Bioturbation and Trace Fossils

"86% of strata in this 430-m succession are unbioturbated" — Lidya Tarhan and eleven co-authors, on a Lower Devonian section in Algeria, Science Advances, 2025¹

"Only a small fraction of the total thickness has an ichnofabric index of 1, while the majority of the total thickness has an ichnofabric index of 3 or greater." — John Woodmorappe, creationist geologist, on 1,151 metres of Cambrian strata, Journal of Creation, 2006²

"However, neither perspective is consistent with the actual rock record" — Carl Froede Jr., creationist geologist, on the diluvial prediction that Flood strata should be little burrowed, Journal of Creation, 2009³

Summary

A burrow is a record of an animal living in sediment after that sediment stopped arriving.  Count the burrowed surfaces in a formation, multiply by the time each one needed, and the result is a minimum duration for the formation that owes nothing to radiometric dating.  That is why bioturbation is a problem for any model that deposits hundreds of metres of rock in days.

Andrew Snelling, whose Earth's Catastrophic Past carries the fullest young-earth treatment of the subject, states the problem himself:  "Because bioturbation requires the passage of time, the degree of bioturbation found preserved in sedimentary strata at numerous levels in the rock record would likewise presumably testify to the passage of perhaps significant amounts of time."⁴⁴  His answer is that the time needed is short, and that most Flood strata are not burrowed anyway.

The verdict.  Bioturbation is unevenly distributed through the column, and the young-earth generalisation is about half right.  Long stretches of lower Palaeozoic rock really are barely burrowed, and mainstream ichnologists say so in print with numbers.  Cretaceous shelf sediments are the opposite:  homogenised through their whole thickness, with reworking outpacing deposition, and they sit inside the Flood year on the young-earth models that put the boundary in the late Mesozoic or the Cenozoic, which is most of them.  The rate literature the creationist side cites does not rescue them.  Its one community-scale measurement is the fast end of a range running to 24 weeks, and no young-earth author has counted the colonisation surfaces in a named formation and shown they fit in a year.  Four young-earth authors have published in their own movement's journals that its answers here do not work.

Nine points, stated first.

1. Mainstream ichnology reports the pattern the Flood side predicts, in parts of the Palaeozoic, and this article leads with it. A 2025 compilation in Science Advances by Lidya Tarhan and eleven co-authors reports that in a 430 metre Lower Devonian succession in Algeria, "86% of strata in this 430-m succession are unbioturbated."¹¹ Frasnian carbonates of the Great Basin and Rockies have "a mean ii of 2.0."¹ Lower Cambrian strata "only rarely exceed ii 3 and are most commonly characterized by ii 1 or 2."¹ None of that came from a creationist source.
2. An unbioturbated bed is not proof of rapid burial, and the standard textbook says so. Richard Bromley lists other causes: "In particular, fluctuation in salinity may severely reduce the biological diversity of a fauna … Another important limiting factor is the availability of oxygen."⁵⁵ Any argument that runs from bare laminae straight to a Flood has skipped a step.
3. Where the creationist literature's own cited survey looked, the rocks are heavily burrowed, and the creationist author reports it accurately. Woodmorappe, writing in Journal of Creation, points his readers at "48 Cambrian stratigraphic sections and 1,151 total metres of sedimentary strata in the western USA" and states: "Only a small fraction of the total thickness has an ichnofabric index of 1, while the majority of the total thickness has an ichnofabric index of 3 or greater."²² His figure checks out against the primary, where Mary Droser and David Bottjer logged 810 metres of inner shelf plus 341 metres of middle shelf.⁶⁶⁶
4. Two young-earth authorities describe the same rocks incompatibly. Snelling writes that "there is so little bioturbation of sedimentary strata in the geologic record" that the record is evidence for rapid deposition.⁴ Woodmorappe puts the majority of 1,151 Cambrian metres at index 3 or above, meaning at least 10% of bedding destroyed and often all of it.
5. The one fast community-scale rate in the creationist literature is the low end of a range, and the range is never printed. Snelling gives "a 12-cm-thick layer of sediment can be 67 percent bioturbated in as little as 5.5 hours."⁴ His source, Woodmorappe, prints 15.5 hours for the same sentence and citation.² The primary, Kurt Grimm and Karl Föllmi in Palaios, gives "Estimated durations range from 15.5 to 4020 hours (~1 day to 24 weeks)" and calls 15.5 hours "the shortest estimated duration."⁷⁷⁷ Neither creationist source reports the upper bound.
6. Measured reworking rates span five orders of magnitude, and the fast figure is a maximum-density model. Murray Gingras and colleagues X-rayed estuarine animals in aquaria and scaled the result to one square metre. Ten filter feeders: 115 years. Ten irregular echinoderms: 42 days. Maximum modelled densities: 61 minutes.⁸⁸ Their verdict on the ichnofacies that dominates the Grand Canyon's Cambrian sandstones is not the 61-minute case: "highly burrowed examples of the Skolithos Ichnofacies reflect high population densities and at least seasonal time spans."⁸
7. The layer animals mix is thin and nearly the same everywhere, so a homogenised column is a stack of thin reworked slices. Bernard Boudreau compiled tracer profiles worldwide and found the mixed layer "is essentially independent of burial velocity with a worldwide mean of 9.8 ± 4.5 cm."⁹⁹ Cambrian mixing was shallower still, 2 to 6 centimetres.⁶ In the lower Cambrian Zhushadong Formation of North China, burrow shafts reach 3.1 centimetres but repeated post-storm colonisation amalgamates into intervals "up to 170 cm thick," roughly 55 successive colonisations of one patch of seafloor.¹⁰¹⁰
8. Woodmorappe's own time budget fails on his own numbers. He allows "one year (8,766 hours) of overall deposition with active sedimentation occurring half of this time," leaving 4,383 quiet hours.² At the 15.5 hours per surface he himself cites, that buys about 283 surfaces, roughly 34 metres of heavily reworked rock for the entire Flood, worldwide. The arithmetic is this article's, run on his figures. The one Cambrian survey he points at covers 1,151 metres.
9. Four young-earth authors have published that the movement's answers fail, and nobody has answered them. Carl Froede Jr., 2009: of the diluvial prediction of little bioturbation and its uniformitarian rival, "neither perspective is consistent with the actual rock record," and one Skolithos horizon took "months, not years or decades."³³³ Chad Arment, 2023: the time needed for Palaeocastor burrows "clearly disconfirms" formation during the Flood.¹¹¹¹ Froede, Jerry Akridge and John Reed, 2022: tracks of living animals run "well past the megasequence-defined Day 150 termination of all terrestrial life."¹²¹²

