STRUCTURE AND STRATIGRAPHY OF THE AJDABIYA TROUGH AREA, EAST SIRT BASIN - LIBYA

Abstract

The structural style within the deepest parts of the Ajdabiya Trough is defined by a system of Early - Late Cretaceous syn-depositional fault blocks bound by normal faults and basement highs devoid of syn-rift sediments, which are buried under a thick succession of Cenozoic post-rift deposits. The range of fault orientations likely reflects the conflicting influences of the ~NE-SW regional extension direction and the dominant ~N-S trending basement fabric. Mainly NW-trending normal faults dissecting Cretaceous and older rocks have been inferred from 2D seismic reflection and potential field data. Other faults trend NE-SW and E-W, and mainly cut Miocene and older strata. Some of these faults have both sinistral and dextral strike slip components and are possibly linked to on-going seismicity in the Sirt Basin and the Cyrenaica Platform. Vertical displacements on these faults are several hundred meters and are defined by large throws on Cretaceous and underlying horizons. Structural mapping confirms the presence of relay ramps associated with overlapping faults developed in the hangingwalls adjacent to west downthrowing normal faults along the eastern margin of the Ajdabiya Trough. The seismic stratigraphic framework is organised into six mega-sequences that correlate to variations in relative sea-level and/or sediment supply during Late Mesozoic and Cenozoic times. The stratigraphic architecture of the trough is largely influenced by relative sea level changes and minimal tectonic effects during the Cenozoic; observed progradation of the Paleocene, Early and Middle Eocene sequences along the trough margin is attributed to relatively rapid sedimentation rates and relatively slow rates of increase in accommodation space. Depositional environments are interpreted using the resultant facies analysis and the characterisation of the seismic reflections indicated that the geological units were deposited in marginal marine, shallow shelf and moderately deep marine environments. Special consideration is given to the principle of seismic sequence stratigraphy analysis of carbonate depositional systems where the facies group took initially place on a homoclinal ramp which later developed into a rimmed platform. This analysis additionally reveals that similar depositional architectures can be divided into systems tracts. The earliest systems tract of the Lower Eocene sequence is interpreted as lowstand prograding wedge distinguished on the basis of the component facies that indicate the dominant depositional regime. Localized debris flow or mass transport complex formed during early highstand systems tract deposition began during the Middle Eocene. The tectono‐stratigraphic analysis of the Ajdabiya Trough reveals that two major extensional pulses controlled the architecture of the trough during continental rifting with crustal stretching (β) factor ranging from 1.3 to 1.5 consistent with subsidence in the Ajdabiya Trough having been controlled by thermal cooling and isostatic adjustments of the crust beneath the trough. Growth strata within grabens and half-grabens denote persistent tectonic subsidence and demonstrate the progressive depocenter locus migration towards the north. In such a context, the current geometry of the Ajdabiya Trough is interpreted to have resulted mainly from rifting cycles and possible renewed continental extension. The investigations of the tectono‐stratigraphic controls reveal that after a period of relative tectonic quiescence, post‐rift tectonic reactivation affected the Ajdabiya Trough almost continuously since the latest Cretaceous to the Miocene. Burial history curves correlated with one-dimensional back-stripping assuming Airy isostasy shows that Cenozoic subsidence in the Ajdabiya Trough can be divided into three episodes of post-rift subsidence characterized by short and long-lived subsidence pulses and rapid sedimentation rates that may lead to development of overpressure by disequilibrium compaction.