Abstract
Recent theories about the sources of silica in bedded and nodular cherts do not fit the origin of cherts from the Kraków–Częstochowa Upland. Since siliceous sponges as a single source of silica is questionable, assumptions about additional sources have to be verified. In order to do so, three samples of nodular cherts and one representative sample of bedded chert were studied by means of 1H LF-NMR 1D and 2D relaxometry and complementary geochemical methods. The results were compared with the literature and standard silica materials which helped to identify five types of 1H signal. The very distinct 1D-T2 spectra of the dried samples indicated the existence of closed pores which, after comprehensive analysis, were identified as inclusions filled with different types of siliceous materials. Saturation revealed the differences between nodular and bedded cherts that were visible mainly in the amount and size of open porosity. The principal component analysis of NMR parameters showed the excellent separation of these two groups of samples and this is well visible on the biplots. Additionally, the estimated pore size distribution revealed that the total porosity of around 2% consisted primarily of mesopores (2–50 nm in diameter) and macropores (diameter >50 nm). In bedded cherts, open porosity is dominated by macropores, while the share of mesopores and macropores is similar in nodular cherts.
Highlights
Cherts are a type of sedimentary, nonporous, highly siliceous rocks composed of quartz
The assumption that siliceous skeletons of sponges were the main source of silica for chert nodules forming horizons in biostrome-like Oxfordian bedded limestones has already been presented in the literature.[34−37] the origin of silica in bedded cherts hosted in calciturbidites from the Oxfordian/Kimmeridgian turn has only been discussed in a single publication by Matyszkiewicz,[38] who suggested the accumulations of radiolarians abruptly buried in sediments by density flows as the source of silica, in accordance with the model after Bustillo and Ruiz-Ortiz.[30]
The concept of siliceous sponges skeletons as practically the sole source of silica for chert nodules hosted in the Upper Jurassic microbial-sponge megafacies of the Krakoẃ -Czestochowa Upland (KCU) has been questioned due to three facts: (i) the lack of a clear correlation between the abundance of siliceous sponges in bedded limestones[39−41] and the accumulations of chert nodules, which only occasionally form horizons in the Middle Oxfordian bedded, biostrome-like limestones,[35,42] (ii) the occurrence of chert nodules within the calciturbidites from the Oxfordian/Kimmeridgian turn, whereas the share of siliceous sponges in in situ sediments of that age is lower than in the Middle Oxfordian sediments and while radiolarians are present in marly sediments overlying the calciturbidites,[38,42] and (iii)
Summary
Cherts are a type of sedimentary, nonporous, highly siliceous rocks composed of quartz. The differentiation of cherts is important from the geological, archeological[2−5] and petrological[6] point of view The origin of both the chert nodules and the bedded cherts in the Fanerozoic sediments has attracted the attention of many authors. These rocks are interpreted as the products of synsedimentary, or early- to late-diagenetic, or epigenetic processes (see, e.g., refs 7−32). The KCU is located in southern Poland and is a part of the Silesian-Krakoẃ Homocline the regional tectonic unit built up of the Triassic, Jurassic and Cretaceous sediments deposited unconformably onto the Precambrian and Paleozoic formations, and transected by the major the Krakoẃ -Lubliniec Fault Zone.[33] Various assumptions as to the origin of silica in bedded and nodular cherts have been made. The concept of siliceous sponges skeletons as practically the sole source of silica for chert nodules hosted in the Upper Jurassic microbial-sponge megafacies of the KCU (as a part of the Tethyan northern margin) has been questioned due to three facts: (i) the lack of a clear correlation between the abundance of siliceous sponges in bedded limestones[39−41] and the accumulations of chert nodules, which only occasionally form horizons in the Middle Oxfordian bedded, biostrome-like limestones,[35,42] (ii) the occurrence of chert nodules within the calciturbidites from the Oxfordian/Kimmeridgian turn, whereas the share of siliceous sponges in in situ sediments of that age is lower than in the Middle Oxfordian sediments and while radiolarians are present in marly sediments overlying the calciturbidites,[38,42] and (iii)
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