The age of Chicxulub event and the K/T-boundary debate

(on base the drilling-cores of Yaxcopoil-1 and Chicx-3A/M007A)

Norbert Brügge, Germany


K/T-boundary definition

The correct definition of the K/T-boundary based on biostratigraphic parameters. Only the biostratigraphically high-resolution processing of the K/T-boundary allows the determination of the absolute duration of the biozones, if they are undisturbed. They allow unlike to radiometric (e.g. 40Ar/39Ar) and magneto-stratigraphic methods a higher resolution. They have lower errors as this radiometric methods (recrystallization, isotopic fractionation, and alteration processes remagnetization).

The boundary between the Cretaceous and Tertiary periods in the sediments around the world are can accurately determine based on such biostratigraphic parameter, unless a hiatus occurs (breaks in sedimentation). In dependence on the respective facies are the K/T-boundary layers worldwide significantly different.

The unusual so-called P0-horizon ("Boundary Tone") documented a catastrophic period with a mass extinction of fauna species. Thereafter is visible a change of the fossil content. Tropical and subtropical Cretaceous foraminifera such Abathomphala mayaroensis or Plummerita hantkenioides quickly disappear on top of the Maastrichtian.
First tertiary species such Parvularugoglobigerina eugubina or P. longiapertura be detected, further tertiary species appear for the first time.

Obviously, there are also directly relations to the K/T-boundary by lithological and geochemical effects - such as the switched "Boundary Tone" and a significant iridium anomaly. Stable isotopes reveal typical late Maastrichtian carbon signals followed by the negative excursion that characterizes the K/T-boundary.

In outcrops of Central America the K/T-boundary can only determined with doubt because in these sections unfavorable conditions prevails for foraminifera.


1. The K/T-boundary debate (Yaxcopoil-1; Unit 794.60 - 794.11 mbsf)

In 2002, the so-called Chicxulub Scientific Drilling Project was conducted. The core hole Yaxcopoil-1 reached a depth of 1511 meters and promoted an almost complete core. The stratigraphic congruent of the Chicxulub event and the K/T- boundary is subject to ongoing discussions since then, because in the borehole Yaxcopoil-1 about 0.50 m thick normal marine sediments of Upper Maastrichtian above the "suevite" were found. From the stratigraphic position would be Chicxulub event and the K/T-boundary no longer congruent. (G. Keller et al, 2004). It was determined a time span between the Chicxulub "impact" event and the K/T-boundary by about 300,000 years.
The published results and conclusions of the group Keller/ Stinnesbeck/ Addate led to violent reactions in the professional world.
Unfortunately, the findings from the borehole Yax-1 in outcrops of Central America could not be confirmed unequivocally. Above all, the layers with spherules are inconsistently and not assignable to a specific event, also not to the Chicxulub event.

