The target rocks are limestone/dolomite. I assume that the CO2 gas in the rock will be released during the impact and force "plasticized" rocks to move out at supersonic speeds. The following textures are common there, but I haven't seen reported in literature from this -St. Martin crater, Manitoba, Canada- or other craters.
Fig 1 shows probable shatter cones (not described in this crater before). What I call "missile rocks" or "umbrella texture", arched up, with rounded tops & concave, clay-rich bottoms in Fig. 2 &3. The piece on the right in Fig2 fits to the bigger piece. Finally, what looks like "expulsion cavities", parallel openings/cavities from expelled gas in Fig 4.
All these rocks were originally part of the gypsum-cemented "crater breccias" that have been released into the soil after the interstitial gypsum has been dissolved by the rainwater.
Hello Michael,
I don't think the specimen you are showing is a shatter cone. This looks more like conchoidal fracturing in homogeneous, micritic limestone. I'd expect to find shatter cones in the drill cores into the Saint Martin crater basement rocks - and in the central uplifted granite exposed at the crater center. Shatter cones may also occur in rock clasts in the impact breccia.
Could the vesicular limestones be small post-impact algal bioherms that formed in a Late Triassic to Jurassic crater lake? I would be surprised if these were carbonate melt rocks (see work by Gordon Osinski on Haughton crater for such features). Such biogenic features are common and well known at the Ries crater in Germany (work by Gernot Arp and colleagues) which is in many aspects similar to Lake Saint Martin. Do these rocks contain any recognizable fossil shells?
Cheers, Martin
@Martin...If these rocks have been recovered from the impacted area as Michael Issigonis has claimed then they can be shock metamorphosed rocks......May not be copy book shatter cones.....I suggest, they can be given a different name but only after confirming by experiments that such features develop due to impact. Cheers!
Dear Michael,
I agree with Martin: I'm quite sure that this specimen is not a shatter cone! In the Steinheim Basin imapct crater, we have a lot of high-quality shatter cones in limestones, but also some poorly develloped "not copy book" shatter cones. But the specimen shown is a typical fracture structure that commonly occurs in limestones.
The "vesicular limestones" are probably of biogenic origin. The specimen shown in Fig. 1 and 2 strongly resemble caliche.
We investigated the problem of degassed limestones in impact craters (Noerdlinger Ries and Steinheim Basin) in the field, in experiments (plasma tunel) and by numerical simulations. Degassing of limestone target rocks is a known process during impact events, but degassed limestones (and carbonate melts) are not stable and decompose quickly. In the Ries and Steinheim craters, hundreds of meters of carbonate target rocks were influenced by the impact (breciated, molten) but we do not find any degasses rock specimen ejected from the craters. Even the most distal impact ejecta of the Ries crater ("Brockhorizon") that were ejected over a distance of 200 km and experienced strong shock and thermal load do not show any signs of degassing (Buchner et al 2007). In the Steinheim Basin, only some very small limestone components with degassed rims are incorporated into the basin breccia (Buchner and Schmieder, 2010). All components that were degassed strongly or completely decomposed.
As a result, the pictures you show most probably do not represent impact metamorphosed limestones.
Best Elmar
E. Buchner, M. Grässlin, H. Maurer, H. Ringwald, U. Schöttle, H. Seyfried (2007) Simulation of trajectories and maximum reach of distal impact ejecta under terrestrial conditions: Consequences for the Ries crater, southern Germany
Elmar BUCHNER, Martin SCHMIEDER (2010) Steinheim suevite—A first report of melt‐bearing impactites from the Steinheim Basin (SW Germany)
Denise Anders, Gordon R. Osinski, Richard A. F. Grieve, Martin Schmieder, Philip Kegler, Elmar Buchner (2012) Geochemical signatures of carbonate impact melts from the Steinheim impact crater, Germany.
