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North Africa Research Group

Basin Evolution and Geochemical Modeling

Characterisation of the El Garia Formation, Tunisia

PhD research by Dr Simon J Beavington-Penney

Background:
Summary of the Ph.D. research of Simon J Beavington-Penney undertaken at Cardiff University and sponsored by BG Group. Simon completed his Ph.D. at the Department of Earth Sciences, Cardiff University, in July 2002. His thesis is largely concerned with the characterisation of Eocene Nummulites accumulations.

 

For more information see: http://www.earth.cardiff.ac.uk/research/basins/

Overview:

Nummulitic limestones are a common component of Eocene circum-Tethyan sediments. They form hydrocarbon reservoirs in North Africa, and are potential exploration targets in the Middle East and the Indian Subcontinent. This study focuses on the Lower Eocene El Garia Formation (Tunisia), and also the Middle Eocene Seeb Formation (Oman).

The North African component of this study is concentrated on El Garia Formation outcrops in north-central Tunisia, and also core from offshore Tunisia gas fields. The outcrops have been studied by numerous authors, largely because proven economical hydrocarbon reserves occur within both the El Garia Fm. in the Gulf of Gabes, and also lateral equivalents in offshore north-west Libya. Consequently, the gross distribution of the Metlaoui Group depositional facies is well known; a highly detailed sedimentological, palaeontological and palaeoecological approach to the interpretation of these limestones, ranging from large-scale studies of their platform location, architecture and geometry, to fine-scale characterisation of Nummulites tests and populations, distinguishes this study from previous ones.

Studies of the ecology of extant relatives of Eocene Nummulites and other larger benthic foraminifera have been used to refine palaeoenvironmental models for the El Garia Fm. To assist the development of such models, a thin section-based classification scheme of test damage has been devised, which enables determination of in situ and transported Nummulites assemblages. This scheme is based on taphonomic experiments using the extant Nummulites venosus, which involved the use of a 'tumbling' barrel and flume tank to simulate transport as part of a storm current-driven 'traction carpet', and also within gravity flows; transport-induced test damage was also assessed from Nummulites within Eocene turbidites from the Pamplona Basin (North Spain).

Main conclusions:
Nummulitic limestones of the El Garia Fm. were deposited on a north-east-dipping, windward ramp on the southern margin of the Tethys Ocean. Water across this ramp was oligotrophic, although nearshore areas may have been relatively mesotrophic due to terrestrial runoff. Detailed taphonomic and palaeoecological study of these limestones suggests that CaCO3 production by Nummulites was at its highest in shallow water (close to or above fair weather wave base), in both nearshore areas and also on isolated, submerged 'highs', where fast-growing, rapidly reproducing megalospheric forms thrived. This challenges the current view that Nummulites produced large quantities of sediment in relatively deep (approximately 30-60 m) water.

Water energy levels and high production rates on the submerged highs, combined with the lack of a reefal rim, meant that much of this sediment was not 'stored' on these isolated platforms; rather the tests were 'exported' into the surrounding deeper parts of the basin by a combination of gravity slides and storm currents. The effectiveness of this offshore transport, combined with the effects of bio-erosion and biogenic sorting of the sediment, resulted in a distal trend of increasingly fine nummulithoclastic sediment away from the 'highs'. This offshore transport, combined with relatively limited sediment production by deeper-dwelling B-form communities in mid ramp environments, and also by elongate A-forms in the deeper euphotic zone, was critical for controlling the ramp-like stratal geometries observed in the field. Subsequent bioturbation of these mid to outer ramp, re-sedimented nummulitic limestones destroyed primary fabrics and stratification.

Nummulites palaeocommunities (i.e. autochthonous and parautochthonous assemblages) identified in this study form low relief, biostromal reef mounds, and exhibit a wide range of A- to B-form ratios, suggesting that the 'normal' 10:1 population hypothesised by Blondeau (1972), and subsequently used by several workers to define in situ and transported assemblages, is incorrect. Indeed, studies of living larger benthic foraminifera suggest that it is impossible to define such a population, and autochthonous or allochthonous populations of Nummulites can only be identified on the basis of taphonomic and biofabric study.

The studied sections of the El Garia Fm. cannot easily be correlated with published global sea level curves, due in part to the deposition of much of this re-sedimented material below the influence of low amplitude sea level fluctuations typical of greenhouse ramps, combined with the effects of local tectonics.

Simon's work is now continuing in the Department of Earth Sciences as a Postdoctoral Research Associate. He is supporting the research of Professor Paul Wright, holder of the BG-sponsored Chair in Carbonate Sedimentology, and is also pursuing his own research interests, including:

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