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Sedimentary history and biota of the Zechstein Limestone (Permian, Wuchiapingian) of the Jabłonna Reef in Western Poland

Peryt T. M., Raczyński P., Peryt D., Chłódek K., Mikołajewski K.

Annales Societatis Geologorum Poloniae

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2016

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Vol. 86, No. 4

379--413

EN

The Jabłonna Reef, one of the reefs formed in Wuchiapingian time in the western part of the Wolsztyn palaeo-High (SW Poland), is characterized by quite irregular outlines and consists of three separate reef bodies (ca. 0.5–1.5 km2 each; the thickness of the reef complex is usually >60 m). It is penetrated by four boreholes, which show two distinct phases of bryozoan reef development during deposition of the the Zechstein Limestone. The first one occurred early in the depositional history and botryoidal aragonitic cementation played a very important role in reef formation. This phase of bryozoan reef development terminated suddenly; one possible reason was that a relative change of sea level – first a fall and then a rise – disturbed the upwelling circulation. Consequently, bioclastic deposition predominated for a relatively long time until the second phase of bryozoan reef development occurred, but the latter was not accompanied by dubious early cementation. During this second phase, reticular fenestellid bryozoans were predominant. Subsequently, microbial reefs developed and abound in the upper part of the Zechstein Limestone sections. The general shallowing-upward nature of deposition in the Jab³onna Reef area resulted in reef-flat conditions with ubiquitous, microbial deposits, in the central part of the Jab³onna Reef. Then, the reef-flat started to prograde and eventually the entire Jab³onna Reef area became the site of very shallow, subaqueous deposition. Five biofacies are distinguished in the Jab³onna Reef sections: the Acanthocladia biofacies at the base, then mollusc-crinoid, brachiopod-bryozoan, Rectifenestella and at the top, stromatolite biofacies. They represent a shallowing-upward cycle, possibly with some important fluctuation recorded as the distinctive lithofacies boundary, corresponding to the Acanthocladia/mollusc-crinoid biofacies boundary. The 13C curves of the Jab³onna 2 and Jab³onna 4 boreholes permit correlation of the trends in the middle parts of both sections and confirm the strong diachroneity of the biofacies boundaries, with the exception of the roughly isochronous Acanthocladia/ mollusc-crinoid biofacies boundary. The presence of echinoderms and strophomenid brachiopods indicates that until deposition of the lower part of the Rectifenestella biofacies, conditions were clearly stenohaline. The subsequent elimination of stenohaline organisms and progressively poorer taxonomic differentiation of the faunal assemblage are characteristic for a slight, gradual rise in salinity. The taxonomic composition of organisms forming the Jab³onna Reef shows a similarity to reefs described from England and Germany, as well as the marginal carbonate platform of SW Poland. Filled fissures were recorded in the lower part of the Jabłonna Reef. The aragonite cementation recorded in some fissure fillings implies that they originated in rocks exposed on the sea floor and are neptunian dykes.

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Folding in the Middle Jurassic Todilto Formation, New Mexico-Colorado, USA

Lucas S., Krainer K., Berglof W.

Volumina Jurassica

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2014

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Vol. 12, no. 2

39--54

EN

The Middle Jurassic (Callovian) Todilto Formation of northwestern New Mexico–southwestern Colorado, USA, is a carbonate/evaporite lithostratigraphic unit that was deposited in a large paralic salina culminated by a gypsiferous evaporitic lake. Intraformational folds of the limestone-dominated lower part of the Todilto Formation (Luciano Mesa Member) range in scale from millimeters to meters, and many of the large folds are the loci of uranium mineralization. A diverse literature has attributed the formation of intraformational folds of the Todilto Formation to several causes, including syndepositional or postdepositional tectonics, soft-sediment deformation due to sediment loading or gravity sliding, diagenetic alteration (primarily the hydration/crystallization of gypsum/anhydrite), the growth of stromatolitic bioherms or the formation of tepee-like structures. We examine in detail two characteristic outcrops of intraformational folds in the Todilto Formation, in west-central New Mexico, to conclude that folds and domal structures present in the Todilto limestone facies at different stratigraphic levels and at different scales have resulted from varied processes that produced dome-like stromatolitic mounds, tepee-like structures, small-scale enterolithic folds and large-scale folds of likely diagenetic origin.

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First find of biogenic activity in the Palaeoproterozoic of the Singhbhum craton (E India)

Loon A. J., Mazumder R.

Geologos

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2013

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Vol. 19, No. 3

185--192

EN

The Palaeoproterozoic succession of the Singhbhum craton in E. India was hitherto considered as almost entirely siliciclastic and partly volcanogenic. Here we describe, from the fine-grained, tidally influenced shale facies of the Palaeoproterozoic Chaibasa Formation (2.1-1.6 Ga), a fine, originally more or less horizontal, wavy to strongly undulating (later locally deformed) lamination. Investigation of these laminae shows that they must be ascribed to the accumulation of fine particles on microbial mats that covered a sandy substrate. The structures must therefore be considered as stromatolites, features that are accepted as proof of the presence of micro-organisms, in this case most probably cyanobacteria. The interpretation of biogenic activity is supported by microscopic analysis. It is the first description of traces left by biogenic activity that took place in the Palaeoproterozoic of the Singhbhum craton.