--> Abstract: A Gulf of Mexico Based Depositional Process Model for Above-grade Slopes, by B. E. Prather; #90933 (1998).

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Abstract: A Gulf of Mexico Based Depositional Process Model for Above-grade Slopes

Prather, Bradford E. - Shell International E&P

Mobile substrates with relatively large amounts of ponded-basin and healed-slope accommodation space characterize above-grade slopes (Fig. 1). There are many above-grade slopes around the world but the Gulf of Mexico (GOM) is arguably the best calibrated. Because of the unique geologic processes attributed to the GOM, however, such as high rates of salt withdrawal (locally > 10 km/Ma) and "melt-water" spikes, the transfer of depositional process concepts to other deep-water provinces may not seem intuitively obvious.

Ponded-basin "fill-and-spill" processes dominate early phases of deposition on above-grade slopes and precede progradation of unconfined slopes. Fill-and-spill processes include (1) ponding, followed by (2) fill, (3) bypass, (4) slope re- adjustment and (5) drape depositional phases. Ponded basins typically have thick, high net-to-gross sands deposited within ponded-basin accommodation space. Numerous reservoir/seal pairs characterize ponded-basin sequences in slope systems with highly mobile substrates where periods of rapid sediment influx punctuate periods of slow sedimentation or condensed zone deposition. Creating additional ponded accommodation space to form stacked reservoir/seal pairs in this way requires periods of little sediment influx in order to allow the local depositional surface to drop below the stepped equilibrium profile.

A stepped-equilibrium profile across a series of ponded basins forms a base for later unconfined slope progradation. Deposition in the unconfined slope occurs in both slope and healed-slope accommodation space. These spaces fill as progradational delta fronts build beyond critical angles and collapse. Sediments tend to be sand lean because the slope angles are generally too high to allow turbidite fan deposition. Sands are generally associated with channels with some scattered, thin ponded fans. Condensed zones at the bases of the ponded fans suggest that the seafloor depressions formed during periods of localized salt withdrawal and low sediment influx. Filling of the depressions force submarine gravity flows to bypass or spill through "bypass" channels down-slope. Sinuous "bypass" channels deposit across lower gradient portions or steps on the slope equilibrium profile whereas straight- to lower-sinuosity channels form across ridges between the steps where sea-floor gradients are higher. Areally extensive mass-wasting also occurs as slopes steepen beyond the angle of repose during deltaic progradation and/or basinward tilting.

Forward models show that an optimal combination of sediment supply relative to uplift of mobile substrates and down-building of intraslope basins is critical to the creation of ponded-basin accommodation space. The introduction of "melt-water" spikes do not radically change the depositional architecture of the slope but they do force turbidite deposition to continue into eustatic sea level rises (Fig. 2). These models also show that: 1) rapid salt withdrawal beneath intraslopes basins is a major control the distribution of accommodation space across the slope and thus it's stratigraphic architecture; 2) basins with more episodic sediment fluxes are more likely to have thick ponded basin successions with numerous reservoir/seal pairs than slope systems with more constant sediment fluxes; and 3) periods of sustained high sediment flux forces progradation of unconfined slopes deposits over earlier ponded-basin successions.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil