Foraminiferal zonation of early Oligocene deposits (Selztal Group, Latdorfian, Rupelian) in the Mainz Basin, Germany

The foraminiferal zonation reported here is valid for the marine Oligocene deposits in the Mainz Basin. Thirteen zones and one subzone are defined in the basin facies, which may be correlated biostratigraphically with the current nannoplankton zones and also lithostratigraphically. The zonation for the beach facies, already described by the present author, is incorporated into the scheme. It is possible, therefore, to generate a high resolution foraminiferal biostratigraphical subdivision of the marine Oligocene deposits of the Mainz Basin.


INTRODUCTION
The Lower Oligocene deposits of the Mainz Basin (sensu Golwer, 1968) are composed of siliciclastic sediments comprising the Pechelbronn Beds, the Bodenheim, the Alzey and the Stadecken formations and the lower part of the Sulzheim Formation, as reported by Grimm et al. (2000). The Bodenheim, Alzey, Stadecken and Sulzheim formations are combined with the Budenheim Formation of Chattian age (Schäfer & Kadolsky, 1998), to form the Selztal Group (Grimm et al., 2000). The stratigraphic sections examined ( Fig. 1) ranged from the Middle Pechelbronn Beds at the base to the Stadecken Formation at the top (see Fig. 2). The marine-brackish to limnic Pechelbronn Beds, first described lithologically by Steuer (1910), in the Mainz Basin have previously not been studied biostratigraphically in detail, but they were investigated by Barth (1969) in the Rhine Graben at the locality of Rot-Malsch.
The biostratigraphical zonation based on foraminifera reported herein summarizes the detailed studies carried out during the last ten years on a collection of samples from more than 100 outcrops and boreholes, and updates this zonation not only to a modern international standard, but also combines a modern lithostratigraphical subdivision of the Mainz Basin with a detailed foraminiferal zonation.

DEFINITION OF FORAMINIFERAL ZONES
The following foraminiferal zones (Figs 2 and 3) are defined as biozones according to Salvador (1994) and Steininger & Piller (1999). In the present study, 13 Biozones and 1 Subzone are established.
The studied stratigraphic section comprises the upper part of the international smaller benthic foraminiferal zone NSR 7a=Zone of Adercotryma agterbergi and the major part of NSR 7b=Zone of Turrilina alsaticaafter Hardenbol et al. (1998). Adercotryma agterbergi, a very small species, is seldom present in the sediments of the middle Pechelbronn Beds of the Mainz Basin because of environmental reasons. Turrilina alsatica is widespread in the deposits of the Mainz Basin, and first appears at the base of the Bodenheim Formation and concomitantly at the base of the Alzey Formation. Hansenisca soldanii was mentioned in older publications but this species has subsequently been assigned to the genus Gyroidina following Kuhn (2000a). The biostratigraphic zonation is correlated with the nannoplankton zones of Martini & Müller (1971). A list of the foraminifera is included in the Appendix and the range of the important species in the biozones is illustrated in Figure 3.

Foraminifera barren Zone
Definition. Due to the limnic character of the Upper Pechelbronn Beds foraminifera are lacking.

Biostratigraphic correlation. By correlation to the Upper Rhine
Graben brackish Upper Pechelbronn Beds the NP22/NP23 boundary is situated within this zone (Martini 1990

Gyroidina soldanii-Turrilina alsatica Assemblage Zone
Definition. This zone is characterized by a low diversity foraminiferal fauna. The abundance of foraminifera is also low. The basal limit is marked by the new appearence of foraminifera after the underlying foraminiferal barren zone. At the upper limit, an increase in foraminifera is observed. Gyroidina soldanii is the dominant species of this zone, and Turrilina alsatica is first recorded at the base.
Biostratigraphic correlation. This zone is a synonym of FM 1 of Grimm (1994Grimm ( , 1998 and is equivalent to the lower part of the NP 23 Zone.

Spiroplectinella carinata-Heterolepa dutemplei Assemblage Zone
Definition. This zone is characterized by a rich foraminiferal fauna and by the presence of a large number of individuals with agglutinated tests. It is defined by the common appearance of Spiroplectinella carinata and Heterolepa dutemplei. Spiroloculina dorsata, Ammodiscus incertus and Karreriella chilostoma are also found. The lower limit is marked by a sudden increase in foraminifera. The upper limit is defined by the disappearence of Spiroplectinella carinata and Heterolepa dutemplei.
Biostratigraphic correlation. This zone is a synonym of the FM 2 Zone of Grimm (1994Grimm ( , 1998 and is equivalent to the NP 23 Zone.

Karreriella chilostoma-Ammodiscus incertus Assemblage Zone
Definition. The occurrence of Spiroloculina dorsata, Ammodiscus incertus and Karreriella chilostoma are characateristic of this zone. The frequency of agglutinated tests is very high. The lower limit is marked by the disappearance of Spiroplectinella carinata and Heterolepa dutemplei, which are both characteristic species of the underlying zone. At the upper limit the numbers of agglutinated tests decreases.
Biostratigraphic correlation. This zone is a synonym of the FM 3 Zone of Grimm (1994Grimm ( , 1998 and is equivalent to part of the NP 23 Zone.

Palmula obliqua-Spiroplectinella deperdita Assemblage Zone
Definition. This zone is characterized by the occurrence of Palmula obliqua and Spiroplectinella deperdita. Spiroplectinella deperdita is also recorded in the lower zones, Palmula obliqua can also be found in lower zones occasionally, but both disappear at the top of this zone. The upper limit is defined by the disappearance of Spiroplectinella deperdita. The lower limit is characterized by the marked decrease in the number of agglutinated tests.

