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

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

introduction Objectives 1.What do the sedimentary processes look like in response to active faulting in these settings? 2.Wetlands can maintain system equilibrium in this instance…but are they? 3.What happens when vertical accretion rates < subsidence rates?

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

geologic setting

methods Aerial photography

methods Sediment core lithostratigraphic correlation

methods

methods Rod Sediment Elevation Tables (RSET)

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

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 , total water area increase of 451.3% on downthrown, compared to 317% on upthrown

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

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

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

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

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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