Geochemistry, geology, and isotopic (Sr, S, and B) composition of evaporites in the Lake St. Martin impact structure: New constraints on the age of melt rock formation

Abstract

[1] We report new Sr, S, and B isotopic data for evaporites (gypsum, anhydrite), carbonates, melt rocks, gneisses, and groundwaters recovered in and around the Lake St. Martin (LSM) impact structure, Interlake Region, Manitoba, Canada. The LSM meteorite impacted Devonian to Ordovician carbonates and sandstones of the eastern Williston Basin, resulting in partial melting of underlying Superior Province ($2.5 Ga) gneisses of the Canadian Shield. Overlying the LSM melt rocks are red beds and evaporites (anhydrite/gypsum/glauberite) previously inferred to have been deposited during the Jurassic. The 87 Sr/ 86 Sr (lowest values cluster at 0.70836) and d 34 S CDT (+23.7 ± 0.9%) of the evaporites, combined with B isotope compositions of associated groundwaters (d 11 B NBS951 = +25 to +28%), are consistent with evaporite deposition within the impact structure near the edge of an ocean-connected salina. The establishment of a marine origin for the evaporites offers a method of age assignment using the secular variation of S and Sr isotopes in seawater. Comparison of Sr and S isotope results with the seawater curves precludes Jurassic deposition for the evaporites or correlation with Watrous and Amaranth formation evaporites, previously considered correlative with those at LSM. The lowest Sr and mean of S isotope values from the LSM evaporites are similar to seawater in the latest Devonian, consistent with conodonts recovered from carbonate breccia overlying melt rocks, and we suggest this as an alternative age of the evaporites. Data presented here preclude a Jurassic age for the evaporites and therefore for the impact event.

The Impact Crater Bandwagon (Some problems with the terrestrial impact cratering record)

“…The popularity of impact cratering and the rapid invoking of this process without proper supporting evidence is further exemplified by the following recent publications: 1) an entirely unsubstantiated claim of recognition of a meteorite impact site at Luna in the Kachchh district of Western India by Karanth (2006) and Karanth et al (2006); 2) the report of a controversial hypothesis that impact of a large comet or asteroid could have been responsible for the demise of the Clovis culture in North America (Dalton 2007;Firestone et al 2007); 3) the claim by Gasperini et al (2007) that a so-called Lake Cheko impact crater related to the Tunguska blast event had been discovered in Siberia; this claim has been refuted by Collins et al (2008); 4) speculations (the actual authors' choice of word) concerning multiple impact events in Antarctica by Weihaupt and Rice (2007); 5) the reference to a "High Rock Lake impact" by Leybourne et al (2007), for which, to my knowledge, no impact-supporting evidence has been provided by these authors nor by Kohn et al (1995) whom they cited.…”

Gypsum-hosted endolithic communities of the Lake St. Martin impact structure, Manitoba, Canada: spectroscopic detectability and implications for Mars

“…The intracrater evaporite member of interest overlies and grades into a red bed member and is a 43 m thick sequence of gypsum/anhydrite, minor glauberite and occasional interbedded clays (Bannatyne 1959; McCabe & Bannatyne 1970). The age of gypsum/anhydrite deposits are unknown, although the regional evaporite upper Amaranth formation (which has been identified as Jurassic), is often inferred to have concurrent deposition (McCabe & Bannatyne 1970); however the evidence for this association is controversial (Leybourne et al 2007). A series of open pit gypsum mines in the area have exposed a continuous ~5 km long section of the gypsum/anhydrite deposit of up to 15 m vertical exposure.…”

“…Present crater depth (from surface to intact bedrock) is *320 m; however, prior to glacial modification, the crater depth may have been up to 610 m (McCabe & Bannatyne 1970). The age of gypsum/anhydrite deposits are unknown, although the regional evaporite upper Amaranth formation (which has been identified as Jurassic), is often inferred to have concurrent deposition (McCabe & Bannatyne 1970); however the evidence for this association is controversial (Leybourne et al 2007). The age of gypsum/anhydrite deposits are unknown, although the regional evaporite upper Amaranth formation (which has been identified as Jurassic), is often inferred to have concurrent deposition (McCabe & Bannatyne 1970); however the evidence for this association is controversial (Leybourne et al 2007).…”

The History of Transgressions during the Late Paleocene-Early Eocene in the Kuqa Depression, Tarim Basin: Constraints from C-O-S-Sr Isotopic Geochemistry

“…The radioactive strontium isotopic ratio of continental gypsum is larger than the strontium isotopic ratio of contemporaneous seawater and plots far from the strontium isotope curve of contemporaneous seawater. For example, the 87 Sr/ 86 Sr ratio of Paleogene continental gypsum in the Dongpu Depression ranges from 0.7116 to 0.7137 [53]. The strontium isotopic ratio of gypsum from the sea-land transition phase is commonly affected by the mixing of seawater and terrestrial water and has the characteristics of both.…”