Peter A. Hochuli1,
(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