Facies in a shelf-edge delta; an example from the subsurface of the Gulf of Mexico, middle Pliocene, Mississippi Canyon, Block 109

Two vertically stacked shelf-edge delta systems, in the middle Pliocene of Mississippi Canyon Block 109, have been described using a three-dimensional seismic survey, 400 ft (122 m) of conventional core, and electric logs. Possible characteristic facies associations and features of these shelf-edge deltas include extensive upper delta-slope deformation; locally ponded, slump-induced turbidites in the upper slope; a complex association of gravity and tractional deposits; relatively thin, preserved mouth-bar deposits; and the absence of delta-plain facies. Six facies have been identified in cores: (1) structureless sands interpreted as high density turbidites; (2) laminated sands; (2.1) sharp-based planar horizontal laminations interpreted as Bouma b- and bc-division turbid tes; (2.2) low-angle discordant laminations interpreted as strong wave activity deposits; (2.3) a complex mixture of lamination styles that may be a product of turbidity currents, waves, or possibly river-fed underflows; (3) thinly interbedded ripple-laminated sands including (3.1) asymmetrical cross-laminations interpreted as unidirectional traction current lamination and (3.2) c-division turbidites, or ripple lamination with mud drapes on foresets, or with a symmetrical form interpreted as wave or possibly tidal-current lamination; (4) contorted, muddy, fine sands and silts interpreted as mud flows and slumps; (5) laminated silts and muds interpreted as low-energy wave deposits or possibly d-division turbidites; and (6) muds interpreted as quiet-water deposits. In the cored intervals o the deltas, turbidite sands immediately overlie muddy slumps in the upper delta slope, suggesting that retrogressive slope failure of developing slump structures tapped back into the mouth-bar causing sandy turbidity currents to develop. The upper delta is represented on seismic data as a series of prograding clinoforms approximately 400 ft (122 m) thick. Electric logs indicate these clinoforms are composed of a lower mud-dominated interval, an upper delta slope dominated by slumps with areally restricted turbidite sands, and coarsening- and cleaning-upward mouth-bar deposits composed of ripple-laminated and parallel-laminated sands interbedded with muds and silts. The lower delta contains similar vertical sequences, but also contains a distinct log and seismic facies that may be either a major shelf-edge failure or a later channel feature incised across the entire delta. Apart from this possible channel, neither delta shows evidence of emergent delta-plain facies.