Chicx-03A (M0077A) drilling results
ESO - Chicxulub K-Pg Impact Crater Expedition 364

Norbert Brügge Germany
Dipl.-Geol.

Update: 10.05.201
7

What can we say when we analyze the drilling report (left):

The Chicxulub crater is smaller than predicted (see below). It is more like a huge volcanic vent with about 70 km diameter. The expected peak or peak ring in the "impact-crater" does not exist.
The indifferent seismic profiles were unfortunately misinterpreted because they were accompanied by a wishful thinking. The ever-used gravity anomaly map shows neither a crater nor a paek ring, but is only the reflection of a former magma chamber.
The "Chicxulub-impact" is a great error. The compact crystalline basement is encountered here in an exceptionally high postion (similar as in the
Chesapeake Bay structure). There are even indications that the so-called "impact melt" in their origin was an andesitic melt. But that must be verified. Some dykes could be intrusions from the dioritic magma chamber. Other dykes could be oldest hypabyssal intrusions and were create long times before the event.*

*   In the meantime we know that a granitic intrusion from the Carboniferous period was drilled and the dykes are different in composition and origin.
     The compositions point to dioritic and granitic origin. Up to now are no detailed results published.


         

 

                   My new geological section through the Chicxulub crater


First Results


Paper "Science" vol. 354, issue 6314, pp. 878-882 (2016, Nov.16)
"The uppermost peak ring is composed of ~130 m of breccia, with impact melt fragments that overlie clastpoor impact melt rock.We encountered felsic basement rocks between 748 and 1334.7 mbsf that were intruded by preimpact mafic and felsic igneous dikes as well as impact-generated dikes.
We recovered one particularly thick impact breccia and impact melt rock sequence between 1250 and 1316 mbsf. The entire section of felsic basement exhibits impact-induced deformation on multiple scales. There aremany fractures, foliated shear zones, and cataclasites, as well as signs of localized hydrothermal alteration. The felsic basement is predominantly a coarse-grained, roughly equigranular granitic rock that is locally aplitic or pegmatitic and, in a few cases, syenitic.
The basement rocks in the peak ring differ from basement in nearby drill holes encountered immediately below the Mesozoic sedimentary rocks, suggesting a source of origin that was deeper than 3 km (???).
Evidence of shock metamorphism is pervasive throughout the entire basement, with quartz crystals displaying up to four sets of decorated planar deformation features. We observed shatter cone fragments in pre-impact dikes between 1129 and 1162 mbsf, as well as within the breccia. Jointly, the observed shock metamorphic features suggest that the peak ring rocks were subjected to shock pressures of ~10 to 35 GPa (?) . No clear systematic variation in shock metamorphism was observed with depth. Impact-melt, which is formed at shock pressures of >60 GPa (?), is also a component of the peak ring.
The formation of the Chicxulub peak ring from felsic basement confirms that crustal rocks lie directly above Mesozoic sedimentary rocks, which is consistent with the dynamic collapse model of peak ring formation (???).
The drilling data confirm that the peak ring rocks have low densities and seismic velocities, as suggested by geophysical models. The density of the felsic basement varies between 2.10 and 2.55 g cm−3, with a mean of 2.41 g cm−3, and P wave velocities vary between 3.5 and 4.5 km s−1, with a mean of 4.1 km s−1. These values are unusually low for felsic basement, which typically has densities of >2.6 g cm−3 (correct is: 2.5 to 2.7 !) and seismic velocities of >5.5 km s−1 (correct is: 4.0 to 6.0)."

It is claimed that the condition of the drilled felsic basement rocks provide the evidence for the existent of a "peak ring" in the Chicxulub crater. These parts of this crystalline basement were shifted several kilometers towards the surface during impact and lie directly above Mesozoic sedimentary rocks. These rocks are cross-cut by dikes and shear zones and have an unusually (?) low density and seismic velocity. Analysis shows that impact generated vertical fluxes and increased porosity.

Again only wishful thinking ! Model simulations are for the trash. Nothing was uplifted and the shear zones etc. can  have emerged during the explosion of the supervolcano, or of times long before the event.
"Shatter cone fragments in pre-impact dikes" in amphibolite facies are even a clear indication that they were created by shock waves in the magma-chamber long time before the event. But, no of the existing hypotheses for the formation of shatter-cones currently is able satisfactory explain the characteristics of this fracturing phenomenon. It is certain, that the presence of shatter-cones as well as the presence of PDFs is not just a criterion for an impact event.


