Pseudofossil artifacts found in the Sinemurian limestones of the Tunežice quarry

Tunežice quarry exposes Sinemurian sandy limestones of the Manín Unit. It was one of the stops during the field trip of the Jurassica VIII conference in 2009 (Schlögl et al., 2009). The limestones are typical by their selectively silicified fossils, mainly oysters Liogryphaea arcuata. Recently ochre-coloured nodules have been found in the quarry (Fig. 1A), resembling fossils of some sessile organisms (Fig. 1B). They were considered to be stromatoporoids, or rather columnar chaetetids due to their radial fibrous structure (Fig. 1C, cf. Stanton et al., 2016, figs. 5-8). The material of the nodules was of unknown origin. First it was supposed to be siliceous, just like in the selectively silicified fossils. However, there was also a similarity with fibrous gypsum. To check this possibility, a simple test was performed. The material was scratched to a thin-section glass and treated by a diluted hydrochloric acid (Rost, 1956). After drying up, typical newly formed needles often arranged in echinoid-like manner Fig. 2A) were revealed which is a typical indicator of gypsum. However, the residuum also contained a silty admixture composed exclusively of zircon grains (Fig. 2B). The latter clearly indicate that the discovered nodules are artifacts which likely originated due to drilling in the quarry. Gypsum drill mud is sometimes used in some special drilling methods; zircon is sometimes used as an abrasive matter. Thin section analysis showed perfectly crystallized gypsum fibres with evenly dispersed zircon (Fig. 2C) which indicates that the silt was stirred before the material solidified. The fibrous gypsum is penetrated by thin veinlets of clear blocky calcite (Fig. 2D-E), indicating some longer time span of crystallization (perhaps several years). Although the described objects are not directly related to Jurassic and geology as such, it is necessary for geologists to be aware of their existence and not to be mistaken as fossils.