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Friday, June 24, 2011

Sediment transport during the Flood: qualitative appendix to a quantitative assessment

Did the Noachian Flood deposit the sediments that comprise a bulk of the geologic column? To most of the general public, this might seem a naïve and unsupported hypothesis that was debunked more than 200 years ago. But recent promoters of Flood geology have worked hard to convince Christians that geology still supports the notion of a recent creation and global flood. John Whitcomb and Henry Morris are perhaps best known for reviving the answer to this question in the affirmative, which they expounded in their 1961 publication The Genesis Flood.

Before Whitcomb and Morris, however, Christians had long postulated (i.e. since the early church) that the Flood might explain fossiliferous sediments found high above sea level today. In fact, the ancient Greeks were torn on the issue of whether fossils were remnants of living organisms—in part because they knew of no mechanism that could turn life into stone—and whether the sea had transgressed the land (how would you argue, if you were in their shoes?). Some of the early church fathers offered Noah's flood as a viable explanation to the Greek dilemma. The debate continued until Steno—who finally proved the biogenic origin of fossils to the satisfaction of the scientific community—but early geologists were still unsure how sedimentation worked, and particularly how sedimentary rocks bearing marine fossils could form on mountains! Consequently, Medieval natural philosophers were also inclined to characterize geologic formations in terms of their relation to a global flood in Noah's day.

Were these early hypotheses about Earth history born out of ignorance or incompetence? Not at all. Natural philosophers from Aristotle to Steno did the best they could with the evidence available to them. The only event in history they knew that could bury so many organisms under so much sediment was the Noachian flood. The fact that we were born, coincidentally, many centuries after the tough questions in geology have already been answered does not entitle us to arrogance, or to take pride in 'knowing better' today.

A modern approach to the age-old question

How much sediment could a global flood actually deposit? Early geologists could not answer this question in a quantitative sense. Neither did they know exactly how much sedimentary rock covered the Earth. But after more than a century of exploration and decades of flume experiments, we can at least begin to offer a quantitative assessment.

Sedimentary Rock Inventory

What is the total mass of sedimentary rock on Earth? The exact figure is unknown, but Drever et al. (1988) offered a reasonable estimate that is widely accepted:

Total Mass of Sedimentary Rock: 2.5±0.4 x 10^21 kg

Of which, 0.12 x 10^21 kg (5% of total) lay at the bottom of the oceans today.

Mudstones comprise the majority of this total, followed by carbonates, sandstones, and evaporites. Since sedimentary rock is recycled at plate boundaries (subduction zones), this mass is primarily comprised of Phanerozoic (Cambrian–Recent) sediments, which are considered to be 'Flood' rocks by most YEC's (e.g. Holt, 1996Froede and Reed, 1999; Oard and Klevberg, 2008).

Could a year-long flood have deposited some 2 billion billion tons of sediment? Let's start small, and answer this question in pieces.

Sediment Transport and the Coconino Sandstone: a quantitative assessment

Since young-Earth geologists must consider 'Flood' rocks as water-lain, formations like the Coconino Sandstone—considered to be an eolian (desert, windblown) deposit—must be reinterpreted to fit the Flood model. In response to an article that appeared in Creation Magazine (Snelling, 1992), Greg Neyman argued strongly that such efforts are misguided and ignorant of the physical evidence (Neyman, 2003). Every detail of the Coconino Sandstone corroborates the conventional hypothesis: the sediments were deposited slowly in a long-lived desert that covered much of the western United States.

Tim Helble, a hydrologist, took the argument a step further (technical article available here). If we assume, for the sake of discussion, that the Coconino Sandstone was deposited under water, we can use sedimentary structures (like cross-bedding) to calculate the rate of sediment transport to the site of deposition. In other words, how long would it take just to move all the sand needed to form the Coconino Sandstone? Find out below, in this well-organized slideshow created by Tim:

Keep in mind that this presentation only raises part of the problem: the transport of sandy sediment to the site of deposition. Additional questions remain, which young-Earth geologists must (but cannot) answer. What was the source of such relatively pure and homogenous, quartzose sand in a heavily vegetated, pre-Flood Earth? Why would such rapid and catastrophic deposition result in a formation that is devoid of body fossils from plants/animals (including teeth, vertebrae, and other sand-sized bones)? How could animals leave trace fossils (footprints) if the water was flowing several meters per second? Why doesn't the Coconino Sandstone contain large rip-up clasts from the underlying Hermit Shale (the contact is flat, and even contains mudcracks filled in by sand from the Coconino)?