What decides the case, and what does not

Speed of digging is the wrong variable.  Bromley's textbook says a Flood-like event can produce fast pioneer burrowing:  after a storm deposit, opportunists "speedily colonize a habitat," making low-diversity vertical dwelling burrows.⁵  Nobody needs to deny that.  What rapid deposition cannot produce is his other category, the climax community:  high trace-fossil diversity, deep tiers occupied, later burrowers cutting across earlier ones.  The question to ask of a disputed formation is not whether an animal could dig that fast, but whether the rock records pioneers or a mature community, and how many times over.

Part 1: How bioturbation is measured

The ichnofabric index

Before 1986 there was no agreed way to write down how burrowed a rock is.  Mary Droser and David Bottjer supplied one in a two-page note in the Journal of Sedimentary Petrology.  They photographed vertical rock faces 50 centimetres by 35 centimetres in lower Palaeozoic carbonates of the Great Basin, measured the disrupted area on each photograph with a planimeter, and cut the results into six bands.¹³¹³  The bands are the ichnofabric index, written ii 1 to ii 6:

The scale is applied with printed cards:  "Enlargements of the schematic diagrams of ichnofabric indexes 1 through 5 (Fig. 1) can be used in the field as 'flash cards' to rank bioturbation."¹³  A section is logged continuously, index against thickness, "analogous to the continuous logging of thicknesses of strata in a section for properties such as rock color or lithology."⁶

Two cautions come from the authors themselves and both help the young-earth side.  The cards were drawn for one kind of rock, so "different schematic 'flash cards' will be necessary to study different types of strata," built on the same definitions "so that the data are comparable."¹³  Comparing Cambrian carbonate with Cretaceous shale is not straightforward.  And the index measures percentage of bedding destroyed:  not depth, not time.  Turning an index into a duration takes a second step, a rate, and that step is where every disagreement in this article lives.

The bioturbation index

A second scale, the bioturbation index or BI, runs 0 to 6 and attaches explicit percentages.  It comes from Andrew Taylor and Roland Goldring in 1993.  That paper could not be obtained for this article.  Its definitions are reproduced verbatim in the methods of an open-access 2017 paper by Li-Jun Zhang and colleagues, and are quoted here from that reproduction:¹⁰

The two scales are close but not identical, and the literature uses whichever the underlying study used.  Nothing below silently converts one into the other.

Tiers, and the layer that actually gets mixed

Burrowers do not work at one depth.  The standard division, as Zhang and colleagues state it, is that "The shallow tier is represented by structures emplaced in the upper 6 cm of the substrate, the mid tier comprises those produced within 6–12 cm of the substrate, the deep tier consists of those formed below 12–100 cm, and the ultra-deep tier is represented by those traces emplaced below 100 cm."¹⁰

Bromley's textbook explains why this matters for reading a rock.  "Deepest structures are the most complete.  Only their upper parts are cross-cut by shallower structures.  Activity within each tier cross-cuts structures produced in shallower tiers and tends to obliterate them."⁵  A preserved ichnofabric is therefore biased towards the deep end of what was there.

Tarhan and colleagues split the same idea into two measurable things:  a mixed layer, the homogenised top, which "extends downward from the sediment-water interface to, on average, nearly 10 cm depth in the seafloor," and beneath it a transition layer of "discrete and actively occupied deep burrows that may, in certain settings, extend to >1 m deep."¹

The mixed layer's thickness is the number the whole argument turns on.  Boudreau assembled published tracer profiles from lead-210, thorium-234, caesium-137, plutonium-239, silicon-32, beryllium-7, tektites and volcanic ash across coastal, shelf, slope and deep-sea settings, and found the depth barely varies:  "the tracer-identified surface mixed-depth … is essentially independent of burial velocity with a worldwide mean of 9.8 ± 4.5 cm."⁹  He credits physics rather than ecology:  "the depth of mixing L is limited primarily by the physical difficulty and increasing energy costs of reworking and excavating deeper than 10-15 cm in a sediment medium."⁹

Boudreau's caveat helps the young-earth reading and belongs here.  The tracer-defined depth is not a ceiling:  "Specific organisms are known to burrow to depths of over 1 m … Most of the values in Table 1 are less than 20 cm.  Therefore, these quoted mixing depths represent the thickness of some zone that is most frequently mixed."⁹  Ten centimetres is the most-worked slice, not the deepest hole.

Part 2: What the strata actually show

Start with the cases that favour a Flood

The strongest evidence for the young-earth position here is in the mainstream literature, and it is recent.

Tarhan and her co-authors compiled Phanerozoic mixed-layer and transition-layer data in 2025.  Four of them are Droser, Bottjer, Boudreau and A. A. Ekdale, so the people who built the ichnofabric index, the mixing-depth compilation and the standard ichnology short course all signed the paper.  For the lower Palaeozoic they report bioturbation that is sparse and shallow.  Lower Cambrian heterolithic and carbonate strata "only rarely exceed ii 3 and are most commonly characterized by ii 1 or 2."¹  Across the early Palaeozoic the mixed layer deepened only "from approximately 0.2 to 1.5 cm in heterolithic siliciclastic settings," which is thin enough that storm beds averaging 1.3 centimetres survived intact because burrowers could not blend across their junctions.¹

The Devonian case is sharper.  In a 430 metre Lochkovian to Emsian succession in the Reggane Basin of Algeria, "86% of strata in this 430-m succession are unbioturbated; 7% are characterized by BI 2, 7% by BI 4, and <1% by BI 5."¹  In Frasnian carbonates of the Great Basin and the Rockies the mean ichnofabric index is 2.0, intensely bioturbated intervals "account for less than 30% of measured strata," and "beds that are vertically mixed rarely exceed 10 cm in thickness."¹

Leonard Brand and Arthur Chadwick, two young-earth geologists, did the field work the movement's rate arguments should have been built on.  They logged bioturbation bed by bed in "a number of rock formations in Utah and western Colorado, quantifying in detail the amount of bioturbation in rocks from Cambrian to Eocene," on their own scale of levels 1 to 4, and found "the vast majority of vertical intervals in all deposits surveyed had no bioturbation or a small amount (level 1), not enough to noticeably disturb the bedding."¹⁴¹⁴  That is the best empirical card the young-earth side holds.

Three qualifications belong beside it.  Their scale is their own, levels 1 to 4, and does not map cleanly onto ii 1 to 6.  Their sample is Utah and western Colorado, heavily Mesozoic and marginal-marine, while the contradictory Cambro-Ordovician data come from Great Basin shelf carbonate.  And the circulated summary of their survey, that 37 formations were measured and 97% of the thickness showed no bioturbation or isolated burrows, appears in no publication this project could find.  Their book gives no percentage at all.  That figure is not used here;  the reasons are in quarantine.

Finally, the textbook caution.  Bromley's chapter on ichnofabric lists the causes of incomplete bioturbation and speed is only one:  "This implies a stress factor that has prevented the completion of the work of the endobenthic community … Clearly, however, other stress factors may inhibit the complete bioturbation of the substrate.  In particular, fluctuation in salinity may severely reduce the biological diversity of a fauna … Another important limiting factor is the availability of oxygen."⁵  He adds the reverse warning too:  where erosion has cut below the burrowed top of a bed, "no bioturbation is preserved, but it would be unwise to assume that the depositional environment was devoid of life."⁵  An unburrowed bed is weak evidence on its own.

Then the Cambrian and Ordovician shelf

Droser and Bottjer logged the Great Basin continuously.  In Cambrian rocks, "A total of 810 m of ichnofabric index data were collected from 34 Cambrian stratigraphic sections (Fig. 2) interpreted to have been deposited in an inner-shelf environment.  The most commonly recorded ichnofabric indices were 3, 4, and 5."⁶  Further offshore the picture is quieter:  "A total of 341 m of ichnofabric index data were collected from 14 Cambrian sections (Fig. 2) interpreted to have been deposited in a middle-shelf environment.  The most common ichnofabric indices recorded were 2 and 3."⁶

The Ordovician is worse for a Flood year.  The average index on the carbonate inner shelf climbs in two steps:  "an increase in average ichnofabric index from 1.02, recorded from the pre-trilobite strata, to 3.1, recorded from the trilobite-bearing Lower Cambrian strata … The second major increase in extent of bioturbation occurred approximately 110 m.y. later between the Middle and Late Ordovician, when the average ichnofabric index increased from 3.1 to 4.5."¹⁵¹⁵  An average of 4.5 means the average metre of Upper Ordovician inner-shelf carbonate has lost 40% to 100% of its bedding to animals.  And "From the Middle to the Late Ordovician, in carbonate inner shelf strata, the percentage of totally reworked beds increased by nearly an order of magnitude."¹⁵

Two details in these papers help the young-earth reading and are printed here for that reason.  The burrowing is shallow:  "Throughout the remaining 65 m.y. of Cambrian time bioturbation increased, although depth of bioturbation remained relatively constant at less than 6 cm."⁶  Middle Cambrian burrows "averaged 2-3 cm; the maximum depth was 5 cm."⁶  Depth stays "consistently less than 6 cm through the Middle Ordovician," reaching "as much as 30 cm" only in the Late Ordovician.¹⁵  A Flood model only has to rework a thin skin at each colonisation.  And the authors consider a rival reading of their own trend:  "This increase in occurrence of more extensively bioturbated beds through the Cambrian could simply be the result of a constant rate of bioturbation coupled with a systematic decrease in sediment accumulation rate."⁶  They reject it on lithofacies grounds, but they raise it, and it is the right kind of objection.