1.1 Petrography of the unit 794.60 - 794.11 mbsf

"This 50-cm-thick interval is between the disconformities at the top of the breccia and the K-T boundary. The interval consists predominantly of laminated micritic limestone, with microlayers or patches of anhedral dolomite crystals and a 5-cm-thick coarse dolomite layer at the base. The micritic limestone indicates deposition under low energy quiet water deposition, whereas the dolomite formed by diagenetic replacement of the precursor limestone with the original laminated texture still visible. In the lower 20 cm of this unit are three thin (1 cm) layers with oblique bedding, which may have formed from weak bottom currents by slightly agitated waters. However, the absence of grain size changes suggests that this may be diagenetic feature or related to settling. Five thin green clayey microclast layers are embedded in the laminated limestones at 794.43, 794.34-794.35, 794.24, 794.19 and 794.11 m of which the latter marks the K-T boundary and a significant hiatus. The insoluble residues of these intervals reveal that the microclasts are of glauconite origin and/or have in situ glauconite coating. No altered glass is present. For comparison, we have analyzed four samples from the breccia at depths of 827.81, 851.02, 861.74 and 876.37 m. XRD analysis of these intervals show the presence of Cheto smectite, which is characteristic of altered glass. Glauconite forms at the sediment-water interface with very slow detritus accumulation. The five microclast layers therefore reveal long pauses in the overall quiet depositional environment with reduced sedimentation and the formation of glauconite followed by sediment winnowing, clast generation and small-scale transport by minor current activity.
Sediment deposition in a normal low energy marine environment over an extended time period is also indicated by the burrowed glauconitic horizons and the heavily bioturbated uppermost 10 cm of this 50-cm-thick interval.
Microfossils within this 50-cm-thick interval also indicate deposition in a normal marine environment over a long time period and characteristic late Maastrichtian assemblages provide critical age control.
Examination of thin sections reveals the laminated micritic intervals to be rich in late Maastrichtian planktic foraminifera, although they are invariably recrystallised and poorly preserved in micritic limestones. The recrystallization process, however, retained the species morphology and the lighter colored shell calcite with respect to the surrounding micrite. A number of species have been identified and date this interval as latest Maastrichtian". Source: Stinnebeck et al. (Geologische Rundschau, 93, 2004)



K8:Glauconitic layer



K9: Bioturbation













K20: Foraminifera

K20a: Foraminifera



Source: Homepage J.Smit (13)

1.2 Biostratigrahic interpretation by G. Keller et al.
(Meteoritics & Planetary Science 39, Nr 7, 2004)

Foraminifera from unit between breccias and Danian layers



"A careful thin section analyses of the laminated limestone revealed a diverse late Maastrichtian planktic foraminiferal assemblage and few low oxygen tolerant benthic foraminifera (mostly buliminellids). The species are invariably recrystallized and poorly preserved, as would be expected in micritic limestones. But, the recrystallization process generally retained the original species morphology, and lighter colored chamber infillings make them clearly recognizable with respect to the surrounding micrite."

"The first diverse and abundant planktic foraminiferal assemblage is present 5 cm above the impact breccia (sample 20, 794.55–794.56 m) and includes species of Globotruncana (G. stuarti, G. insignis), Rosita contusa, Abathomphalus mayaroensis, Rugoglobigerina (R. rugosa, R. macrocephala), Plummerita hantkeninoides, Globotruncanella petaloidea, Heterohelix, Hedbergella, and Globigerinelloides. Most of these species can also be identified in the other laminated intervals, including sample 9 (794.13–794.14 m) at 2 cm below the green clay that marks the K/T boundary but not above it. Planktic foraminiferal assemblages are, thus, of high diversity with large and small, fragile and robust species present, and all are characteristic of the late Maastrichtian. Such uniform assemblages cannot be explained by backwash and crater infill, but they are consistent with in situ deposition in a normal low energy marine environment."

"The age of this late Maastrichtian assemblage can be further narrowed based on the presence of the biozone CF1 marker Plummerita hantkeninoides, which spans the last 300 kyr of the Cretaceous. This is consistent with the magnetostratigraphic results at Yax-1 and indicates that the laminated limestone and glauconite layers between the breccia and K/T were deposited sometime during the last 300 kyr of the Maastrichtian. The K/T boundary is marked by a 2-3 cm-thick dark gray-green marly limestone with a 3 to 4 mm-thick green glauconite clay that marks an erosional disconformity. Most Cretaceous planktic foraminifera disappear at this interval. Only very rare early Danian foraminifera are preserved in the green clay (as a result of downward reworking) that marks the K/T hiatus."

"The most significant evidence of deposition in lowenergy environments is revealed by five green clayey glauconite layers embedded within the laminated limestone at 794.43, 794.34–794.35, 794.24, 794.19, and 794.11 m, with the latter at the K/T boundary. The microclasts of these layers are of glauconite origin and/or have in situ glauconite coating, as revealed by environmental scanning electron microscope (ESEM) and XRD analyses."


