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Drilling debates: Is the Chicxulub crater the K/T
boundary killer or Not? A reaction to the
statements by Keller et
al, posted on the
GeolSoc forum "Old
evidence, partial interpretation"? Not a single
piece of evidence holds up that Chicxulub is older
than the K/T boundary
Chicxulub Drilling debates: Is the Chicxulub crater the K/T boundary killer or Not?
A reaction to the 4-page statements by Keller et al, posted on the GeolSoc forum
"Old evidence, partial interpretation"? Not a single piece of evidence holds up that Chicxulub is older than the K/T boundary
Keller et al's lengthy riposte of 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 unscrew in the field every bit and piece of these "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 what I have neither written nor suggested. For instance, I wrote that only Mendez beds were fluidized, yet Keller et al devote a whole section to argue that I have written that the so-called "sandy limestone layer" is fluidized, what I never did.
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 by confusing the layers with marl clasts and spherules with the hard "sandy limestone layer" in their fig 6b, NOT me!
Another inconsistency: "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 &endash; calling them scratches, wasp nests and mud-filled rootlets". Instead I wrote "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. I will try to clarify this terminological mess further down.
Burrows in units 1 and 2
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 "
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 labeling 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 that figure 3b (reproduced from Ekdale and Stinnesbeck, 1998). To clarify the subject, I show here this figure (figure1), now with the labeling 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".
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.
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.
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 ".
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.
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 unconceivable- than the burrows could not be filled with Tertiary fossils. The "simple test" is therefore not as simple as Keller et al. think.
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 meters 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 Kellers 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)
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 (http://wwwrz.rz.uni-karlsruhe.de/~de57/Geo/Staff/Schulte/), where he graphically displays a model for the deposition and syn-sedimentary deformation of the clastic beds. I completely underscribe his model, but one should be aware that the situation may differ from outcrop to outcrop.
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.
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).
In Figure6b 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.
Of all the levels I have sampled (figure6) polished thin sections were prepared, and coated with carbon and analyzed 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.
Keller et al's conclusions (and comment to each of those)
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."
1) These extra layers are all anomalous and discontinuous, and can be deposited in a time span of<10years
"The spherule layers are interbedded in undisturbed bedded marls of the Mendez Formation, which reveals the age of deposition. "
2) These Mendez beds are not undisturbed.
"Spherule layers are correlatable over great distances."
3) Even the so-called basal original layer cannot be correlated over more than ten meters.
"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."
4) The "lowest" layer is highly variable in composition, and differs from outcrop to outcrop.
"Subsequent spherule layers contain variable amounts of reworked clasts indicating erosion and re-deposition."
5) Erosion and redeposition should be expected from large tsunami waves, or tsunami triggered gravity flows, and are not an argument pro or con.
"The absence of major tectonic disturbance, including major slumps, faults or fluidized sediments."
6) The slumps and fluidization of the topmost Mendez beds are there for all to see, in all localities with "multiple layers" or oversteepened channel walls.
"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."
7) Backwash from a tsunami wave, or from a tsunami triggered gravity flow (turbidite) is expected to bring shallow water debris.
"A sandy limestone layer within spherule unit 1 indicates that deposition occurred in two phases separated by hemipelagic deposition. "
8) Nonsense. The " Sandy limestone layer", or calcarenite, is current transported sandstone, not hemipelagic sediment.
"The K/T boundary, Ir anomaly and mass extinction occurs above the siliciclastic deposit and represents the true K/T impact event."
9) The iridium anomaly occurs in the top of the clastic beds, not above. The mass-extinction coincides with the deposition of the clastic beds, because there are no Cretaceous hemipelagic beds above the clastic beds.
2. "The Chicxulub impact-tsunami hypothesis is invalid"
This hypothesis was designed to explain the presence siliciclastic deposit between the K/T boundary above it and the Chicxulub spherule ejecta below. This hypothesis is invalid for many reasons, but the major ones include:
There are various horizons of bioturbation within units 1, 2 and 3 of the siliciclastic deposit, which indicate repeated colonization of the ocean floor during sedimentation and hence rules out a tsunami deposition event.
10) Units 1 is devoid of burrows, unit two contains only the one questionable "burrow" shown by Keller, which can be explained, because it is so unclear, by various other mechanisms (e.g. as a flame structure). The various levels of bioturbation at unit 3 can all be explained by one colonization episode after deposition of the clastic beds
There are various fine-grained layers, often bioturbated within unit 3 that indicate normal hemipelagic sedimentation alternating with rapid deposition.
11) These layers are all silty, and different in texture from the underlying Mendez and overlying Velasco hemipelagic marls, that represents the normal hemipelagic sedimentation in eastern Mexico
Two bentonite layers in unit 3 are correlatable across NE Mexico and indicate periods of volcanic influx and normal deposition.
12) That evidence solely rests on the occurrence of zeolites, not an unequivocal volcanic indicator. Those layers in unit 3 are also very different from the many true bentonites that occur in the Mendez. It is no surprise to find volcanic material in the clastic layers reworked from those benthonites.
A sandy limestone layer within spherule unit 1 is burrowed and also indicates a period of hemipelagic deposition.
13) One burrow does not make a "bioturbation", and the "sandy limestone (=calcarenite) is certainly not hemipelagic
3. The age of the Chicxulub impact breccia at Yax-1 predates the K/T boundary.
Critical evidence is in the 50cm interval between the impact breccia and the K/T boundary and includes the presence of:
Low energy laminated micrites and dolomitic limestones between the impact breccia and the K/T boundary.
14) This micrite (=usually nannofossil ooze) does not exist, and the sandstone textures do not imply low-energy.
Four thin layers of glauconite formation within this interval that indicates very low sedimentation over a very long time (l05 yrs).
15) The glauconite particles are texturally very similar to the bubbly altered tektites from all over the Gulf of Mexico, and not a unique indicator of shallow water environments.
Bioturbation within these sediments that indicates an active bottom dwelling fauna during deposition.
16) Bioturbation is restricted to the interval 794.11-794.19m, and indicates a hardground at level 794.11, indicating indeed missing time!
Late Maastrichtian planktic foraminiferal assemblages of zone CF1, indicative of deposition during the last 300Ka, similar to NE Mexico.
17) Plummerita hantkeninoides, the index for CF1, is extremely rare in normal foraminiferal associations, and can be distinguished from Rugoglobigerina by its delicate spines. Even in well-preserved material it would be a miracle that the spines are visible in thin section, let alone in this dolomitic sand.
Palaeomagnetic chron 29r that marks the last 500Ka of the Maastrichtian.
18) No argument. All possible events occur in chron 29R
Carbon isotope values characteristic of late Maastrichtian sediments and without evidence of erratic changes that would indicate reworking.
19) I question this interpretation. Carbon isotope values are from either the dolomite crystals, or from the well-crystallized interstitial diagenetic calcite.
Absence of impact breccia clasts, or reworked clasts from lithologies below the impact breccia.
20) The glauconite pellets are from the impact breccias.
Absence of reworked fossils from older sediments.
21) Arz et al found Albian fossils
Absence of high-energy deposition, backwash, slumps, crater infill.
22) Those cross-bedded and parallel-bedded sandstones are high-energy deposits
Absence of Cheto smectite that would indicate presence of altered impact glass.
23) Impact glass does not necessarily alter to cheto-smectite.
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.
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.
Look here for the Yaxcopoil-1 core segment 793.85 to 794.60 m: the transition of the impact to post-impact crater infill,