Why the Chicxulub-impact was an explosive supervolcano

(An asteroid impact that killed the dinosaurs, there were not)

Norbert Brügge, Germany
Dipl.-Geol.
mailto: B14643@aol.com

Update: 24.12.2016

The Cretaceous/Tertiary boundary characterizes one of the five great mass extinction in Earth's history. It is believed that these faunas-cut was caused by the impact of an asteroid on the Yucatan Peninsula 65 million years ago (Chicxulub-impact). Exposed sediment series in the Cretaceous/Tertiary transition in the world were associated with this scenario. In many outcrops at the K/T-boundary were detected worldwide ejecta (glassy spherules) and elevated iridium concentrations. As type locality (Global Standard Stratotype Section and Point, GSSP) defines the K/T-boundary the profile El Kef (Tunisia) with a so-called "Boundary Tone".

Since the results of biostratigraphic analysis of borehole Yaxcopoil-1 on Yucatan (2004) were published, began a controverse debate about the causal relations between Chicxulub event and K/T-boundary.
The group Keller/Stinnesbeck/Addate doubt this context. They are convinced that they have detected in the core from the borehole Ya-1 above the ejecta (breccias) about 0.50 m Cretaceous sediments with foraminifera in situ. That would mean that the Chicxulub event has to do with the K/T-boundary nothing. The advocates of causal relations between Chicxculub event and K/T-boundary try to discredit these results.

Apart from the K/T debate, the author doubts that the Chicxulub crater in Yucatan documents the impact of an asteroid with massive global effects. The detection of an andesitic melt in the center of the crater (with dioritic magma chamber in the underground) suggest the explosion of a supervolcano. It is absolutely inconceivable that an andesitic melt is compatible to molten dolomites or limestone as target-rocks by an impact. Gravimetric investigations on Yucatan, suggest that the former magma chamber is not limited under the center of the crater. Possibly a large magma chamber in the crust reach far to south. The presumed "peak ring" in the center of the crater is an illusion and was undetectable by drilling. Finally, the arguments of the advocates for an impact origin of the Chicxulub are not convincing, more shaky, because:

  • Iridium is not only an indicator of "young" exoterrestrial material. It is been since the creation of the planet exists in its liquid core.
  • Planar deformation or shock-generated features are equal indicators of volcanic eruptions, or shock waves by magma movements.
  • As with other supposedly large impact structures, it was also in this case not possible to reach a geochemical correlation between target-rocks and "impact"-melt.
    (therefore also the "Tektites" are of volcanic origin)
  • The so-called impact-melt in the Chicxulub is clearly of magmatic origin (pure andesite in the depth of about 1400 m). The significance of this fact is completely ignored.
    The found glasses are either geochemically compatible with the andesitic melt or are a mix with molten bedrock.

Due to the andesitic melt in the crater there have been only a influences from inside to outward. Probably a huge gas explosion took place, by this the bedrock was destroyed and the debris were widely distributed. The strewn field extends to Belize, Guatemala and southern Mexico. The detected andesitic melt ascended after the gas explosion and is secondarily penetrated in parts of the breccia and the bedrock (Veins, Dykes). It is also likely that falling-back debris are sunken primarily in the melt and were partially melted.

The effect of Chicxulub event is limited. In the Caribbean or Central America in the studied outcrops are no clear evidence of Tsunami sediments that have something to do with the Chicxulub event. The strewn field of the Chicxulub breccia is relatively limited. There is no evidence of eruptive melt in breccias outside the crater on Yucatan. The presence of glassy spherules and volcanoclasts in the studied outcrops in Central America and Caribbean is very diverse and marked a longer period of the Upper Cretaceous to Paleocene. For this and for the iridium concentrations, a persistent regional volcanism may be considered.
 

 

A. The Chicxulub structure

1. Drilling programs
Drilling programs have been conducted as part of the PEMEX oil exploration surveys, the UNAM Chicxulub Program, the Federal Commission of Electricity exploration program (FCE-UNAM) and the international Chicxulub Scientific Drilling Project (CSDP).
Several exploratory wells were drilled by Petroleos Mexicanos (PEMEX) in and around the structure. Three of the boreholes (Yucatan-6, Chicxulub-1, and Sacapuc-1) penetrated a melt rock that was interpreted to be an extrusive (volcanic) andesite. Further examination of samples from the Yucatan-6 borehole, however, revealed shocked quartz in a polymict breccia that overlays a thick melt unit in the interior of the structure, indicating that the Chicxulub structure is an impact crater (and this was an error). Because Yucatan-6 was an oil exploration borehole, and much of the cored material has been lost. The same situation exists for two other exploration boreholes drilled within the rim of the structure (Chicxulub-1 and Sacapuc-1) and five boreholes drilled outside the structure (Ticul-1, Yucatan-1, -2, -4, and 5A.
The first effort to obtain such cores was initiated by the Universidad Nacional Autonoma de Mexico (UNAM), which began a shallow drilling program that has recovered additional samples of "impact" breccias in 3 boreholes outside the rim of the crater. Recovery depths range from ~60 m to ~700 m.
However, several deep boreholes are still needed to penetrate melt or breccia lithologies within the center of the crater. The first of these is called the Chicxulub Scientific Drilling Project (CSDP), which was drilled as part of the International Continental Drilling Program (ICDP) in a coordinated effort with UNAM.

               
 


IODP Expedition 364 aims to drill into the Chicxulub impact structure to recover cores from, and above, the "peak ring". The expedition will drill a single 1200 m deep offshore borehole (Chicx-03A) at site about 30km northwest of Progreso, Mexico, on the Yucatan shelf, Gulf of Mexico. Date: 1st April to 31st May 2016.
Working hypothese
Chicx-03A will sample the material that forms a topographic peak ring. It will test the working hypotheses that peak rings are formed from: 1) overturned and uplifted upper, mid, or lower crustal basement rocks, 2) mega-breccias, or 3) some other material. One intrinsic feature of the peak ring is that it is a seismic low-velocity zone, despite some models predicting it consists of lithologies from the formerly deeply buried basement of the Yucatan. Sampling these lithologies will test the hypothesis that peak rings consist of lithologies that are deeply sourced but heavily brecciated and, hence, have higher porosities than surrounding impact and target rocks. This hypothesis, if correct, has consequences for questions of peak rings serving as habitat for exotic microbiology, due to the potential existence of macro-porosity, in the presence of significant hydrothermal circulation.
Chicx-04A will penetrate the enigmatic dipping reflectors (1200 – 1400 m) that run from the outer edge of the peak ring and dip inwards. It will test whether the dipping reflectivity beneath the peak ring is: 1) a lithologic boundary between uplifted basement lithologies and younger Mesozoic sediments, 2) is a thrust fault formed during peak ring emplacement, 3) is the result of vigorous hydrothermal circulation in the wake of peak ring emplacement.
 

