Early Jurassic (Pliensbachian-Toarcian) Dinoflagellate Migrations and Cyst Paleoecology in the Boreal and Tethyan Realms

Abstract

The discrepancies in the stratigraphical ranges of selected dinoflagellate cysts recorded in the Boreal and Tethyan realms have revealed two migrational events during the Early Jurassic. The first event occurred at the early-late Pliensbachian boundary and consists of mutual biotic exchanges between the two realms. This is linked to a major Early Jurassic transgression which improved marine communications between the Boreal and Tethyan areas. The second dinoflagellate migrational event occurred during the mid Toarcian and was driven by paleoenvironmental factors. The numerous available Lower Jurassic dinoflagellate cyst data from the Boreal and Tethyan realms indicates that phytoplankton distribution was profoundly affected by paleoecological factors. Information pertaining to the life strategies and the paleoecological requirements of the genera Luehndea, Nannoceratopsis and Valvaeodinium has also been determined. INTRODUCTION The distributions of dinoflagellate cysts are primarily related to paleoenvironmental regimes. Certain dinoflagellate cyst taxa exhibit heterochroneity over wide areas because of differences in paleoecological preferences and overall paleoenvironments (Goodman 1987). This situation has implications for the application of dinoflagellate cysts to intercontinental biostratigraphy (Bucefalo Palliani and Riding 1997a). During the Jurassic two faunal provinces, the Boreal and Tethyan realms, have been distinguished partly on the basis of different ammonite, foraminiferal, brachiopod and calcareous nannofossil assemblages (Arkell 1956; Gordon 1970; Voros 1977; Bown 1987). Biotic differences between these realms may be characterized by relative abundances rather than by the presence or absence of specific taxa (Hallam 1969). The Boreal Realm occupied the northern part of the Northern Hemisphere; the Tethyan Realm lay to the south (Hallam 1969). During the Early Jurassic a broad transitional area with both Boreal and Tethyan biotic characteristics was developed. This intermediate belt comprises southern France, Hungary and Portugal, and was also delineated on the basis of ammonites, calcareous nannofossils and dinoflagellate cysts (Zeigler 1980; Geczy 1984; Cariou et al. 1985; Gardin and Manivit 1994; Baldanza et al. 1995). Within the Tethyan Realm, the macrofaunas allow the Mediterranean and sub-Mediterranean bioprovinces to be distinguished. The former comprises the northern margin of the Tethys Ocean, thus is of European affinity; the sub-Mediterranean is of African affinity, being on the southern margin of Tethys (Pavia and Sarti 1987). Recently these two Tethyan provinces have been differentiated using calcareous nannofossils (Baldanza and Mattioli 1992; Baldanza et al. 1995; Mattioli 1995). The majority of publications on Lower Jurassic dinoflagellate cysts are from the Boreal domain; there are significantly fewer published data from north-west Tethys. The biogeographical distributions of Lower Jurassic dinoflagellate cysts have not been investigated in detail. On the basis of data by Davies (1985) from central Portugal, Tethyan Lower Jurassic microplankton assemblages are broadly similar in generic/specific content to more northerly floras. However, there are several taxa whose stratigraphical ranges differ profoundly from those in the Boreal Realm (Poulsen 1996; Riding and Ioannides 1996). Recently, palynological studies have been carried out on ammoniteand calcareous nannofossil-dated Lower Jurassic Tethyan sections in order to define the stratigraphical ranges of dinoflagellate cysts and to compare them with their ranges in the Boreal Realm (Bucefalo Palliani 1996). The aims of this study are to distinguish and interpret the stratigraphical discrepancies between the two paleogeographical realms, to utilize the stratigraphical data to identify the areas where the main Pliensbachian-Toarcian speciation events occured, to recognize the most important migrational events and consequently the biogeographical factors which drove the dinoflagellate cyst distributions. MATERIAL Samples were collected from several European Pliensbachianearly Toarcian successions (text-fig. 1). The correlations are based on the ammonite, calcareous nannofossil and dinoflagellate cyst content. The Portugese sections, Peniche, Rabacal and Brehna, are outcrop localities in central Portugal, north of Lisbon (text-fig. 2). The Peniche section ranges in age from the early Pliensbachian (Carixian) to the early Toarcian. It consists of intercalations of marls and more calcareous marls with interbedded black shales and detrital crinoidal limestone. The samples studied are from the Lower Pliensbachian (text-fig. 3). The late Sinemurian (Lotharingian) to early Bajocian Brenha road cutting section was dated by Mouterde et al. (1972). The Lower Pliensbachian portion of the section, made up by grey marls with marly limestone beds, has been investigated during this study (text-fig. 3). micropaleontology, vol. 45, no. 2, pp. 201-214, text-figures 1-12, appendix 1, 1999 201 This content downloaded from 207.46.13.51 on Sun, 19 Jun 2016 05:55:42 UTC All use subject to http://about.jstor.org/terms Raffaella B. Palliani and James B. Riding: Early Jurassic dinoflagellate migrations and paleoecology, Boreal and Tethyan realms