Shelf Margin/Upper Slope Sedimentation Patterns of the Lower Cristellaria “I” Depositional Sequence, Southeast Louisiana
Paul N. Lawless, Michael M. Ledet, 2013. "Shelf Margin/Upper Slope Sedimentation Patterns of the Lower Cristellaria “I” Depositional Sequence, Southeast Louisiana", 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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The lower Cristellaria “I” depositional sequence is a type “2” third-order sequence composed of seven fourth-order sequences, which has elements of a type “1” sequence. The lower three fourth-order sequences are confined within the upper slope submarine canyon section and thin downdip. The upper four fourth-order sequences are not confined and have extremely progradational, shelf margin systems-tract architecture. The sand-rich submarine-fan section extending many miles basinward to upper- and middle-slope positions correlates with this upper, unconfined section. The submarine canyons cut down as much as 1400 ft into underlying lower Miocene strata creating, in places, a spectacular angular unconformity. This angular unconformity is the lower boundary of the third-order sequence. Its dramatic character also makes it an excellent supersequence (second-order) boundary candidate on reflection seismic. Superficially, this fits with the Hardenbol et al. (1998) cycle chart that shows this section to be the first third order depositional sequence within the Middle/Upper Miocene Supersequence. However, when the stratigraphic architecture and the stratigraphic occurrence of downdip slope sand deposits are considered, the original Haq et al. (1988) placement of this sequence at the top of the Lower Miocene Supersequence seems a better fit. Additionally, the lower Cristellaria “I” third-order depositional sequence is shown to have avulsed from a main lower Miocene depocenter positioned in southwest Louisiana into southeast Louisiana (Fillon and Lawless, 2000). Although transgressive sea-level change has caused the avulsion and main depocenter shift, local sediment instability related to underlying salt reactivation plays a larger role in creating the spectacular nature of the basal unconformity.
Viewed in well log cross sections, the canyon fill section expands from a 200 ft, very serrated blocky sand updip, to a very shaley 1400 ft thick interval of section on the upper slope. Most sands are thinly bedded and exhibit crescentic shaped log patterns. Well-developed blocky sands are rare. Viewed on seismic data, the three confined fourth-order sequences in the most updip parts of the canyons contain sharply defined channel-scale features. Downdip, however, strata are seismically dim, and channels the size of canyons cut into each other giving a nested look. Together these three confined sequences form the third-order slope fan for the lower Cristellaria “I” depositional sequence. They thin basinward and little section is deposited farther downdip in the study area.
The four younger, unconfined fourth-order sequences are aggradational and form the prograding complex and transgressive systems tract of the third-order depositional sequence. This section is typified by rapidly prograding shelf margins coupled with a locally very sandy bypass section. A typical shelf-margin signature on seismic consists of an onlapping bypass toe-of-slope reflector, which is downlapped by shelf-edge clinoform reflectors. In Lafourche Parish, where more salt-displacement accommodation is available, the prograding clinoforms are sigmoidal and associated transgressive systems tracts are well developed. In St. Charles and northern Jefferson parishes, where less salt-displacement accommodation exists, the prograding clinoforms are shingled and associated transgressive systems tracts are very thin.