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This page continues the Chicxulub Discussion from Discussion Page 1. Newest contributions at the top.

Jan Smit writes: Keller et al.'s lengthy riposte (December 8, 2003) contains a few items of substance that I can comment on.

Those items remain important for the question that is at hand here at the Geolsoc debate: is there plausible evidence that the Chicxulub crater predates the K/T boundary?

1. Burrows in units 1 and 2
2. Significance of the 'sandy limestone layer'
3. Multiple spherule layers
4. Foraminiferal evidence from Yaxcopoil-1
5. Keller et al.'s conclusions

In their last response (8 December 2003) Keller et al. repeatedly mention (op. cit.): 'detailed empirical evidence', 'comprehensive evidence', 'large body of contrary evidence', 'enormous amount of new data', 'irrefutable evidence', 'empirical evidence', 'is multifaceted and very strong', that would underscore the hypothesis that the Chicxulub crater is older than the K/T boundary. However, time and again all these 'data' and 'evidence' are interpretations, not facts. 'The large body of contrary evidence to the K/T impact-tsunami hypothesis' contains very little substance as well.

However, I am more than willing to go back in the field with a bunch of knowledgeable sedimentologists, (better still, a truckload of curious and critical geology majors) and discuss (and unpick) in the field every piece of these alleged 'large bodies of evidence'.

Communication is a difficult matter, and apparently I do not get my message across to Keller et al. in a number of instances. Keller et al. consistently (mis)read what I have not written, or conclude that which I have neither written nor suggested. For instance, I wrote that only Mendez beds were fluidised, yet Keller et al. devote a whole section to argue that I have written that the so-called 'sandy limestone layer' is fluidised, which I never said.

Also, I said in my riposte (ironically) that their 'Fig 6b is not a 'sandy limestone layer', but instead 'Mendez marl clasts embedded in spherules'. By this I meant that Keller et al. made the mistake of confusing the layers with marl clasts and spherules with the hard 'sandy limestone layer' in their fig 6b (NOT me)!

Another inconsistency: They say: 'Mineralogical data also unequivocally show that these deposits are not tsunami events. I fail to see that mineralogical data can replace sedimentological field observations. What are the unequivocal 'mineralogical' data that are supposed to achieve this? If Keller means clay minerals and other phyllosilicates such as glauconite, Illite and smectite, than I am amazed that she finds this clay mineralogical can of worms 'unequivocal evidence'.

Keller in the same sentence writes: 'By definition, glauconite is a poorly crystallized illite enriched in K and Fe. And it is NOT an illite'. I am now totally confused, is it an illite or is it not?

Another slip of the pen: 'Smit denies that there are multiple horizons of burrowing within the fine-grained layers of unit 3 - calling them scratches, wasp nests and mud-filled rootlets'. What I actually wrote was: 'no burrows were found in Mimbral in the lower levels of units 1 and 2, nor in any other of the outcrops in eastern Mexico, but burrows do occur in unit 3, near the upper part'.

Furthermore, I have great reservations about the definitions Keller et al. use for the terms 'micrite', 'sand' and 'sandstone'2. I will try to clarify this terminological mess further down.

1. Burrows in units 1 and 2 - J-Shaped burrows.

Keller et al. write: "Smit reproduced these images and absurdly claims that they represent mirror images of one and the same burrow, one from the hand specimen and one from the field outcrop. This is nonsense"

Figure1a: Figures shown in Kellers riposte of Nov 19 of a J-shaped "burrow " from the "sandy limestone" from el Penon. The labeling of the Units is from Keller. It is clear that the four images are from the same burrow-like structure. Images W-X are from the counterpart of images Y-Z. Features such as grain size both in- and outside the J-shape are the same in each of the images, although that can be only properly judged if Keller would submit a sharper print of these images.

Here Keller is playing poker, and I called her bluff. I tried to say cautiously that Keller probably had multiplied one 'burrow' to look like four, and moreover, that in the labelling she placed these four 'burrows' in three different units: 1, 2 and 3. Now Keller says that: 'Smit is obviously confused about the J-shaped spherule infilled burrows in units 1 and 2. Contrary to his claim, we never found these burrows in unit 3'. That is not my claim, but Keller's, and I assume she made an error in figure 3b (reproduced from Ekdale and Stinnesbeck, 1998). To clarify the subject, I show here this figure (figure1), now with the labelling from Keller's reply of 19 November in the image.

