Origin of major cyclicity of evaporites and bituminous rocks: An actualistic model

Abstract

Cyclic changes in basin depth occur in an evaporite basin if the constant rate of subsidence is intermediate between the slow rate of pre-evaporitic and the high rate of halite sedimentation. In a basin located in the arid parts of the trade-wind belt with the entrance area downwind, the cyclic changes in depth result in cyclic changes in the hydrological regime. The wind-driven quasi-estuarine circulation, which prevails in the pre-evaporite phase over the whole basin as long as the depth is small, gives way to an anti-estuarine circulation when the depth becomes great, but keeps predominance over the sill. This combination provides a mechanism which leads to high salinities, since it prevents an outflow of highly saline water. After a rapid shoaling in the halite phase the wind-drift regains predominance over the whole basin implying a decrease in salinity. The boundaries, imposed by the continental land-masses upon the anticyclonic flow of air and water in low latitudes, result in an eastward down-slope of the temperature inversion in the air, and to an eastward up-slope of the permanent thermocline in the sea. If the coast is to the right of a paralleling wind in the Southern Hemisphere the boundaries result also in coastal divergences in air and water, which imply subsidence in the atmosphere and upwelling in the sea. The low position of the air temperature inversion and maximum air subsidence in a rainless climate which favours deposition of evaporites. The high position or disappearance in the process of upwelling of the thermocline causes extreme fertility, which favours anoxic conditions and deposition of bituminous sediments. In this way a frequent association of evaporites with bituminous rocks becomes comprehensible. In a basin bituminous sediments are deposited in the pre-evaporite phase when the quasi-estuarine circulation dominates; the black colours tend to disappear when the anti-estuarine circulation gains predominance and evaporites are being precipitated.