--> Trans-Atlantic correlation of basinal organic-rich sequences around Devonian-Carboniferous boundary

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Trans-Atlantic correlation of basinal organic-rich sequences around Devonian-Carboniferous boundary


During the Devonian-Carboniferous transition (D-C), global scale organic carbon generation and burial in low latitudes is evidenced by the formation of organic- and phosphate-rich mudrocks. The Woodford Shale Formation (WSF) in southern USA and their equivalents in Chattanooga Barnett, Bakken, Sappington, New Albany shales, and chert-rich Arkansas and Caballos novaculites are best examples of such a type of D-C succession. Similar facies known as the Hangenberg Black Shale (HBS) occur in the Rhenohercynian Basin in Europe (RHB), e.g. in the Holy Cross Mountains (HCM) in Poland. Because the HBS is considered worldwide as a valuable shale gas exploration target, in this study we try to compare two deep-water, equatorially-located sections enclosing the D-C transition in the HCM in Europe and the Ouachita–Arbuckle Mts. The HBS is excellently exposed in the Kowala quarry but, however, in other HCM sections an hiatus including the D/C boundary manifests as hard grounds, erosional unconformities, and phosphatic concretion lag with glauconite, i.e. features similar to the D/C boundary in WFS from the northern limb sections of the Arbuckle Mts. (Schwartzapfel and Holdsworth, 1996, Cushman Found. Foram. Res., Sp. Pub. 33, 1-275).

The D-C transition hiatus in WSF and some sections in HCM could be explained by narrowing a basin can cause winnowing of fine-grained materials by strong bottom-currents including the pelagic realm (Noble, 1993; Geology. 21, 315-318). Nutrients derived by rivers and eolian transport stimulate the primary productivity in the water column.

Pyroclasts are present in both eastern USA (e.g. Chattanooga from the Appalachian Basin) as well as European RHB (Kowala section in HCM). A lack of tuffites in WFS could be caused by (i) winnowing-related hiatus across the D/C boundary, (ii) more remote/distal position in relation to volcanic activity centers, (iii) position outside of main trade wind courses transporting the volcanic ashes.

Minor amounts of kaolinite (up to 5%) were determined in the WSF mudstones, indicating a low-topography passive margin deeply weathered under humid conditions as one of the clay material sources. In the HBS of HCM Kowala section, however, kaolinite is restricted only to middle, smectite-dominated interval, interpreted as tuffite rich in terrestrial wildfire charcoal (Marynowski et al., 2012, 3Palaeo 346-347, 66–86). This unusual, exclusive co-occurrence of bentonitized ash and land-derived kaolinite and charcoal could be explain as fast kaolinitization of fresh volcanic glass fallen on humus acid-rich swamp and marshes along shore belt (similar way to the paralic tonstein formation process). Then a partially kaolinized material with terrestrial plant detritus were washed out into deeper, offshore part of basin where it was mixed with analogous ash layer in situ altered into a smectitic clay. Due to high topography of the NE basin rim in the Kowala section and short transportation path to the basin made the detritus no chance to reside long enough to be altered into kaolinite. Only a highly reactive volcanic ash could have been partially kaolinized in the near-shore swamps.

Trans-Atlantic correlation shows general similarity of studied sections across D/C boundary with some restriction connected with local hiatuses, slightly different geotectonic position and detritus provenances. In both basins we consider bottom water-column anoxia, high organic productivity, organic-walled and siliceous microbiota including. This is the argument for global glacieustatic transgression connected with end of the Late Famennian glaciation which is knows in Gondwana and Appalachian Mountains.

AP and GR acknowledge financial support from the Polish National Science Centre grants 2011/03/B/ST10/04602 and 2013/08/A/ST10/00717.