Finally, the Coconino Sandstone is just one example from the American southwest, and comprises a minor portion of the regional stratigraphy. Mesozoic examples from the Colorado Plateau include the much larger Navajo Sandstone, as well as the Wingate and Dakota sandstones. Why do young-Earth geologists (e.g. Morris, 2010) still insist that these giant formations were laid down in a matter of days, contrary to all the evidence? Because the young-Earth, biblical hermeneutic is remarkably rigid and unrelenting. Though designed to protect the faith from modernism and liberalism, it rather prevents YEC's from witnessing the full beauty of God's creation, while leaving modernism/liberalism unscathed.

Andrew Snelling 'responds'
Although Snelling (2008) does not respond directly to the challenges above, he did provide some comfort to his readers in the form of misdirection. Snelling conveniently ignores the problem of sediment transport by attempting to turn it around on "slow and gradual" geologists. He argues that since the Coconino Sandstone contains detrital zircons from the Appalachians, the situation "poses somewhat of a dilemma...because no known sediment transport system is capable of carrying sand across the entire North American continent during the required millions of years."

Ironically, Dr. Snelling proves the impossibility of such transport with a geographic map of modern North America, on which the Mississippi River drainage basin currently covers most of the continental U.S.! As it turns out, water tends to move downhill, and rivers are perfectly capable of carrying sediments from distant mountain belts to low-lying deserts and coastal plains. During the late Paleozoic and Mesozoic, the Colorado Plateau was near sea-level, and rivers flowed from the Appalachian Range out west toward Nevada (opposite of today). But Snelling continues:

"It must have been water over an area even bigger than the continent. All they can do is postulate that some unknown transcontinental river system must have done the job. But even in their scientific belief system of earth history, it is impossible for such a river to have persisted for millions of years."

Why must water cover an area bigger than the continent for mineral grains to be transported cross-country? Fragments of the Rocky Mountains can be found in the Mississippi River delta today. But I've driven across Colorado and am happy to report: it's not submerged in water!

Sand-sized grains in modern deserts (e.g. Sahara, Gobi, Great Victorian) are commonly sourced from both near and far. The discovery that the Coconino and Navajo sandstones derived some of their material from hundreds of miles distant is hardly surprising. Furthermore, river systems are not always a necessary transport mechanism, since prevailing winds may accomplish the same under favorable conditions (if even from one side of the desert to another). Lastly, I would challenge Dr. Snelling to explain why a continental river system (e.g. Mississippi, Nile, Amazon) could not persist for millions of years. This statement is complete fluff to divert readers' attention from the real issue: even a global flood could not deposit the great sandstones of the Colorado Plateau in less than a year.

Rapidly deposited stratigraphy at Mt. St. Helens

The landscape evolution around Mt. St. Helens has been a favorite topic of young-Earth geologists. In fact, Dr. Steve Austin and others recently led a GSA field trip to the volcano (summary here from Dr. Austin; response here from one attendee), in part to show how rapidly sedimentation and erosion can take place. The unstated argument is as follows:

The eruption at Mt. St. Helens proved deposition and erosion are not always gradual. How effectively, then, could a global flood account for Phanerozoic sediments and subsequent erosion (e.g. Grand Canyon)?

Austin and others thus relied on a subtle 'hook, line, and sinker' tactic to reel others into their own way of thinking (the tactic is more explicit in creationist literature, of course). Nonetheless, they did not fabricate the data. In less than 30 years, Mt. St. Helens did produce thick bodies of fine-laminated sediments, bury entire forests, and carve out a large canyon. So why are 'conventional' geologists yet unconvinced by the arguments of Austin (1986)?

Greg Neyman (2005) answered this question succinctly and accurately: the events at Mt. St. Helens are easily incorporated into uniformitarian models of Earth history, because volcanic eruptions are an 'everyday' part of Earth cycles, geologically speaking. Geologists do not rule out the possibility of rapid deposition/erosion a priori, but rather use the scientific method to treat each case individually (gathering data to falsify or corroborate the initial hypothesis). Consequently, many examples of rapid deposition are known from the geologic column—from tsunami and submarine landslide deposits to meteor impacts to megafloods. On the other hand, young-Earth geologists do rule out the possibility of slow deposition a priori, and commonly distort the facts to corroborate their misguided antithesis.

So how do we tell the difference? Austin (1986) reported:

"[Mt. St. Helens] deposits include fine pumice ash laminae and beds from one millimeter thick to greater than one meter thick, each representing just a few seconds to several minutes of accumulation. A deposit accumulated in less than one day, on June 12, 1980, is 25 feet thick and contains many thin laminae and beds. Conventionally, sedimentary laminae and beds are assumed to represent longer seasonal variations, or annual changes, as the layers accumulated very slowly."