The Mesozoic reverses the picture

Tarhan and colleagues describe the Cretaceous shelf in terms that no Flood-year model can absorb:  "throughout the Cretaceous, the majority of offshore and shelf sediments were completely homogenized by biogenic mixing … even in comparatively high-energy shallow marine settings, rates of bioturbation commonly outpaced rates of sediment accumulation."¹  Also:  "Cretaceous chalks deposited in epicontinental settings are commonly completely bioturbated," and "Primary sedimentary structures are relatively rare in Cretaceous and Cenozoic shallow marine deposits."¹¹  In 48 deep-sea cores of Cenozoic age, "rates of bioturbation appear to have, under most circumstances, outpaced sediment accumulation rates."¹

Depth rises with extent.  Deep-tier transition layers make up 34.1% of the Palaeozoic sample, 37.1% of the Triassic, 57.1% of the Jurassic and 65.9% of the Cretaceous, and median maximum burrow depth climbs from 8.0 centimetres in the Triassic to 17.6 in the Jurassic, with a Cretaceous 75th percentile of 40 centimetres.¹

This is where the young-earth position takes its real damage, and the reason is stratigraphic.  Most young-earth models put the Flood boundary in the late Mesozoic or the Cenozoic, so the Cretaceous chalks and shelf carbonates fall inside the Flood year.  The Chalk itself is treated in Chalk, Limestone, and the Flood.

The Grand Canyon, where a Flood model has to move fastest

The Tonto Group is the first thick pile most young-earth models deposit.  The published ichnology, compiled by Anne Miller and colleagues for the National Park Service, records burrowing in all three formations.  In the Tapeats Sandstone, Hagadorn and others "reported the abundance of Arenicolites in the Tapeats Sandstone creating highly bioturbated beds at outcrops within eastern and western GRCA."¹⁶¹⁶  Diplocraterion is common there too and carries spreiten, which "indicates adjusting behavior to maintain a connection with the changing sediment-water interface."¹⁶  A spreite records an animal re-cutting its burrow to keep pace with sediment arriving in small increments, with living time between them.

In the Bright Angel Shale "21 ichnogenera have been reported and classified into 15 architectural designs," a deposit-feeder assemblage assigned to the Cruziana Ichnofacies, "typical of shallow-marine settings with moderate to low energy," where "Low rates of sedimentation and erosion favor the preservation of locomotion traces."¹⁶¹⁶  The primary behind that count describes the formation the same way.  Christopher Baldwin and colleagues:  "The Middle Cambrian Bright Angel Shale in the eastern Grand Canyon contains a depauperate normal marine fauna, but trace fossils and palynomorphs are abundant throughout the formation."¹⁷¹⁷  Their reading of those traces is a steady estuarine setting, not one burial event:  "Initial correspondence between organic matter content in mudstones and feeding type and intensity (as indicated by traces) is consistent with an estuarine setting for this deposit."¹⁷  Only their abstract could be obtained.  In the Muav Limestone, "bioturbation is extremely common within calcareous-dominated sediment and is responsible for the mottled appearance."¹⁶  The rock's texture is burrowing.

One honest gap:  no continuous ichnofabric log of the Tapeats, Bright Angel or Muav was found for this article.  The descriptions above are qualitative, and no percentage should be attached to Tonto Group bioturbation until somebody logs it.

The cleanest single test in the literature

Zhang and colleagues logged 34, 53 and 28 metres of the lower Cambrian Zhushadong Formation at three sections in North China.  The beds are storm beds, laid down fast, which is the depositional style a Flood model wants.  What sits on top of them is the problem:  "Maximum penetration of individual burrow shafts is 3.1 cm, but bioturbated zones are amalgamated forming up to 170 cm thick bioturbated intervals.  Also, some of the shafts connect burrow systems formed at different depths, indicating that these are multi-layer colonizers able to penetrate more than one event bed.  In these cases, maximum burrow depth is 32.4 cm.  Beds were most likely deposited by discrete storm events.  Thalassinoides records post-storm colonization and intense bioturbation has locally obliterated bedding boundaries, resulting in the amalgamation of bioturbated zones."¹⁰

A 170 centimetre interval assembled from 3.1 centimetre increments is about 55 separate colonisations of the same patch of seafloor, each following a separate storm bed.  That is the pattern a Flood year has to compress, and it is not one bed worked once.

The mainstream disagrees with itself here, and the disagreement is worth stating.  Zhang and colleagues find deep-tier burrowing in the lower Cambrian of North China;  Tarhan and colleagues find lower Cambrian fabrics mostly at ii 1 to 2.  Neither position is anywhere near a Flood year.  The dispute is whether early Cambrian seafloors were churned to 3 centimetres or to 30, over intervals both sides measure in millions of years.

Part 3: How fast burrowers really rework sediment

A single animal digging is not a bed being reworked

Snelling's rate case runs to eleven numbered notes, and only two of them measure a community rather than an animal.  The rest are single-animal burial times:  he lists "annelid worms, numerous kinds of bivalves, certain razor clams, a pelecypod, several different kinds of gastropods, many crustaceans, and various crabs," each with its own citation.⁴  Woodmorappe's list is the same kind.  A clam burying itself in seconds says nothing about how long a square metre of seafloor takes to lose its bedding.  That depends on how many animals there are, how much each moves per hour, and how deep they work.

Snelling's fifth citation is a population-scale study, and it does not report what he takes from it.  He writes that "certain urchins are known to rework the upper 5 cm of sediment in three days," citing Lohrer and colleagues 2005.⁴  That paper is a probabilistic computer model of the heart urchin Echinocardium australe at six shallow subtidal sites in northern New Zealand.  Its third parameter, urchin movement rate, was obtained "from one time and place only (Site OB5)," and OB5 "had the highest sediment reworking rate of all sites," so the authors offer the output as "a good upper bound estimate."¹⁸¹⁸  The three-day figure is that upper bound:  "The volume of sediment displaced by Echinocardium populations reached 20,000 cm³ m⁻² d⁻¹ at OB5, suggesting that surface sediment is reworked about every 3 days at sites where Echinocardium is abundant."¹⁸  The same abstract carries a result the creationist versions drop:  "Experimental work with a fluorescent tracer at OB5 suggested limited downward particle movement as a result of Echinocardium bioturbation."¹⁸  A dense urchin herd furrowing the sediment surface is not the same thing as destroying bedding at depth.  No "5 cm" figure appears anywhere in the abstract or in the published section snippets;  the full text is paywalled and was not obtained, so where Snelling's depth came from is unknown.

Gingras and colleagues did that scaling in 2008, collecting bivalves, arthropods and echinoderms from the Ogeechee estuary in Georgia and X-raying them through the walls of thin aquaria.  Froede, summarising their methods for a creationist audience, records that the tanks were "filled with alternating layers of sand and heavy minerals with each layer being approximately 5 to 10 mm thick" and imaged "at 1, 6, 24, 72 and 144 hour intervals."³  The abstract reports the result:  "Burrowing rates ranged between 0.01 and 0.15 cm³/h for suspension-feeding animals.  Deposit-feeding animals moved between 1 and 10 cm³ of sand per hour, approximately 10 to 100 times more sediment than the suspension feeders moved over similar times.  Neoichnological experiments show that ten filter-feeding individuals could take as long as 115 yr to churn a 1 m² plot of sediment, by indexing the measured burrowing rates to realistic animal population densities.  Ten such mobile deposit feeders as irregular echinoderms could bioturbate the same sediment in just 42 days.  Under the maximum population densities modeled, the animals could bioturbate the sediment plot in 61 min."⁸

Three answers for one plot:  115 years, 42 days, 61 minutes.  The difference is not the biology of digging but how many animals are assumed and what they eat.  The full text could not be obtained;  everything here is from the abstract, verified by two routes that agree word for word.

The authors' own translation to the rock record is the part the creationist literature leaves alone:  "highly burrowed examples of the Skolithos Ichnofacies reflect high population densities and at least seasonal time spans.  Highly burrowed examples of the Cruziana Ichnofacies may represent moderate population densities and short time spans."⁸  Skolithos piperock is exactly what the Tapeats Sandstone carries.  A Flood year cannot supply a season of high population density at every horizon that shows one.

The one community-scale measurement, read in full

Grimm and Föllmi published the only community-scale reworking calculation the creationist literature uses.  Their subject is the Oligo-Miocene San Gregorio Formation of Baja California Sur:  laminated, organic-rich, anaerobic basin mud containing 41 isolated horizons of Thalassinoides and Gyrolithes burrows, each tied to a phosphatic turbidite.⁷  Their thesis is in the title, "Doomed pioneers":  the burrowers were carried in by the turbidity currents, dug in an anoxic basin that could not support breeding, and died.

For one 12 centimetre bed they measured burrow volume on serial slabs, 67 plus or minus 2% over 250 counts, and divided the resulting 80,400 cubic centimetres by several published density and rate combinations.  The result is a range:  "Estimated durations range from 15.5 to 4020 hours (~1 day to 24 weeks).  Of course, these values provide only the loosest constraints upon the duration of the burrowing interval for this bed.  However, it is clear that extensively bioturbated horizons do not necessarily require protracted intervals of time for their development."⁷

The fast end is flagged by its own authors:  "calculated duration = 80,400 cm³/(58 cm³/hour/hole x 89 holes/m²) = 15.5 hours (this method yielded the shortest estimated duration)."⁷  A second published estimate for the same bed, using a measured population reworking rate rather than a per-hole rate, comes out at 1,073 hours, about 45 days.⁷  And their conclusion is about density, not speed:  "it seems reasonable to conjecture that some extensively bioturbated horizons may record an unusually high density of redeposited tracemakers which inhabited the substrate, rather than an extended interval of bioturbation."⁷

The two rates the creationist literature uses disagree with each other

Gingras measured deposit feeders at 1 to 10 cubic centimetres per hour.  The callianassid figure behind Grimm and Föllmi's 15.5-hour minimum is 58 cubic centimetres per hour per entry hole, six times faster than any animal Gingras measured.  Both cannot be typical, and any argument that prints one point value misrepresents the state of measurement.

Part 4: The number the young-earth case rests on, and where it came from

A chain of three documents

Snelling's chapter on bioturbation gives one community-scale rate, and it is the only one in the young-earth literature:  "Indeed, depending primarily upon the density of fast-burrowing organisms such as callianassid shrimp, a 12-cm-thick layer of sediment can be 67 percent bioturbated in as little as 5.5 hours."⁴

His source is Woodmorappe's 2006 paper, which prints the same sentence with a different number:  "Moreover, depending primarily upon the density of fast-burrowing organisms such as callianassid shrimp, a 12-cm thick layer of sediment can be 67% bioturbated (ichnofabric index of ~5, figure 1) in as little as 15.5 hours".²

Woodmorappe's source is Grimm and Föllmi, who print 15.5 hours as the shortest of an ensemble running to 4,020 hours.  So Snelling's figure is nearly three times too fast against his own chain of custody.  That is a transcription slip, not dishonesty, and it should be read as one.  Both men reproduce the authors' sentence about extensively bioturbated horizons not necessarily requiring protracted intervals.  Neither reproduces the sentence immediately before it, which gives the range.

The omission is the substantive defect, not the digit.  A Flood-year timetable can absorb a day.  It cannot absorb 24 weeks at even a few horizons.  The two worked examples the paper prints in full come out at 15.5 hours and 1,073 hours, and 15.5 is the one the creationist literature carries.

The setting is the opposite of the one the argument needs

The San Gregorio beds are isolated burrowed horizons in otherwise laminated anoxic mud, made by animals transported in that could not reproduce.  That is the pattern a Flood model predicts:  rare, thin, event-driven burrowing between undisturbed layers.  Using this paper to explain a formation burrowed continuously through hundreds of metres inverts what it shows.  A turbidite dumping an unusual density of animals cannot be invoked for a whole shelf logged at ii 3 to 5 for a kilometre of section.

The arithmetic nobody on the young-earth side has run

Woodmorappe supplies the time budget himself, in note 103:  "As an upper limit of episodicity, assume one year (8,766 hours) of overall deposition with active sedimentation occurring half of this time.  Theoretically speaking, there was time available for the construction of 4,383 superimposed 1-hour bioturbating events or 52,596 superimposed 5-minute bioturbating events.  Of course, the foregoing example is extreme; intentionally so for the purpose of illustration."²

Take the extreme case at face value and put his own rate into it.  A one-hour event does not produce an ichnofabric index of 5;  by the figure Woodmorappe himself cites, that takes 15.5 hours.  So 4,383 quiet hours divided by 15.5 hours per surface gives 283 surfaces, and at 12 centimetres each that is about 34 metres of heavily reworked rock for the whole Flood, on the whole planet.  The calculation is this article's, run on Woodmorappe's stated inputs;  he does not perform it.  At the other published estimates for the same bed the budget buys 4 surfaces, roughly half a metre, or at the slowest, one.

Against that, set the thicknesses in the literature.  The Cambrian survey Woodmorappe points his readers to logged 1,151 metres, most of it at ii 3 or above.²  One lower Cambrian formation in North China has a single amalgamated interval 170 centimetres thick built from 3.1 centimetre increments.¹⁰  The Cretaceous shelf is homogenised over most of its area.¹

A second route gives the same order of answer.  The mixed layer is about 10 centimetres worldwide and Cambrian mixing depth was 2 to 6 centimetres, so a thoroughly reworked column is a stack of thin slices, each of which had to be the seafloor separately.⁹⁶  Divide a thick homogenised formation by its own measured burrow depth and the required seafloors run into the thousands.  Spread over 371 days, that leaves a few hours each, at or below the fastest rate anyone has measured.

No young-earth publication does this arithmetic for a named formation.  That is the largest single hole in the position, and it is a hole that could be filled with a spreadsheet and a published ichnofabric log.

Part 5: Where creationist authors have conceded the point

Concessions inside the argument

Woodmorappe's paper is the most careful young-earth treatment of the subject, and it concedes more than any critic has extracted from the outside.  He states the difficulty:  "Very thick sedimentary lithologies that have been extensively bioturbated present more of a challenge to a Flood model, even if they occur at or near the top of the local lithostratigraphic column.  Thick sequences of sedimentary rock, characterized by high ichnofabric indices, and occurring in undoubted superposition, may have had insufficient time during the Flood for them to be explained by one-horizon-at-a-time bioturbation."²

His escape is the PPLC hypothesis, short for promptly-precipitated lithified crust.  Thin cemented crusts form above buried animals, shield them from the overburden, and let several horizons be worked at once, giving "a thick but cryptically composite, highly bioturbed interval."²  He also suggests thick bioturbated sequences "may be at least partly illusory" because such horizons are often discontinuous.²  And he ends honestly:  "The efficacy of the PPLC hypothesis requires experimental validation."²  Twenty years on, no such experiment was found for this article, in the creationist journals or outside them.

