative beverage options all provide easy excuses for why one isn’t drinking. By bringing beer into the posters halls, AGU successfully opens up the range of people who will stay and participate:


Some people are going to drink an afternoon beer. By providing it during the afternoon posters, the AGU captures those people and keeps them engaged in the event. Instead of people hiding away in private conversations tucked away at a pub, those conversations are happening in a brightly-lit conference hall. This makes them accessible to undergraduates who are primarily under the drinking age of 21. The other beverage options make the event inclusive of non- or light-drinkers, or the un- and under-employed on limited budgets who would feel pressured to make over-priced purchases at a pub. And it reduces the chances of awkward moments when someone in a conversation thinks that heading off to a poorly-lit alcohol-centric social venue meant that the relationship wasn’t so professional anymore (and keeps around a large crowd of people and a very present staff to intervene in a rescue if alcohol-alone is enough to create that situation).

Beverages creates an excuse to loiter and engage in less-directed conversations, while posters provide starting-points to initiate a conversation when networking with strangers. That the choice of beverage options includes an intoxicant might test some people’s professionalism, but it also takes networking opportunities out of private pubs and into the conference venue.

Edited to add: The best bit about the free beer, coffee, tea, and everything else on offer? You don’t need to drink any of it, and you’ll still be a geoscientist.

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Hedgehog Running Pace

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

Pet hedgehogs run a lot. Some people install odometers on their wheels to track how long and how far they run each night. Informal surveys of on hedgehog forums suggest the marathoners of the hedgehog world run up to 12 miles (20 km) per night.

Ichnologist Tony Martin told me that for quadrupedal mammals, walking pace stride length is 1.1 to 1.25 times the acetabular distance (length from hip to shoulder). Acetabular distance varies hedgehog, but is somewhere around 5-10cm. Dividing the total distance run by the stride length produces how many pitter-pattering little footsteps a hedgehog manages in a night:

footsteps = odometer distance/(1.2 x acetabular distance)

Hedgehogs are marathon runners.

Hedgehogs are marathon runners.

My tiny friend ran 6.7 kilometers in 2.75 hours last week. The distance from his hip to shoulder is 7 centimeters, so after converting units that’s (6700 m / 1.25 x 0.07 m) = 76,571 footsteps.

For a human, it’s about 2,000 footsteps per mile, so 1,250 footsteps per kilometer. If I were to match my hedgehog step-for-step, I’d need to run just under 61 kilometers.

Marathons are 42.195 kilometers (26.22 miles) long, with the current world records set at 2 hours, 3 minutes for men or 2 hours, 15 minutes for women. Scaled for size, my hedgehog ran just under 1.5 marathons, and for a single marathon, crossed the line at 1 hour, 54 minutes, a record-holding pace.

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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.

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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.

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