Seis mi cally-in duced soft-sed i ment de for ma tion in cre vasse-splay microdelta de pos its (Mid dle Mio cene, cen tral Po land). Geo log i cal Quar terly, 63 (1): 162-177, doi: 10.7306/gq.1456 As so ci ate ed i tor: Anna Wysocka Cre vasse-splay microdelta de pos its and their soft-sed i ment de for ma tion struc tures (SSDS) are de scribed from a tec toni cally ac tive lig nite-bear ing area. These strongly de formed siliciclastic de pos its, sit u ated be tween two lig nite benches, are typ i cal of a cre vasse-splay microdelta. They ac cu mu lated in the overbank zone of a Mid dle Mio cene river sys tem (backswamp area) where shal low ponds or lakes oc ca sion ally ex isted. The de for ma tion takes the form of de formed lam i na tion and load (load casts and flame struc tures) struc tures as well as seis mic brec cias within the first Mid-Pol ish lig nite seam. Duc tile de for ma tion struc tures were gen er ated first by liq ue fac tion and then the brec cia was formed un der brit tle con di tions. The brecciation followed a sud den tec tonic col lapse re sult ing in an in crease in pore pres sure re lated to up ward wa ter move ment. The oc currence in a tec tonic graben and char ac ter is tic mor pho log i cal fea tures sug gest an or i gin of these deformational struc tures with seis mic shocks; thus, they can be called seismites. Hence, we pro vide strong ev i dence for ac cu mu la tion of cre vasse-splay sed i ments in the stand ing wa ter of a backswamp area, and for tec tonic ac tiv ity in cen tral Po land as the Mid dle Mio cene lignite ac cu mu lated.

No abstract

Sedimentary structures discussed in the present study are genetically linked to ripples that consist of pure sand or alternating sand and mud layers. All types of ripple-related structures, such as climbing-ripple cross-lamination and heterolithic bedding, i.e., flaser, wavy and lenticular (nodular), have been identified for the first time in fluvial strata that have been characterised previously as commonly massive. These small-scale bedforms, produced by migrating ripples, have been documented in a fluvial channel of late Neogene age in central Poland. The abundance and co-occurrence of the structures discussed and their spatial distribution provide evidence of their formation under very low-energy conditions, when flow velocity changed markedly, but was often significantly less than 0.5 m/s. Therefore, these ripple-derived sedimentary structures are here recognised as typical of channel fills of an anastomosing river.

The present study focuses on the upper Neogene deposits, called the “Poznań Clays”, that cover more than 75,000 km2 of Poland. They are situated between the first mid- Polish lignite seam and the glaciogenic deposits of the Pleistocene age. Lithostratigraphically, the “Poznań Clays” belong to the uppermost portion of the lignite-bearing Grey Clays Member and the whole Wielkopolska Member (Poznań Formation). The examined fine-grained sediments include mud-rich floodplain deposits with palaeosol remnants and large sandy-muddy or muddy palaeochannel bodies. Therefore, taking into account facies analysis, cross-sectional geometry, and the planform of the palaeochannels, it can be stated that the “Poznań Clays” formed in the environment of a late Neogene anastomosing river.