Linking Shelf-Edge Deltas to Deep-Water Sheet Sand and Channel Turbidite Reservoirs: Three Examples from the Miocene-Pleistocene, Gulf of Mexico
Lawrence D. Meckel, III, Michael P. Dean, Mike J. Harris, Michael F. Medeiros, Dean Christensen, James R. Booth, 2013. "Linking Shelf-Edge Deltas to Deep-Water Sheet Sand and Channel Turbidite Reservoirs: Three Examples from the Miocene-Pleistocene, Gulf of Mexico", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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Large-scale shifts in the shelf-edge location of deltaic depocenters have caused temporal and spatial fluxes in sediment supply that have exerted a significant control on the third-order reservoir stratigraphy of the Mars-Ursa, Auger-Macaroni, and Brutus-Bullwinkle intraslope basins, among the most productive areas in the deep water Gulf of Mexico(Fig. 1). Each basin has a producing interval characterized by a remarkably similar up-section transition from more sheet-like turbidite deposits to more channelized deposits, despite locations hundreds of miles apart, and different geologic ages. The transition is directly related to the changing position of the deltaic depocenter with respect to the basins, and the associated increase in sediment supply to the slope with respect to accommodation, independent of fluctuations in eustasy.
Previous workers have described the transition from sheets to channels: Prather et al. (1998) describe a calibrated up-section change in Plio-Pleistocene sediments from a “ponded” seismic facies assemblage to a “bypass” seismic facies assemblage, Booth et al. (2000) describe a similar transition for the Pliocene Auger-Macaroni Basin, and Meckel et al. (2002) describe such a transition in the late Miocene-early Pliocene stratigraphy of the Mars-Ursa area. However, the ubiquitous nature of the transition and its spatial dischroneity has not been previously described, nor has the relationship between the deep-water reservoirs and their coeval shelfedge deltaic depocenters been made explicit.
Paleogeographic reconstructions of the position of the shelf-edge systems (Winker and Booth, 2000) show that the deltaic depocenter migrated westward from a location updip of the Mars-Ursa basin in the late Miocene to a location updip and northwest of the Auger-Macaroni basin by the beginning of the late Pliocene. From the late Pliocene to present, the depocenter has migrated eastward, returning to a present-day shelf-edge position very close to where it was during the late Miocene. The depocenter passes updip of the Brutus-Bullwinkle area sometime between the latest Miocene and early Pliocene while migrating westward, and again in the late Pliocene to early Pleistocene, as it migrated eastward.
When the deltaic depocenter was in an up-dip, proximal position with respect to each basin, laterally extensive, high net-to-gross sheet sands dominated deposition there. In the Mars-Ursa area, sheet sands were deposited from 9.5 Ma or before until 7.5 Ma. This period correspondeded to deposition of the latter part of the Atwater Unit (terminology of Winker and Booth, 2000). In the Auger-Macaroni area, sheet sands were most prevalent in the section from 4-2.95 Ma, corresponding to deposition of the Keathly Unit (terminology of Winker and Booth, 2000). In the Brutus-Bullwinkle area, sheet sand deposition dominated from 3.5-1.95 Ma, during the early deposition of the Sigsbee Unit (terminology of Winker and Booth, 2000).
When the depocenter abandoned one fairway and migrated to a location more distal with respect to a given basin, less continuous, lower net-to-gross channels and overbank deposits dominated deposition. In the Mars-Ursa area, channelized deposition dominated from 7.5-4 Ma, when the depocenter had migrated westward. In the Auger-Macaroni area, channelized deposition dominated from 3-2 Ma, when the depocenter had migrated eastward (corresponding to the sheet sands in the Brutus-Bullwinkle area). As the depocenter continued migrating eastward, channelized reservoirs were deposited in the Brutus-Bullwinkle area from 1.95-1.04 Ma.
The transition from sheet sand deposition to channelized deposition occurs at different times in each basin-7.5 Ma in the Mars-Ursa area, 2.95 Ma in the Auger-Macaroni area, and 1.95 Ma in the Brutus-Bullwinkle area-yet the similarity of the transtion argues for a common explanation. The links between sheet dominated, delta-proximal conditions and channel dominated, delta abandonment conditions across space and time implies a fundamental genetic link between the updip and downdip systems that cannot be coincidental. Glacioeustatic changes in sea level and other commonly invoked mechanisms of cyclicity, such as climate or tectonics, are inadequate to explain the observed transitions. Such factors are regional to global in nature, and would result in a more synchronous transition between the areas in question.
Furthermore, the magnitude, frequency, and timing of eustatic changes in particular do not correspond to the observed transitions in a meaningful way. Thus, as alternate explanations are neither convincing nor sufficient to explain the data, we conclude that the changing sediment supply associated with shelf-edge depocenter migration is the most reasonable explanation for the transition.
In our model, increased sediment supply associated with proximal shelf-edge deltaic systems overwhelmed other possible contributing factors, resulting in sheet sand deposition. Assuming that salt withdrawal created a relatively constant rate of creation of accommodation, the sandy turbidites deposited at this time were able to efficiently fill existing (and newly created) space. The decrease in sedimentation rate that occurred when the depocenter switched locations resulted in channelized deposition that was less efficient in filling the basin with continuous sands. Short-term fluctuations in relative sea level and topography within the overall supply dominated succession (or lack thereof) might have caused higher-frequency (fourth-and fifth-order) alternations between sheets and channels that appear to be another similarity among the basin fills.