BIOSTRATIGRAPHIC ZONATION WITHIN A BRACKISH LACUSTRINE SYSTEM, CENTRAL PARATETHYS, LAKE PANNON

Imre Magyar
MOL Hungarian Oil and Gas Co.

Lake Pannon was a large, long-lived, brackish lake that existed in the intra-Carpathian basins of the Central Paratethys during the Late Miocene and earliest Pliocene. In terms of its origin, size, depth, composition of the fauna and water chemistry, the best modern analogue for Lake Pannon is the Caspian Lake. It formed by isolation from the sea, maintained a significant concentration of salts in its water, and became the site of an extraordinary endemic radiation of molluscs (>900 species), ostracods (>500 species), and other groups of the biota.

The lack of foraminifers, endemism of dinoflagellates, and scarcity and endemism of calcareous nannoplankton in Lake Pannon deposits made biostratigraphic correlation with the marine sequences impossible. Because Lake Pannon deposits are important source of hydrocarbons, water, lignite, etc., and because they may exceed 4 km in thickness in some basins, there has long been a demand for an intrabasinal, regional biostratigraphic scheme based on endemic forms (primarily molluscs). The development of such a system, however, had many pitfalls.

Biostratigraphic study of Lake Pannon deposits started in surface outcrops in the 19th century; the first comprehensive successions of molluscs were described in the first decade of the 20th century. These successions, however, have almost never been recognized in any single locality; they included a composite set of outcrops from different geographical regions within the lacustrine basin. The first deep drillings, performed in the 1910’s, were expected to demonstrate the “real” stratigraphic successions of fossils. The enormous thickness of the Lake Pannon sedimentary sequence penetrated by the drillings surprised geologists, but even these thick sequences failed to provide the expected evidence for several successive biozones; one, two, exceptionally three units were recognized only. In addition, long-distance horizontal correlation of the recognized biostratigraphic units between the wells was not possible. Many stratigraphers concluded that the endemic creatures of Lake Pannon are simply useless for biostratigraphic purposes, because their presence or lack in any location was controlled solely by the paleoenvironmental conditions (“facies-dependent” fossils). This view was widely held even in the 1990’s. In practice, the subdivision and correlation of Lake Pannon deposits was mostly based on lithological characteristics, and lithostratigraphy was clearly preferred to biostratigraphy in assessing a chronostratigraphic framework.

In the last few decades, a growing amount of evidence from the area of subsurface sedimentology, paleontology, and, most importantly, seismic stratigraphy revealed that the basin of Lake Pannon was filled up by prograding deltas of rivers that charged into the lake in the NW and NE. As progradation proceeded from N to S, the lacustrine basin was gradually turned into alluvial plains. The basin fill geometry in seismic reflection profiles is thus characterized by a succession of sigmoids; the sediment packages (systems tracts) follow each other in lateral direction. As a consequence, the sublittoral to profundal environment of the lake gradually shrank, and the littoral environment more or less continuously shifted from N to S. Because the overwhelming majority of mollusc species lived in littoral and shallow sublittoral environments, an outcrop or borehole in any location within the lake’s basin would yield the characteristic fauna of that time only when the shallow sublittoral environment and the shoreline stayed in the area. Other time intervals in the same profile are represented by non-deposition or strongly condensed deep water deposition or fluvial deposition. Contrary to the traditional approach, temporal, chronostratigraphic successions have to be looked for in horizontal directions, whereas vertical successions in any location can be regarded as almost coeval, even if they include very (environmentally) different fossil assemblages.

This basin fill geometry implies that the boundary of lithostratigraphic units is often strongly diachronous; the lithological formations are not usable for chronostratigraphic correlations. A need for a reliable and high-resolution biostratigraphic scheme rose again.

Our system (Figure 1) is partly based on earlier stratigraphic observations, but we carefully separated the mollusc assemblages of various paleoenvironments and, where it was possible, based the biozones on anagenetically evolving species. The average resolution of mollusc biozones in sublittoral and littoral facies is approximately 1 m.y. Resolution is considerably worse in the deep basinal environment, where molluscan evolution appears to have been relatively slow. In this environment, however, the dinoflagellates offer good stratigraphic markers; in turn, they are not applicable in littoral environments.

K/Ar age determinations from interbedded volcanics, magnetic polarity profiles from reference boreholes, and vertebrate biostratigraphy are the tools by means of which the Lake Pannon stratigraphic system can be connected to the global geochronological scale. There is a difference in the amount of available data and, consequently, in the reliability of our correlations between the lower (older than approximately 9.5 million years) and upper parts of Lake Pannon deposits; the former is well-established, whereas a scarcity of radio-isotopic ages from the upper part of the sequence results in more uncertainty. In addition, the biostratigraphic subdivision of the last ca. 3 million year interval of the lacustrine sequence is almost entirely lacking. This work is ongoing, and more data on the evolution of molluscs and dinoflagellates as well as on radiometric ages are expected to result in a more detailed biochronostratigraphy.

Figure 1.