Peter A. Hochuli¹, André Strasser²
(1) Department of Earth Sciences, ETH, CH-8092 Zürich, Switzerland
(2) Institut de Géologie, Université de Fribourg, CH-1700 Fribourg, Switzerland

Abstract: Sequence-stratigraphic expression of major environmental changes in the Early Aptian of the Western Tethys

The Scaglia Variegata Formation from the Southern Alps (N-Italy) represents a pelagic environment typical of the Western Tethys during Early Aptian times. In this interval, major environmental changes including the Nannoconus crisis and a black shale event (Selli Unit or Oceanic Anoxic Event 1a) find a clear lithological expression. Detailed studies of organic facies, palynology, and stable isotope geochemistry of the Scaglia Variegata Formation in the Cismon section indicate drastic changes in climate and/or oceanic circulation pattern. The Selli level represents a relatively short episode (about 500ky) of elevated oceanic productivity under "greenhouse" conditions, most probably induced by high volcanic activity. Following this event, cooler and more humid climatic conditions and/or a reorganisation of the Tethyan oceanographic circulation pattern are indicated by paleontological and geochemical evidence.

The high-resolution time frame calibrated by ¹³C and ¹⁸O isotope stratigraphy and biostratigraphy allows precise correlation of the Selli Unit with equivalent levels in other areas. Sequence boundary Ap3 of de Graciansky et al. (1998) and the corresponding maximum-flooding interval can be identified in all studied sections. The levels of highest organic content generally correspond to the transgressive, maximum-flooding, and early highstand phases of this depositional sequence. However, high-frequency climatic and sea-level fluctuations superimposed on this general trend of sequence evolution as well as local topographic effects modifying oceanic circulation, created a strongly differentiated sedimentary record.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana