Oligo-Miocene coal in a microtidal environment reworked under Quaternary periglacial conditions (Western Falkland Islands/Isla Gran Malvina) – Coal formation and natural sand processing

Abstract

The Western Falkland Islands are characterized by landforms and sediments of Quaternary age originating from the interplay of periglacial and marine processes. These have left their imprints on siliciclastic sediments and subbituminous coal, peatland, and podzolic soil. The white quartz sand and a subbituminous coal seam are in the focus of investigations (coal petrography, carbon isotope analysis, sedimentary petrography, clay mineralogy, granulometric, morphometric studies), and geochemistry. The study of the coal-sand couple may sheds some light on two different parts of geology. Firstly, it broadens the knowledge of the regional geology at the southern tip of South America, how coal seams may be emplaced in a periglacial-microtidal environment. Secondly, the study is also relevant for applied geology, how sand is processed in a coal-bearing paralic regime. The coal-sand couple evolved during 6 discrete stages. 2 Miocene under a warm tropical climate organic matter was accumulated in a paralic environment giving rise to subbituminous coal. Stage III: During the Pleistocene alternating erosion and deposition in a wave-dominated coastal-marine regime gave rise to the built-up of the coal layer (allochthonous) interbedded into sand. The hydrodynamic regime reduced the grain size to the level of medium to fine sand, it enhanced its sorting coefficient and caused the separation of minerals according to their specific gravity (mechanical processing).Stage IV: The subbituminous coal seam has been reworked by the last glacial progradation.Stage V: Wind and water were accountable for the morphology of grains to gradually approaching the sphere and the content of heavy minerals to increase along the backshore and in the dune belt. Stage VI: Organic acids derived from the coal promoted the chemical leaching. It significantly contributed to the decrease of the labile constituents of arenites (feldspar, carbonates minerals, and calcareous tests of faunal remains). Chemical alteration was grinded to a halt by another sea level rise, which gradually destroyed the coal layer. The depositional environment most effective for the processing of sand is located in the microtidal setting. In the meso-to macrotidal hydrodynamic regime the chemical leaching becomes the only efficacious tool to improve the quality of the footwall rocks. Due to the predominance of argillaceous sediments there, the quality of clay is enhanced and, e.g., kaolin deposits may come into existence.

Vitrinite reflectances and mineralogy of coal clasts in the Late Carboniferous sequences in the two-deep research wells from the Kozlu coalfield (Zonguldak Basin, NW Türkiye)

“…Peat beds and coal-bearing sequences could be eroded due to several reasons (e.g., flooding events and mass movements during peat formation or fluvial and marine influence after coalification) (Petersen et al, 1998;Lindsay and Herbert, 2002;Geršlová et al, 2016;Izart et al, 2016;Martínek et al, 2017;Bicca et al, 2020). As a result, coal clasts and/or coal-placers could be observed within the synchronous siliciclastic sediments and marine carbonates of coal seams and in modern marine sediments (Littke et al, 1989;Hower et al, 2001;Pešek and Sýkorová, 2006;Dill et al, 2017Dill et al, , 2021Zhang et al, 2019;Yang et al, 2020). Coal clasts are commonly found in Carboniferous coal-bearing sequences and range in size from a millimetre to tens of centimetres.…”