Update: 13.04.2017

1.1 PEMEX
PEMEX developed and carried out a drilling program starting in 1952.The boreholes drilled a sequence of sediments with variable thickness. Boreholes Chicxulub-1 (1,581 m), Sacapuc-1 (1,530 m) and Yucatan-6 (1,645 m) drilled into a unit of igneous-textured rocks. The melt unit presents a minimum thickness of about 250 m. The breccias section is about 250-400 m thick in these boreholes. The breccias section was also cut in boreholes outside the basin at increasing distance away from the crater center in boreholes Ticul-1 (3,175 m), Yucatan-2 (3,488 m), Yucatan-5A (3,003 m), Yucatan-1 (3,226 m) and Yucatan-4 (2,425 m). The breccia sequence drilled in boreholes outside the basin present considerable thickness up to 400 to 600 m. The Paleozoic crystalline basement is drilled in boreholes Yucatan-1 and Yucatan-4.

Breccia and melt from Yucatan-6 well, core sections N13, N14, N16, N17 and N19, and from the Chicxulub-1 well, core section N10 have been analyzed. Yucatan-6 N13, N14, N16, N17 and N19 came from: 1,100-1,103 m, 1,208-1,211 m, 1,295.5-1,299 m, and 1,393-1,394 m, respectively. Chicxulub-1 N10 melt samples come from interval 1,393-1,394 m and show fine-to-medium grained coherent microcrystalline matrix.  Breccias contain fine-grained altered melt clasts dispersed in a medium-to-coarse grained matrix of pyroxene and feldspar. Clasts contain fragments of basement silicate material (granitic gneiss, amphibolite). Melt and breccias in Yucatan-6 extend from about 1,100 m to more than 1,400 m; sequence is well-sorted with an apparent gradation. They also reported observations on shock mosaicism and diaplectic glasses. Samples from Chicxulub-1 N10 and Yucatan-6 N19 were analyzed for iridium contents. Iridium abundances were relatively high with variable contents. However, these both factors are not only relevant as evidence of an impact event.


Unique "melt-rock" image (andesite) from Y-6 (N19),  Source: Homepage J. Smit


 
Another rare photos of andesite from the borehole Y-6 (N-19)


Breccia-rich melt from Y-6 (N-14) with enclosed anhydrite
Source: Homepage J. Smit



 Breccia-rich melt from Y-6 (N-14), Source: Homepage J. Smit



Y6 (N-14): Breccia-rich melt with basement debris (such of A-granite, B-gneis and C-amphibolithe), Source: Kettrup & Deutsch, 2003


Analysis of the Chicxulub-Andesite

by P. H. Warren et al.
Geological Society of America, Special Paper 307, 1996
We have studied seven thin sections of "impact melt-derived rocks" (read andesite) from Chicxulub: one from core C1, position N9, a short distance (≤4m) above the C1/N10 sample of Sharpton et al. (1992) and Schuraytz et al. (1994); and three each from positions N17 and N19 of core Y6, the same (to within a few m) as the original positions of samples described previously by Hildebrand et al. (1991), Kring and Boynton (1992), Sharpton et al. (1992), and Schuraytz et al. (1994). Our samples came from ~1,390 (C1/N9), ~1,297 (Y6/N17), and ~1,378 (Y6/N19) m below sea level.
The matrix portions of all six Y6 thin sections are aphanitic and are dominated by grains <10μm across, albeit leucocratic, texturally diverse but generally much coarser grained lithic clasts are incongruously sprinkled throughout (~20 vol%). The extraordinarily wide range of feldspar compositions in the Y6/N19 samples "tends to confirm the textural evidence for origin as a clast-rich magma". Pyroxene compositions show considerable variation between matrix portions of two nearby Y6/N19 samples. The Y6/N19-10 pyroxenes are remarkably rich in Al2O3.
Among our samples, the coarsest matrix by far is that of C1/N9, with grains mostly ~0.3 mm in maximum dimension. Texturally and mineralogically, this sample strongly resembles C1/N10 (Schuraytz et al., 1994). Only six clasts are discernible, all very fine grained (feldspar and pyroxene ~20μm), totaling ~2 vol% of the rock. A minor proportion (1%) of the matrix consists of isolated uncommonly coarse (up to 1.6 mm) grains similar to those described as phenocrysts (up to1 mm) in C1/N10 by Schuraytz et al. (1994). We retain this term for C1/N9 grains coarser than twice the prevalent 0.3 mm size, including three of the most diopside-rich pyroxenes analyzed.
C1/N9 and C1/N10 are the coarsest Chicxulub "impact" melt products yet sampled. Most other "impact" melt-like rocks (from elsewhere in the C1 and Y6 cores and from the S1 core) are described as “glass” or “microcrystalline” (Hildebrand et al., 1991; Quezada Muñeton et al., 1992; Kring and Boynton, 1992) or as “fine- to medium-grained coherent crystalline” (Sharpton et al., 1992). Sharpton et al. (1992) described Y6/N19 as having a “medium- to coarse-grained melt rock matrix,” but a more detailed recent description by three of the same authors (Schuraytz et al., 1994) indicates that their sample from Y6/N19, like the two we studied, consists mainly of an aphanitic matrix with grains ~10 μm across.
The finer grain size of the Chicxulub samples is probably not a consequence of systematically smaller dimensions of individual Chicxulub “meltbodies,” because according to Lopez Ramos (1975) the Chicxulub cores C1, S1, and Y6 all contain layers of pure  extrusive andesite  over 200 m thick. The Y6/N17 samples came from near the middle of a 60-m-thick continuous  extrusive andesite  layer, and the Y6/N19 samples came from ~12 m below the top of a continuous layer ~220 m thick (Meyerhoff et al., 1994).