Images W and X are, feature by feature, clearly the same. Similarly, Y and Z are the same, grain-by-grain, crack-by-crack. W-X and Y-Z are almost certainly mirror images, so I suggested that w-x is photographed from a counterpart hand specimen, because Y-Z is photographed in the field.

Am I wrong? I don't think so. Keller et al. claim that the 'burrow' in unit 2 is twice the size as the one in unit 1. Although the quality of images Y-Z is not the best (I hope Keller will show a few better ones) the visible grain sizes in the resized pictures from both the matrix and the spherule filing of the 'burrow' of w-x and y-z appear the same.

I repeat here: All of these pictures strongly suggest that they represent one and the same 'burrow' (if it is a burrow at all, what I contest), consistent with the conclusions from ichnospecialist Tony Ekdale (Ekdale and Stinnesbeck, l998): 'that Unit 1 does not contain burrows in any site, and that in Unit 2 only in Penon a few poorly defined burrows occur, which could have been excavated very quickly'.

Figure.1b Overview of the Penon outcrop showing a hard calcareous sandstone (labeled by Keller et al as Sandy limestone layer). Both Keller et al and we have searched the layer (see inset) for burrows. We found here (figure 1c) only one possible burrow-like feature, which turned out to be a rusty crack.

We very carefully searched the same outcrop (figure1b) and found only one 'burrow-like' feature (figure 1c), that on close inspection turned out to be an iron-oxide lined crack.

Figure 1c Burrow-like feature in the hard sandstone of figure 1b, that turned out to be a crack lined with iron oxides

Keller et al. show nice burrows from the Rancho Canales site (their figure16). My colleagues and I have also seen these burrows and agree (!) with Keller et al. that these are burrows. However, our interpretations are clearly different. I described (Smit, 1996) from the nearby site el Penon very similar 'glauconite' lined Ophiomorpha burrows, which I attributed to crabs/shrimps that were capable to burrow more than 70cm deep into sandstone (figure2a-c). We observed at el Penon more or less vertical bundles of tubes penetrating from the bioturbated surface of the clastic beds (unit3) (figure2c) to 70-100cm below the surface where the burrows spread out horizontally in a radiating and bifurcating pattern in one of the finer silt layers (figure2b). The glauconite lined burrows Keller et al. show from Rancho Canales are very similar to these vertical tubes from Penon, and I assume also those at Rancho Canales penetrate up to the surface of the unit 3 beds, although I could not observe that due to overlying beds. They are probably not truncated as Keller et al. claim.

Figure 2a Block-diagram of unit 3 at el Penon, showing the various ichnofabrics. The ichnofabric tiers in the upper levels of unit 3 are diverse and contain Zoophycos, Chondrites, Planolites and deeper burrowing ophiomorpha-like structures. The vertical bundle of tubes can penetrate up to 100 cm down into unit 3 from the top of the clastic beds. These vertical tubes are very similar to those as shown in Keller et al's figure 16.

Keller et al. show in their figure 17 a few U-shaped features from the base of the clastic beds at los Ramones. They claim these to be 'common U-shaped Rhizocorallium burrow sticking out of the lower surface of a sandstone layer'.

Figure 2b Images of the horizontal Ophiomorpha bifurcating burrow-tubes radiating from the vertical bundle of tube structures that reach 70cm up to the surface of the clastic beds.

Figure 2c Close up of the vertical bundle of ophiomorpha burrows. A) View on the bedding plane where the bundle of over 20 tubes disappears to deeper levels. B) Vertical cross section through a current rippled bed of the top of unit 3, showing a few "glauconite" lined tubes of a vertical bundle of burrows.

