Monday afternoon: Paleogene Canyons and Wilcox Sands: Pros and Cons of the Gulf of Mexico Draw-Down Hypothesis
Published:December 01, 2007
2007. "Monday afternoon: Paleogene Canyons and Wilcox Sands: Pros and Cons of the Gulf of Mexico Draw-Down Hypothesis", 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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The Wilcox Group contains most of the documented large submarine canyons and slump scars within the northern Gulf of Mexico Paleogene section. Indeed, comparable abundance and scale of canyons is not seen again in the Gulf until the late Neogene. Four styles of shelf margin excavation morphologies can be differentiated based on published examples:
Simple slump scars.
Submarine canyons created by retrogradational slumping.
Graded, mature submarine canyons.
Valley-form cross-shelf gorges.
These styles appear to be part of a continuum; many examples share elements of two or more types. However, most canyons have been mapped using limited well control, and their illustrated morphology may largely reflect the interpreter’s use of fluvial morphologic analogues as much as data constraints.
Wilcox canyons typically occur in geographically localized clusters, which are centered beneath the central and upper Texas coastal plain and the central Louisiana coastal plain. Two of the clusters occur on the flanks of the lower–middle Wilcox Houston delta system; the third lies on the progradational front of the lower Wilcox Holly Springs delta system. Canyon clusters appear to occur at or near major tectonicstratigraphic domain boundaries of one or more Wilcox deposodes. Several of the largest canyons, including the Yoakum canyon, correlate with two thin, regional marine flooding units, the Yoakum and Big shales.
However, known canyons also occur at several additional stratigraphic positions at the base of and within the Paleocene lower Wilcox genetic sequence. Recurrent shelf margin mass wasting events could have been triggered by seismic shocks initiated along the Laramide tectonic front, which lay along the west and northwest margin of the Gulf basin.
Detailed analyses of the Yoakum and Lavaca canyons showed that they were excavated (at least in their late stage of development) across the Wilcox shelf and deltaic platform during times of local to regional transgression by processes of headward slumping and erosion by submarine currents. Canyon cutting and filling occurred sufficiently rapidly that steep (up to thirty degree) unstable canyon walls consisting of unconsolidated slope and prodelta muds were buried and preserved. Infilling occurred during subsequent progradational advance of the shore line. The obvious canyons were largely mud filled. However, sand bodies were present within the fill of both the Yoakum and Lavaca canyons. Fill of the Lavaca canyon was extensively cored and consists of turbidite channel and levee facies suspended within volumetrically dominant, muddy debris flow deposits. Intact slump blocks of canyonbounding delta front successions were also found.
Updip reaches of canyons typically included a lower onlap fill and a superimposed, mud-dominated progradational fill.
The presence of clusters of canyons along the updip Paleocene continental margin suggests a genetic relationship to turbidite channel and lobe sand bodies that constitute the reservoirs for the recent deep-water Paleogene discoveries. Successive canyons would have served to collect and focus sediment transport across the shelf and down the nascent Cenozoic continental slope, creating a submarine canyon—fan couple. Such highly evolved sediment transport systems would allow efficient separation of bed load (sand) from suspended load (silt/clay) as sediment gravity flows traveled from the shelf margin and upper slope, through mature canyons, and onto large, abyssal plain fan systems with well segregated upper, middle, and lower fan provinces. The high net/gross sections of channeled turbidite lobe sand bodies penetrated in the Mississippi Fan and Perdido fold-belt fairways likely have accumulated in the sand-rich middle fan. Paleogeographic reconstruction places the sand-rich middle fans more than 200 km basinward from the slope toe. Although impressive, such dimensions are typical of many sandy Quaternary abyssal fan systems.