John Whitmore and Paul Garner, setting out criteria for recognising Flood strata, rank bioturbation as their weakest indicator, a 3 on a scale where 1 is diagnostic:  "we would expect extensively bioturbated horizons, with the resultant disruption of internal stratification and sedimentary structures, to be less common in Flood-deposited sediments.  Before and after the Flood, as more time became available between depositional events, we would expect bioturbation to become much more abundant."¹⁹¹⁹  Half of that prediction succeeds.  Bioturbation really does increase up the column, from a mean index of 1.02 in the pre-trilobite Cambrian to 4.5 in the Upper Ordovician, with deep tiers rising from a third of Palaeozoic samples to two thirds of Cretaceous ones.  It fails on where the increase happens:  the most homogenised rocks are Cretaceous and Cenozoic, and those lie inside the Flood on the models that make the prediction.

Kurt Wise uses bioturbation as a mainstream sedimentologist would.  Testing whether a fossil forest grew where it is found, he writes that "the sediment substrates of modern communities are heavily bioturbated, usually completely obliterating original bedding.  In contrast, the Flood usually involved a sediment production rate that was too high to permit extensive infaunal bioturbation," so an in-place assemblage "should display much more substrate bioturbation than will be displayed in allochthonous assemblages."²⁰²⁰  He rules Fossil Grove transported because its laminae survive.²⁰  The method is sound.  Applied consistently, a thoroughly bioturbated Cretaceous chalk or Ordovician inner shelf is an in-place community by Wise's own criterion, and those rocks are Flood deposits on his own stratigraphy.

Three published statements that the answers fail

Carl Froede Jr. reviewed the Gingras aquarium results for a creationist audience in 2009 and reported the awkward part first.  Some diluvialists expect little bioturbation because deposition was fast, others expect the record to alternate between intensely burrowed and unburrowed intervals:  "However, neither perspective is consistent with the actual rock record."³  Of a Skolithos horizon in the Meridian Sand at Campbell Mountain, Alabama, he wrote:  "Conditions were optimum for trace maker activity and the amount of time necessary to create these traces could be measured in months, not years or decades."³  Months, inside a year that also has to deposit the Phanerozoic.

Chad Arment, in Answers Research Journal in 2023, argued that Palaeocastor helical burrows in the Miocene Arikaree Group mark post-Flood strata, and rejected the standard young-earth alternative by name.  Michael Oard had written that such structures "represent animal activity during the Flood," made on "briefly exposed diluvial sediments."²¹²¹  Arment answers:  "Could Palaeocastor burrows have been formed during the Flood?  The amount of time necessary for burrow formation, then plant growth and burrow infiltration, then fossilization, at multiple vertical levels, clearly disconfirms this as a possibility."¹¹  And directly:  "Clearly, extensive plant growth, episodic and vertically staggered burrow formation, and a large terrestrial beaver population demonstrate that this phenomenon didn't 'briefly' occur during the Flood."¹¹  The root infiltration in that argument is the same evidence treated in Buried Soils.

Froede, Jerry Akridge and John Reed attacked Tim Clarey's megasequence model in 2022 using the same logic.  Genesis has land life dead by Day 150, so tracks made by living animals should stop there.  They do not:  "They are found well past the megasequence-defined Day 150 termination of all terrestrial life, as conveyed by Scripture."¹²  Their premise is that traces cannot be moved:  footprints "must be formed in situ."¹²  The megasequence framework they are attacking is described in Megasequences;  the vertebrate trackways themselves belong to The Coconino Sandstone.

An unresolved contradiction inside the movement

Woodmorappe's mechanism requires that burrowers were transported to where their traces are found.  Three other young-earth authors deny that traces can be transported at all.  Riley Isaacs writes that "ichnofossils are primarily autochthonous and only rarely transported."²²²²  Arment writes that burrows "by definition are non-transportable" and "Fossilization had to have occurred in place."¹¹  Froede, Akridge and Reed say tracks "must be formed in situ."¹²  Nothing found for this article adjudicates the disagreement, and the three in-place positions remove the mechanism the fourth depends on.

Snelling's other answer, that hardgrounds were cut, cemented, encrusted by oysters and re-eroded "within a few hours" by tidal surges, has the same shape:  a rate is asserted for the cementation and none for the encrusting animals' growth.⁴  Hardground borings are covered in Flat Gaps.

Objections and responses

"Most of the column is unbioturbated, so the Flood is fine."  Partly true, and the best version comes from mainstream work:  86% of a 430 metre Devonian section in Algeria, a Frasnian mean index of 2.0, lower Cambrian fabrics mostly at ii 1 or 2.  It fails as a generalisation.  The same compilation describes Cretaceous offshore and shelf sediments as completely homogenised, with reworking outpacing deposition, and on the young-earth models that put the Flood boundary in the late Mesozoic or the Cenozoic, which is most of them, those rocks are Flood deposits.  A model does not survive by fitting the quiet stretches.