We have also observed these features (figure3a, b), and conclude that these cannot be 'Rhizocorallium burrows' for two reasons:

1. Rhizocorallium burrows are common in shallow water coastal environments, and invariably occur at the top of a sandstone layer. In other words, the Rhizocorallium animal prefers to burrow into sand. At Los Ramones they occur on the underside of sandstone layers. This means that if they are Rhizocorallium, they first had to be excavated in the underlying Mendez shale, and subsequently filled (as cast) by the sand from the sandstone. This seems very unlikely regarding their living environment. But even if these features would represent burrows, they would then be of Mendez (=upper Cretaceous) age, preceding the deposition of the sandstone beds, NOT during the deposition of the beds, and therefore irrelevant for the discussion.
2. The 'Rhizocorallium burrow' shown by Keller et al. are not burrows, but flute casts (figure3 a, b). We have observed many flute casts at the Ramones locality, curiously enough cast on the top of 10-50cm sized clasts of Mendez shale. The large clasts are now eroded, so what remains are the holes where they once existed, and the flute casts are lining the holes. The u-shaped features we observed are all flute casts, and although the figure17 of Keller et al. is not the best quality, we assume that the U-shaped features Keller et al. claim to be Rhizocorallium are actually also flute casts

Figure 3a Flute casts gauged into a Mendez shale boulder in the channels axis of the channel at Los Ramones. The U-shaped flutes are reminiscent of the U-shaped "burrow like feature" Keller et al show in their figure 17 (inset)

At last, Keller et al. claim that there is 'a simple test that can establish whether the burrowing community lived in Tertiary sediments', because the burrows would be filled with Tertiary sediments. 'Tertiary' sediments are defined by index fossils of Tertiary age, but it is well known that in the first few thousands of years of the Tertiary, in the base of the P0 zone, such Tertiary fossils do not exist yet. Therefore, if the burrow-fabric in the top of Unit 3 was formed at that time, before or during deposition of the base of P0 -not inconceivable- than the burrows could not be filled with Tertiary fossils. The 'simple test' is therefore not as simple as Keller et al. think.

Figure 3b Stereo pair of images of flute casts that frequently show up on the Mendez boulders that occur in the base of the clastic beds in the channel axis at Los Ramones. We did not find true Rhizocorallium at this lower level, but some u-shaped burrows were observed at the rippled and bioturbated top of the clastic beds at this locality.

2. Significance of the 'sandy limestone layer'?

Keller et al. claim that the so-called 'sandy limestone layer' is a hemipelagic limestone: 'Hemipelagic limestone in spherule unit 1'? 'This SLL indicates that spherule deposition within unit 1 occurred in two phases interrupted by long-term hemipelagic sedimentation'.

I strongly question that this sandy limestone layer is a hemipelagic sediment:

1. It is a lithified sandstone, usually a grainstone, with foraminifers and a few lithic grains as principal components, sometimes with well-preserved spherules (Figure 4a,b)
2. Hemipelagic means that the components are derived from the fine-grained pelagic rain, such as the hundreds of metres thick adjacent hemipelagic Mendez and Velasco formations. The 'sandy limestone' is a laminated, sometimes cross-bedded and often channel filling sandstone, and therefore completely different from the Mendez or Velasco formations. That is confirmed by Keller's own granulometric analysis shown in her figure 13 (figure4c)
3. Hemipelagic beds are, by their very nature, continuous and continuous in thickness over large distances. The 'sandy limestone layer' is not even continuous over more than 10m (cf. Figure1c)

Figure 4a - Images of the "sandy limestone layer" at El Mimbral, from the outcrop as described in Keller et al. (1994) as such. This layer contains many spherules with bubble cavities that are characteristic for the impact glass particles at the KT boundary in and outside the Chicxulub crater. The white particles are limestone fragments, probably also ejecta because these co-occur only with the bubbly spherules.

Figure 4b - Detail (A) and transmitted light thin section image (B) of the "sandy limestone layer" of the location of 4a. Sand-sized grains, mostly planktic foraminferal tests filled with a green clay mineral, demonstrate that the limestone is a calcarenite with a sparry calcite matrix, with interspersed spherules (although these spherules may not occur within similar layers in other outcrops). Therefore this layer qualifies as high-energy sandstone, not a hemipelagic limestone.

Figure 4c - Laser grain size analyses of the clastic beds at El Penon. The two analyses are quite comparable, although there are differences in detail. The "sandy limestone layer" (SLL) is clearly coarse grained, and cannot be a hemipelagic layer.

3. Multiple spherule layers

Keller et al. try to downplay the evidence for slumping and soft-sediment deformation as explanation fo the occurrence of multiple spherule layers. However, I am not the only one that is aware of this widespread mechanism. I reproduce here two figures (Figure 5) from the excellent PhD thesis of Peter Schulte, where he graphically displays a model for the deposition and synsedimentary deformation of the clastic beds. I completely support his model, but one should be aware that the situation may differ from outcrop to outcrop.

