ABSTRACT: Responses of a Deltaic System to Minor Relative Sea Level Variations (Middle Jurassic, Cleveland Basin, England): Consequences on the Reservoir Geometry

R. Eschard, Ch. Ravenne, Heresim Group

In the Middle Jurassic series of the Cleveland basin (England), an accurate three-dimensional reconstruction of depositional sequences from outcrops, cores, and wireline logs show that minor sea level variations control the evolution of a deltaic system. Because of low subsidence and sediment supply rates, rapid sea level rises induce several landward shifts of the shoreline, whereas minor sea level drops produce the incision of small paleovalleys.

The deltaic series, 200 m thick at the basin depocenter, are subdivided into five depositional sequences. During the first two sequences, the sediment supply rate exceeds the relative sea level rate and the delta progrades. The delta aggrades during the third depositional sequence and retrogrades in the fourth depositional sequence when the relative sea level rise rate balances then exceeds the sediment supply rate. Finally, a relative sea level drop induces the incision of a paleovalley progressively infilled during the following sea level rise.

Several orders of bathymetric cycles are deduced from detailed correlations of the unconformities and from the comparison of the vertical facies succession between low and high subsiding areas. The general evolution of the delta is related to a 5-m.y. bathymetric cycle due to a eustatic sea level rise and fall. Pne to 3-m.y. bathymetric cycles control the main aggradation patterns of the delta. These patterns are related to third-order eustatic cycles or to local tectonic events. Less than 0.5 m.y. bathymetric cycles are registrated by parasequences or minor depositional sequences which are correlated all over the basin. The controlling parameters of the sequences are discussed. Minor bathymetric cycles (0.2 m.y.) reflect small-scale sea level oscillations or autocyclic evolution of t e sedimentary bodies. We focus on the reservoir geometry within each bathymetric cycle.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990