Biostratigraphy, palaeoecology and palaeogeography of the mainly marine Ager Formation (Upper Paleocene — Lower Eocene) in the Tremp Basin, Central-South Pyrenees, Spain

During the greater part of the Palaeogene the Tremp Basin was an area which underwent rapid subsidence as compared with the axial zone of the Pyrenees to the north, and the Ebro Massif to the south. As a result the sea occupied this area for a long time and deposition of the Ager Formation took place during the Late Paleocene and Early Eocene in a bay forming an appendix of the Atlantic Ocean. At the maximum of the transgression probably a connection with the Tethys existed. During this entire development the Tremp Basin was tectonically separated from the more southerly Ager Basin by the Montsech High which underwent much less subsidence. The southern side of the Tremp Basin, under the influence of this high, subsided less rapidly than the central part. This resulted in a considerably thinner succession along the southern side, consisting mainly of limestones, and a much thicker pile in the central part mainly built up of marls. Sediments deposited along the northern side of the basin for a greater part have been eroded. Where they have been preserved, they are less limy than those along the southern side. The northern margin moved in the course of time in northern or southern direction, in contrast to the stable Montsech High which formed a stationary southern margin. The correlations within the formation are based on alveolinids and on lithostratigraphical characteristics. Hottinger’s alveolinid biozonation undergoes one modification which has important consequences for the chronostratigraphy of the Mediterranean Palaeogene. The .Fasciolites oblongus Zone namely does not appear to be a higher zone than the F. trempinus Zone, for F. oblongus lived contemporaneously with F. corbaricus and F. trempinus; F. oblongus lived in deeper water than the latter two, which explains that they are rarely found together in one sample. The occurrence of F. oblongus in the Ager Formation proves that the Ilerdian coincides for a large part with the Ypresian (Cuisian). Consequently the usually accepted Paleocene-Eocene boundary comes to lie within the Ilerdian, so that this stage cannot be maintained. Data from all alveolinid species together have resulted in a better comprehension of the evolution of this group. The number of species and lineages proves to be smaller than has been thought. From the palaeoecology of the alveolinids it can be deduced that three possibilities were realized by the successive species of a lineage: (1) they remain restricted to some specific, shallow marine environment during their entire evolution, (2) they adapt themselves to many different, but mainly shallow marine environments and remain so during their further evolution, or (3) they migrate from shallow to deeper marine environments. Fifteen facies types and 51 subfacies types are distinguished on palaeontological and/or sedimentological characteristics. Larger foraminifera, other microfossils and various groups of macrofossils are used for this, together with sedimentary structures and lithology. A large variety of facies types from fluvial and swamp deposits to marls of the deeper shelf occurs. Important facies types to be mentioned are lagoonal and tidal deposits, coral and algal reefs and related facies and various open marine facies. Apart from some minor fluctuations the sea gradually extended over a progressively larger area during the deposition of the lower half of the Ager Formation becoming deeper at the same time. The relief of the sea bottom simultaneously became steeper. Half-way through the formation the transgression reached its maximum, after which the sea again became shallower and the area occupied by it smaller. The relief of the sea bottom then again was reduced. The maximum reef development occurred in the lower part of the F. corbaricus Zone, and was most extensive along the southern side of the basin. In the upper half of the same biozone reef growth was more and more hampered due to important influxes of clastic material. In the lower half of the formation the occurrence of sand and silt is always local, whereas it is found nearly everywhere in the upper half. Most of this material came from NE, E and SE directions. The fauna and the flora point to a tropical climate. The Tremp Basin was situated at the northern margin of the tropics (zone of marginal reef growth). The maximum depth of the sea in the area studied was between 100 and 150 m. An important aid to the determination of the sea depth of various facies is the occurrence of glauconite which cannot have been formed above ca. 50 m depth in the Tremp Basin.