SPECIAL SESSION: IODP-ICDP EXPEDITION 364 TO THE CHICXULUB IMPACT CRATER; March 21, 2017
 

Macro- and microscopic Evidence of impact Metamorphism in Rocks from the Chicxulub peak ring
L. Ferričre et al. -- Lunar and Planetary Science XLVIII (2017); 1600.pdf

The M0077A core was subdivided into three main lithological units: a (post-impact) section (from 505.7 to 617.3 mbsf), an upper (peak ring) section of "suevite" and melt rocks (from 617.3 to 747.0 mbsf), and a lower (peak ring) section mainly consisting of granitoid rocks (with aplite and pegmatite dikes) intruded by different types of subvolcanic dikes, and intercalations of millimeter to decameter thick "suevite" and  melt rock units (from 747.0 to 1334.7 mbsf).
The upper (peak ring) section consists of 104 m of  "suevite"  (polymict lithic breccia with mm to over 25 cm in size melt clasts and lithic mineral and rock fragments) on top of ~25 m  melt rock (dominantly clast-poor but with clast-rich intervals). The matrix of the "suevite" is calcitic (ranging from micritic to sparitic). Clasts include a variety of more or less shocked mineral and rock fragments (sedimentary [including isolated fossils], metamorphic, and igneous lithologies, with carbonate and granitoid being the most abundant rock types) and melt fragments withaltered (green to brown in color in plane-polarized-light; clay minerals) glassy to microcrystalline textures. Many of the melt fragments have flow textures and are occasionally vesicular. They often contain relic mineral clasts (dominated by feldspars and quartz) and shocked lithic rock fragments, or are, themselves, coated with an additional layer of melt. Quartz grains show planar fractures (PFs) and/or (decorated) planar deformation features (PDFs), with up to 3 sets. A few toasted quartz grains were noted. Silica glass, generally recrystallized, with a chert-like appearance (and with ballen silica also occur. Other minerals also exhibit shock features, especially plagioclase and alkali-feldspar (with PFs and PDFs). Possible coesite (??) was observed in a large silica-rich melt fragment. The melt rock from the lower part of the upper (peak ring) section is green to black in color, with flow banding, and in some cases vesicular. The green and black melts are locally intermixed, forming schlieren of green material (altered melt?) in a black-colored melt. A large variety of clasts are present (as in "suevite", with the exception of sedimentary rock clasts that were not found), in some cases so heavily shocked and/or hydrothermally altered that it was difficult to identify them. Shock features similar to those in "suevite" were observed in the melt rocks, such as PFs and PDFs in quartz, toasted quartz, and a variety of shock features in other minerals.
The lower (peak ring) section consists mainly of pervasively deformed granitoid basement rocks (granite to syenite), overall coarse-grained, with locally cm to dm thick aplitic and pegmatitic sections. All main rock-forming minerals, i.e., alkali-feldspar, plagioclase, quartz, and biotite, show signs of shock deformation. In the case of quartz, locally PFs were even visible as a result of preferential hydrothermal alteration. Almost all quartz grains are shocked, with PFs, feather features (FFs), and/or (decorated) PDFs ; up to 4 sets of PDFs are seen. Kinkbanding was also observed for some quartz grains. Similar shock features were observed in alkali-feldspar and plagioclase. Biotites and chlorites are often kinked. Based on qualitative evaluation of the thin sections no noticeable shock attenuation with depth was observed. Preliminary quantitative results seem to indicate little or no shock attenuation with depth.
For the first time shatter cones were found at Chicxulub. They are well-developed in some of the subvolcanic dikes intruding the granitoid basement rocks, such as in aplite at 777.2 and 777.4 mbsf and in phonotephrite dikes at 1125.1, 1131.7, 1137.5, and 1138.3 mbsf. A possible poorly-developed shatter cone was also noted in a coarse-grained granitoid sample at 909.6 mbsf. Finally, "suevites"and melt rocks also occur in the lower (peak ring) section in the form of small dikes and large bodies, such as the ~100 m thick occurrence between 1215 and 1316 mbsf. Similar shock features as in "suevites" and melt rocks from the upper (peak ring) section were observed.
 

