Characterizing Sedimentary Responses to Coastal Faulting Using High-Resolution Geochronology and Sedimentology: East Matagorda Peninsula, Texas

The structural framework of the northern Gulf of Mexico coastal zone is characterized by numerous growth fault systems. Neotectonic processes in coastal marshes in this region have been shown to be important drivers of relative sea-level rise as well as having significant influence on marsh accretion processes. An apparent historical acceleration of movement along some of these coastal faults is believed to be largely a result of the regional onset and intensification of subsurface fluid withdrawal from the 1930's to the present. One active growth fault breached the surface of East Matagorda Peninsula, Texas as early as the 1960's and displacement there is ongoing, leading to significant wetland losses over the past several decades. To characterize the Holocene behavior of this fault and the consequent sedimentary responses, a suite of fallout radionuclides (⁷Be, ¹³⁷Cs, ²¹⁰Pb) and radiocarbon (¹⁴C), supplemented by sedimentological data have been used to determine sediment mixing depths, rates of sediment accumulation, and sediment geochronology. These tools allow for testing of the hypothesis that the fault at Matagorda has been recently reactivated, leading to surficial deformation and alteration of sediment accumulation processes, particularly on the downthrown side of the fault. Correlation of time-equivalent stratigraphic boundaries reveals a maximum total Holocene fault offset of ~1 meter. Determination of fault slip rates from these values reveals a linear trend of displacement as a function of distance along the fault trace with maximum slip occurring to the southwest (seaward) and minimum slip to the northeast. Mean fallout radionuclide-derived sediment accumulation rates for the past ~100 years are relatively uniform across the fault trace. However, rates from the downthrown station nearest to the fault trace display a dramatic increase over the last 30 years. This increase is likely a response to fault-induced increased accommodation space on the downthrown side of the fault, and is another line of evidence supporting recent acceleration of fault movement. Sediment bulk density and grain size data suggest an interaction between fault-driven geomorphic change and sedimentation where a migrating land-water interface (marsh margin) has influenced the type of sediment accumulation here.