46th Lunar and Planetary Science Conference (2015) 1915.pdf

ROCHECHOUART AS NATURAL IMPACT LABORATORY: A REVIEW. P. Lambert, Sciences & Applica- tions, Bd Albert 1er, 33800 Bordeaux, France. E-mail: [email protected].

Introduction: The planetary importance of impact shock studies and preliminary gravimetric investiga- as a natural process is well acknowledged today, but tions [2,7]. The initial crater must have been larger. the recognition and studies of impacts are very recent. Recent work by [8] agrees with the 40-50 km diameter Fundamental aspects such as how large impacts get proposed earlier by [3]. 2-Position of the center of the their final shape are still not well understood. Also we crater: The largest coherent melt-rich impactite occur- are still at the onset of discovering and evaluating ma- ring at Montoume is interpreted by [8] as the center of jor incidences such as emergence of life both in the the crater, 6 km south of the center admitted so far Early Earth and on other Planetary Bodies. Research on from the distribution of shock in both the target and the the above mentioned issues has focused so far on theo- breccia deposit [2] and from preliminary gravimetric retical approaches and mathematical models. These studies [7]. Points 1 and 2 call for combined detailed need to be constrained and validated by ground truth gravimetric, petrographic and micro-meso-tectonic data. In that frame, terrestrial impacts are and will re- studies outside the breccia deposits where most of the main by far the most accessible data reservoir. This research has been focusing until today. Studies should reservoir is certainly “under studied”, and within this cover at least a 50 km zone centered on the current reservoir, some objects are definitely “under studied”. breccia deposit and extend up to the border with the It is precisely the case of Rochechouart. Mesozoic deposits at the western margin of the conti- Bibliographical record: Since Rochechouart has nent at the time of impact. 3-Age and projectile identi- been recognized as an eroded impact structure in 1969 fication issues: These have been controversial, ac- [1] only 32 full length papers have been published in counting for about 40% of the total literature on peer review literature. The number drops to 22 for pub- Rochechouart. It is particularly stringent for age as the lications exclusively devoted to Rochechouart, 25 most robust data as of today makes the impact concom- when including the 3 University thesis on the structure. itant with Triassic-Jurassic mass extinction [5,9]. 4- For comparison, volume 75 of Geologica Bavarica Representativity? The Rochechouart crater floor is (1977) alone contains 34 full length papers on Ries. On known to be extremely flat [1,3,5] and shock below the Rochechouart, combined geological, geophysical and crater floor is highly variable at all scales [2]. These geochemical studies represent only 1/3rd of the biblio- features depart from the general figure we have for the graphical record dominated by specific investigations wide majority of impact craters on planetary surfaces (age, projectile contamination and shock related stud- resulting from “single” projectile collision. Could ies). Rochechouart be an exception? The hypothesis of a What makes Rochechouart so interesting: tight cluster of projectiles resulting in a single flat Rochechouart is among the most accessible impact crater has been raised [10], yet never addressed. Com- sites of its size on Earth. It is located in a politically paring the terrestrial record with other planetary sur- stable country close to two international airports (1 and faces, the probability for such a case in a population of 2 hours drive respectively). A very dense array of 185 objects tends towards 0. One can then ask if it is farms and tiny villages is so that any place within the not the current understanding of impact crater floor whole impact structure is never more than a few hun- geometry at large impact that need to be revised, or at dred meters from a road or a trail accessible by car. least amended. Rochechouart happens to be the only Relief is smooth, no step cliffs, no lakes, no big cities case in the terrestrial record, where complete detailed masking the record. Erosion has removed all the mate- geometry and petrology of the readjusted crater floor is rial that covered the impact deposits in the crater [2,3]. readily exposed. All of what we know from elsewhere, Later cut thru river valleys gives access to the full se- comes from drillings, from geophysics or from theory. quence of fall back ejecta, including late fine grained It is far to be as precise and “real” as what is exposed deposits (impactoclastites) [3-5] down to below the in the field at Rochechouart. 5-The apparent deficit of crater floor, in a zone equivalent of the whole inner deposits within the crater [3,5], 6-The origin and dis- crater at Ries, although about twice larger [3,5]. tribution of impactoclatites [4-5], 7- The incidence of Rochechouart is just explored and essentially all de- the littoral setting of the impact on its characteristics tailed study remains to be done. Fundamental issues and the potential for enhanced pyroclastic activity due needing special attention include: 1- Size of the origi- to sea water intrusion at the bottom of the crater [5], 8- nal crater: The official 23 km [6] is known to have no The distal consequences [9]. Beyond local issues, sev- phenomenological signification [3,5] relating to the eral fundamental issues common to the whole impact size of the structure as set some 40 years ago from and meteorite community are particularly well ad- 46th Lunar and Planetary Science Conference (2015) 1915.pdf