Emplacing impact melt in the Chicxulub peak ring
David A. Kring et al. -- Lunar and Planetary Science XLVIII (2017); 1213.pdf

Distribution of Melt in the Core: After penetrating (post-impact) sediments (Unit 1), the top of the (peak ring) was encountered at 617.33 mbsf, beginning with a 104-m-thick polymict, melt-bearing breccia (Unit 2) with a calcitic matrix that may represent a plume of carbonate ash. The most abundant clasts in the breccia are melt fragments. That unit has been sub-divided (2A, 2B, 2C) based on sedimentary and matrix features. A melt rock, Unit 3, extends ~26 m to a depth of 747 mbsf. It is dominantly a clast-poor melt rock, but clast-rich intervals occur at ~722, ~732-734, and ~744 mbsf. It has been subdivided to reflect a change from green schlieren-bearing black melt (3A) to a basal ~9.5 m-thick coherent black melt unit (3B). Those units cover granite and related basement lithologies. Thin, <1 m-thick melt horizons were logged within the granite. A thicker (~4 m) series of melt and melt-bearing breccia horizons were logged at ~1000 mbsf and ~58 m of melt and melt-bearing breccias dominate the lower 100 m of core. The total thickness of the basement interval sampled by the borehole is 588 m.

Clast Content: There is a diverse array of sedimentary, metamorphic, and igneous clasts within those units. Sedimentary lithologies are carbonate, chert that in many cases is visibly associated with and derived from carbonate, shale, sandstone, and red silt-stone. Metamorphic lithologies are gneiss, mylonite, schist, amphibolite, and quartzite. Marble was also logged, but thin-section studies are needed to determine if it is a target unit or shock-modified carbonate. Igneous lithologies include granite, granodiorite, diorite, dacite, felsite, and mafic clasts that were variously logged as gabbro, diabase, and dolerite. Carbonate and granite are the most abundant lithologies.
Sedimentary, metamorphic, and igneous lithologies are found in the uppermost units, but carbonate was not logged in the bottom eight cores (from ~722 to ~747 mbsf) of the Unit 3 melt rock, and granite dominates the clast assemblage at the base of that interval. Thin melt horizons within the granite only have clasts of (locally derived) granite. However, the horizon at ~1000 mbsf contains clasts of melt, granite, granodiorite, dolerite, and gneiss. The basal interval of melt and melt-bearing breccias also has a diverse array of metamorphic and igneous clasts, but no sedimentary clasts.

Discussion: For the melt-bearing horizons within the granite, we consider four hypotheses. (1) The "impact" melts in the granite may have been injected into the walls of the transient crater and transported with the bounding granite during uplift and collapse into a peak ring. This seems unlikely, however, because the granite is heavily sheared while the impact melt rocks and melt-bearing breccias are not. The melts were introduced after most (albeit not all) of the deformation of the peak ring had occurred.
The "impact" melts in the granite may have instead been (2) produced by melting along shear planes and faults in the basement. That is a likely source of the thin, <1 m melt horizons, which only contain clasts of granite. However, there is a wide variety of clasts in the thicker melt-bearing units at ~1000 mbsf and at the base of the borehole that would require transport of multiple lithologies (granodiorite, gneiss, dolerite) an unknown distance along an intrusive conduit and emplacement in granite.
We cannot discount, however, (3) the infusion of melt from adjacent melt pools along open fractures in the peak ring. The melts at the base of the borehole lie at a greater depth than "impact" melt in the adjacent crater trough, although that source may require transport through fractures over a distanceof at least 2 km, without quenching, while mixing with clasts from multiple peak-ring lithologies.
An alternative (ludicrously) hypothesis (4) is prompted by the lack of sedimentary clasts in the deeper "impact" melts. If the dynamic collapse and Displaced Structure Uplift models are correct, then as the peak ring collapsed and deformed outward, it may have overrun and flowed over surficial melt-bearing components before the fallback breccia with its sedimentary clast components landed. The repetition of "impact" melts within the granitic sequence implies there was internal shearing within the granite as it displaced outward, allowing it to cover breccias at least twice.
 

Preliminary chemical Data for drill cores of the Chicxulub impact structure's peak ring
A. Wittmann et al. -- Lunar and Planetary Science XLVIII (2017); 2075.pdf

Upper "Peak Ring"
The 50 samples from the 130 m thick section of "suevite" and (impact) melt rocks between 618.22‒744.07 mbsf show pronounced vertical variation in their major and minor element concentrations. Based on these chemical variations, the upper (peak ring) section can be subdivded into an upper sorted "suevite" section from 617.34‒684 mbsf that is grossly chemically homogenous. A lower sorted "suevite"section from 687‒718 mbsf shows more chemical variation compared to the subsection above, while average concentrations are grossly similar.
The lowermost section between 720‒744 mbsf is dominated by  melt rocks and is chemically distinct with higher concentrations of SiO2, TiO2, Al2O3, FeO compared to the "suevite" subsections and notably, oxide totals that are typically 20 wt% higher than those of the "suevite" subunits above.

