Deep-Water Depositional Trends of Mesozoic and Paleogene Strata in the Central Northern Gulf of Mexico
Published:December 01, 2007
Joseph C. Fiduk, Lynn E. Anderson, Thomas R. Schultz, Andrew J. Pulham, 2007. "Deep-Water Depositional Trends of Mesozoic and Paleogene Strata in the Central Northern Gulf of Mexico", The Paleogene of the Gulf of Mexico and Caribbean Basins: Processes, Events and Petroleum Systems, Lorcan Kennan, James Pindell, Norman C. Rosen
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Isochron maps between the four structural surfaces reflect the underlying structure and depositional trends of the interval. Thus the basement to Cretaceous isochron shows thick Jurassic infill, Cretaceous drape in the grabens (Fig. 2), and thin to no cover over highs in the rifted basement topography. The Cretaceous to Wilcox isochron has a broad lobate form that thins gently from west to east. A very subtle down-lapping pattern is visible within the Wilcox interval on Figure 2. Deviations from this pattern occur primarily where basement structures produce isolated thins. The Wilcox to Oligocene interval shows a regional gradient of north to south thickening and only a slight influence from deeper structure. Down-lapping and thinning to the north strongly suggest a southerly source for the Oligocene interval.
Beneath the allochthonous salt of the Sigsbee Escarpment, all surfaces deepen northward and show much greater local variability. Basement is only occasionally visible as it generally lies below the fifteen kilometer limit of the available PSDM data. The deepest area mapped is in Green Canyon where the top Oligocene approaches twelve kilometers depth, the top Wilcox approaches thirteen kilometers, and the top Cretaceous almost fourteen and one half kilometers. These surfaces shallow to less than eight kilometers deep on the abyssal plain. Three coincident lows roughly oriented north-south suggest preferred sediment pathways and possibly areas of thicker original autothonous salt. A change on the structure and isopach maps from smooth broadly spaced contours on the abyssal plain to highly variable tightly spaced contours suggests the location for the original limits of salt deposition in this area. This location often lies close to but not exactly in line with the present day Sigsbee Escarpment (Fig. 1).
Pre-existing basement highs have caused the Wilcox to be thin or absent around those structures. Although basement topography is mostly smoothed over by the end of the Cretaceous, a few large structures still influenced deposition in the Wilcox on the abyssal plain beyond the Sigsbee Escarpment.
Salt nappes and salt pillows have caused thinning of Wilcox strata over those structures. Our interpretation indicates multiple kilometer thick salt nappes extruded beyond the limits of the original salt basin during the Cretaceous (Figs. 4 and 5). Inflated salt pillows associated with the nappes lay along the boundary of the salt basin. Though now deflated, the presence of these salt pillows and other salt pillows updip are recorded by the depositional thinning of Wilcox strata above them. These allochthonous bodies provided the core structure over which Wilcox and Miocene reservoirs are folded or draped at Chinook, Atlantis, Das Bump, and other important deepwater discoveries. The location of allochthonous salt at the onset of Wilcox deposition is apparently coincident with the pronounced increase in northerly dips of the Mesozoic and Paleogene strata. This relationship is consistent with originally thick autochthonous salt above the deepest mapped basement.
Sites of continued salt withdrawal from the autochthonous level into growing salt structures directly affected Wilcox sediment thickness. Such sites would have been primary candidates for the location of Wilcox sediment fairways. Identification and elimination of salt feeders would help in refining/defining these pathways.
Deposition of the Wilcox strata can be broadly divided into two paleogeographic domains: (A) a relatively complex north-westerly region characterized by pre-existing, elevated sea-floor, salt-cored structures and sites of contemporaneous salt evacuation, and (B) a relatively simple south-easterly region characterized by a near flat and smooth sea-floor rarely punctuated by unburied basement structures. The transition between these two regions should mark changes in Wilcox depositional styles.
In the more complex topographic region, Wilcox depositional events were forced to interact with relatively rapid changing sea-floor dips. Whereas in the more simple region to the southeast, a much more unconfined sea-floor presented limited impediment to widespread expansion of depositional events exiting the more complex region to the north-west.
Drilling of Wilcox strata to-date has been mainly in the simpler south-easterly region and in the transition zone to the more complex Wilcox geometries towards the north-west.
Figure 4 shows an example of one salt nappe and its contractional deformation front that lies in close proximity but basinward of the Sigsbee Escarpment. Thrust relationships suggest that the nappe continued to move/inflate until the end of the Cretaceous. An inflated salt pillow associated with the nappe is present through the Oligocene but then deflates during the Miocene. This interpretation is supported by the thin but depressed Wilcox and Oligocene section behind the nappe today. We predict that the edge of the salt basin lies behind the nappe, below where the Wilcox and Oligocene intervals begin dipping to the north.
Figure 5 shows another example of a salt nappe that lies in about thirty kilometers inside of the Sigsbee Escarpment. This nappe does not have a deformational front associated with it. But an inflated salt pillow is associated with this nappe as in Figure 4. Similar to Figure 4, the interpretation is supported by a thin but depressed Wilcox section behind the nappe. In contrast, evacuation of the pillow begins in the Oligocene, as evidenced by the Oligocene age turtle structure. Evacuation continues into the Miocene until the pillow is completely deflated. The nappe remnant is all that remains of this salt body. Unique to these two examples, but possibly typical of most salt pillows around the edge of the salt basin, loading has forced salt backwards (updip) into the salt basin. In Figure 4, the reversal of salt movement is about ten kilometers. In Figure 5, the reversal of salt movement may be twenty to twenty-five kilometers.