Sedimentary responses to active growth-faulting in coastal marshes of the northern Gulf of Mexico

1. Sedimentary responses to active growth-faulting in coastal marshes of the northern Gulf of Mexico Phil Wolfe GLY 730 Spring 2012

2. introduction • Coastal marshes…why do we care? • Productive ecosystems • Important habitat for both aquatic and terrestrial plants and animals • Northern Gulf of Mexico • Wetland losses • Due to large scale and localized processes • Deep-seated growth fault movement and autocompaction • Local submergence due to vertical displacement along active normal growth faults • Localized processes can have farther-reaching impacts • Surface hydrology and vegetation -> accretion processes

3. introduction • Objectives • What do the sedimentary processes look like in response to active faulting in these settings? • Wetlands can maintain system equilibrium in this instance…but are they? • What happens when vertical accretion rates < subsidence rates?

4. geologic setting • Northern Gulf of Mexico • Numerous growth fault systems • Growth fault – syndepositional, normal listric-type fault that often exhibits increasing throw at depth and expanded equivalent stratigraphic thickness on the downthrown side • Emplaced during Cenozoic • Recent reactivation • Anthropogenic processes = subsurface fluid withdrawal and reservoir compaction • Natural processes = migrating salt structures and movement of deep-seated growth faults

5. geologic setting

6. methods • Aerial photography

7. methods • Sediment core lithostratigraphic correlation

8. methods • Sediment core lithostratigraphic correlation

9. methods • Rod Sediment Elevation Tables (RSET)

10. sediment geochronology • Short-lived fallout radionuclides • 137Cs and 210Pb • Provide maximum temporal resolution of ~100 years

11. results and discussion • Cline et al. (2011) • Results: • Time series aerial photography revealed observable trend between fault scarp proximity and land cover change • Suggested that faulting and land cover loss are correlated • Land cover changes on downthrown extent more rapid and of greater magnitude • From 1948-2008, total water area increase of 451.3% on downthrown, compared to 317% on upthrown

12. results and discussion • Feagin et al. (2010), Morton et al. (2001), and Yeager et al. (2012) • Results: • Marsh facies and foram assemblages on downthrown side of fault suggest accelerated displacement • Marsh forams at surface indicate that vertical displacement is ongoing and out-paces sediment supply

13. results and discussion • Feagin et al. (2010), Morton et al. (2001), and Yeager et al. (2012) • Results: • RSET station monitoring over three-year period (2007-2010) • More sediment accumulation on down-dropped block due to increased accommodation space • These rates unable to keep pace with local RSL rise

14. results and discussion • Feagin et al. (2010), Morton et al. (2001), and Yeager et al. (2012) • Results: • Sediment geochronology (210Pb and 137Cs) show increased sediment mass accumulation rates on downthrown side • Again, vertical displacement outpaces sediment supply -> wetland submergence Matagorda, TX Pearl River, LA

15. results and discussion • Feagin et al. (2010), Morton et al. (2001), and Yeager et al. (2012) • Results: • Fault induced subsidence and subaqueous erosion on downthrown side of fault • Vertical accretion << rates of vertical displacement and RSL rise = open water conversion

16. conclusions • Accretion rates are typically higher on the downthrown side of the fault relative to the upthrown side • Vertical accretion rates in many coastal marshes of the northern Gulf of Mexico are not able to keep pace with rates of fault-driven subsidence • Permanent land cover changes in coastal marshes has been occurring and appears to be ongoing

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