Despite his effort to the contrary, Dr. Austin thus provides good reason to accept conventional age assignments for the geologic column, and interpretations regarding its deposition. Consider the following:

1) Deposits around Mt. St. Helens are rich in volcanic glass and minerals, unlike a majority of fine-laminated mudstones in the geologic record. No geologist denies that volcanic ash can be deposited rapidly—in a matter of hours or even minutes.

2) Fine-grained sand and mud is also known to accumulate rapidly under favorable circumstances (e.g. submarine density flows). But the result is a thin-bedded mudstone that forms unique contacts with the underlying sediment (resembling flame structures) or even antidunes. Shale, on the other hand, forms when relatively flat clay minerals are preferentially oriented as they accumulate slowly in still waters. Both examples are found in the sedimentary rock record (though more commonly the latter), so geologists are not challenged by Austin's claim.

3) Sedimentary structures aside, rapidly deposited mudstones cannot be traced over large distances (such as in the Green River Formation, or even the Bright Angel Shale). Instead, thin beds and laminae will 'pinch out', or disappear laterally, as seen in the Mt. St. Helens' deposits or the experiments of Berthault (summarized by Snelling, 1997).

4) Sometimes, thin laminae result from seasonal effects on the basin (e.g. stratification and overturn of a large lake). But geologists do not simply assume this to be the case. Rather, they use mineralogical, paleontological, and geochemical criteria to distinguish seasonal varves from other phenomena. For example, the concentration of organic matter will vary significantly in varves, along with the carbon, nitrogen, and oxygen isotopic signatures. Major and trace element concentrations should also vary, reflecting a difference in climatic conditions and sediment source during each season (sediments from oxic, river water vs. sediments in the oxygen-poor, deep water). The example of Austin (1986) has absolutely nothing to do with varve interpretation.

5) A 25-foot deposit is impressive, especially for having formed in less than a day, but the sediment transport mechanism is unique to volcanic eruptions: a high-density ash flow. As Tim Helble's presentation demonstrates, similar transport rates in submarine conditions will not produce the sedimentary structures (esp. cross bedding) seen throughout the geologic column.

6) Once again, 25-feet is a lot of sediment. But in some places, Phanerozoic sediments are more than 5 miles thick, not including the sediment removed by unconformities! Austin's explanation would require deposition to occur at several times the rate seen at Mt. St. Helens, every day, for an entire year, while sorting thousands of fossil assemblages into precise, stratigraphic intervals. The stratigraphy of Mt. St. Helens does not provide a viable analog for this speculative, impossible scenario.

Heterogeneity of the Geologic Column: a qualitative appendix

The two examples above provide YEC's with the best possible data to explain sedimentary strata in terms of rapid deposition. Large sandstone units like the Coconino and Navajo contain obvious evidence of fluid flow, which is ubiquitous throughout the formations. Likewise, sedimentary structures in deposits around Mt. St. Helens are relatively homogenous. Whether slow or fast, a single mechanism seems to be responsible for the deposition of each unit.

By mechanism, I am actually referring to what is called the flow regime. A flow regime is defined by the physical characteristics of the fluid: how deep was the fluid and how fast was it moving? Based on flume experiments, geologists can interpret the flow regime using grain size and sedimentary structures (e.g. as was done for the Coconino Sandstone in the slide presentation above). Conversely, variations in grain size and sedimentary structures indicate when the flow regime changed during deposition, and by how much.

A quick look at strata in the Grand Canyon reveals that even if deposition were rapid and catastrophic, the flow regime must have changed many times. Below is a picture of the Supai Group, comprised of alternating limestone, shale, siltstone, and sandstone.

View from the South Rim. Late Paleozoic strata from the Supai Group in the foreground.

Whatever mechanism was responsible for deposition, the depth, direction, and rate of flow must have changed many times to produce such a heterogeneous body of sediment. Moreover, the respective sediment sources must have been geographically distinct (i.e. limestone from one region; quartz sand from another). There is no reason, hydrodynamically, that carbonate sediments would separate from siliciclastics, particularly because carbonate grains range in size from clay (micritic mud) to sand (ooids) to pebbles (shells, intraclasts and oncoids). But if the flow regime and sediment source were highly variable over the duration of the Flood, why do we find sand, shale, and carbonate bodies like these that can be correlated across continents?

