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Abstract: Bottom- (Tractive-) Current Reworking of Turbidites - An Example from a North Sea Field

C. Cazzola, V. Kolla, J. Gerard, G. Druillon

The field example from the North Sea consists of a Jurassic interval deposited in a half-graben basin. In the axial part of this basin, the Jurassic interval contains frequently thick, structureless (except for subtle normal grading), fine-to very fine-grained, amalgamated sandstone beds. Such massive sandstone beds are less thick along the marginal highs of the basin. On the other hand, in these marginal areas, there are commonly, several meters-thick, very fine-grained sand-intervals that are characterized by an abundance of thin parallel laminations, small-scale cross laminations, flaser-like stratification, bidirectional wavy-ripple bedding, alternating sequences of horizontal and ripple laminations with internal truncations, starved ripples with lenticular geometry, ud drapes and reactivation surfaces, reverse grading and extensive bioturbation. These intervals are associated with normally graded but otherwise structurless turbidite beds whose tops may be truncated by erosional surfaces. These diverse sedimentary structures are much less common in the axial portions of the basin. It is possible that some of these structures might have been caused by tractive currents within the high-dilute turbidity currents subsequent to the deposition of massive sand beds. However, all the above sedimentary structures, taken together, suggest reworking of turbidites and subsequent redeposition by bottom currents, unrelated to the turbidity-current evolution. Of possible types of water-movements, thermo-haline circulation, tidal currents and wind-driven circulation, we believe that the wind-driven currents might have penetrated to the deep but still probably relatively shallow Jurassic sea-floor, especially along the marginal high areas of the basin, and reworked the turbidites. Distinction of bottom-current from turbidity-current depositional facies is critical to the prediction of deep-water reservoirs.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France