NSF-GLD Update – July 12, 2013 Kevin Yeager Phil Wolfe University of Kentucky

sedimentary responses to growth faulting in a salt marsh: East Matagorda Peninsula, Texas NSF-GLD Update – July 12, 2013 Kevin Yeager Phil Wolfe University of Kentucky

Introduction/Background • This research is part of a multi-disciplinary effort to investigate the responses of coastal marshes to active growth faulting • Sedimentology (Kevin Yeager, UK), spatial analysis/ecology (Rusty Feagin, TAMU), bio-stratigraphy (Charlotte Brunner, USM) • Our part involves characterizing sedimentary responses to vertical displacement in these settings • 210Pb and 137Cs geochronology, 14C geochronology, litho-stratigraphy, physical sedimentology (grain size, POC, etc.)

Introduction/Background • Overall project hypotheses include the following: H1 Accretion in Spartinaalterniflorawetlands matches the amount of displacement on either side of growth faults, when not limited by sedimentation H2Spartinaalterniflorawetlands falter when the amount of displacement exceeds a given threshold, due to an accretionary limit H3 Increases in accretion rate are coupled with increases in the RSL rise rate, up to a given threshold

Introduction/Background • Our tools allow us to characterize fault behavior and sedimentary responses over the Holocene (<10,000 y) • Our objectives relative to the project hypotheses include: • Determine time-equivalent intervals (14C) to measure total fault offsets, and derive fault slip rates • Use short-lived radionuclides to establish chronology and sediment mass accumulation rates over last ~100 y • Compare these rates to long-term rates (14C)

Introduction/Background • Holocene sedimentary evolution Wilkinson and Basse, 1978

Introduction/Background • Holocene sedimentary evolution 6,000 y BP – gray estuarine mud 4,500 y BP – red-brown prodelta mud Wilkinson and Basse, 1978

Introduction/Background • Holocene sedimentary evolution 3,000 y BP – gray estuarine mud Present – shelly, subaerial sand Wilkinson and Basse, 1978

Lithostratigraphy Transect 3

Lithostratigraphy Observed lithostratigraphyconsistent with Wilkinson and Basse, 1978 interpretation of Matagorda Bay Holocene sedimentary evolution Transect 3

Lithostratigraphy/Radiocarbon Transect 1

Lithostratigraphy/Radiocarbon Dotted line: displacement and deformation based on basal contacts of subaerial sand Dashed line: displacement and deformation based on 2,500 y BP isochron(14C) Transect 1

Lithostratigraphy/Radiocarbon 0.36 mm/yr Transect 1

Lithostratigraphy/Radiocarbon Non-uniform slip: Distance along fault measured from park access road (east, 0), to location of transect three. Additional, time-bracketed slip rates will be added to these data. Relationship suggests a focus of slip to the southeast, and likely translate into complex “twist” as fault motion proceeds along plane of failure

Results (so far) Fault  Transect one Fault Transect two  *Rates calculated from uppermost 14C age at each station (typically between 1,500-2,500 y BP)

Results (so far) Transect three Fault Avg Avg

Results (so far) Upthrown Downthrown 14C-based SMARs over time: Data shown exclude stations that are either immediately distal to surface expression of fault, or have less than three 14C-based rates

Results (so far) Upthrown Downthrown Further refinement: NSF_07B excluded due to edge effects; NSF_10B excluded as it is likely beyond the effects of fault-driven displacement (elevation [0.44 m above NAVD 88] and distance from fault [93 m])

Results (so far) Wedge core 210Pbxs SMARs Supports that sediment supply is likely not a limiting factor Increasing rates = response to regional subsidence + eustatic sea level rise over last ~100 y A third wedge core will be added to this data set

Results (so far) 210Pbxs-derived SMARs vs Time Station farthest from fault trace (~50 m) Station closest to fault trace (~10 m) Brown plots represent upthrown stations Transect 1

Results (so far) 210Pbxs-derived SMARs vs Time Intermediate station (~31 m) Station closest to fault trace (~14 m) Blue plots represent downthrown stations Transect 1

Results (so far) 210Pbxs-derived SMARs vs Time Station farthest from fault trace (~77 m) Intermediate station (~32 m) Brown plots represent upthrown stations Transect 2 Station closest to fault trace (~5 m)

Results (so far) Fault Mean rates are higher on upthrown side. This is interpreted as an “edge effect”, where geomorphic change driven by faulting has resulted in marsh platform becoming marsh edge = enhanced erosion

Results (so far) Fault These upthrown side rates are all below measured rates of regional RSL rise (0.47 cm/y in Rockport, TX; 0.91 cm/y in Galveston, TX; psmsl.org)

Results (so far) Fault Note:Unit with largest difference in thickness across the fault (sub-aerial sand) is the 2nd youngest (after modern marsh [muddy fine sand with grass fibers]), suggesting likely enhanced fault motion syndepositionally

Results (so far) Fault Note:Lack of obvious differences in unit thickness across the fault suggest later fault initiation and/or a lower rate of slip (which we know from 14C)

Results (so far) Fault Note:As with transect one, the unit with largest difference in thickness across the fault (sub-aerial sand) is the 2nd youngest, suggesting enhancedfault motion syndepositionally

Results (so far) Mean interval thicknesses on upthrown and downthrown sides. Faciesabove the shell hash unit exhibit slightly greater thicknesses on the downthrown extent

SER2L lab analyses Transect One Transect one analyses are complete!

SER2L lab analyses Transect Two Red – In queue Black - Completed

SER2L lab analyses Transect Three Completion of these cores will take a while longer...

Conclusions…to date • Observed lithostratigraphy is consistent with Wilkinson and Basse (1978) interpretation of Matagorda Bay Holocene sedimentary evolution • Using calibrated 14C ages, minimum fault slip rates of 0.36, 0.24, and 0.14 mm/y since ~2500 yrs BP have been determined for transects one, two, and three, respectively • Calculated long-term SMARs (14C-derived) are consistent with late Holocene sea-level models

Conclusions…to date H1 Accretion in Spartina alterniflora wetlands matches the amount of displacement on either side of growth faults, when not limited by sedimentation • Based on the observed lithology, the facies above the shell hash exhibit slightly greater thicknesses on the downthrown extent • Observed temporary “pulses” in SMARs between 30 and 80 y BP could represent sedimentological responses to acceleration of fault movement during these periods

Conclusions…to date H2Spartina alterniflora wetlands falter when the amount of displacement exceeds a given threshold, due to an accretionarylimit • However, overall 137Cs and 210Pbxs derived SMARs show decreased short-term (<100 y) rates on the downthrown stations at transect one • This might suggest that fault-induced increased accommodation space is not the major influence of SMARs here but rather, the creation of an erosional marsh “edge effect”

Conclusions…to date H3 Increases in accretion rate are coupled with increases in the RSL rise rate, up to a given threshold • Wedge-core 210Pbxs SMARs suggest accelerated regional subsidence coupled with eustatic sea level rise and subsequent increased rates of sedimentation occurring over the past ~100 years

NSF-GLD Update – July 12, 2013 Kevin Yeager Phil Wolfe University of Kentucky