Hello All,
Thank you for your contributions. It is strange that good shatter cones have not been described from this crater. I will take Martin's suggestion and look at the core, it could have been missed - it was logged a long time ago when this phenomenon was newly described.
From what I have seen at St. Martin's, the phenomena at Haughton and Ries - referred to by Martin -are different. I have seen nothing resembling fossil shells.
The rocks shown in figures 2 & 3 are calcite-rich with flattened cavities. A petrographer on similar, in-situ, "amygdaloidal -type" rock found it having-besides carbonates- "pervasive alteration with clay and quartz, of cherty, chalcedonic character in parts resembling a siliceous sinter or hot-spring deposit".
What intrigued me, to ask the question, was the peculiar shape of the blocks (arched-up). To me it looks like some force has been applied. Could the explosion at St. Martin's have been less violent that those at Ries and Steinheim, or alternatively, some late-stage explosions kept the thrown out debris inside the crater?
Hello Michael,
it is hard to say from the fotographs aloone, but an Interpretation of the rocks shown in Fig. 2 and 3 as sinter or hot spring deposits seem plausible. Impact induced hydrothermal systems are known from many imapact structures, even from small craters like the Steinheim Basin.
For me, it is not important if the rocks were found inside or outside the crater and if the Impact process was more or less violent. Signs of degassing of limestones look different and degassed carbonates are unstable in any case. We never observed "expulsion cavities", neither in nature nor in experiments. Degassing always affects the entire rim of the limestone components uniformly.
Best Elmar
I agree with Martin about the specimen you show, it is common to see this type of fracturing in the micritic limestone....in Jordan you can see this in a distinct Formation called Wadi As Sir Formation. Figs 2,3 and 4 are not clear in the pictures ?
See our work about v
NEW LABORATORY RESULTS ON FIELD SECTIONS AT THE IMPACT CRATER OF ARAGUAINHA (MT, GO, BRAZIL). AREA OF PROXIMAL AND DISTAL IMPACT EJECTA, INCLUDING MICROSPHERULES DATED FROM THE END OF PERMIAN.
J. M. Théry1, A. Crosta², E. Veto Akos3, E. Bilal4, K. Gal-Solymos5 and E. Dransart6. 1 9,
Introduction: The Araguainha Impact crater is located in Brasil in the northeastern part of Parana Basin: latitude -16°77’, longitude –52°58’. Its diameter is 40 Km.
W.Engelhardt1 (1992) and A.P. Crosta2 (1981) are amist the first with Dietz and French (1972) to recognize, this crater as astrobleme. We have to give a right credit to all geologists, who worked there before.
On the impact crater at Araguainha and neighbouring areas [1], we have obtained new data during our field survey carried out in July 1998 with A. Crosta.
The purpose of the present paper is to give an outline of our research. The samples collected within the crater and in neighboring localities have provided a clear understanding of the stratigraphy of the Permian-Triassic boundary (PTB) at the time of impact. As a result of a careful choice of twelve new sample locations in the area of the crater itself, we have been able to discern some of the nature and constitutions of the breccia and dikes. Subsequently, the constituents were studied thoroughly by petrographic analyses. The layered nature of the breccia allowed us to establish an order in the deposits. The results from petrographic thin sections and Scanning Electron Microscopy/EDAX studies were compared. We systematically analyzed the Landsat MSS and the Landsat Thematic Mapper images (already studied in [2], and [3]), from the outcrop areas, in order to obtain a correct map base for our interpretations.
Results:
The most important results have been obtained from the use of the petrographic diagrams of [4] and [5].
Plots in the Total Alkali Silicates diagram (TAS) are consistent with the QAPF modal classification of granites and of eruptive rock and proposals published before were revised and updated.
In this framework, some of the alkali-rich K granites collected at Araguainha are in reality rhyolites with all their various constituents, rhyo-dacites and andesites. It has become evident that the uplifted K granite of the target rock (basement dated at present 500 to 530 Ma, the red dikes and the breccias ejected and deposited around the area are of the same age.
