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

摘要

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 in 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.