SEDIMENTARY BASINS BASIN TYPES ACCORDING TO TECTONIC. by Prof. Dr. Abbas Mansour

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1 SEDIMENTARY BASINS BASIN TYPES ACCORDING TO TECTONIC by Prof. Dr. Abbas Mansour

2 B. Convergent plate margins 10. STABLE CONTINENTAL INTERIOR BASIN E.g. Lake Eyre Basin Intracratonic (= within stable continental crustal mass) Long term stability Flat topography River, desert, lake environments & facies Mature basement derived sed. ± evaporites

3 Simple basins are divisible into two groups. Intracratonic basins lie within the continental crust. Epicratonic basins lie on continental crust but are partially open to an ocean basin. These two types often occur adjacent to one another with little fundamental difference in genesis or fill Intracratonic basins are the classic type of sedimentary basin. Notable examples include the Williston, Michigan and Illinois basins of North America, the Maranhao basin of Brazil, and the Murzuk and Kufra

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5 The Williston basin is the classic example of an intracratonic basin. It contains some three kilometres of rocks of all periods from Cambrian to Tertiary, with notable gaps only in the Permian and Triassic. Sedimentation spanned a range of environments including fluvial and marine sands, reefal carbonates, evaporites and subwave- base pelagic muds. Deep-sea, turbidite and deltaic fades, igneous activity and shallow syndepositional faulting are all absent.

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8 The lengthy history, diverse facies and structural simplicity of the Williston basin are also found in the other basins cited as examples of intracratonic basins. It is important to note that the sedimentary facies, though diverse in lithology and environment, are seldom indicative of deep water or abrupt subsidence of the basin floor. Deposition took place close to sea level. Subsidence was thus a gradual, if erratic event, with sedimentation being sufficiently rapid to keep the basin nearly Filled at any point in time.

9 Basins of this type are found well within the present limit of the continental margins, but it is obvious that they were frequently connected to the sea. This is shown by their intermittent phases of marine carbonate and evaporite sedimentation. During these periods they might, therefore, be more truly termed embayments rather than basins.

10 Modem analogues include the Hudson Bay and the Baltic Sea, which lie on the Canadian and Scandinavian shields respectively. Intermittent uplift of the open rim of the embayment closes the basin off from oceanic influence. Evaporite or continental sedimentation follows. An interesting example of this is provided by the Murzuk basin of Libya.

11 An interesting example of this is provided by the Murzuk basin of Libya. The northern rim of this basin was separated from the Tethyan Ocean by the Gargaf arch, a tectonic feature which controlled deposition through much of Palaeozoic and Mesozoic time. Nevertheless, palacocurrent analysis of fluvial Cambro- Ordovician and Mesozoic strata indicate a northerly palacoslope over the Gargaf arch (Fig. 140). These data show that the Murzuk basin alternately fluctuated from embayment to basin until attaining its present structurallyclosed intracratonic basin shape.

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13 Epicratonic basins Epicratonic basins are those which lie on the edge of continental crust. Though subcircular in plan, by definition, they tend to be embayed and open towards the adjacent ocean basin. The axis of an epicratonic basin may plunge to the floor of the ocean or be interrupted by a sill-like feature at the rim of the continental margin. Examples of epicratonic basins include those of the Mississippi Gulf coast, the Niger basin and the Sirte basin of North Africa

14 Epicratonic basins The Mississippi and Niger basins are very similar. Both originated towards the end of the Mesozoic and continued to be sites of active sedimentation until the present day. Both basins contain a basal layer of salt, which diapirically intrudes younger sedimentary rocks. The Louann salt of the Gulf basin is of Jurassic age. The basal salt of the Niger basin is believed to be of Albian-Aptian age (Mascle et at., 1973).

15 Epicratonic basins The basins are infilled by prisms of terrigenous elastics which were deposited in a range of environments. On the landward sides of the embayments alluvial deposits predominate. These pass basinward into diverse shoreline facies, which include both barrier and deltaic deposits. These thin and grade seawards into marine slope muds, with some development of turbidite sand facies at the base of the slope. Geo- physical data suggest a gradual seaward thinning of the continental crust beneath both the Mississippi Gulf coast and Niger basins

16 Epicratonic basins The Sirte basin of North Africa shares many features with the Gulf coast and Niger basins. It too is essentially an embayment which opens out to an oceanic basin (the Mediterranean). The Sirte basin originated in the end of the Cretaceous and was infilled more or less continuously throughout the Tertiary.

17 Epicratonic basins By contrast with the previous, two examples, the Sirte basin was predominantly a site of carbonate sedimentation. Basal sands and thin evaporites are overlain by deep-water Upper Cretaceous and Palaeocene shales. These are thickest in intrabasinal troughs, while reefal carbonates were deposited on adjacent horsts. Throughout the Eocene the Sirte embayment was infilled by nearly a kilometre of interbedded carbonates and evaporites. The final phase of basin infilling during the Oligocene and Miocene involved terrigenous and carbonate sedimentation in both marine and continental environments. The active history of the basin was concluded by a bout of basaltic volcanism in the Pleistocene.

18 Comparison between both types These brief reviews of three epicratonic basins show how they differ from the previously described intracratonic examples. Epicratonic basins tend to be much less stable than intracratonic ones, due to their situation at continent margins. Initial basin subsidence can be rapid, resulting in an early phase of deep-water sedimentation. The floor of the Sirte basin was extensively faulted and there was some igneous activity. Like intracratonic basins, however, epicratonic embayments can be infilled by both carbonates and terrigenous sediment. This differentiation is a function of the degree of uplift of the adjacent crust.