1.3 Biostratigrahic interpretation by J.A. Arz et al. (Meteoritics & Planetary Science 39, Nr 7, 2004)


Heterohelix globulosa (6);  Guembelitria trifolia (7)

Rugoglobugerina scotti

Planomalina buxtorfi

Hedbergella planispira

Praeglobotruncana delrioensis

8-10:        794.31 m
7, 16-18:  794.33 m
13-15:      794.37 m
6, 11-12:  794.44 m

"Higher up in the K/P cocktail unit, and overlaying the redeposited suevite, there is a 46 cm-thick dolomitic calcareous sandstone unit with common amalgamated dolomite crystals. We studied 13 samples from this unit (794.60 m to 794.14 m), where three subunits can be recognized. The lower subunit consists of a 24 cm-thick (794.60 m-794.36 m) crossbedded body of very dolomitized, calcareous sandstones with some microconglomerate layers intercalated. The cross-lamination and the microconglomeratic layers indicate high current activity during its deposition. The middle subunit consists of a 17 cm-thick (794.36 m-794.19 m), predominantly laminated, dolomitic calcareous sandstone layer. The upper subunit consists of a 5 cm-thick (794.19 m-794.14 m) bioturbated, dolomitic calcareous sandstone bed, the top of which represents an omission surface. In spite of the fact that we intensively scanned the sediment in this interval, foraminifera are practically absent in the 13 samples recovered from the dolomitic calcareous sandstone unit, though some reworked specimens were found. The dolomitization processes affecting these rocks might be one of the reasons for the scarcity of foraminifera in this unit. A 3-4 cm-thick (794.14 m-794.11 m) laminated marly clay layer with ichthyoliths overlies the dolomitic calcareous sandstone unit. Sample 794.11 m is barren of planktic foraminifera, and it yields only few benthic specimens".
"In the 46 cm-thick dolomitic calcareous sandstone unit overlaying the redeposited suevite (794.60-794.14 m) at Yaxcopoil-1, we found only 11 planktic foraminiferal specimens that belong to the species Guembelitria cretacea, G. trifolia, Heterohelix globulosa, H. navarroensis, Praeglobotruncana delrioensis, Hedbergella planispira, Planomalina buxtorfi, Rugoglobigerina scotti, and Globigerinelloides volutus. They represent reworked, mixed assemblages containing species from the Albian-Turonian (P. delrioensis, H. planispira, P. buxtorfi) and Campanian-Maastrichtian (H. navarroensis, R. scotti)."