Microfacies of Carbonate Rocks

Austin (1990), and a feedback article from Creation magazine, posited that thick layers of carbonate rock (limestone, dolostone) were also deposited rapidly. Austin (1990) said that prevailing currents during the Flood may have transported massive quantities lime mud and its constituents from the sea to the continents. He even cites evidence of fluid flow that is meant to contradict that conventional interpretation of limestone formations in the Grand Canyon. His understanding seems to be, however, that according to conventional geology, all limestone was deposited in "calm, placid seas."

This generalization is misleading, since most limestones are interpreted to have been deposited in very shallow water (less than 200 meters, but usually less than 10 meters). Carbonate sediments are just as susceptible, therefore, to waves and currents. Consequently, most ancient limestones contain sedimentary structures like cross-bedding, horizontal bedding, mud drapes, and rip-up clasts. Others contain evidence of subaerial exposure (dissolution cavities, mudcracks) or long-term interaction with microorganisms (microbialites, stromatolites, thrombolites, oncoids, bioturbation). Carbonate rocks are thus divided into microfacies, based on composition, structure, and fossils. Each microfacies reflects different conditions (water depth, velocity, salinity, etc.) at the time of deposition.

Below is a combination of two photos taken near Timpanogos Cave, Utah (my apologies for no scale; each photo covers a little more than 1 square foot). In the photo on the left, the upper layer contains rip-up clasts (>2 cm diameter) of lime mud. The name of this rock is intraclastic rudstone. For mud clasts to be incorporated, the sediment had to be deposited 1) in a current; and 2) within proximity to a layer of weakly cemented (and dried) lime mud. But if the current were too strong, the finer-grained matrix (i.e. the dark material between mud clasts) would have remained in suspension. If the fragile mud clasts came from a long distance, they would have been destroyed during transport. Modern, shallow, carbonate platforms provide a viable analog for this type of deposition (specifically, the intertidal zone). Flood geology, on the other hand, finds itself in a conundrum. Why does the intraclastic rudstone comprise such a thin layer, distinct in every physical aspect from the surrounding carbonate sediments, if sediments were being deposited so rapidly? What is the source of these fragile mud clasts if the whole land was submerged?

Carbonate rocks near Timpanogos Cave, Utah, illustrating the microscale, sedimentological diversity of carbonate rocks. Each photo is approximately 1 square foot.
In the photo on the right, thin bedding is visible in the top and bottom layers, indicative of gentle currents that sorted the fine-sand-sized particles. In the middle, a coarse shell hash represents higher energy conditions, but with little sorting of the grains, which range from less than 1 mm to more than 2 centimeters across. The dark-gray/tan bed near the bottom is an oolitic packstone—a somewhat muddy, carbonate beach sand. Each facies suggests deposition in the shallow subtidal zone (<10 meters water depth at any time; below low tide), where sand bars constantly prograde across the shallow platform. Once again, Flood geology cannot account for the sedimentological diversity of this rock, because rapid deposition cannot separate these sediments with such precise detail in a matter of minutes.


Only long-term, prevailing currents can deposit homogeneous sediments (like the Coconino) across the face of a continent in a global flood scenario. In response, YEC's propose that large-scale, regional currents were responsible for depositing extensive, tabular beds of sandstone, shale, and limestone. But such currents cannot account for the heterogeneity found in the layers of the Bright Angel Shale, Supai Group, and other formations. Consequently, YEC's must also argue that repeated transgression, regression, and periods of 'stand-still' occurred amid the flood, wherein sediments of differing clast size and composition could be deposited between larger waves. But if continuous, prevailing currents are not sufficient even to carry the sediment required even for the 150–500 foot-thick Coconino Sandstone within a full year, how can Flood geologists explain the remaining miles of sediment in the Colorado Plateau?

The challenge grows immensely when one examines the microscale heterogeneity in sedimentary rocks (carbonates in particular). Catastrophic, sediment-choked currents would have had zero time to slow down, change directions, or stop completely. Therefore, Flood geology cannot satisfactorily explain the range of geological data as a unified theory. But unfortunately, Flood geologists continue to mislead amateur readers by explaining various phenomena in isolation from the relevant data. The result is a confused populace, seeking only to reconcile their faith with the facts of nature. Such misplaced trust is unhealthy, in my opinion, for the future of public/private education, the scientific community, and especially for the church.

References Cited (but not linked):

Drever, J.I., Li, Y.-H., and Maynard, J.B., 1988, Geochemical Cycles: The Continental Curst and the Oceans, in Gregor, C.B., Garrels, R.M., Mackenzie, F.T., Maynard, J.B., [editors], Chemical Cycles in the Evolution of the Earth: John Wiley & Sons, New York, 276 p.

1 comment:

  1. Good stuff. I enjoyed the slideshare as well. Thanks for posting.