High-frequency sea-level fluctuations and plant habitats in Cenomanian fluvial to estuarine succession: Pecيnov quarry, Bohemia

The succession of the Peruc and Korycany Members in the Pecínov quarry, Bohemia, reflects the interplay of 4th-order sea-level fluctuations, of approximately 100–120 ka periodicity, and a long-term (3rd-order) sea-level rise of the late middle through early late Cenomanian. The succession studied includes deposits of a shallow, gravelly braided river (unit 1), tide-influenced braided river (unit 2), supratidal marsh (unit 3), tidal flat (unit 4) and ebb-tidal delta to estuary mouth fill (unit 5). Units 1–5 are interpreted as parasequences of a single depositional sequence; units 1–4 together make up a transgressive systems tract and unit 5 is a highstand deposit, capped by a sequence boundary. esence of deciduous plants attests to a certain degree of seasonality of the climate. The mixing of thermophile and temperate vegetation, together with the presumed palaeolatitudinal position of the Bohemian Massif, suggests a generally slightly semi-arid (i.e., seasonally wet) regional climatic conditions during the deposition of units 1–5; substrates in local habitats in alluvial valleys and estuary margins were damp due to high local water table. ant habitat of the alluvial valley (units 1 and 2) was characterized by the Myrtophyllum assemblage, which included an angiosperm-dominated gallery forest and shrubs on braid bars and islands. The transition between fluvial and coastal environments was characterized by a taxodioid backswamp forest (the Ceratostrobus assemblage). Coastal vegetation (i.e., the marsh habitats of unit 3 and the upper part of unit 5), changes distinctly between the two units, although both represent similar facies of high intertidal to supratidal origin. The Frenelopsis assemblage of unit 3, representing shrubby marsh vegetation growing seaward of the Ceratostrobus forest, differs strongly from the more xerophytic Sphenolepis assemblage of unit 5, characterized by herbaceous pteridophytes growing in the marsh in front of a coastal taxodioid forest. We explain the differences between units 3 and 5 by the different sequence-stratigraphic position of the two units: a high groundwater table and eutrophic substrate of the transgressive systems tract (unit 3) compared with a lower or falling regional water table and oligotrophic substrate during the late highstand systems tract (unit 5). Thus, temporal changes in coastal plant assemblages of the Pecínov section do not merely reflect lateral shifts of facies belts, but show that similar depositional environments in different systems tracts hosted very different floras.