Print this pageFossil Sclereids: A New Tool for High-Resolution Biostratigraphy of Santonian (Upper Cretaceous) Reservoirs of Offshore Southeastern Brazil
The sandstones constituting the main Santonian reservoirs in the Santos and Campos basins of offshore southeastern Brazil were deposited under unique marine conditions. In onshore areas contemporaneous wildfires prevailed, caused by repeated incendiary volcanic activity, as indicated by alkaline intrusions and volcanic ash layers in Santonian strata. The scarcity of land vegetation favoured the high input of terrigenous material that produced a turbid water mass. This inhibited the effective development of most marine micro-organisms such as foraminifera and calcareous nannoplankton. Moreover, the scarcity of land vegetation induced by fires resulted in very low input of terrestrial palynomorphs (spores and pollen grains). The acid-resistant organic assemblage hosted by the fine siliciclastics intercalated in the Santonian sandstones is characterized by a predominance of fine black burned debris with very few useful palynomorphs. This situation has created difficulties in the development of a reliable high-resolution biostratigraphy. On the other hand, the assemblage includes a component of distinctive organic structures attributable to sclereids, which are cells of sclerenchyma (tissue whose principal function is support and protection of vascular plants). It seems that fossilization of the lignin-rich sclereids was preferentially favoured by the elevated fire-induced temperatures. In the present study, fossil sclereids were identified during optical microscopy of strew slides prepared for routine palynological analysis. Thirteen radially symmetrical morphotypes (formae; plural of forma, latin) of sclereids are recognizable, as follows: aculeata (sharply pointed), brevis (short, compact), callosa (thickened, callus-like), crenulata (undulate), crucis (cross-shaped), elongata (elongate), furcata (branched), perforata (perforated), rotunda (rounded), simplex (simple, unbranched), spinosa (spiny), tenuis (tenuous), and truncata (truncate). All belong to the group of astrosclereids, which have star-like configuration. Among these, the crenulata, perforata, simplex, spinosa, and truncata morphotypes evidently have their LADs (Last Appearance Datums) in the reservoir interval. Hence, they may prove useful as biostratigraphic markers in this otherwise virtually unfossiliferous stratigraphic section.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil