Harry F. L. Williams

Stratigraphy, Sedimentology, and Microfossil Content of Hurricane Rita Storm Surge Deposits in Southwest Louisiana

Abstract Hurricane Rita generated a 4–5-m-high storm surge along the southwest Louisiana coast. The storm surge and accompanying waves transported sand and mud into woodland and freshwater marsh environments located immediately landward of sandy beaches. The resulting storm surge deposit is up to 0.5 m thick and extends at least 500 m inland. Analysis of the stratigraphy, sedimentology, and microfossil content of the deposit suggests two distinct phases of deposition: a thin layer of finer sand and mud and an overlying thicker layer of coarser sand. The thin layer of finer sand and mud is characterized by planar laminae and calcareous foraminifera, characteristic of shallow marine environments. The layer drapes preexisting surfaces, extends at least 500 m inland, and abruptly overlies woodland and marsh soils. These findings suggest deposition from suspension of offshore sand and mud in an early stage of storm surge inundation. This layer is overlain by up to 0.5 m of coarser sand with prominent foreset laminae. Foraminiferal tests are rare in this part of the deposit. The coarser sand layer has an abrupt termination 100–150 m inland. These findings suggest the sand is a traction load deposit, formed at a later stage of storm surge inundation. The storm surge deposit has a distinctive geologic signature in the study area, raising the possibility of extending paleotempestology studies to this and other similar nearshore environments. The more distal inland part of the deposit has characteristics commonly associated with tsunami-laid sand, suggesting that in studies of coastlines vulnerable to extreme storms and tsunamis, and where the sedimentary evidence is fragmentary, the geologic records of storms and tsunamis may not be easily differentiated.

Foraminiferal zonations on the Fraser River delta and their application to paleoenvironmental interpretations

Abstract Well-defined elevational zonations of foraminiferal species were found over the intertidal surface of the sand-rich Fraser River delta. A zonation of foraminiferal species according to depth is also apparent on the delta foreslope, although data here are more limited. Application of these findings to the interpretation of depositional environments within drill-cores from the delta provided independent evidence of a 4 to 5 m lower sea-level during the mid-Holocene. This finding is in good agreement with a sea-level curve developed for the Fraser Lowland region.

Assessing the impact of weir construction on recent sedimentation using cesium-137

Ten cores were obtained from a marsh developed along Mad Island Slough, Texas, USA, upstream of a weir constructed in 1948. The cores were analyzed for cesium-137 to identify time-stratigraphic marker horizons and calculate recent sedimentation rates. The cesium-137 analysis provided a 1954 marker horizon in nine of the ten cores. A second marker horizon, present in all ten cores, consisted of an abrupt downcore change in lithology from dark organic-rich muds to grey organic-poor sands. This transition was tentatively identified as coinciding with 1948 and the beginning of marsh sedimentation. Resulting sedimentation rates show that surprisingly little sedimentation has occurred behind the weir, averaging only 27 cm in almost 50 years. Sedimentation rates in the marsh declined from an average of 2.4 cm yr−1 in 1948–1954 to 0.32 cm yr−1 in 1954–1994. A similar trend of declining sedimentation has been documented for adjoining Mad Island Lake, suggesting that land-use changes in the lakes watershed have reduced the sediment supply in recent decades. The results also suggest that the weir is not a very efficient sediment trap in this watershed.

Palynology of Holocene top-set aggradational sediments of the Fraser River Delta, British Columbia

