STRUCTURAL STYLE AND TECTONIC EVOLUTION OF THE NORTHWEST SIRT BASIN–CRETACEOUS- TERTIARY RIFT, LIBYA

Abstract

The Cretaceous-Tertiary northwest-trending Sirt Basin system, Libya is a rift/sag basin formed on Pan-African to Paleozoic-aged basement of North Africa. This study is an investigation of the rift-basin architecture and tectonic framework of the western Sirt Basin. A combination of several geological and geophysical methods (remote sensing interpretation supported by surface geologic maps, borehole data analysis, from about 300 deep wells, extensive seismic reflection profiles and maps, and paleostress analysis), have been used to construct geological cross sections, and surface and subsurface geology maps that have proved particularly useful in identifying various regional and local structures in the study area. Indication of the relative timing of structures and movement along faults has been determined where possible.
We recognize six lithostratigraphic sequences (phases) in the area's stratigraphic framework. A Pre-graben (Pre-rift) initiation stage involved the Pre-Cretaceous sediments formed before the main Sirt Basin subsidence. Then followed a Cretaceous to Eocene graben-fill stage that can divided into four structurally-active and structurally-inactive periods, and finally a terminal continental siliciclastics-rich package representing the Eocene post-rift stage.
Late Eocene rocks exposed in the western part of the basin exhibit a complex network of branching segmented normal and strike-slip faults, generally with a NNW-SSE structural grain. Many surface structural features have been interpreted from satellite images which confirm sinistral strike-slip kinematics. Relay ramp structures, numerous elongate asymmetric synclines associated with shallow west limbs and steeper dipping east limbs are developed in the hangingwalls adjacent to west downthrowing normal faults. These structural patterns reflect Cretaceous/Tertiary extensional tectonics with additional control by underlying pre-existing Pan-African basement fabrics and ENE-WSW trending Hercynian structures.
The interpretation of a large volume of two-way time seismic data has allowed the construction of long regional seismic cross sections and resultant time structure contour and thickness maps, thus enabling an understanding of the structural and stratigraphic relationships on a regional scale.
Six major fault systems were identified initially from the remote sensing study and was subsequently expanded to seven on the basis of interpretation of 2D seismic data. These fault systems dissect the study area into geomorphological elevated blocks and depressions. Most of the oil fields present in the study area are associated with structural hinge zones and adjoining highs.
A new kinematic and structural conceptual model is proposed here for the evolution of NW Libya (Jifarah Basin and the Nafusah Uplift) and Hun Graben during Late Eocene times.
The multidisciplinary approach used in this study has resulted in good surface and sub-surface structure correlation and future petroleum prospectivity has been identified in this study.
We relate the Sirt Basin rift development as exemplified in our study area to the break-up of Gondwana represented by the structural evolution of the West-Central African rift system and the South and Central Atlantic, the Tethys and the Indian Oceans.