dressed at Rochechouart: A-Thermal evolution and faith of PGE in relation with cooling condition and fluid behavior of impact structures: This is of critical hydrothermal alteration. importance in the context of their possible influence on Ground truth data mining opportunities: The emergence of life both in the Early Earth and on other recent establishment of the Rochechouart Astrobleme Planetary Bodies. This is largely related to hydrother- as a protected area in the scheme of National Natural mal mechanisms at work during the cooling of the hot Reserve (NNR) and its collaterals [14] are providing materials produced and deposited inside impact craters. the opportunity to use Rochechouart as a full scale la- Yet most of the research in this field is focused on the- boratory for impact related studies on planetary surfac- oretical approaches and mathematical models [11]. es. If sampling is now subject to prior approval by Within the terrestrial reservoir, only one third of the NNR, digging and excavating rocks is not prohibited, 185 impact sites currently identified on Earth are suffi- at the contrary. Yet it has to be flagged to NNR for ciently large to have initiated an hydrothermal cell sim- conservative sampling. This way, NNR will progres- ilar to that advocated for producing water lakes on sively build up and curate a unique impact rock collec- large Planetary bodies including Mars and the Early tion at the scale of the whole structure and counterbal- Earth. Within that population, essentially all are either ance the limited outcropping due vegetal cover. To too young and not eroded enough (such as Ries or enable such a curatorial development the local authori- Chesapeake Bay) or too old and too eroded to expose ties are installing on site a small dedicated “Research the material needed to constrain the key processes of Center” comparable to “ZERIN” at Ries. This entity temperature evolution and water availability at a large will handle the scientific management of the collected impact site. To do so, continuous 3D access to material material and the exchanges with the scientific commu- at/across the crater floor limit is required. It must cover nity at large encouraged to take advantage of these “on the hot impact melt deposits above, down to autoch- shelve” material for studies. Beyond passive sampling, thonous para-autochthonous target below, with prefer- NNR has set drilling the crater as the core of its new 5 ence to crystalline rocks target to enable planetary years term management plan [14]. Budget foreseen comparisons. Rochechouart precisely fits all these re- would allow 300 m of cumulated cores. The number, quirements and is also known for having developed the depth and location of the drillings is planned to be required hydrothermal cell [3,8,12]. B-Cratering me- tuned according to specifications of the scientists who chanics: The mechanism(s) by which the material be- will apply for getting samples. A call for expression of neath transient crater is displaced over kilometers at the interest is open (form and information are available at readjustment stage, as well as the figure on how this [email protected]). An interna- happens are still poorly constrained and only known tional advisory committee will survey both the drillings from theory. Experiments are not able to reproduce and the Research Center [14]. these mechanisms and ground truth data have not yet References: [1] Kraut F. (1969) Geologica Bavarica been made available. Damage, fracturing and plastic 61, 428–450. [2] Lambert P. (1977) Earth and Plane- deformation at all scales induced by the impact are tary Sci. Letters 35, 258-268. [3] Lambert P. (2010) indeed difficult to recognize and interpret at natural GSA Special Paper 465, 505–541. [4] Lambert P. and terrestrial sites, owing to convergence with features Reimold W.U. (2009) MAPS 44, A-119. [5] Lambert, which are usually also present at the initial stage in the P. and Goderis, S. (2014) MAPS 49, Issue s1. Abstract target. Yet this can be done, providing a wide access #5182. [6] www.passc.net/ EarthImpactDatabase. [7] and detailed studies at and below the bottom of a large Pohl J. et al. (1978) Meteoritics 13, 601–604. [8] Sa- impact crater, as exposed at Rochechouart. C - Limita- pers, H.M. et al. (2013) MAPS 49: 2152-2168. [9] tions of PGE inter-element correlations in identifying Schmieder et al. (2010) MAPS 45: 1225-1242. [10] meteorite and projectile contamination: Phase chemis- Lambert P. (1982) Meteoritics 17(4), 240–241. [11] try of contamination carriers is poorly constrained in Naumov, M. (2005) Geofluids 5, 165-184. [12] Lam- all these materials. Relative mobility of PGE during bert P. (1977) In Impact and Explosion Cratering, cooling and post impact history on planetary surfaces Roddy D.J. et al. edts, 449–460. [13] Lambert P. starts to be realized and calls for ground truth investi- (1982) GSA Special Paper 190, 57–68; [14] Lambert gations. With the insurance that impact and post impact P. et al. (2014) MAPS 49, Issue s1. Abstract #5171. alterations have fully developed in Rochechouart and with a well established geochemical projectile over- print in all the crater fill deposits, including in the non- melted materials [12], we have there an excellent op- portunity for investigating the phase chemistry of pro- jectile contamination carriers and characterization of