by P. Claeys et al.
Lunar and Planetary Science, XXIX. [1361.pdf]
Yucatan melt rock samples, from the well Chicxulub-1 = C1) and from the well Yucatan 6 = Y6, were analyzed and seem to show significant differences.
The melt rocks in C1 (C1-N9: 1390 mbsl; C1- N10: 1393 mbsl) have a very coarse-grained, aphanitic matrix containing approximately 5 vol% quartz (10 μm), 25 vol% K-feldspar (50 μm), 64 vol% plagioclase (100 μm), 5 vol% augitic pyroxene (100 μm in scale) and 1 vol% opaques minerals (< 10 μm, ilmenite, pyrite). No clasts were found.
In contrast to C1, the melt rock in well Y6 (Y6-N17: 1295.5- 1299 mbsl; Y6-N19: 1377-1379.5 mbsl) is more clast rich (Note: rim of the vent !). Quartz, feldspar, anhydrite and carbonate clasts, range in size between 0.4 and 2 mm. All clasts except anhydrite and carbonate, are surrounded by a corona of augitic pyroxenes and K-feldspars. The clasts are usually subrounded, many are broken and their fractures are filled with matrix. Here to, many basement clasts are partly digested by the matrix. Vermicular anhydrite is also present in vein and cavities, suggesting pore filling (either from a melt or vapor phase) during the melt´s major cooling phase. The melt rock in well Y6 is more altered than its C1 counterpart and new mineral parageneses such as zeolites (stilbite, laumontite) and secondary calcite have often formed in cracks and holes.
The Y6 fine grained hypocrystalline matrix is composed of quartz (8 vol%), alkali feldspar: (13 vol%), plagioclase (66 vol%), augitic pyroxene (12 vol%) and minor constituents such as magnetite, ilmenite, sphene, zircon etc. (1 vol%). The matrix grain size is always below 10 μm, much finer than in C1 melt rock. Locally, the matrix even appears glassy or cryptocrystalline. The feldspar in the matrix consists of remnants of K-rich feldspar, mantled by a more Ca-rich feldspar phase, indicating a chemical change of the melt to a more Ca-rich composition. The abundance of Ca-rich fluids is also corroborated by the strongly augitic composition of the pyroxenes.
  

 
Diagram of the analyzed data

  Analyzes of "andesitic melt" from PEMEX boreholes
(Schuraytz et al., 1994 & Hildebrandt et al., 1991, Swisher et al., 1992)

 

Bohrung Yucatan-6

Chicxulub-1

 

N-17 N-19 N-10 "Andesite"
SiO2 62,30 61,90 63,20 60,50 54,80 57,40 61,20 58,50 61,70 58,30 57,60 64,60 59,71-58,45
Al2O3 14,60 13,10 12,60 13,60 15,30 16,41 14,70 15,50 13,70 13,70 15,50 14,90 15,84-13,77
MgO 2,90 3,20 3,10 3,20 3,15 2,96 2,74 2,75 2,55 2,92 3,05 2,75 5,42-4,06
CaO 8,80 10,40 10,20 10,50 11,90 10,18 9,30 10,55 10,01 12,10 11,41 5,50 11,24-9,35
Na2O 2,10 4,40 4,00 4,70 3,25 3,10 2,75 3,41 2,54 3,41 3,62 3,71 4,60-4,01
K2O 2,50 1,90 1,90 1,90 1,75 2,47 2,91 2,19 2,27 1,06 1,76 2,72 2,42-2,07
FeO 4,80 4,80 4,50 5,00 5,50 4,80 3,90 3,56 3,83 3,70 4,24 4,60 4,36-3,49
TiO2 0,50 0,40 0,40 0,40 0,56 0,54 0,40 0,40 0,36 0,39 0,43 0,53 0,13
MnO 0,10 0,10 0,10 0,10 0,13 0,12 0,09 0,11 0,06 0,11 0,11 0,09 0,14

  Weight fractions in %
 

Thin sections of andesite melt with grains of feldspar and pyroxene

 Y-6 N17

Y-6 N19

Y-6 N19

 

 

C-1 N10

Thin sections of andesite melt with grains of feldspar (plagioklase)


The observed "suevite" of borehole Yucatan-6 is clearly stratified in terms of composition, grain size, type of matrix and concentration of melt material. Three distinct types of "suevite" breccia can be identified:

  • An upper "suevite" (carbonate-rich). Small densely packed carbonate clasts clearly dominate over crystalline basement fragments. The clasts are embedded in a porous, 10 micron-size matrix. This "suevite"  represents a fall-back material.
  • A middle "suevite" (clast-rich). The silicate basement clasts and altered silicate melt fragments increase in proportion with depth. The clasts are suspended in a more compacted and much less porous matrix.
  • A lower "suevite" (thermometamorphic). This is composed of basement and evaporite clasts and abundant silicate melt fragments. The matrix is completely crystalline and consists of euhedral feldspar and pyroxene grains.

The lower "suevite" can be viewed as an intermediary unit between middle "suevite" and the underlying the melt rocks, already described by Kring and Boynton (1992), Schuryatz et al. (1994), and Warren et al. (1996). It appears to have reacted with the underlying hot andesitic melt-rock matrix. Samples of melt clasts, glasses, melt rocks and melt matrix separates were analyzed for major oxides (Sharpton et al., 1992; Sigurdsson et al., 1991; Izett et al., 1990; Claeys et al., 2003).



