ABRUPT CLIMATIC AND SEA LEVEL CHANGES ACROSS THE PALEOCENE-EOCENE BOUNDARY, AS RECORDED IN AN ANCIENT COASTAL PLAIN SETTING (PYRENEES, SPAIN)

The effects of Paleocene-Eocene boundary events have been studied in an ancient alluvial plain and in its adjacent shallow marine carbonate platform in extensive outcrops of the South Pyrenees, Spain. We find that an important sea-level fall occurred just before the onset of the Initial Eocene Thermal Maximum (IETM), recorded by a generalized incision of valleys in the alluvial plain and by the subaerial exposure of the carbonate platform. The valleys were filled-up already during the latest Paleocene, by a combination of a rise of sea level and an increase in sedimentary yield caused by a climate somewhat more humid than the semiarid one prevailing during most of the Paleocene.

The onset of the IETM is defined by a remarkable unit that we have named the Claret Conglomerate (CC). This is a laterally extensive (at least 30x10 km) but comparatively thin unit (up to 8 m, but usually less than 4 m thick) of coarse-grained conglomerate (clasts up to 0.7 m in size), with minor sandstone and clay intercalations. The CC records the development of a vast braidplain during the first thousand years of the IETM. We link the CC to a short period of seasonal extreme storms causing frequent and powerful flash floods in an otherwise generally dry landscape. The top of the CC is sharp and irregular, often preserving the topography of gravely bars, evidence of a substantial and rapid decrease of storms and consequently of discharge and stream power.

The bulk of the IETM is represented by a cumulate soil, yellowish-orange in color and containing abundant small-sized (ca. 1 cm) carbonate nodules that yield δ13C values of -12 to -14 ‰. In nearshore sections this soil is less than 1 m thick and is immediately overlaid by marine Ilerdian deposits, whereas in alluvial plain sections it is up to 20 m thick and is overlaid by continental deposits. These facts, and field data, demonstrate that the cumulate soil records the aggradation of the alluvial plain during a rapid rise of the sea level coeval with the IETM.

In alluvial plain sections the yellowish-orange soil is overlaid by a red-colored soil that includes carbonate nodules with δ13C values ca. -8 ‰ and variable amounts of gypsum. This red soil records the end of the IETM event and the return to semiarid conditions in the study area.