Sedimentary Characteristics of an Estuarine Marsh System and the Roles of Storm Overwash, Micro-tidal Currents, and Organic Production: Big Branch Marsh, Louisiana
John Andrew Fleming
The goal of this project is to document sediment composition in Big Branch Marsh National Wildlife Refuge, located on the northern shore of Lake Pontchartrain in St. Tammany Parish, Louisiana, to discern the relative influences of fluvial, tidal, and storm overwash transport dynamics. The study area is adjacent to the mouth of Bayou Lacombe and is exemplary of other micro-tidal estuarine marsh systems common along the northern Gulf Coast. Sediment samples were collected from tidal channel beds, pond bottoms, and marsh surfaces, and were analyzed for organic content and particle-size distribution. Average organic content and particle-size parameters of each sub-environment group were compared to find differences that would represent changes in relative transport energy and depositional processes. The overall median particle size for the study area was 0.043 mm-average organic content was 26.0%. Bayou Lacombe bed-material samples yielded a median d50 of 0.051 mm, average sand fraction of 21.7%, clay fraction of 6.7%, and OM of 12.4%. Bed-material samples from tidal channels yielded a median d50 of 0.055 mm, average sand fraction of 19.2%, clay fraction of 7.4%, and OM of 17.6%. Pond bottom samples yielded a median d50 of 0.052 mm, average sand fraction of 23.3%, clay fraction of 12.0%, and OM of 33.3%. Overall marsh surface samples yielded a median d50 of 0.036 mm, average sand fraction of 11.5%, OM of 28.7%. Whereas some results adhered to the model of coarser sediments in relatively high-energy sub-environments, there were some anomalies that suggested that such a simple correlation cannot be used to predict the sediment distribution for the study area. These anomalies are explained by the combination of two different sediment supply mechanisms: (i) storm overwash from Lake Pontchartrain and (ii) daily micro-tidal currents. Relatively coarse-grained sediments are uniformly deposited in all sub-environments during overwash events. Subsequently, micro-tidal currents introduce relatively fine-grained sediments to the immediate Bayou Lacombe corridor and proximal marsh sub-environments but their affects diminish toward the distal areas of the system. Thus, coarse sediments deposited by storm overwash in the interior marsh are not effectively 'masked' by micro-tidal processes. These two depositional processes along with vertical accretion of organic matter are the mechanisms by which micro-tidal marshes are sustained. Sedimentary variability within the marsh system can be explained by position relative to micro-tidal channels. These results should inform coastal restoration efforts, notably activities to introduce additional sediment into marsh environments to offset relative sea-level rise and shoreline erosion.
AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013