Figure 5 - Coherent models for the deposition of the clastic beds in the Gulf of Mexico. (from Schulte, 2003). These models are completely compatible with deposition related to the Chicxulub impact and related phenomena, such as earthquakes, tsunamis and gravity flows.

In my reaction to Markus Harting (17.12.03) I showed from el Penon in Image 2 two 'layers' of spherules, the lower one inclined and discontinuous. Superficially, there are multiple spherule layers in many outcrops in eastern Mexico and the US Gulf coast (Brazos river, Moscow Landing), but those layers are all discontinuous, and not separated by normal hemipelagic Mendez layers. Digging a trench may encounter such additional layers, but it is impossible to verify any lateral continuity this way.

4. Foraminiferal evidence from Yaxcopoil-1

Keller still maintains that the rhomboid crystal forms she showed in figure 22 are foraminiferal tests embedded in a micritic matrix. Although Arz found some foraminiferal tests in the same interval, they are not the same as the ones shown by Keller; the features Keller shows are dolomite rhombs. To support my argument I show here a few more backscatter SEM micrographs from these, what Keller et al. claim, 'micritic intervals' (figure5a).

Figure 6a SEM backscatter images of five samples from the transition impact to post-impact rocks of the Yaxcopoil-1 drill hole, drilled within the Chicxulub crater. (Click on the sample numbers to get a detailed image). If foraminiferal tests, consisting of calcite, were present, they would show up as light reflections just as the interstitial sparry calcite.

In Figure 5b the backscatter SEM image is combined with X-ray Ka mapping of Mg, Si and Ca.

Micrite is defined as "an abbreviation of 'microcrystalline ooze', or as "microcrystalline calcite" (Folk, 1959) or an "aggregate of CaCO3 crystals less than 4 microns in size?, and in a wide sense as: ?finely crystalline calcium carbonate of almost any sort". As is clearly shown on figure 5a-c, there is no micrite present, and the vast majority of the grains are made of dolomite. The whole texture of this interval 794.11-794.85 is one of a coarse to fine sandstone, not of a pelagic micrite.

We can bring this round of the debate to a solution, by performing a simple test.

Figure 6b SEM backscatter image of sample 315, with X-ray Ka mapping of Mg, Si, and Ca of the same area. The dark zoned rhomboid crystals are clearly dolomite. The light areas are sparry crystals of calcite, which fill the space between the dolomite crystals. Hexagonal crystals within the calcite are low-temperature quartz, diagenetically growing inside the calcite. The darkest areas between the dolomite and calcite show Si Ka reflections, most likely phyllosilicates, such as smectite, or "glauconite". If micrite were present, it would show up as light reflections like the calcite.

Figure 6c - SEM backscatter overview of samples 316(A), 314 (B), and enlargement of 314 (C). Dark areas are dolomite, black areas are filled with phyllosilicate, and light areas are calcite. The light-grey areas are feldspar.

Of all the levels I have sampled (figure 6) polished thin sections were prepared, and coated with carbon and analysed on the SEM and on the Electron microprobe. Those thin sections are available to anyone who wants to look for foraminifera and dolomite crystals, and with SEM in backscatter mode it is easy to distinguish dolomite from calcite. Foraminiferal tests are made of calcite, not dolomite, and can be easily distinguished by this technique. I don't know if Gerta Keller has prepared (polished) thin sections without cover slip, but our technicians can easily remove the cover slip, and polish the surface carefully without destroying the thin section. After carbon coating it should be an easy matter to relocate the forams/dolomite rhombs shown by Keller et al. in their figure 22 (and the other plates she has shown on her website http://geoweb.princeton.edu/people/faculty/keller/chicxpage2.html#28 )

And distinguish the dolomite rhombs from a calcitic foraminiferal test.

I hope someone will rise and act as impartial moderator to perform this test.

Figure 7 Image of the core-segment of the Yaxcopoil-1 drill hole that represents the transition from impact to post-impact rocks. Smit (306-325) and Keller (K1-K21) have analyzed samples labeled. The red labels indicate the samples shown in figure 5a, from similar levels as those analyzed by Keller (green labels) and claimed by Keller to be rich in foraminifers. As can be clearly seen on figure 5a, no cross-sections of foraminifers are visible.

The interval 793.85-794.11m is uncontested Paleocene micritic hemipelagic wackestone, rich in Paleocene foraminifers. The interval 794.11-794.19m represents a hardground, strongly burrowed. The interval 794.19-794.60m represents cross-bedded and parallel-bedded sandstones, mainly composed of dolomite rhombs as can be seen in figres5a-c.

5. Keller et al.'s conclusions - numbered comment on each

The age of the oldest Chicxulub impact ejecta spherule layer in NE Mexico predates the K/T boundary by 300,000 years.

• Evidence includes multiple spherule layers with the oldest one near the base of zone CF1, which spans the last 300 kyr of the Maastrichtian

• The spherule layers are interbedded in undisturbed bedded marls of the Mendez Formation, which reveals the age of deposition

• Spherule layers are correlatable over great distances

• The stratigraphically lowest and oldest spherule layer consists of almost pure spherules and only rare clasts, which indicates rapid deposition after the impact and no significant bottom currents

• Subsequent spherule layers contain variable amounts of reworked clasts indicating erosion and re-deposition

• The absence of major tectonic disturbance, including major slumps, faults or fluidised sediments

• The stratigraphically highest and youngest spherule layer occurs just below the siliciclastic deposit and is known as spherule unit 1. It contains the most abundant reworked shallow water debris and mud clasts, indicating transport from shallow shelf areas

• A sandy limestone layer within spherule unit 1 indicates that deposition occurred in two phases separated by hemipelagic deposition

• The K/T boundary, Ir anomaly and mass extinction occurs above the siliciclastic deposit and represents the true K/T impact event

• The Chicxulub impact-tsunami hypothesis is invalid

• There are various fine-grained layers, often bioturbated within unit 3 that indicate normal hemipelagic sedimentation alternating with rapid deposition

• Two bentonite layers in unit 3 are correlatable across NE Mexico and indicate periods of volcanic influx and normal deposition

• A sandy limestone layer within spherule unit 1 is burrowed and also indicates a period of hemipelagic deposition

• The age of the Chicxulub impact breccia at Yax-1 predates the K/T boundary

• Four thin layers of glauconite formation within this interval that indicates very low sedimentation over a very long time (l05 yrs)

• Bioturbation within these sediments that indicates an active bottom dwelling fauna during deposition.

• Palaeomagnetic chron 29r that marks the last 500Ka of the Maastrichtian.

• Carbon isotope values characteristic of late Maastrichtian sediments and without evidence of erratic changes that would indicate reworking.

• Absence of impact breccia clasts, or reworked clasts from lithologies below the impact breccia.

• Absence of reworked fossils from older sediments.

• Absence of high-energy deposition, backwash, slumps, crater infill.

• Absence of Cheto smectite that would indicate presence of altered impact glass.

All of these arguments raised by Keller et al. can be easily countered.

But Keller et al. do not have an answer on the fact that in coal swamp deposits over the entire US western Interior a clay layer with iridium, shocked quartz and shocked zircon with Chicxulub characteristics, lies directly on top of a layer with spherules identical to those spherules in the Gulf of Mexico even Keller et al. consider as derived from Chicxulub.

To squeeze 300kyr between those two amalgamated layers in all Western Interior outcrops requires a bizarre miracle.

Therefore, the K/T boundary impact and the Chicxulub impact solidly remain one and the same.

Read this page on Jan Smit's website

References

Ekdale, A. A. and W. Stinnesbeck (1998). Palaios 13(6): 593-602.

Keller, G., W. Stinnesbeck, et al. (1994). "Age, deposition and Biotic effects of the Cretaceous/Tertiary boundary event at Mimbral, NE Mexico." Palaios 9: 144-157.

Schulte, P., 2003, The Cretaceous-Paleogene transition and Chicxulub impact ejecta in the northwestern Gulf of Mexico: Paleoenvironments, sequence stratigraphic setting and target lithologies [PhD thesis]: University of Karlsruhe, 204 p.

Smit, J., T. B. Roep, et al. (1996). Geol. Soc. of Amer. Sp. Pap. 307: 151-182.