According to field observations on sample locations 6 and 8 (E Veto Akos, written information), the impact related breccias, present in the impact products, are mixed with rhyolitic debris. A volcanic eruption was caused by the impact event at PTB.
We have established the geological buildup of the target rocks at the time of the impact event, the severe modification of this build up after the impact and the erosion and weathering phases since the Permian time by studying the signs of shock waves observed during our field trip and from aerial photographs.
A large, 3 to 4 km in diameter, iron-rich bearing meteorite, fell at Araguainha at the end of the Permian, very close to a mega structure known as the Tranbrasilian Lineament [6] at the boundary between the Guapore Plate (U. Precambrian) and the Goias Plate (Archean to Precambrian). Shearing and thrusting took place at the boundary of an old Cambro-Ordovician granitic trend and the Arnica Mts. The latter were folded during the Guandacol- and subsequent Ocloyica orogenies, the Famatinian cycle, probably at the end of the Ordovician but definitively at the end of Lower Devonian time ([7], [8] and [9]).
According to detailed maps of the DNPM of the state of Goias (1986), the Serra Negra Rift in the Araguainha area and at Piranhas near Bom Jardim of Goias was formed along this line of weakness in Western Gondwana. This event took place as early as Lower Llandovery [10]. This tectonic process was initiated by the collision of the Precordillera of Argentina, separated from Laurentia, against the western margin of Western Gondwana [11].
With the ejection of molten granite and rhyolites, a layer of breccia was formed with the largest particles and ejecta near the front of the Arnica Mts. Already formed in Devonian times. Those old ranges are at present eroded but retain their former outlines and fold axis. The finer sized ejecta are believed to have been thrown in a southern direction in accordance with the scheme of [12] and revised by [13], following simulation studies of the growth and movement of particles. As a last stage, we assume a process of volcanism followed
by a hydrothermal stage at the northeastern periphery of the crater (sample locations 11 and 12) with a characteristic fluid phase (OHmol, F, CI).
Microspherules Within The Proximal And Distal Ejecta:
The microspherules ejected from the crater are probably included within the smallest ejected particles.We have many examples of this within the Araguainha crater itself and surrounding areas, e.g. at Balneario 60 km away from the crater location and in Montividiu, 196 km. towards the southeast. The investigations on proximal ejected particles were carried out at a hill site in the breccia belt identified by the presence of a TV antenna. Here, we have collected some samples (Nb 5) which show the presence of Ni-rich) when analysed by ionic probe IMS (1500 ppm Ni), SEM and elemental spectra. At the same emplacement Hipperttet et Lana (1998 MPS 33, 1303-1309) observed a Rh and Ir enrichment (4x) relative to the indigenous target sediments, where Ir content is only 0.4 ppb. In Balneario near Alto Garças (MT), just above the thick- stromatolite beds [14], we found nice PDF with 1-2 microns-thick laminae. According to E. Dransart they contain small fluid inclusions, less than 1 micron in size. We have also observed oval quartz, 30 micron in size forming a "ballen", structure. They are "crackled" [15] and provide evidence of an impact origin of the Araguainha crater. They are considered as small ejecta included in the deposits and mixed with gravels and probably pseudotachylites [14]. We have also found diaplectic glass along a dike close to the TV hillside (written information from thin section studies by E.Veto Akos). According to Jean Dejax (Museum d'HistoireNaturelle, Paris ) and Rodolfo Dino (Petrobras, Rio, Brazil),
a palynological assemblage found in a recent determination, precises the top of Tatarian to Lower Triassic at Montividiu (GO) (written pers.com). We identified, precisely the extraterrestrial microspherules, the shocked quartz and the detailed stratigraphy of the PTB. In addition, these data confirm the age of 247-251 Ma for the distal ejecta in Montividiu (GO), which had been formerly determined by datation of observed fungal event [16].