Spectrum of analyzed silicate melt particles

 

Analyzes of silicate melt particles (Yucatan-6)
(Claeys et al., 2003)

 

Upper "suevite"

Lower "suevite"

Middle "suevite"

 

N13 N13 N13 N14 N15 (av) N15 (min)  N15 (max) N15-10 N-14 (av) N-14 (min) N-14 (max) N14 N14  N14-5 N14-10 N14-11 N14-14 N14-15
SiO2 42,8 45,2 32,7 44,3 62,3 60,7 64,0 59,6 64,7 63,9 66,0 67,9 60,9  52,00 50,90 50,9 50,2 49,8
Al2O3 9,8 9,5 7,7 14,1 16,6 15,8 17,4 13,1 18,1 17,7 18,8 16,9 16,6 11,2 11,0 11,7 11,1 11,0
MgO 5,4 4,9 4,3 15,5 2,0 1,5 2,4 4,2 2,9 2,7 3,2 2,5 5,4 4,0 3,9 4,2 4,0 3,5
CaO 16,2 17,0 25,7 3,1 3,2 2,5 3,9 7,7 2,7 2,6 3,0 1,7 1,9 12,0 12,5 11,5 13,1 13,5
Na2O 2,4 2,5 2,1 2,6 4,6 3,2 5,4 5,1 6,5 6,0 7,3 0,6 0,9 3,4 3,3 3,5 3,6 3,6
K2O 1,3 1,8 1,1 0,7 5,6 4,1 7,6 2,2 1,6 0,6 2,6 2,7 1,6 1,7 1,7 1,7 1,7 1,7
FeO 4,1 4,0 2,9 13,2 5,5 4,7 6,00 5,2 3,9 3,5 4,6 5,2 9,1 4,3 4,4 4,7 4,4 3,9
TiO2 0,4 0,4 0,3 - - - - 0,5 1,2 1,2 1,2 0,7 1,0 0,4 0,4 0,5 0,5 0,4

MnO

0,1 0,1 0,1 - - - - 0,1 - - - 0,07 0,09 0,1 0,1 0,1 0,1 0,1

                     Weight fractions in %

Chemically different silicate melt particles occur in all parts of the "suevite". The amounts of  SiO2, K2O and Na2O are variable and not compatible with the andesitic melt below . These melt fragments document a mixed melt at the contact of magma and bedrock.  Only two analyses show andesitic-dacitic composition.


1.2 UNAM

The UNAM drilling program incorporated continuous coring in eight boreholes distributed within and immediately outside the crater rim, with three boreholes (UNAM-5, UNAM-6, UNAM-7) cutting the carbonate-"impact" breccia contact. The boreholes sampled the "impact" breccias, with the carbonate-"impact" breccia contact lying at varying depths between 222 and 332 m. "Impact" breccias are characterized by carbonate, melt and crystalline basement clasts supported in a carbonate-rich or melt-rich matrix. Two breccia units, have been cored, where upper breccias are rich in carbonate clasts and lower breccias are rich in melt and basement clasts.
In the Santa Elena (U5) borehole the contact of "impact" breccias and Paleogene carbonates lie at 332 m. The"Suevitic" breccias present a thickness greater than 146 m. The basal carbonates up to 30 m above the "impact" breccias contact are characterized by white creamy limestones, with several thin clay layers and variable content of clay lenses.

 

 "Impact" breccias are characterized by carbonate, melt and crystalline basement clasts supported in a carbonate-rich or melt-rich matrix. Two breccia units have been cored, where upper breccias are rich in carbonate clasts and lower breccias are rich in melt and basement clasts. Upper breccias have high magnetic susceptibilities, low seismic velocities, low density and high porosities and permeabilities. Lower breccias in contrast show low susceptibilities, variable high seismic velocities and lower porosity and permeability.


Source: Homepage J. Smit (2)


Breccias from UNAM-5 with crystalline basement clast

 

1.3 CFE-UNAM
As part of the Comision Federal de Electricidad (CFE) program, exploratory boreholes have been drilled near Merida, Huhi and Valladolid.
The program includes three boreholes (BEM1, BEH1, BEV4) drilled with continuous core recovery system, which allows investigation of the stratigraphy in this zone.

Only the borehole Valladolid BEV-4 cored a  34 m thick section of breccias between 250 to 284 m deep, which is part of the proximal ejecta blanket deposits. Breccias are characterized by abundant clasts of limestone, dolomite, gypsum and anhydrite.

 

1.4 CSDP (Yaxcopoil-1)

The Yaxcopoil-1 well, drilled as part of the CSDP international project. Continuous coring recovered cores from 404 m down to 1511 m of Paleogene sediments (400 m), "impactites" (100 m) and Cretaceous carbonates (1000 m). The 100 m thick "impactite" sequence is formed by six distinct units, which record variable conditions of emplacement and "post-impact" alteration. The "impactite" sequence in the Yaxcopoil-1 borehole was cored from 795 m to 895 m.
The section is formed by six distinct units. Subunits are characterized by distinct textural and compositional differences in size, type and relative abundance of clasts and melt-rich or carbonate-rich matrix types.
  •  Upper Sorted  “Suevite” (795-808 m)

  •  Lower Sorted “Suevite” (808-823 m)

  •  Upper “Suevite” (823-846 m)

  •  Middle “Suevite” (846-861 m)

  •  Brecciated Melt Rock (861-885 m)

  •  Lower “Suevite” (885-895 m)

 


USS:800m


LSS:818m


US:828m


MS:857 m


BMR:866m

 

  

Analyzes of silicate melt particles (Yaxcopoil-1)
(Tuchscherer et al., 2004)

 

Upper "suevite" (827m)

 Middle "suevite"
 (860m)

Brecciated melt rock
 (878m)

Lower "suevite"
 (892m)

 

n=20 n=14 n=33 n=25 n=16 n=20

SiO2

60,51 56,97 50,09 58,43 51,78 59,35

Al2O3

18,70 20,10 15,24 21,23 15,85 18,00

MgO

1,23 0,33 6,52 0,57 3,76 0,19

CaO

4,36 4,32 0,75 5,43 4,39 0,88

Na2O

1,31 2,34 0,47 3,87 1,47 1,18

K2O

11,17 1,76 3,83 7,10 7,05 12,66

FeO

1,05 4,23 5,19 1,67 2,43 5,05

TiO2

0,73 0,77 0,48 0,28 0,71 0,52

MnO

- 0,03 0,01 - - 0,03

                                                                                Weight fractions in %

Chemically different silicate melt particles occur in all parts of the "suevite". The amounts of  SiO2, K2O and Na2O are variable and not compatible with an andesitic melt. These melt fragments document again a mixed melt of magma and bedrock.
 

