Seismically-induced soft-sediment deformation in crevasse-splay microdelta deposits (Mid-Miocene, central Poland)

Abstract

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.

Seismically induced soft-sediment deformation in crevasse-splay microdelta deposits (Middle Miocene, central Poland) – reply

“…We ex pected that our pa per on the cre vasse-splay microdelta (Chomiak et al, 2019) would arouse the in ter est of other research ers for at least two rea sons. First, this is the first such palaeoform dis cov ered and de scribed within the Mid-Mio cene lig nite seam in Po land.…”

“…The cre vasse-splay sed i ments dis cussed are sit u ated within the first Mid-Pol ish (first Lusatian) lig nite seam, which be longs to the lower part of the Poznañ For ma tion -the youn gest Neo gene lithostratigraphic unit in cen tral Po land (Widera, 2007). This lig nite ac cu mu lated in the backswamp area of a Mid dle Mio cene low-ly -ing mire, as ev i denced by, inter alia, the oc cur rence of de pos its of a cre vasse-splay microdelta (Chomiak et al, 2019). In gen eral, the study area cov ers the shal low fault-bounded Kleczew Graben, where the Me so zoic bed rock, built of car bon ate rocks (lime stone and marl), is lo cated sev eral tens of metres deep in the ax ial zone of the graben.…”

“…In gen eral, the study area cov ers the shal low fault-bounded Kleczew Graben, where the Me so zoic bed rock, built of car bon ate rocks (lime stone and marl), is lo cated sev eral tens of metres deep in the ax ial zone of the graben. It seems im por tant, in the con text of the sed i mentary and deformational or i gin un der dis cus sion, that the ac cu mu lation of the lig nite/peat in the study area was co in ci dent with tectonic sub si dence (synsedimentary tec ton ics) of up to 20 m (Widera, 2007;Chomiak et al, 2019). Ob vi ously, this is small com pared to the Kleszczów Graben men tioned by the in ter loc utor, where the depth of the tec tonic de pres sion reaches 550 m, and the con tin u ous thick ness of the main lig nite seam is over 250 m (Piwocki, 1992;Widera, 2013).…”

“…The co-oc cur rence of such de pos its and de for ma tions is unknown from the Pol ish lig nite-bear ing ar eas and prob a bly from other coal/lig nite-bear ing re gions in the world. There fore, we con cluded that they were worth de scrib ing and in ter pret ing (Chomiak et al, 2019). Opin ion con vinced us of the need of our in ter loc u tor, van Loon, who claimed that these sed i ments are "…a type of sed i ment that seems largely un der val ued in sedimentology.…”

“…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) -re ply. Geo log i cal Quar terly, 63 (2): 429-433, doi: 10.7306/gq.1477As so ci ate ed i tor: Anna WysockaWe ex pected that our pa per on the cre vasse-splay microdelta (Chomiak et al, 2019) would arouse the in ter est of other research ers for at least two rea sons. First, this is the first such palaeoform dis cov ered and de scribed within the Mid-Mio cene lig nite seam in Po land.…”