Distinct types of crevasse splays formed in the area of Middle Miocene mires, central Poland: Insights from geological mapping and facies analysis

Abstract

This paper focuses on subaerial- and subaqueous-type crevasse splays in the lignite-bearing Mid-Miocene succession near the town of Konin in central Poland. During the formation of these floodplain forms arose on the surface of highly compactable peat, then transformed into lignite, in territories of at least intermittently active tectonics. Hence, both of these major types of crevasse splays are consistently divided into deformed and undeformed subtypes in this paper. All these types and subtypes are found within the first mid-Polish lignite seam (MPLS-1), which is mined for electricity production in the study area. The numerous borehole data and ongoing exploitation of the MPLS-1 allow the geological mapping and detailed facies analyses of the crevasse-splay sediments to be conducted in most cases.The crevasse-splay sandbodies studied in the Tomisławice and Jóźwin IIB lignite opencasts are small as they cover an area of <0.075–0.63 km2. Their maximum length reaches up to 0.9 km, whilst their maximum thickness obtained from the borehole is 5.3 m, and ca. 1.4–3 m as observed in the field. These sandy deposits were accumulated during overbank flooding on the Mid-Miocene mire (backswamp) surface under subaerial and/or subaqueous conditions. In the latter case, ‘prograding splay deposits’ are typical of crevasse-splay microdeltas. Due to the tectonic activity and high compaction properties of peat/lignite, the interseam sands sometimes underwent both ductile and brittle deformation. This allowed subtypes of the crevasse splays that split the currently mined MPLS-1 to be distinguished. In addition to the cognitive value, a better knowledge of the crevasse-splay sediments, their spatial distribution, and deposition mechanisms can be of great use for lignite extraction planning and operations.