Chronostratigraphy. Rupelian, Oligocene.
Biostratigraphic correlation. This zone is a synonym of the FS 1 Zone of Grimm (1991Grimm ( , 1994 and is equivalent to part of the NP 23 Zone. Chronostratigraphy. Rupelian, Oligocene.
Biostratigraphic correlation. This zone is a synonym of the FS 2 and FS 3 Zones of Grimm (1991Grimm ( , 1994 and is equivalent to part of the NP 23 Zone.

Chronostratigraphy. Rupelian, Oligocene.
Biostratigraphic correlation. This zone is a synonym of the FS 2 Zone of Grimm (1991Grimm ( , 1994 and equivalent to part of the NP 23 Zone.

Bathysiphon tauriensis-Cyclammina placenta Assemblage Zone
Definition. This zone is marked by a generally impoverished foraminiferal fauna. A few species (Bathysiphon taurinensis and Cyclammina placenta) extend throughout the zone. The lower limit is characterized by the disappearance of the rich foraminiferal fauna of the underlying zone. The upper limit is characterized by the sudden rise of a more diverse fauna. The Bathysiphon tauriensis-Cyclammina placenta Assemblage Zone can be recognized in species-richness diagrams from its reduced species diversity (Grimm, 1994).

Chronostratigraphy. Rupelian, Oligocene.
Biostratigraphic correlation. This zone is a synonym of the FS 4 Zone of Grimm (1991Grimm ( , 1994 and is equivalent to part of the NP 23 Zone.

Bolivina beyrichi-Gyroidina soldanii Assemblage Zone
Definition. The zone is marked by a brief increase in diversity within an otherwise impoverished section. Occasionally Vaginulopsis hauerina may be recorded. The lower limit is characterized by the increase, and the upper limit by the decrease in foraminifera.

Chronostratigraphy. Rupelian, Oligocene.
Biostratigraphic correlation. This zone is a synonym of the FS 5 Zone of Grimm (1991Grimm ( , 1994 and is equivalent to part of the NP 23 Zone.

Bathysiphon tauriensis Assemblage Zone
Definition. This zone is very poor in foraminifera. Only rare specimens of Cyclammina placenta and Bathysiphon taurinensis occur. The lower limit is defined by a sudden decrease in foraminfera. The upper limit is characterized by the disappearance of Bathysiphon taurinensis and the beginning of a more diverse foraminiferal fauna.
Biostratigraphic correlation. This zone is a synonym of the FS 6 Zone of Grimm (1991Grimm ( , 1994

Rotaliatina offenbachensis Range Zone
Definition. The lower limit of this zone is defined by the appearance of Rotaliatina offenbachensis, with the upper limit marked by the disappearance of Rotaliatina offenbachensis and Vaginulopsis hauerina. Pulsiphonina sonnei first appears in this zone. Furthermore, this zone is characterized by a moderately diverse benthonic foraminiferal fauna, which is much increased after the impoverished fauna of zones 8 to 10.

Chronostratigraphy. Rupelian, Oligocene.
Biostratigraphic correlation. This zone is a synonym of the ORT 1 Zone of Grimm (1994) and is equivalent to the base of the NP 24 Zone.

Spiroplectinella carinata-Cibicides ungerianus Assemblage Zone
Definition. This zone is characterized by the common occurrence of Spiroplectinella carinata and Cibicides ungerianus. The lower limit is marked by the disappearance of Rotaliatina offenbachensis and Vaginulopsis hauerina and the upper limit by the appearance of a reduced fauna with small individuals. Older reworked foraminifera coming from the Alps appear within this zone.

Chronostratigraphy. Rupelian, Oligocene.
Biostratigraphic correlation. This zone is a synonym of the ORT 2 Zone of Grimm (1994) and is equivalent to part of the NP 24 Zone.

Globigerinidae-Miliolidae Abundance Zone
Definition. The Globigerinidae-Miliolidae Abundance Zone is characterized by large numbers of Globigerinidae and Miliolidae. Many of the Globigerinidae are represented by specimens of Globigerina bulloides. Among the Miliolidae, Quinqueloculina impressa is the most frequent taxon. The entire foraminiferal fauna is neither abundant nor diverse. Pulsiphonina sonnei disappears within this zone. Polymorphinidae may also occur in large numbers. This zone is also characterized by the appearance of older reworked foraminifera (see above).

BEACH FACIES
For the beach facies the following two biozones were established by Grimm (1998).

Planorbulina difformis-Cibicides lobatulus Abundance Zone
Definition. The Planorbulina difformis-Cibicides lobatulus Abundance Zone is characterized by large numbers of Planorbulina difformis (>40% of the foraminiferal assemblage) and Cibicides lobatulus. The lower limit is defined by the appearance of foraminifera in the beach facies of the Mainz Basin. The upper limit is characterized by the increase in individuals of the Miliolidae.

Biostratigraphic correlation.
A correlation with the basin facies is possible, so this zone corresponds to FS B and perhaps to ORT 1 of Grimm (1991Grimm ( , 1994Grimm ( , 1998

CONCLUSION
A detailed biostratigraphical subdivision for the Mainz Basin is established, based on foraminifera. The foraminiferal fauna reflects not only the stratigraphic succession but also the marked changes in ecology. This zonation is only valid for the small area of the Mainz Basin that experienced fully marine conditions during the Latdorfian and Rupelian. This has been correlated accurately with the nannoplankton zones of Martini & Müller (1971). Spiroloculina dorsata Reuss, 1866: Grimm (1994)