1.4 Ripostes by J. Smit et al. (Meteoritics & Planetary Science 39, Nr 7, 2004)
"The core interval 794.60-794.11 m contains the critical transition from the  impact  to  post-impact  lithologies. The transition is visually sharp and marked by a dark 2 cm-thick clay layer at 794.11 m. It is tempting to equate this level to the K/T boundary, but that is not correct."
"The unit probably represents a hardground. The rock is strongly indurated. Several round, non-collapsed burrows filled with dolomite, crystalline lithoclasts, or smectite blebs can be observed. As all burrows in the overlying sediment are strongly compressed due to compaction. Reworked bioclasts containing indeterminate foraminifers increase upward. The top 2 cm grades into a dark clay at 794.11-794.10 m. This interval displays an increasing number of horsetail laminations."
"But, is the Yax-1 section really complete? Several lines of evidence indicate that it is not. Therefore, the Yax-1 drill core cannot be used to test the above mentioned hypotheses. The crossbedded interval from 794.60-794.19 cm contains several levels (at 794.52 and 794.40 m) where overlying crossbedded sediments onlap on a previously lithified surface. This suggests a hiatus at these levels."
"Several burrows are round and non-compressed, suggesting that they were made by lithofagous borers after lithification and remained open when they were filled by clay and fragments of green clay blebs and dolomite clasts. The overlying 2 cm-thick clay layer seems to be a residual clay, left after dissolution of carbonate. The typical horsetail laminae, similar to stylolites, suggest strong dissolution, and the abundance of fish remains also suggests the presence of an omission surface that is often enriched in residual fish debris. The clay layer itself, if comparable to the global K/T boundary clay, would be enriched in siderophile elements and might contain microkrystites."
"These lines of evidence suggest that the clay is a residual clay, overlying a hardground surface, both indicative of a significant hiatus rather than being the equivalent of the K/T boundary clay. The magnitude of the hiatus is difficult to assess. The foraminifers in the first  post-impact  background infill are strongly dissolved, and the interval may be condensed."
"Since part of the top of the ejecta sequence in Yax-1 is eroded, and a hiatus of up to 330 kyr is present at the top of the ejecta, the hypothesis Stinnesbeck et al. (2001) cannot be accepted or rejected on the basis of Yax-1 data. The suggestion by Keller et al. (2004) that the crossbedded interval 794.6-794.11 contains an indigenous Cretaceous planktic fauna is highly unlikely for several reasons:
 1) we did not find Cretaceous taxa but found zoned dolomite rhombs instead. Arz et al. (2004) found just a few badly preserved specimens in samples from the same levels. The size of the rhombs and the thickness of the zonal overgrowth of the dolomite rhombs is comparable to the thickness of a foraminiferal shell, creating the illusion of foraminiferal shells when two adjacent dolomite rhombs are positioned together.
 2) even if Cretaceous taxa are present, their presence within crossbedded sandstone layers suggests that they are reworked from older units, and therefore, they cannot be used for a biostratigraphic age."

1.5 My opinion to the debate
PEMEX investigators of the l970’s (Meyerhoff et al and Lopez-Ramos) reported limestones with diverse planktic foraminiferal assemblages of upper Maastrichtian age overlying the breccia in wells Chicxulub-1 and Yucatan-6. An interval of 18 m-thick Maastrichtian marls overlying the "impact" breccia at the C-1 well. It was identified the planktic foraminiferal species Globotruncana rosetta, G. ventricosa, G. lapparenti, G. fornicata, Pseudoguembelina excolata, Heterohelix globocarinata, Pseudotextularia elegans, Planoglobulina carseyae, and Globigerinelloides volutus in this interval and conclude that the Chicxulub event occurred before the K/T-boundary.
By Keller et al (2004) in Yaxcopoil-1, unit 794.60-794.11m, detected Maastrichtian foraminifera are unfortunately dolomitic crystallized and give rise to discussion. Smit interpret these forms as dolomite rhombs.
Nevertheless, the conclusions of Keller et al. are right, and the Chicxulub event has nothing to do with the K/T-boundary. The K/T-boundary is biostratigraphically in profile clearly placed 0.50 cm above the breccias. The discussed sediments in the unit above the breccia are no catastrophic or hardground deposits (in sense Smit), they in contrary are layered in a normal marine milieu with moderatly movements during a longer period of time (300 000 years ?).

After the event the devastated land surface, after a time gap, was flooded again by a marine transgression. The first marine deposits consists of fine material (volcanic mud?) that may have been transported mostly fluviatile. A portion of the sediments shows millimeter fine laminations. This indicates slightly increased water-energy for the deposition on ground. Moreover, in the sediments was bioturbation detected. Striking are inserted thin layers of green clay (or glauconite) indicate altered sedimentation conditions. The last "green clay" layer (794.12m) is markedly and could be the equivalent of the "Boundary Tone". The bubble-like inclusions in the greenish layer could be former spherules from the widespread volcanism after the Chicxulub event.

The conclusions by Arz et al. (2004) for the Yaxcopoil-1 unit 794.60-794.11m are not right. They confirmed unintentionally the thesis of Keller el al., because the detected Cretaceous foraminifera are not reworked. They are in good condition. A reworking during the postulated catastrophic crater infilling (where are breccias) would have destroyed these filigree casings. Furthermore, how it should be possible these originally in the limestone embedded casing undamaged to isolate after a short time. The postulated mixed reworking assemblage of foraminifers (Albian-Turonian and Campanian-Maastrichtian) should be evaluated with caution. There are also long-lived Cretaceous species therein.