Abstract Palynological study of radiocarbon-aged top-set Holocene sediments of the Fraser Delta, British Columbia, reveals a record of tidal marsh, river marsh and river swamp environments which indicate former sea-level positions and rates of sea-level change. Interbedded silts and fine sands, containing Pinus and Cyperaceae pollen and monolete fern spores, occur between 15.1 and 14.2 m depth and are interpreted as freshwater-dominated delta-front marsh deposits. Organic-rich silts, containing grass and skunk cabbage pollen and horsetail spores, between 14.2–7.8 m depth, represent an emergent delta-top marsh environment, regularly flooded by the Fraser River. Arboreal pollen types pine, spruce and alder dominate organic-rich silts between 7.8 and 5.8 m, but Rosaceae pollen rises to dominance in peatier sediments between 4.7–5.8 m. These two zones record natural vegetation succession from marsh to swamp, due to increased relative elevation of the site. An increase in arboreal pollen in organic-rich silts between 4.7 and 2.3 m, signals a drop in relative elevation and return to regular flooding. Declines in percentages of arboreal pollen and increases in Rosaceae and skunk cabbage pollen, indicate preliminary stages of bog development and disappearance of fluvial influence above 2.3 m. Organic-rich silts formed mainly in a fluvially-dominated marsh setting, as top-set aggradation kept pace with sea-level rise between approximately 7960 and 4410 yr B.P. An apparent slowing or cessation in sea-level rise allowed peaty sediments to form around 6000 yr B.P. The rate of sea-level rise again declined at 4410 yr B.P., causing organic-rich silts to be replaced by peat deposition. The study provides insights into the history of sea-level change and aggradation in the Fraser River Delta.

Contribution of Hurricane Ike storm surge sedimentation to long- term aggradation of Southeastern Texas coastal marshes

ABSTRACT Williams, H.F.L. and Denlinger, E. Contribution of Hurricane Ike storm surge sedimentation to long-term aggradation of Southeastern Texas coastal marshes Hurricane storm surges transport littoral sediment inland and may represent an important mechanism for long-term aggradation of coastal marshes. Hurricane Ike made landfall near Galveston, Texas, on September 13th 2008, generating a large storm surge that flooded 100 km of coastal marshes in southeastern Texas between Galveston Bay and the Louisiana border at Sabine Pass. The objective of the study was to measure the contribution of the hurricane-derived sediment input to long-term marsh aggradation. A field survey of the storm surge deposit in southeastern Texas coastal marshes was conducted in January 2009 and repeated in January 2010. The results show that the deposit is well-preserved and forms a landward-tapering wedge of sandy to muddy sediments that extends hundreds to thousands of meters inland. Cesium-137 dating was used to estimate decadal-scale sedimentation rates at seven coastal marsh sites positioned along two transects beginning near the coastline and extending inland, aligned with the storm surge direction. Long-term sedimentation rates, including the sediment input from Hurricane Ike, vary from about 0.5 to 1.6 cm/year and decline with distance inland along both transects. The storm surge deposit of Hurricane Ike contributed between 42 % and 73 % of overall sedimentation at these sites in the 58-year period 1950–2008. The study findings suggest that hurricane storm surges may be the predominant mechanism for long-term aggradation of coastal marshes in the study area.

Assessing the Effectiveness of Coastal Foredune Barriers in Reducing Hurricane Washover Sedimentation

ABSTRACT Williams, H., 2018. Assessing the effectiveness of coastal foredune barriers in reducing hurricane washover sedimentation. This study tests the effectiveness of physical barriers in blocking or reducing washover sedimentation in coastal marshes. It is widely assumed that natural and artificial physical barriers, such as foredunes and roads, block or reduce storm overwash. However, there appears to be little documented assessment of this effect. This topic has gained significance recently because of concerns over sea-level rise induced by global warming creating a larger potential threat from storm overwash. Nearshore physical barriers act to protect populations and infrastructure farther inland from storm surge, but they may also reduce sediment delivery to coastal marshlands, slowing their vertical accretion and contributing to their submergence by rising sea level. Consequently, there is a critical need for more information on the effect of physical barriers on marsh sedimentation. Washover sediment beds were examined along two transects across marshes in McFaddin National Wildlife Refuge in southeastern Texas, one in the lee of a higher and wider foredune barrier and one where foredunes are lower and narrower. Washover sediment beds deposited by Hurricanes Rita (2005) and Ike (2008) were found on both transects. Comparison of washover sedimentation along the two transects showed an average 40% reduction in sediment volume and an average reduction in inland extent of deposits of 505 m in the lee of the higher and wider barrier. These results show that even relatively subtle increases in barrier dimensions can cause substantial reductions in the magnitude of washover sedimentation in marshes. These findings have implications for paleotempestology studies that use the magnitude of washover sedimentation in the lee of dynamic coastal barriers as geologic proxies of storm intensity. Coastal management agencies contemplating the use of physical barriers to block storm surge should take into consideration the impact of such barriers on the sustainability of coastal marshes faced with rising sea level.