Messinian Sedimentation in the Offshore Sirt Basin of Libya

Tim J. Green and Teresa Sabato Ceraldi

The Messinian section in BP’s offshore Sirt 3D area is interpreted to be underlain by fine-grained clastic, Oligo-Miocene sediments, deposited in a slope environment and later deformed by gravity sliding, slumping and mass transport . The base of the Messinian is marked by a widespread regional unconformity, which erodes progressively more deeply into the underlying older Tertiary section towards the basin margins. In the south and east of the Sirt 3D area there is a prominent scarp slope, which offsets the Messinian reflectors, and appears to have divided an area of more positive relief to southeast from a more basinal area to the northwest. In the basinal area, halite deposits, characterised by their transparent character and rapidly-changing thicknesses, occur immediately above the basal unconformity. In between the halite bodies there are low-amplitude chaotic and deformed seismic packages which connect upwards to mud ridges on the seafloor, and these are interpreted to consist of mobilised mud derived from the underlying, relatively unconsolidated , and slumped Oligo-Miocene shale.

These are overlain by a sequence of sub-parallel, high-amplitude reflectors interpreted as interbedded anhydrite (or gypsum) and shales of late Messinian age. This sequence is thickest (300 – 400 msec TWT) in the west of the survey area and it thins and onlaps eastward to become only a single seismic cycle thick in the east of the area. The western area is interpreted to have been a restricted basin with episodes of variable salinity, and subject to episodic drying-out and re-flooding. This period of lacustrine (or hypersaline lake) deposition may indicate that more humid conditions occurred in late Messinian times. A series of channels, possibly of late Messinian or Plio-Pleistocene age, subsequently cut deep incisions into these lacustrine sediments and may indicate that this humid phase was accompanied by increased clastic sediment supply.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013