Unit 5 (BMR)
"The green melt is generally massive in appearance, but contains flow lines on both macroscopic and microscopic scales that are suggestive of incompletely mixed and rapidly quenched melt. No fresh glasses were observed. The melt is dominated by microcrystalline, Ca-rich pyroxene, plagioclase and alkali feldspar. Granitic clasts up to 4.5 cm and mafic clasts up to 6.5 cm were observed. In thin section, small amounts of shocked and unshocked clasts are entrained in the melt, including quartz and quartzite with planar deformation features and ballen structures after cristobalite, isolated altered feldspar and mafic minerals, and mafic lithics. Inclusions within the thin section are aligned, as in a trachytic texture, implying that the melt was flowing before solidification. The green melt is also brecciated and highly altered along its margins where the contacts were conduits for carbonate-rich fluids. While the core was being recovered, a 10 m-thick carbonate-charged and brecciated green impact melt unit (unit 6) with larger clasts, including a 34 cm granite, was logged below the principal 24 m-thick green melt unit (unit 5).
Unit 4 (MS)
A 15 m-thick breccia was logged immediately above the green melt unit with abundant and sometimes very large (up to 20 cm) clasts of banded melts. The melt is dominated by microcrystalline, pyroxene, plagioclase, and alkali feldspar. Similar to the green melt, although the color (shades of rose) is different. The melt entrained small amounts of shocked and unshocked clasts of quartz, feldspar, sandstone, metaquartzite, and granite. These melt fragments exist in a breccia that is variously clast and matrix supported, the latter of which appears to have been a conduit for fluids and is now charged with secondary alkali feldspar and subordinate carbonate.



Yax-1; 836 m (US)



Yax-1; 862 m (BMR)



Yax-1: 864 m (BMR)

Glass fragments and breccias in crystalline matrix

Unit 3 (US)
This 23 m-thick melt-rich unit of breccias is dominated by fragments of altered silicate melt, generally with microcrystalline textures. Although some fragments appear to have been partly to wholly glassy before being replaced by phyllosilicates and calcite. Primary minerals in the microcrystalline melts include pyroxene, plagioclase and alkali feldspar. Some of these silicate melts contained immiscible carbonate melt, gas vesicles (some of which were subsequently filled with secondary calcite and silicates), and flow-aligned crystals. In general, these melt fragments are more vesicular than those in the overlying suevitic units. The melt fragments in this breccia section entrained feldspar, quartz, magnetite, lithic metaquartzites, micritic carbonate, shale, and crystalline mafics.
The textural distinction between solidified silicate melt with immiscible carbonate melt and solidified silicate melt with gas vesicles subsequently filled with calcite is subtle. Ellipsoids of carbonate protruding from the edges of silicate melt fragments into the breccia matrix indicate that the carbonate was solid before the fragmentation of the melt fragment and incorporation into the breccia. In this case, the carbonate is a primary phase and was immiscible when the melts were molten. Ellipsoids of carbonate completely bounded by silicate melt could also represent immiscible carbonate melt but, in these cases, could instead be ellipsoids of secondary carbonate filling gas-evacuated vesicles."
Source: Kring et al. (Meteoritics & Planetary Science 39, Nr 6, 2004)

 

"The breccia-unit is a 100-m-thick suevite impact breccia with erosive lower and upper contacts. Clasts are angular to subrounded and average a few cm in diameter, though larger clasts up to 1 m in diameter are present. It is a polymict breccia that contains clasts of dolomite, limestone as well as rare sandstone and anhydrite.
In addition to undigested clasts of the Mesozoic sediment cover of northern Yucatan, there are minor but significant constituents of melt rock, and crystalline rocks of continental basement origin, such as granite, granodiorite, gneis, quartzite, albite-green schists and micaschists.
Within these deposits, grey-green fragments are common along with spherules of glass altered to well-crystallised smectite. Some silicate melt clasts show fluidal textures and domains of variable chemical compositions, while others are cryptocrystalline and show spherulitic textures typical of devitrification. Quartz and feldspar xenocrysts with planar deformation lamellae are also common and indicate shock deformation. Limestone and dolomite clasts frequently show thermal alteration, i.e. altered rims, and still others show no alteration. Glass is always devitrified and the entire unit shows hydrothermal alteration subsequent to deposition as also indicated by the presence of K-feldspars, albite, apatite, zeolites and pyrite.
The top 15 m of the breccia unit was deposited by current transport as indicated by layered breccias with alternating layers of small and larger subrounded to rounded clasts, fining upward and sorting of clasts. Devitrified glass is abundant and often characterised by pumice-like structures.

The general upward-fining trend recognised in the upper 15 m of unit ranges from maximum clast diameters of 3-5 cm at the base of the member to about 2-5 mm near the top. Horizontal layering and coarse cross-bedding structures in the uppermost metre suggest reworking, as also indicated by alternating friable grey sand layers (clasts of approx. 2 mm) with slightly larger (clasts 2-5 mm) microconglomeratic horizons. This 15-m-thick layered and upward fining breccia sequence reveals characteristic features of reworking and transport by currents after deposition of the impact breccia."
Source: Stinnebeck et al. (Geologische Rundschau, 93, 2004)

 


K/T-boundary


Upper sorted "suevite"


Lower sorted "suevite"


Upper "suevite"


Middle "suevite"


"Suevite" intruded melt


Lower "suevite"


K/"suevite"-boundary


Dyke "suevite"

Source: Homepage J.Smit  

 

Smit et al. (2002), Wittmann et al. (2003) and Kenkmann et al. (2003)  reported numerous "pseudotachylite" veins and dykes of "suevite" and melt rock in the "pre-impact" sequence. In fact, in over 600 m of "pre-impact" sediments, only two "suevitic" dykes at 909 and 915 m depths, and two possible thin injection veins (at 1,398.5 and 1,348 m) were detected. These injection veins formed by lateral forcing of melt (with breccias) into cracks or shear zones in pre-existing sediments.
 

 
2. My opinion to the origin of the Chicxulub structure

The whole debate about the genesis of drilled melt and breccias in the Chicxulub crater is conducted with wrong assumptions. The Chicxulub "impact" crater is in fact the caldera of a supervolcano that is created by a huge gas explosion. The volcanic origin is by drilled pure andesite in the center of the structure no doubt proved. The bedrock was blasted in a great extent and widely distributed.

Gravimetric investigations on Yucatan, suggest that the former magma chamber is not limited under the center of the Chicxulub crater. Possibly the dioritic magma chamber reach far to south. It is possible that the crater to south is open (no boreholes). That would be one explanation for the thick breccia units in Guatemala and South-Mexico.

The ascending andesitic melt in the center of the explosion had only a low eruptive potential. There is no evidence of eruptive melt in the breccias outside the crater on the Yucatan peninsula (questionable Y-5A). The caldera after the gas explosion was immediately covered with falling-back debris. The detection of dykes or veins in the bedrock (Ya-1) indicates lateral intrusions from the yet not cooled andesitic melt body (mixed with breccia). In two wells (C-1, Y-1) were found "ash with glass" in depths between 1300 and 1400 m. This is mysterious and would be worth a new analysis.
There are two different types of breccias from the ruined bedrock. There are breccias above the intact bedrock and breccia above the igneous andesite. The breccias above the bedrock has no contact with the melt and are unchanged. The breccias above the andesite (S-1, C-1, Y-1 wells) are melted or are in contact with melt, in decreasing intensity upwards. Possibly the lowest parts of falling-back debris are already fallen in the andesitic melt. A sorting the breccias (called units) make not sense because convective movements in the melt are expected. In the upper part of the breccia-package are smaller components of falling-back debris concentrated and have already stratification. Glass fragments in the melted breccias-package are geochemically different. These fragments belong as the breccia to the falling-back debris after the explosion. Presumably, these glasses come from a mixed melt of igneous andesite and bedrock on top of the eruptive vent. All the others in Central America found glassy spherules and glass fragments have probably not a connection with the Chicxulub event (see below).
The occurrence of shock-generated features is not only an indicator for an impact event, because this is also proved by finds in volcanic structures.Very exceptional shock structures, that caused by volcanic shock waves, can you here seen: "Mysterious columnar sandstone"

 

B. The Chicxulub K/T-boundary debate

1. 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.

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

2.1 Petrography of the unit 794.60 - 794.11 m

"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)
  


320


K8:Glauconitic layer


318


318a


K9: Bioturbation


316


316a

 
315


315a


314


314a


313


312


311


310


309


308


K20: Foraminifera


K20a: Foraminifera


307


306

Source: Homepage J.Smit (13)

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



Foraminifa from unit between breccias and Danian layers

 

Argumentation

"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."

 

2.3 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
 

 

Argumentation
"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)."
"Nevertheless, if these planktic foraminiferal assemblages were autochthonous. Therefore, it is more probable that this Chicxulub-1 interval also contains reworked Cretaceous specimens similar to those found in the Yaxcopoil-1 dolomitic calcareous sandstone unit."

2.4 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."
 


3. 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). A portion of the sediments shows millimeter fine laminations. This indicates slightly increased water-energy for the deposition of sediments. Moreover, in the sediments was bioturbation detected. It is also normal that at the beginning various reworked material of limestone and dolomite debris was deposited mainly. 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 bubblelike inclusions in the greenish layer could be spherules from the widespread post-Chicxulub volcanism. A significant time gap is possible because the following marine sediments suddenly documented a changed marine milieu.

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.
 

 

 

C. The distribution and significance of spherules-layers in Central America


The many layers with glassy spherules in outcrops of Central America -- which include the time of Late Cretaceous, the K/T-boundary and youngest Tertiary -- are a particular problem. A correlation is difficult and probably not useful to find the biostratigraphic K/T-boundary. The equivalent of "Boundary Tone" that marks the K/T transition, is with such spherules layers possibly not to find. This can only succeed with support of the biostratigraphy and that was due to the particular facies (fine clastic sediments) or a gap in the sedimentation not been convincingly possible *).
In many outcrops in Mexico, Guatemala, Belize and Haiti are spherules-layers from the late Maastrichtian (zone CF1) and the Danian (zone P1a) documented, and an attempt at a correlation has been subjected. A summary of the results presented Gerta Keller et al. in several publications. It is striking that the geochemical composition of the spherules varies, what is missing is a plausible explanation.


*)
The special fine clastic sediments of Mimbral and Penon (no determinable fossils) at the transition between the Cretaceous and Tertiary are controversial interpreted as Tsunami deposits (Smit et al.) or as normal marine sedimentation (Keller et al.) after the Chicxulub event. Depending on, this sequence between the zones CF1 and P1a would be for the time insignificant or significant (300 000 years).



Outcrops with spherules layer, but without breccias in Mexico



Outcrops with breccias and/or spherules layer on Yucatan

The spherules in recurrent strata -- from late Maastrichtian (zone CF1) until early Paleocene (zone P1a) -- are registert as reworking of older layers of zone CF1 (eg Mendez marl). The lower layers with spherules should be derived from the Chicxulub event, the others should contain reworked spherules of lower layers. But that should be doubted, it is easy to explain everything with "reworking". The spherules layers fit well into an ongoing scenario of volcanic activities in the region, was possibly was only accompanied by the Chicxulub event and was steadily present until the early Paleocene. This includes recurrent Iridium-anomalies, that never indicate impact events. The Ir-peaks documented maxima in an ongoing volcanic scenario. The layers with glassy spherules are both stratigraphically and geochemically very unevenly distributed and should not be assigned to a single event. A connection with the Chicxulub event can mandatory not be detected. A more regional volcanism should be considered. The suspected volcanic scenario also includes the lapilli or ash from Guayal in other outcrops.