Summary:
The presence of distal ejecta at the south of the Araguainha impact point at the PTB up to 10-60 Km and even 196 km distant, suggests a large area of distribution of characteristic fallout ejecta. In addition, according to map data, three to four secondary craters are believed to exist around the main crater in the form of a cluster 25 to 35 km in size. It shows the importance of the Araguainha impact at PTB, already well known as the time of the most important mass extinction of flora and fauna. This worldwide catastrophic event can be emphasized by the impressive belt of extraterrestrial material of the same age around the old Tethys Ocean ([17] and [18]) and if we consider all material containing exraterrestrial microspherules known at Sassayama [19] (Japan), Meishan [20] (South China) and in the Bukk Mountains (hungary)[21], as impact fallout material at the end of Pemian time. We can also add one more recently discovered location in the western Caucasus (pers. com) where we found -Ni/Cr spinels at the Pemian Triassic boundary.
References:
[1] Engelhardt W. et al. (1992) Meteoritics, 27, 442-457.
[2] Crosta A. P. et al. (1981) Rev. Bras. Geosciences, 11, 139-146.
[3] Crosta A. P.and Thomé Filho (2000) Post Congr. field trip, Aft 47, Rio de Janeiro.
[4] Mc Donald et al. (1992) US Geol. Survey Prof. Paper 1523.
[5] Le Bas et al. (1986) J. of Petro., 27, P3,
745-754.
[6] Brito Neves and Cordani (1991) Precambr.
Res., 53, 23-40.
[7] Rapela et al. (1992) in Gutierrez et al.,
Merida, Extramadura; 21-68.
[8] Pankhurst and Rapela (1998) in “The proto Andean Morgin of Gondwana”. Geol. Soc. Special Publ. 142; 383p.-London..
[9] Thery JM. (1978) inter. rept. unpublished. S.N.P.A. 120p. Trompette R., 1994 A.A. Balkema Edit. Rotterdam/Brookfield 350p.
[10] Gray J. et al. (1982) Geology, 13, 7, 521-525.
[11] Astini R. A. (1996) XIII Congr. Geol. Argentina, III Congr. Expl. Hidrocarburos, Actas V, 509-736.
[12] Stoeffler D. (1974) Forschritte Mineralogie, 51, 256-289.
[13] Artiemeva (2002) IX Int. Conf. on Moldavites, Imp. Glass, and Imp. Proc. Frantiskovy-Lazne, Abstract.
[14] Théry et al. (2002) Ibid.,Frantöskovy-Lazne.
[15] French B. M. (1998) LPI Contr. 954, Houston.
[16] Visscher et al. (1996) US Nat. Acad. of Sciences Proceed. 8, 93, 2155-2158. [17] Retallack G. et al.
(1998) Geology, 26, 11, 979-982. [18] Chateauneuf et Stampfli (1978) Notes Labo Paleont Univ. Genève,
Fasc. 2, l-l3.
[19] Miono et al. (1973), Nuclear Methods in Physics Research B75 p 435-439.
[20] Jin Y.G. et al. (2000) Science, 289, 432-437.
[21] G.H.Bachmann et H.W. Kozur ,Conf. 19 int. 2002 Frantiskovy, abstact.
it is a simple fine-grained massive calcareous rock, samples of which you can find by the thousands and more. You may call it micritic or mudstone. Rock strength, inhomogeneties and the build-up of tension during consolidation and compaction are the cause for these structures well-known to field geologists. To what extent the sulfate cement has provoked such structures is difficult to say but it could have been a catalyst´s effect for the structure. Part of the carbonate was original enriched in Fe which now shows up in its trivalent state as FeOOH (goethite) giving the rock a rusty outward appearance. This may be due to the fact that a higher dolomite content was present in the original rock since dolomite tends to accommodate a higher rate of bivalent Fe into its lattice. That is all what I can say based upon the photograph as they stand.
Best regards
Harald G. Dill