Abstract
Siliceous concretions (nodules), from two different geological settings—the Apulian platform margins in Kefalonia island, and the Ionian Basin in Ithaca, Atokos, and Kastos islands—have been studied both in the field and in the laboratory. Nodule cuttings are mainly characterized by the development of a core, around which a ring (rim) has been formed. Mineralogical study, using X-ray powder diffraction (XRPD) analysis, showed that the rim is usually richer in moganite than the core. Homogeneous concretions, without discernible inner core and outer ring, were observed generally in both settings. Mineralogical analysis of the selected siliceous concretions from Kefalonia island showed the presence mostly of quartz and moganite, while calcite either was absent or participated in a few samples in minor/trace abundances. Moganite was generally abundant in all the samples from Kefalonia island. Concretions from the Ionian Basin showed a variation in the quartz, moganite, and calcite contents. Mineralogical differences were recognized both between the different studied geodynamic settings and internally in the same setting, but with different stages of development. The above-mentioned differential diagenesis on nodules evolution could be related to the presence and/or abundance of stylolites, later fluid flows, restrictions from one area to another due to synchronous fault activity, and the composition of substances dissolved in fluids. Moreover, the development of concretions produced secondary fractures in the surrounding area of the nodule-bearing rocks.
Highlights
Both bedded and nodular siliceous concretions are common in both deep-water and basin margins of Cretaceous limestones of the Ionian Basin (IB) and the Apulian platform margins (APM) (Pre-Apulian zone) of western Greece [1,2]
As submarine landslide deposits and turbidites are common on the basin margins, there is an opportunity to examine the evolution of lithified chert in relation to synsedimentary deformation as turbidite deposition produces stable redox boundaries [4], and thick calciturbidites may be susceptible to chert formation [1]
There are great mineralogical differences between APM and IB, siliceous concretions that could be related to many different conditions: 1. the basin shape and configuration, during rift stage of the IB, producing sub-basins with many margins (Figure 28a) [27]; 2. the underlying Triassic evaporites and the several synsedimentary faults in IB (Figure 28b) [31]; 3. the abundance of a stylolites network, which mostly characterizes the Paleocene deposits of APM, and increased the fluid flows; 4. the mineralogical composition and differences between the different geotectonic settings; and 5. the different mineralogical composition between sedimentation processes of different ages
Summary
Both bedded and nodular siliceous concretions are common in both deep-water and basin margins of Cretaceous limestones of the Ionian Basin (IB) and the Apulian platform margins (APM) (Pre-Apulian zone) of western Greece [1,2]. Siliceous concretions were initiated due to precursors in limestone during very early burial diagenesis and are related to redox-controlled boundaries [1,3]. Nodules can progressively form nearly concentric parts, i.e., a rim, a main body, and a core. Their size ranges from a few centimeters to several tens of centimeters, and their morphology can include either subspheroidal masses or lenses. When primary deposits exist (as many jasper and radiolarites reveal), they occur as thin beds
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