2. The K/T-boundary debate (Chicx-1A/M007A;
Unit 617.33 - 616.58 mbsf)

Site M0077: Post-Impact Sedimentary Rocks
J. Morgan, S.Gulick et al. -- IODP publications 2017;

"Between the disconformities at the top of the breccias and the fixed K-T boundary is a 75-cm-thick interval of "boundary cocktail" deposited (Subunit 1G).  It could correspond in time to the 50 cm thick deposits in the well Yaxcopoil-1, which was there classified by G. KELLER on the basis of foraminifera in the zone CF1 of the Upper Cretaceous.

Subunit 1G occurs from 616.58 to 617.33 mbsf and consists mainly of dark brown to dark grayish brown lime mudstone to wackestone (mud/wackestone). Subunit 1G was initially described as siltstone during visual core description. The subunit is complex, with several different lithologies that generally fine upward. It contains postdepositional pyrite nodules that disrupt bedding. The upper portion of the subunit has two thin intervals that display Cr enrichment based on μXRF mapping. The uppermost Subunit 1G is slightly deformed, with one millimeter-scale greenish marlstone interbedded with the lightest gray mud/wackestone in Section 40R-1/616.58–616.60 mbsf. Bedding is indistinct in the upper mud/wackestone and is obscured by soft-sediment deformation in Section 40R-1/616.73–616.83 mbsf. In Section 40R-1/617.24 mbsf. Subunit 1G contains millimeter-bedded couplets of dark brown and dark grayish brown mud/wackestone that overlie centimeter-bedded mud/wackestone. In Section 40R-1/617.24 to 617.33 mbsf, are five thin intervals: (1) a 3 cm thick light gray mud/wackestone laminated at the submillimeter scale with local greenish laminae, (2) a 1.8 cm thick mud/wackestone that is slightly darker gray and has some greenish laminae and clasts, (3) a 3.1 cm thick light gray mud/wackestone laminated at the submillimeter scale (similar to 2) that also contains a calcite-filled convoluted feature that crosscuts itself, and (4 and 5) two normally graded beds of packstone with (impact) glass and a variety of carbonate grains, each less than 1 cm thick. Beds grade upward from packstone with greenish sand-sized clasts, similar to those in the underlying suevite, to dark gray millimeter-thick laminae similar to the overlying mud/wackestone. The base of the subunit is a sharp, stylolitized contact in Section 40R-1/617.33 mbsf, that overlies the upper suevite of Subunit 2A.


Two samples of planktic foraminifers were taken for paleontological analysis in the brown mud/wackestone interval 40R-1/616.66 mbsf, and 40R-1/617.21 mbsf. Both samples contain
reworked Cretaceous  foraminifers, including members of the genera Heterohelix, Laeviheterohelix, and Rugoglobigerina.
First specimes of Paleocene planktic foraminifers (Parvularugoglobigerina eugubina) were found in 40R-1/616.56 mbsf  and 39R-3/616.24 mbsf."

The Recovery of Life in the Chicxulub Crater following the end Cretaceous Mass Extinction
C.M. Lowery et al. -- Lunar and Planetary Science XLVIII (2017), 2156.pdf

"Syn-impact suevite breccia is immediately overlain by a dark brown calcareous silt-stone between 616.58-617.33 mbsf. This unit contains a suite of reworked Maastrichtian microfossils well known from the K/Pg boundary in the Gulf of Mexico, termed the K/Pg Boundary Cocktail. The top of this unit is immediately overlain by limestones that contain the planktic foraminifer Parvularugoglobigerina eugubina, the first new planktic species to evolve in the Danian, less than 30 kyr post-impact. This horizon is also characterized by a bloom of the photosynthesizing nannoplankton Braarudosphaera bigelowii and the calcareous dinoflagellate Thoracosphaera spp. The latter fossils represent resting cysts that can lay dormant for years, and Thoracosphaera are most common in the modern ocean in regions with low nu-trient concentrations.

