Author	Stover, L.E. and Hardenbol, J.
Year	1994
Journal	Bulletin de la SociÚtÚ belge de GÚologie, T.102 (1-2), 1993, pp.5-77
Title	Dinoflagellates and depositional sequences in the Lower Oligocene (Rupelian) Boom Clay Formation, Belgium

Abstract Stover, L.E. and Hardenbol, J., 1994; Dinoflagellates and depositional sequences in the Lower Oligocene (Rupelian) Boom Clay Formation, Belgium. Bulletin de la SociÚtÚ belge de GÚologie, T.102 (1-2), 1993, pp.5-77 The Boom Formation in northwestern Belgium consists of approximately 67 meters of alternating silty clay and clayey silt layers, which represent deposition during a major transgressive-regressive cycle (relative sea-level event). The base and the top of the formation are delimited by major sequence boundaries and a minor sequence boundary near the middle of the formation separates the older RU-1 and younger RU-2 sequences. The silty clay - clayey silt alternations in the Boom clay show a high frequency signal, while relative sea-level changes, rate of sediment supply and longer grain size trends are low frequency variables. In all probability eustasy was instrumental in both the low and high frequency trends, but its effect cannot be separated readily from that of subsidence or climate, respectively. Detailed lithostratigraphic studies such as those of Vandenberghe (1978) and Vandenberghe and van Echelpoel (1987) demonstrate the applicability of sequence stratigraphic principles to offshore sections; our results support their sequence stratigraphic interpretation for the Boom Formation. Palynologically, this study provides an accounting of the dinoflagellate cysts, as well as their distribution and relative abundances in the Boom and underlying Zelzate Formations. In the Boom clay, as exemplified in the Antwerpen area, a marked and rather abrupt influx of dinoflagellate taxa occurred in the transgressive systems tract of the RU-1 sequence and, conversely, a general and gradual reduction of taxa took place in the middle and upper part of the highstand systems tract of the Rupel-2 sequence. Relatively few species appeared or terminated in the intervening section. An impressive number of dinoflagellate bases and tops are positioned in the RU-1 and RU-2 sequences that were deposited in 6 Ma. The majority of bases are found in transgressive deposits whereas the majority of tops occur in highstand deposits. On the basis of mainly quantitative differences, 12 dinoflagellate associations are recognized; five each from transgressive and highstand systems tracts and two from strata at or near downlap surfaces. Although the correlation value of the associations outside Belgium remains to be determined, it appears that dinoflagellate associations at or near downlap surfaces are characterized by a significant reduction in the number of taxa, or the number of specimens, or both relative to adjacent associations. An analysis of the dinoflagellate taxa by eco-groups (following the approach of K÷the, 1990) indicates that most eco-groups appear to be systems tract tolerant. However, a few may have a moderate preference for certain sequence-related locations. Comparison of the dinoflagellate distribution in the lower part of the Boom Formation (Belsele-Waas Member) and the Zelzate Formation (Ruisbroek, Watervliet and Bassevelde Members) in Belgium with that in the Priabonian sections in Italy as reported by Brinkhuis and co-workers (1993) shows a seemingly reliable dinoflagellate correlation. The dinoflagellate data raises concern about the placement of the Late Eocene - Early Oligocene boundary in the boundary stratotype at Massignano Section in Italy. This horizon appears to correlate in Belgium to a position well below the base of the Boom Formation. Twelve new species and one new genus identified during this study are described and illustrated. The new species Achomosphaera expansa, Caligodinium endoreticulum, Charlesdowniea limitata, Elytrocysta breva, Impagidinium torsium, Impletosphaeridium machaeroides, Membranophoridinum connectum, M. intermedium, Vozzhennikovia cearaichia, V. spinula, Xenicodinium conispinum and X. echinoferum. Spiniferites cornutus (Gerlach 1961) Sarjeant 1970 is transferred to the new genus, Spiniferella, as its type species.