HIGH-FREQUENCY SEA-LEVEL FLUCTUATIONS AND PLANT HABITATS IN CENOMANIAN FLUVIAL TO ESTUARINE SUCCESSION - PECINOV QUARRY, BOHEMIA

The succession of the Peruc and Korycany Members in the Pecinov 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 s uccession studied includes deposits of a shallow, gravelly braided riv er (unit 1), tide-influenced braided river (unit 2), supratidal marsh (unit 3), tidal flat (unit 4) and ebb-tidal delta to estuary mouth fil l (unit 5). Units 1-5 are interpreted as parasequences of a single dep ositional sequence; units 1-4 together make up a transgressive systems tract and unit 5 is a highstand deposit, capped by a sequence boundar y.The presence of deciduous plants attests to a certain degree of seas onality of the climate. The mixing of thermophile and temperate vegeta tion, together with the presumed palaeolatitudinal position of the Boh emian Massif, suggests a generally slightly semi-arid (i.e., seasonall y 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. The plant habitat of the all uvial valley (units 1 and 2) was characterized by the Myrtophyllum ass emblage, which included an angiosperm-dominated gallery forest and shr ubs on braid bars and islands. The transition between fluvial and coas tal environments was characterized by a taxodioid backswamp forest (th e Ceratostrobus assemblage). Coastal vegetation (i.e., the marsh habit ats of unit 3 and the upper part of unit 5), changes distinctly betwee n the two units, although both represent similar facies of high intert idal to supratidal origin. The Frenelopsis assemblage of unit 3, repre senting shrubby marsh vegetation growing seaward of the Ceratostrobus forest, differs strongly from the more xerophytic Sphenolepis assembla ge of unit 5, characterized by herbaceous pteridophytes growing in the marsh in front of a coastal taxodioid forest. We explain the differen ces between units 3 and 5 by the different sequence-stratigraphic posi tion of the two units: a high groundwater table and eutrophic substrat e of the transgressive systems tract (unit 3) compared with a lower or falling regional water table and oligotrophic substrate during the la te highstand systems tract (unit 5). Thus, temporal changes in coastal plant assemblages of the Pecinov section do not merely reflect latera l shifts of facies belts, but show that similar depositional environme nts in different systems tracts hosted very different floras. (C) 1997 Elsevier Science B.V.