Lower "Peak Ring":
We analyzed 194 samples from the 586 m thick section of granite, subvolcanic dikes, "suevite" and melt rocks between 747.89‒1332.75 mbsf. The granite lithologies in this section are remarkably similar in composition, suggesting compositions of granites and syenites in the TAS diagram and metasedimentary protoliths. Subtile differences are present, though, for example Na2O concentrations from 748.89‒948.39 mbsf are lower than in the section between 948.39 and 1332.75 m. Very low incompatible trace element compositions suggest a volcanic arc setting for the emplacement of these granites, which is also supported by their magnesian character.
Compared to the granitic rocks, the most common intrusive rock, a dark, aphanitic, subvolcanic lithology, has much lower SiO2 and K2O contents, while MgO, FeO, CaO, MnO, TiO2 are significantly enriched. A less common, brown, fine-grained subvolcanic lithology that exhibits shatter cones displays a chemical affinity to the dark subvolcanic dike lithology in that it shows similar depletion and enrichment trends for the major, minor and trace element concentrations compared to their granite host rocks. In the TAS diagram it plots in the field for phonotephrite, while the dark subvolcanic dikes plot in the fields for foidite and basanite.
Intercalcations of "suevite" and melt rocks show variable compositions but tend to be relatively depleted in K2O and enriched in TiO2, CaO, FeO, MgO, and MnO compared to the granitic rocks.
 

Ages and Geochemistry of the Basement Granites of the Chicxulub impact crater
L. Xiao et al. -- Lunar and Planetary Science XLVIII (2017); 1311.pdf

The granitic rocks are intruded by three or more types of subvolcanic dikes or dike swarms including felsite/phonotephrite, dacite/trachyite and diabase/dolerite. Major rock-forming minerals of the granitoid are alkali feldspar, plagioclase, quartz and biotite. Accessory minerals observed in thin section include zircon, apatite, sphene, and opaques. Shock metamorphism is extensive throughout the core; planar deformation features developed in quartz and plagioclase at mineral scale.
Five samples taken from depths of 829mbsf, 927mbsf, 979mbsf, 1076mbsf and 1200 mbsf were dated. Their 206Pb/238U ages are 304+10Ma, 321+7.7Ma, 313+14Ma, 325.9+7.5Ma and 340.8+9.9 Ma. A total of 147 analyses yielded major element oxide contents for SiO2 of 64wt% to 79wt%, Al2O3 of 10 to 17wt%, Na2O of 3.5wt% to 7wt%, and K2O of 2wt% to 8wt%. In the TAS diagram, most samples plot within the granite field, while some are in the quartz monzonite and syenite field.
The zircon dating results suggest that the granites were formed mostly around 300-340 Ma ago. This large age variation of the five samples may indicate that this was not a single granitic intrusion during the Carboniferous period."

Note
: There remain at least 2300 m of basement rocks underneath, which were not drilled and which can be much older.

Here some pictures, published in the net
 

                

Felsic basement rock types

Shatter-cone fragment from an amphibolite clast in the breccia (708.5 mbsf)



Above the crystalline basement: "Green melt" with crystal grains (?)
Photo published on May 10


Breccia in condition of solution-penetration (690 mbsf ?)



Breccias



 



Filled fissure in the granite



Melt rock with granitic clasts



"Granite cut by mixture of "impact" melt (green vs black)"


Seems to be black andesite melt !? (depth unknown)



Core 104R: Granite with melt filled fissures



Crystalline basement rock, and lots of it



Different "granite"


          
Dark mafic melt and pink granitic basement rock


     
Pre-event welded shear-fissure in the basement rock


About 1165 mbsf: Pre-"impact" dyke filled with crystals and clasts, perhaps Lamprophyre
 (finds of shatter-cone fragments in amphibolite facies)


"In two days of drilling (24th - 25th) we recovered a complicated sequence of melt breccia. This sequence has some physical properties that are different to the previous melt breccias found further up the hole"



About 1265 mbsf : No "impact" melt breccia
perhaps
amphibolite clast from Lamprophyre dyke