Seismic-based paleoenvironmental analysis of the Paleocene carbonate shelf in Ajdabiya Trough, north-central of Libya

Abstract

Abstract The results of the seismic and well logs interpretation, together with previous literature, were used to construct a generalized model of the development of the Paleocene reefs, and a depositional model for the Paleocene carbonate shelf in Ajdabiya Trough that focuses on the initiation, growth, and demise of the reefs in this basin. The Intisar reefs began as algal-foraminiferal banks. Growth of the reefs was halted by a short-lived transgression that drowned reefs and led to shale deposition of Kheir shales. The inversion of 3-D seismic data to acoustic impedance has allowed for a better determination of the main facies in Intisar reefs. These facies reflect different depositional conditions and different ages. They are: 1) algal-foraminiferal packstone/grainstone; 2) bioclastic grainstone/packstone; 3) coralline floatstone/bindstone; 4) reefal boundstone and; 5) bio-lithoclastic talus. The Upper Sabil consists of a prograding carbonate shelf with ramp platform morphology, and its differentiated deposits indicate four main depositional facies tracts: 1) inner shelf margin; 2) barrier reefs; 3) pinnacle reefs and; 4) outer shelf margin. The proposed model for the growth of the Paleocene reefs in the Ajdabiya Trough represents consecutive stages of development: 1) the Lower Sabil Carbonate was deposited during the very early Upper Paleocene time; 2) the influx of the Sheterat Shale during the major marine transgression; 3) the sea-level began a very slow fall due to variable subsidence of the underlying strata (a deposition of micrite over the Sheterat Shale, the lower part of the Upper Sabil Carbonate); 4) the rise in sea level is evident from an increase in foraminifera and algae constituents over the high areas of the lower part of the Upper Sabil Carbonate; 5) the isolated Intisar reefs were born under the proper local circumstances. The rate of subsidence was very slow, and the coral reef had significant upward growth and; 6) a major transgression happened at the end of the Paleocene, and the consequent deepening of the water in the area drowned and killed the Intisar reefs and capped it with Kheir Shale.