Zone Pα, which is defined by the occurrence of P. eugubina, is approximately 32 cm thick, although it covers less than 200 kyr of time and is therefore relatively expanded compared to most deep sea sites. Above this, a condensed record of the rest of the early Paleocene, ranging from planktic foraminifer Zones P1a to P3b (65.72 to 60.73 Ma) occurs in the limestones ranging from 608.02 to 617.33 mbsf. Planktic foraminifera appear relatively normal, and show a normal succession of species originations. Calcareous nannoplankton, meanwhile, are clearly stressed, as indicated by the continued dominance of disaster bloom taxa and the absence of common early Paleocene species. Benthic foraminifera are also generally rare and lack diversity, suggesting seafloor conditions which were not conducive to benthic life. Thus, the limestone records a divergent story about how local conditions during the recovery impacted different trophic levels, with phytoplankton are generally stressed and slow to recover, and zooplankton diversifying at the expected rate."

After the event, the devastated land surface, after a time gap, was flooded again by a marine transgression. The first marine deposits consists of fine material (volcanic mud?) that may have been transported mostly fluviatile. A portion of the sediments shows millimeter fine laminations. This indicates slightly increased water-energy for the deposition on ground. Moreover, in the sediments was bioturbation detected.

Rapid Recovery of Life at ground zero of the end-Cretaceous Mass Extinction
C. M. Lowery et al. -- Nature: International Journal of Science (2018)

"The boundary between the suevite and overlying earliest-Palaeocene pelagic limestone is in core 40-1, and comprises a 76-cm-thick upward-fining, brown, fine-grained micritic limestone that we term the ‘transitional unit’. The lower portion of the transitional unit is laminated below 54-cm core depth and contains no trace fossils . The laminations are thin, graded beds with sub-millimetre-scale cross-bedding that indicates bottom currents, and are likely due to the movement of wave energy—including tsunami and/or seiches—in the days after the impact. The fine grain size (primarily clay to silt, with some sand-sized grains concentrated in the graded beds) suggests that much of the material in the transitional unit was deposited from resuspension and settling. The transitional unit is overlain by a white pelagic limestone. The lowermost sample taken in this limestone (34 cm core depth) contains the planktic foraminifer Parvularugoglobigerina eugubina (which marks the base of Zone Pα), other foraminifer of the same genus (P. extensa, P. alabamensis) and Guembelitria cretacea. Because many other species that originate within Zone Pα first appear a few centimetres higher in the section (31–32 cm), we conclude that the base of the limestone lies very near the base of this zone, 30 kyr after the impact.
The transitional unit microfossils are dominated by clearly reworked Maastrichtian foraminifera and nannoplankton, known across the Gulf of Mexico and Caribbean as the Cretaceous/Palaeogene (K/Pg) boundary cocktail.
Although overall foraminiferal abundance is high at the base of the unit, species known to range across the boundary (survivor species) are rare in the lower transitional unit and become more common up-section even as total foraminifera decline. Survivor species, here defined as G. cretacea, Muricohedbergella monmouthensis and Muricohedbergella holmdelensis, dominate a depauperate assemblage in the upper 20 cm of the transitional unit, coinciding with the first appearance of trace fossils."


Foraminifera from the "boundary cocktail" between Barren and Danian Pα/P1a units

....... are similar to those in Yaxcopoil-1 and were as there deposited in-situ. A reworking of these well-preserved specimens (as postulated) is incomprehensible. It is unimaginable that all these so structured and undamaged foraminifera are come from limestone-breccias of the older bedrock.

Heterohelix, Laeviheterohelix, and Rugoglobigerina

Globigerinelloides sp. and  Heterohelix sp.

Guembelitria cretacea and  Muricohedbergella sp.