Sequence Stratigraphy

SEPM Sequence Stratigraphy Research Group
Annual Meeting Report
 
When: Monday, April 3, 7:00-10:00 pm
Where: Whitney Room (2nd floor) of the Four Seasons Hotel
(1300 Lamar St, Houston, TX 77010 - Hotel Location)
 
2017 Organizers
 
   This group had a good attendance of mostly students with a handful of more experienced folks to add perspective and contribute to lively debate. The subject at hand was the role of sediment supply variation in the creation of depositional sequence geometries. Jinyu Zhang of the University of Texas in Austin shared his thesis research into the topic “Can sediment supply drive shoreline migration and sequence developments?” and “if so, can supply-driven cycles create similar stratigraphic pattern like accommodation-driven cycles?”

   This work has been documented in a recent AAPG Bulletin article on the Paleogene Wilcox Group of the Gulf of Mexico (Zhang et al., 2016). Using regional well log cross sections, Zhang documented basinward and landward trends of the shoreline in a manner that recalled Galloway’s work in the late 1980’s on his genetic sequence stratigraphy. The shoreline movement observed by Zhang was proposed to be driven by climate-induced sediment supply variation in a greenhouse climate. To test this hypothesis, Zhang walked the assembled group through a series of Dionisos diffusion-based forward models, varying sea level, sediment flux (Qs) and water flux (Qw) to see what resulting geometries would form on a subsiding substrate.

   The results presented showed highly varying geometries. The sea level varying model to produced significant onlapping and degradationally stacking strata when Qs/Qw was low. When Qs/Qw was high and sea level change was low, a pattern of progradation to aggradation followed by aggradation to progradation was seen. Interestingly, topset aggradation was observed when the model input sea level fall. In the discussion, the effects of relative changes in sea level from convolving tectonic subsidence with low magnitude eustasy came up, especially as it might vary around active tectonic features as have been documented by Gawthorpe et al around the Gulf of Corinth or in the Jurassic of the North Sea.

   Coastal plain aggradation in response to sea level change was discussed and Keith Shanley was in the audience to share his experiences modeling nonmarine systems and the strong role variations in accommodation play. Zhang next showed models with variable Qw that produced variations in progradational geometries.

   The discussion that followed highlighted the fact that basinward shifts in coastal onlap are only produced in models that consider variations in accommodation. While the signal in fully nonmarine successions is complicated by the degree of coupling with the fluvial system and shoreline trajectory, it was agreed that a combination of factors is needed to explain the geometries seen in nature. Further discussion focused on the importance of an observation-based method to build a framework that can be subsequently tested against changes in the main factors controlling depositional sequence formation and variability.

   Plans for continued discussion into these topics, including the role of tectonics and possibly carbonate environments, at the 2018 ACE meeting in Salt Lake was proposed.
 
Questions? Please contact one of the organizers (listed above) or Hayley Cooney at SEPM headquarters: hcooney@sepm.org

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