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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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.

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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.

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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…

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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.

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