Über die Frühgeschichte (Campan bis Eozän) der südlichen mittelameri­kanischen Landbrücke

Abstract

Die südliche mittelamerikanische Landbrücke ging aus der Subduktion der ozeanischen pazifischen Platte unter die ozeanische karibische Platte hervor. Ihre Entstehung läßt sich in eine präcampane ozeanische Vorgeschichte, eine Frühgeschichte mit inselbogenartigen Verhältnissen (Campan bis Eozän) und eine jüngere Geschichte (Oligozän - Rezent) unterteilen, in deren Verlauf der Isthmus seine heutige Form erst ziemlich spät (Pleistozän) erhielt. Die vorliegende Arbeit beschäftigt sich in der Hauptsache mit der bislang fast noch unbekannten Frühgeschichte. Im Obercampan existierte ein nichtvulkanischer Basaltinsel-Archipel, auf dessen Insularschelfen sich Rudisten-Biostrome ansiedelten. Der größte Teil dieses Basaltinsel-Archipels versank schon wieder am Ende des Campans und wurde im Verlaufe des Paleozäns und Eozäns unter dem Schutt des sich langsam herausbildenden vulkanisch aktiven andesitischen Inselbogens begraben. Erkenntnisse über die Beschaffenheit der insularen bzw. kontinentalen Environments dieser Epoche lassen sich nur aus zeitgleichen Abhangsedimenten gewinnen. Dies ist der zentrale methodische Ansatz in dieser Studie.

The southem part of the Central American isthmus resulted from the subduction of the oceanic Pacific plate beneath the oceanic Caribbean plate. The rise of this landbrigde from the pre-Campanian ocean floor is characterized by a Campanian to Eocene island arc stage, an Oligocene to Miocene swell stage as well as a Pliocene to Recent mountain chain stage. The present study deals mainly with the island arc stage. The first island ares emerged towards the beginning of the Late Campanian: a non-volcanic archipelagos of basaltic islands bounded by strike-slip fault scarps. Where insular shelves did exist, they were colonized by Rudist biostromes; the talus originated mainly by rock-falling and avalanching. As early as at the end of the Late Campanian, the greater part of this basaltic archipelagos subsided. During the Paleocene and the Eocene, it became more or less deeply buried under hemipelagic sediments deriving from the active andesitic island arc which formed at the si te of the actual mainland. The borders of this andesitic island arc were oversupplied with elastics due to excessive erosion. Thus, carbonate shelves could hardly extend and were restricted to some patchy areas where communities of larger foraminifera and red algae developped during the Middle/Upper Eocene. Dueto local uplift, however, a relatively small isolated Bahama-type carbonate platform established in the western Bajo Tempisque area, producing the (probably Eocene) Barra Honda formation. This carbonate platform both supplied slide blocks to the adjacent basins and was surged up and bypassed by turbidites deriving from the andesitic island are. Paleovertebratological data evidence that from both island chains only the Campanian one permitted an exchange of terrestrial animals between North and South America. This early connection must have been disrupted, from the latest Campanian onward, by transform displacements and/or late rotations. In this context it may be annotated that the present-day geological configuration in the Costa Rican area provides only incomplete paleogeographical profiles. A hypothetical section fitting the Peninsula de Nicoya and Bajo Tempisque area with the Quepos - Fila Costeila - Talamanca area would join the missing links. This configuration, however, requires NW - SE lateral displacements which, at the moment, can neither be lo- calized nor be proved due to the lack of paleomagnetical data.