BENALIOULHAF, SAID, and B. C. SCHREIBER, Department of Geology, Queens College (C.U.N.Y.), Flushing, NY; and R. P. PHILP, School of Geology and Geophysics, University of Oklahoma, Norman OK

ABSTRACT: Reconstruction of Depositional Conditions During Restricted and Evaporitic Sedimentation

In order to find a good model for the study of the relationship between organic matter and evaporitic sedimentation, we have chosen to study the Neogene sediments of south-eastern Spain. Here there are several intermontane basins that contain marine sequences that become increasingly restricted over a comparatively short period of time, and are capped by gypsum and halite. The basins of Sorbas, Lorca, Mula, and Fortuna to name a few, contain such evaporites and intercalated organic-rich sediments (up to 25% TOC), but because of the ease and completeness of sampling, we have chosen to study the Lorca Basin.

In this basin the depositional section includes a wide range of normal marine lithofacies together with several types of evaporative carbonates and "true" evaporites, and it has been examined in great detail in terms of sedimentation, diagenesis and organic geochemistry. Once the depositional microfacies and early diagenetic overprints had been determined, the concommitant organic materials were then analyzed. The detailed study of the biomarkers contained in the saturated and aromatic fractions of samples from the pre-evaporitic and evaporitic sequence of the Lorca basin provide different well-defined groupings of samples. These distinctions are based on the origin of the organic matter and the distinctions inferred concerning the original conditions of deposition (Eh and salinity of the water) that are carried by the biomarkers (n-alkanes, isoprenoids, hopanoids, steroids, sulfur compounds, and chromans). The sample groupings tally with corresponding lithologic groupings which have been deposited under specific conditions within the formative water body.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.