The few relevant geochemical analyzes of spherules in Central America documented regional differences. The Danian spherules of Beloc (Haiti) correspond to an andesitic magma. K-rich spherules from layers of Sierrita (Mexico) correspond to a phonolitic magma, but the most of them can not be allocated due to their low SiO2% contents. Spherules of Mimbral (Mexico) are different: Andesitic-dacitic and basaltic-trachybasaltic. Spherules in the Brazos (Texas) outcrops correspond with a basaltic magma.



El Mimbral: Chaotic spherules layers in Mendez marl



El Penon spherules


Selection of geochemical analyzes of spherules

 

Brazos (Texas)

Mimbral (Mexiko)

Sieritta (Mexiko)

Beloc (Haiti)

Keller et al

Smit et al 1992

Schulte et al 2006

Sigurdson et al 1991

Izett et al 1990

Stueben et al 2002

 

Yellow layer (n=250) (n=250)     Ca-rich (n=21) (n=12) (n=61) min max  (n=11) min max dark  yellow Smectite Smectite
SiO2 47,67 50,13 51,10 62,99 66,20 52,20 50,23 50,12 63,09 60,30 67,90 63,30 60,30 67,60 66,85 60,47 61,98 65,26
Al2O3 10,28 12,73 19,63 15,73 18,73 12,40 29,15 29,55 15,21 13,70 15,30 14,40 13,70 15,30 14,94 14,21 9,82 11,12
MgO 3,52 4,18 3,23 3,01 2,64 3,90 2,35 2,14 2,74 2,20 3,80 2,80 2,40 3,80 2,75 3,16 3,92 4,64
CaO 1,34 1,70 3,35 6,88 0,84 22,96 0,57 0,56 7,26 4,50 10,90 7,10 4,70 10,90 5,38 11,45 1,06 0,88
Na2O 0,08 0,09 0,11 3,34 0,84 2,02 0,10 0,10 3,63 2,40 3,70  3,30 2,40 3,60 1,96 2,78 0,05 0,06
K2O 4,51 4,89 0,45 1,50 3,68 0,58 7,25 7,14 1,59 1,00 1,80  1,50 1,00 1,80 1,53 1,40 1,13 1,10
FeO 18,07 15,55 2,40 5,32 5,67 4,23 1,88 1,49 5,44 4,60 5,70 5,30 4,70 5,70 5,31 5,37 5,07 4,81
TiO2 0,08 0,11 0,38 0,70 0,02 0,56 0,19 0,15 0,67 0,48 0,84 0,80 0,70 0,80 0,55 0,65 0,81 0,34
MnO 0,02 0,02 0,02 0,13 0,00 0,14 - - 0,14 0,00 0,18 0,20 0,10 0,20  -  - - -

          Weight fractions in %


Danian Beloc glass


Beloc glass and others


Two different varieties of Mimbral glass (Bell & Sharpton, 1996)


Mimbral glass

 

D. What suggest the spherules-bearing layer below the breccia-unit in the Albion Island quarry (Belize)

 
"The Cretaceous-Tertiary boundary stratigraphy in north (Albion) and central (Armenia) Belize encompasses the Maastrichtian dolomite and overlying diamictite layer with basal spheroid bed. The northern Albion spheroid-bearing clay layer rests directly upon Maastrichtian dolostones and consists of brown to rust-colored clay, which is locally rich in oblate, pebble-size dolomitic spheroids. Near the bed base, it is rich in irregular red or green clay clasts. This whole clay layer ranges from 15 to 150cm thick. The basal contact is sharp and is usually marked by a paper-thin hematitic rind on the underlying Maastrictian dolostone. The spheroid-bearing clay layer is highly deformed. Studies of associated red and green clay clasts suggest that they are glass fragments that have been altered. This green clay clasts (clay spheroids) are smectites.

The diamictite unit rests directly upon the spheroid-bearing clay layer and consists of light tan, moderately indurated and matrix-rich, poorly sorted breccia. The diamictite unit contains at least 5 individual beds. Each of these beds ranges from 2 to 7 meters thick. Within individual beds, crude grading (with floating boulder clasts near or at the top of beds) is common.  As part of the crude grading, boulders located at the base of some beds show imbrication and clast support. Other internal structures are crude flow laminations, which are located in the upper one to 2 meters of each bed. Some beds have rare megaboulders  near their bases or tops. Other beds contain rare matrix-coated boulders up to 3 m in diameter. Diamictite matrix is mainly highly comminuted dolomite in the silt-clay size range. Matrix content commonly equals approximately 50 to 70 percent of the rock and matrix is more abundant near the tops of most beds. This matrix contains some red and green clay clasts, similar to those found within the underlying spheroid-bearing clay layer. These clay clasts are thought to have a similar origin to the ones mentioned earlier from within the spheroid-bearing clay layer. Red and green clay clasts are much more common in the lower 2 to 3 meters of the diamictite unit matrix. Comparison of underlying Maastrichtian dolomitic facies and lithologies of diamictite clasts shows significant differences."
Source: Homepage D. King

 


Albion Island quarry


Albion spheroid-bearing layer


dito


Dolomite spheroids


Dolomite spheroid (lapilli)


Boulder


"Diamictite" unit Albion


Boulder

 

"Belize has figured prominently in the Cretaceous-Tertiary boundary controversy as a result of the discovery of clay spheroids and diamictite in the Albion Island and Armenia outcrops.
In the Albion Island quarry the spheroid layer overlies Maastrichtian dolomite and is between 1 and 2 m thick with some bedding features. This layer is characterized by abundant rounded to subangular bodies (or clasts) of dolomite and clay up to 2 cm in diameter, which float in a fine-grained matrix of friable dolomite and calcite-silt. The diamictite above the spheroid layer contains rounded to angular carbonate clasts in clay matrix with some relic glass preserved. The spheroids and diamictite layers have been interpreted as altered impact glass based on their smectite-rich clay composition. There are significant differences in the clay mineral compositions of the Maastrichtian dolomite, spheroid and diamictite units.
The spheroid clay layer is characterized by a single clay mineral phase, which consists of extremely well-crystallized smectite characterized by very high intensity, and a webby morphology with the major element a typical Cheto Mg-smectite (Si, Al, Mg, with minor Fe and K). This Cheto type Mg-smectite indicates a single origin for the spheroid layer and the alteration of glass.
In the diamictite above the spheroid layer, the clay composition is similar, with extremely abundant smectite (90%), but contains also minor amounts of palygorskite, kaolinite and illite. Smectite is slightly less well crystallized. These features suggest a multiple origin for the diamictite constituents, including erosion of carbonate sediments, soils and glass alteration."
Source:  Keller et al. (Journal of the Geological Society, 160, 2003)
 

