Geotechnical Properties

Some physical properties of geologic materials are important in geotechnical engineering, but are not useful for geophysical interpretation. These properties are the dry strength, dilatancy plasticity, and toughness of the material. All four properties may be easily quantitatively categorized in the field by handling the materials.

Dry Strength

Dry strength is how strong the material is when it is dry. The dry strength is categorized in the field by the engineer modelling a small ball, adding water if necessary until the material has the consistency of putty, then applying finger pressure. If the ball crumbles with the pressure of handling, it has no dry strength. Categories progress through low, medium, and high, with very high dry strength indicating a material that cannot be broken with the pressure applied by squeezing the sample between a thumb and a hard surface.

Dilatancy

Dilatancy is how the volume of a cohesion less soil will expand under loading or shear deformation. The dilatancy of a sample can be qualitatively categorized in the field using basic tools. The field technician will need to mould the specimen into a small ball, adding water until it is soft, then smoothing the surface with a blade or spatula. Then, he or she will shake the ball horizontally, striking the side of one hand against the other, and observing how quickly water appears on the surface of the ball. Then the engineer squeezes the balled sample by closing his or her hand, or by pinching the material, and observing if and how quickly the water disappears back into the ball. The dilatancy is then categorized as none, slow, or rapid depending on how quickly the water appears and disappears when the ball is manipulated.

Plasticity

Plasticity is how far a material may be deformed under constant stress, without cracking or dilatancy. The plastic limit can be determined by rolling the material into a thread, then folding and re-rolling the material until the thread crumbles. After reaching the plastic limit (when the thread crumbles), the field technician kneads the material into a lump, continuing to knead until the material crumbles. The plasticity is qualitatively categorized by how the material behaves with more or less water than the plastic limit, particularly how long it must be rolled to form a thread, then how a lump of the material behaves as it dries out.

Toughness

Toughness is the ratio of the plasticity index to the flow index. It is tested concurrently to the plasticity test. Qualitatively, it is categorized into low, medium, and high by how much pressure is required to rolling the material into a thread and kneading it into a lump during the plasticity test, and the stiffness of the thread and lump.

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

Detecting Physical Properties (Fieldwork)

Geophysical field surveys are used to investigate the subsurface. Geophysics may utilize passive techniques, measuring and mapping changes in the ambient field, or as active techniques that measures how the geological environment responds to an input signal. In either instance, the geophysical signal is processed and interpreted to indicate particular physical properties of geological materials. Those physical properties are used to determine a geological interpretation for subsurface materials and structures that would plausibly result in the observed geophysical signals.

The advantage of geophysics surveying is that passive techniques are non-destructive, that measurements are taken close (or at) the area of interest, and that an appropriate survey can provide a wealth of information about the subsurface in a cost-effective manner. The disadvantage of field surveying is that active techniques can be destructive (depending on the type of source providing an active signal), that environmental conditions (weather, access, injury) can increase survey time beyond estimates, and that in-situ properties do not always match properties determined in controlled laboratory settings.

Passive techniques include any survey where sensors measure the ambient field. Examples include gravity and magnetic methods, and some resistivity techniques. Active techniques include any survey where a source provides an active signal, and sensors measure how that signal is altered by the environment. Examples include induced polarization and artificial-source seismic techniques.

Borehole Logging

Boreholes are a circular cross-section made in soil or rock, that is either cased with piping or left bare. A borehole log is the record of the depth, geologic units, sample recovery (if any), water level, and any other significant facts related to the drilling including geophysical data collected through borehole sondes. A sonde is a long, tubular object containing instruments that is attached to an armoured cable, lowered into the drill hole with a pulley and winch system. The sonde is lowered to depth, then data is collected during a slow pull up the hole.

The advantages of a borehole logging with geophysical probes is that we can obtain in-situ measurements close to the target area, can use core or drill chipping to guide geological interpretation of the geophysical data, and, for some types of surveys, obtain measurements for the undisturbed surface away from the borehole. The disadvantages of borehole logging is that drilling is expensive (particularly depending on the location, subsurface characteristics, depth, and if core is preserved), destructive, and may induce fracturing or breach impermeable layers spreading contamination plumes.

A sonde may contain one or more geophysical systems, most commonly sensors to measure resistivity, acoustic velocity, ground penetrating radar, spontaneous potential, neutron-neutron, natural gamma, gamma-gamma, temperature, fluid flow, gravity, or magnetic properties. A survey may consist of in individual hole in isolation, or a cross hole survey with the source in one hole and receivers in another.

Data extracted from geophysical borehole probes is interpreted in conduction with other information from the site. Two additional borehole probes are commonly used in conjunction with the geophysical sondes are calipers (or callipers for those across the ocean), providing a record of borehole diameter, and an optical televiewer, creating an oriented digital image of the borehole wall. Additional information on site conditions, elevation, diameter and angle of hole, material used during drilling or casing, observations on the extracted core or chip pings, and the people involved in the project are all part of standard field logs that can aid geophysical interpretation.

