Why the Chicxulub-impact was an explosive supervolcano

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

Norbert Brügge, Germany
Dipl.-Geol.
mailto: [email protected]

Upload: 2013
Update: 24.04.2018

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" extraterrestrial 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 overpressure 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 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.

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.

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).

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.

1.4 CSDP (Yaxcopoil-1)

B. The Chicxulub K/T-boundary debate

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.

........  more here 

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).

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.