"Burrows form in hours, so bioturbation is no clock."  Individual burrows do, and Bromley's textbook says pioneers colonise a fresh storm bed quickly.  The clock is not one burrow.  It is the number of times a surface has to be colonised, worked and buried to build a thick reworked interval, times the length of each cycle.  The one community-scale measurement in the creationist literature gives 15.5 to 4,020 hours for one 12 centimetre bed.

"Animals fleeing the Flood burrowed into briefly exposed sediment."  This is Oard's BEDS proposal, and it predicts shallow, single-episode, low-diversity escape structures on bedding planes.  It predicts against deep tiers, against burrows re-cut to follow a rising sediment surface, and against high trace-fossil diversity.  The Bright Angel Shale has 21 ichnogenera in 15 architectural designs, and the Tapeats has Diplocraterion with spreiten.  Arment, a young-earth author, rejects BEDS in print for the case Oard applied it to.

"Ichnofabric depends on oxygen, salinity and food supply, not on time."  It depends on all of them, and Bromley says so.  That is why a bare laminated bed is weak evidence for speed, and why this article does not argue from unburrowed rock.  The argument runs the other way.  A thoroughly homogenised bed needs animals, oxygen, food and time, and removing any of the first three explains an absence of bioturbation, not a presence of it.

"Woodmorappe's cemented crusts let several tiers be worked at once, so the surfaces need not be counted one at a time."  It is the best idea the movement has produced here, and it is untested.  Its author wrote that it "requires experimental validation," and twenty years later none was found for this article.  The pass condition is clear:  show that an early carbonate crust of achievable thickness deflects enough overburden for an animal beneath it to keep working, and that the resulting fabric can be told apart from stacked colonisation.

"This was settled by Droser and Bottjer in the 1980s, so why revisit it."  Because the mainstream position has moved, and it has moved towards the young-earth observation.  The 2025 compilation by Tarhan and co-authors, which includes Droser and Bottjer, reports large parts of the Palaeozoic as lightly mixed and shallowly burrowed, and states plainly where its own data are thin:  "with limited exception (e.g., the lower Paleozoic, the Permian–Triassic transition, and recent intervals of the Cenozoic), the data necessary to robustly quantify mixed layer depths are sparse to nonexistent."¹  Anyone citing the 1980s alone is citing an argument the field has since qualified.

"Ichnofabric index counts are subjective field estimates, not measurements."  Fair as far as it goes.  The scale was calibrated by planimeter but is applied by eye against printed cards, and its authors warn that cards drawn for one lithology do not transfer to another.  The disagreement here is not about a band or two.  It is between "the majority of the total thickness has an ichnofabric index of 3 or greater" and "there is so little bioturbation of sedimentary strata in the geologic record."

Issues and unanswered questions

• Five sources were not obtained in full. Taylor and Goldring 1993, the origin of the bioturbation index, is closed everywhere tried, so its scale is quoted only as reproduced by Zhang and colleagues. The Ekdale, Bromley and Pemberton 1984 short course is not online. Gingras and colleagues 2008 is closed, so only its abstract is used. Lohrer and colleagues 2005 is closed too; its abstract and Elsevier's published section snippets were obtained, and they are enough to show that Snelling's version of it overstates a modelled upper bound, but the full text and the origin of his "5 cm" are still out of reach; see quarantine. Baldwin and colleagues 2004 on Bright Angel ichnoguilds and Hagadorn and colleagues 2011 on Tapeats Arenicolites yielded abstracts only, and neither abstract carries an ichnofabric or bioturbation index; Raisanen and Hasiotis 2018 on Morrison Formation burrows was not chased.
• No continuous ichnofabric log of the Tonto Group appears to exist. The Grand Canyon evidence here is qualitative. Somebody should log the Tapeats, Bright Angel and Muav the way Droser and Bottjer logged the Great Basin.
• Brand and Chadwick's survey has never been published with numbers. The book describes it and gives no percentages. If a CRSQ, Origins, ICC or GSA version exists, it would materially change what this article can say.
• The colonisation-surface count has not been done by anyone, on either side. The inputs exist: published ichnofabric logs, measured burrow depths, and a measured range of reworking times. Producing the full grid rather than a point estimate would settle more than another round of quotations.
• The disagreement between Zhang and colleagues and Tarhan and colleagues about early Cambrian burrow depth is live and unresolved.

Verdict

Bioturbation is a real clock, and the young-earth literature agrees that it is.  Snelling says burrowed layers record "at least a few hours."  The dispute is only over how many hours, at how many levels.

The young-earth answer works for the rocks it was built on.  Large parts of the lower Palaeozoic are lightly and shallowly burrowed, and the best evidence for that is mainstream and recent.  The answer fails for the Mesozoic, where shelf sediments are homogenised through their thickness and mixing outpaced deposition, and those rocks fall inside the Flood year on the young-earth side's own stratigraphy.

The rate literature does not close the gap.  The single community-scale measurement in circulation is the fast end of a range reaching 24 weeks, taken from isolated burrowed horizons in laminated mud, which is the opposite of the setting needing explanation.  Run it through the movement's own time budget and it yields about 34 metres of heavily reworked rock for the entire Flood.

And the sharpest objections are already in print, in creationist journals, from Froede, Arment, and Froede with Akridge and Reed.  Nobody has answered them.

Sources