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

Characterizing Physical Properties (Labwork)

Measurements are made to determine the physical characteristics of geological materials. This is done by placing a homogeneous sample in specialized equipment designed to measure that particular property under either controlled or in-situ conditions.

Extracting an intact sample is essential for testing of geological materials in controlled conditions.
The material – rock or soil – is obtained using a set standard of care intended to preserve the in- situ properties (such as structure, water content, or density) that are relevant to the particular tests. This is frequently accomplished through some variety of core drilling machine such as that manufactured by The Testwel Instruments Company.

The advantages of laboratory testing is the ability to measure the particular characteristics of the sample uncontaminated by ambient noise. The disadvantage of laboratory testing is that some physical properties change when removed from their in-situ environment, and extracting a sample is destructive. Particular care must be taken when determining the correct sample dimensions for size-dependent mechanical properties, like strength, where the specimen size strongly influences the characteristic.

Examples of physical characteristics that are measured in a laboratory setting, and the machines used to make those measurements Matest Hoek cells used to measure permeability, the Barrington Instruments MS3 for measuring magnetic susceptibility, and triaxial cells for measuring volumetric elasticity to determine the bulk modulus of a material.

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

How Physical Properties relate to Geological Materials

The physical properties of geological materials are what link geophysical data gathered at the surface to interpretation of subsurface geology. These properties are determined by the microscopic and macroscopic characteristics of the materials and their environments.

Measurements of physical characteristics are taken in controlled laboratory settings, or on site in the field under in-situ conditions. Controlled measurements in laboratory may be complicated by lack of homogeneity in samples, or by changes in physical properties caused by isolating the sample from their environment. On site measurements by their nature incorporate more of the in-situ environmental conditions that would impact a geophysical field survey data, but it is then more difficult to isolate environmental factors that may alter the measurements at one site versus another. As a result, the measured physical properties of a geological material may vary greatly between particular samples, and the environmental conditions of the sample while it is being tested.

Some materials demonstrate heterogeneity, anisotropy, or the physical characteristics inherently dependent upon environmental factors such as saturation. Thus, even in ideal conditions, particular environmental conditions may substantially alter the measured property. Thus, the physical proper- ties of geological materials are more accurately a range of values most commonly associated with a specific material under specific conditions.

In addition to the physical properties themselves demonstrating variability, our ability to measure those properties is also limited. Measurement limitations in the lab or in the field are anything that reduce the ability to quantify a sample, including resolution, sensitivity, precision, bias, repeatability, reproducibility, and uncertainty. Bias in measurements may be quantified by inter-laboratory or round robin testing programs of materials with a known value, but cannot be quantified in materials without accepted reference values. ASTM International1 establishes and maintains a series of standards for consistent practices to take measurements of physical properties in order to increase the repeatability of the tests, and to reduce the impact of variation from human factors in the process.

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

Peer Review

The first time I was asked to peer-review an article, I was intimidated. How could I possibly cover all aspects of confirming if research was sound and ready to enter the academic literature?

To start, as with many situations, I called on friends for advice. While I’ve gone on a career path that mixes industry with teaching but doesn’t continue up the academic ladder, members of my various cohorts over the years are now snuggling into post-doc and even professorships, and reviewing papers is a basic part of their jobs. It doesn’t matter that they aren’t all in the same discipline as me — although the evolution of culture and landslides don’t share much overlap, reviewing papers in psychology and geoscience follow the same techniques applied to different fields.

The advice I was given is solid:

1. Why are you being asked to review the article? Are you a specialist in a particular model (for me, DAN-W and DAN3D), a technique, or a theory? Sometimes the editor will tell you directly, other times you need to guess. Evaluate that part particularly strongly, particularly checking if they applied the model correctly, did their math properly, or interpreted a theory consistently.

2. What is it for? A conference paper can reviewed more gently than an article for a prestigious journal.

3. By reading this paper, could you replicate the procedures or processes? Do you have confidence you understand exactly what they did and how they measured it? Is the process sound, or do they use an inappropriate combination (for example, using a container that is known to leach into the sample, or using the wrong model for the type of event)?

4. Would you cite this paper? Why not?

5. Copyediting is a very minor aspect of reviewing, but can be important when correcting the usage of technical vocabulary, especially for papers where the author is not writing in their native language. Be careful not to confuse style preferences with grammatical issues, but feel free to suggest alternate phrasings or correct typos.

Although I finished graduate school years ago, a good advisor-relationship can continue even after leaving academia. My advisor and I have a clear overlap in our academic interests, so any paper I am asked to review is one he will eventually read to stay current on the literature anyway. This means asking him to read it early and meeting with me to chat about it isn’t a big imposition on his time, so that’s exactly what I did. The biggest message he had for me was to relax and not stress about giving a flawless review. I wasn’t the only reviewer, and considering that peer review is debatably effective anyway, the entire academic body of landslide studies will not crumble if I make a mistake.

How do you go about peer reviewing articles?

Posted in Practice of Science | Tagged | Leave a comment

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).

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

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?

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

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.

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

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.

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

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.

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