Geochemical analyzes of Albion spherules by Pope et al. (1999)

 

Spheroid layer

Diamictite layer

 

n=12 max min n=11 max min

SiO2

58,21 59,85 56,58 57,33 58,48 56,18

Al2O3

16,70 17,28 16,12 16,42 16,76 16,08

MgO

9,65 10,01 9,29 8,56 8,82 8,30

CaO

0,13 0,24 0,02 0,13 0,15 0,11

Na2O

0,06 0,07 0,05 0,06 0,08 0,04

K2O

0,36 0,40 0,32 2,00 2,53 1,47

FeO

3,43 3,68 3,18 6,02 7,06 4,98

TiO2

0,65 1,22 0,08 0,93 1,32 0,54

MnO

0,02 0,03 0,01 0,01 0,02 0,00

                     Weight fractions in %

 


My opinion
The spheroid-bearing clay layer (smectite) in the Albion Island quarry (northern Belize), immediately above the eroded Maastrichtian bedrock, are older altered volcanic deposits, and include a time gap to the following Chicxulub event. The clay spheroids are altered volcanic glass, and the dolomite spheroids are accretionary lapilli. The overlying >15-m-thick diamictite are falling-back material from the Chicxulub event. The breccias unit containing rare altered glass, large accretionary blocks, striated, and impacted cobbles. The localities in Belize are unique because here is documented a volcanic activity before the Chicxulub event (right).

In southern Belize, these deposits are completely missing. Here are directly above the bedrock, with a time gap, deposits of the Paleocene, which are interspersed with reworked breccias (Cayo diamictite), other clasts and volcanic glasses. This is the result of a widespread volcanism at that time in the entire Caribbean region.
The situation is similar in the location Beloc/Haiti. About eroded Maastrichtian limestones follow on a time gap deposits of the Paleocene, which are interspersed with volcanic glasses.

The partly massive breccia packages of Guatemala and southern Mexico (Actela, Trinitaria, Bochil, Guayal, Campeche drilling field) again are falling-back debris from the Chicxulub event. It seems that the whole region was in time of the Chicxulub event in a non-marine phase. The biostratigraphically documented Paleocene sedimetation follows after a time gap.

In Central Mexico a widespread volcanism is noted in the Maastrichian period that started before the Chicxulub event and has continued even after the event. Analyzes of glasses documenting different volcanic origins. The thesis of reworked spherules should be discarded.

The spheroid-bearing layer in the Albion Island quarry could be the "missing link" for all older spherules-layers in Central America. Here is my suggestion:

 


Alvaro Obregon, Belize


Albion, Belize


Spheroid bed, Belize


Spheroid beds, Haiti


Spheroid beds, Mexico



Thickness of Chicxulub
ejection breccias

 

E. Some sources
Homepage Gerta Keller
Homepage Jan Smit
Homepage David King

Arz et al. (2004) --
Foraminiferal biostratigraphy and paleoenvironmental reconstruction at the Yaxcopoil-1 drill hole, Chicxulub crater, Yucatán Peninsula. Meteoritics & Planetary Science 39(7), 1099–1111.
Claeys et al. 2003 -- The Suevite of drill hole Yucatan 6 in the Chicxulub impact crater. Meteoritics & Planetary Science 38(9), 1299-1317.
Claeys et al.           --
The two different melt rocks of the Chicxulub impact crater and .......  Lunar and Planetary Science, XXIX. [1361.pdf]
Hildebrand et al. (1991) --
Chicxulub crater: a possible Cretaceous/Tertiary boundary impact crater on the Yucatan Peninsula, Mexico. Geology 19, 867-871.
Keller et al. (2003) --
Spherule deposits in Cretaceous–Tertiary boundary sediments in Belize and Guatemala. Journal of the Geological Society, 160, 783–795.
Keller et al. (2003) -- Multiple impacts across the Cretaceous–Tertiary boundary. Earth-Science Review, 62, 3-4, 327-363.
Keller et al. (2004) --
More evidence that the Chicxulub impact predates the K/T mass extinction. Meteoritics & Planetary Science 39(7), 1127-1144.
Keller, G.  (2012) -- The Cretaceous-Tertiary mass extinction, Chicxulub impact, and Deccan volcanism. Earth and Life, International Year of Planet Earth, 759.
Kring et al. (2004) --
Impact lithologies and their emplacement in the Chicxulub impact crater: Initialresults from the Chicxulub Scientific Drilling Project, Yaxcopoil, Mexico. Meteoritics & Planetary Science 39(6), 879-897.
Pope et al. (1999) -- Chicxulub impact ejecta from Albion Island, Belize. Earth and Planetary Science, 170, 4, 351–364.
Schuraytz et al. (1994)
-- Petrology ot impact-melt rocks at the Chicxulub multiring basin, Yucatan, Mexico. Geology, vol 22.
Sharpton et al. (1993) --
Chicxulub multiring impact basin: Size and other characteristics derived from gravity analysis. Science 261, 1564-1567.
Smit et al. (2004) -- Is the transition impact to post-impact rock complete? Meteoritics & Planetary Science 39(7), 1113-1126.
Stinnesbeck et al. (2004) - Yaxcopoil-1 and the Chicxulub impact. Geologische Rundschau, 93, 10423-1065.
Urrutia-Fucugauchi et al. (2011) --
The chicxulub multi-ring impact crater, Yucatan carbonate platform, Gulf of Mexico. Geofisica International, 50-1, 99-127.
Warren et al. (1996) --  Mega-impact melt petrology (Chicxulub, Sudbury, and the Moon. Geological Society of America, Special Paper 307