Full document / COSOC-W-86-002 - the National Sea Grant Library
Full document / COSOC-W-86-002 - the National Sea Grant Library Full document / COSOC-W-86-002 - the National Sea Grant Library
424 Estuarine andCoastal Management • Toots of the Abstract only Trade. Proceedingsof theTenthNationalConference of TheCoastalSociety. October1215.1986. New Orleans.LA. Copyrightby TheCoastalSociety 1987. GEOMORPHOLOCY AND SEDIMENTARY FACES OF AN EPHEMERAL WASHOVER BREACH/TIDAL INLET, CAMINADA-MOREAU HEADLAND, SOUTHEAST LOUISIANA JohnB. Wagnerand DavidL. Pope LouisianaGeological Survey Coastal Geology Program BoxG UniversityStation BatonRouge. LA 70803 This study examines the origin, development, geomorphic history and stratigraphyofand ephemeral washover breach/tidalinlet located0.381 km east of Bay Champagne on the Caminada- Moreau headland of Lafourche Parish, southeastern Louisiana. Strike and dip oriented stratigraphic cross-sections were constructed from eight (8) vibracores averaging 3.5 m indepth. High altitude aerial photographs of the study area,taking in 1960,1965,1973, and 1985, as well as oblique aerialphotographstaken priorto andjust afterHurricane Juanin 1985 were utilized to determine the relative ratesofbeach erosion and vertical accretionofsand by washover processes in the inlet Through macroscopiccore analysis,three(3) distinctsedimentaryfades were recognized. From bottom to top, these fades are:(1) Bay Fill;(2) Marsh;and (3) Washover. 1. The Bay Fill fades is characterized by alternating layersofthin sandsand silry sands with silly clays and clays. An increasein sand content and bed thickness is noticeable towards the top of the sequence, representing a gradual increase in energy and proximity to a source of sediment supply. These sediments appearto have been deposited as crevasse splays from the Bayou Lafourche distributary. Sedimentary structures associated with thisfaeies include parallel laminationsand cross-stratified sandsand silts with a varietyof climbing ripplesandtroughcrossbedding. Mica and thin layersoforganicdebrisarecommon alongbedding plane surfacesin this fades. 2. The Marsh fades is characterizedby clays with extensive plant (root) and animal burrow traces which increasetowardsthe top. Organic particles arealso incorporated into the clays, andoccuras both thin layersanddiscreteparticles within the clays. Some sand-filledroot burrows ore also seen, as well as some siderite bands and nodules. This faeies also contains some oyster shells at the top ofindividual units. 3. The Washover fades is characterized by thin fine-grained sand blankets of several superimposed washover deposits. Each washoverdeposit can be ascribed to severe storm or hurricane events, and contains a shell-rich layer with an erosive contact overlain by horizontallylaminated sands.
High altitude and oblique aerial photographsof the study area indicate that the inlet is active only during storms, winter cold fronts, and hurricanes when the beach is breached by waves. The photographs alsoconfirm thatthe inlet hasnot migratedlaterally (at least since 1960). This may in partbe due to the transgressive (retrogradational) natureofthe headlandin responseto erosion because of a lack of longshore sediment transport This theory is also supported by the beachprofiledatawhich indicatesverticalaccretion of sandin the inlet due to washoverdeposition. The inletrepresents a topographic low (less than 1 m above msl) compared to the surroundingbarriershoreline;it is easily recognized as a "gap" in the barrier,extending from the ocean to lagoon without interruption as a sandy, vegelationless low plain. The inlet thalwag or channel is slightly sinuous in nature, and as stated above, is only active during shore periods of time in response to storms. This inlet is thereforeclassified asephemeraldue to its limited role as an active inlet and its non-migratory nature. Additionally, its preservation potential is considered minimal due to rapidshorelineretreat, erosion,andreworkingofthe shoreface. 425
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High altitude and oblique aerial photographsof <strong>the</strong> study area indicate that <strong>the</strong> inlet is<br />
active only during storms, winter cold fronts, and hurricanes when <strong>the</strong> beach is breached by<br />
waves. The photographs alsoconfirm that<strong>the</strong> inlet hasnot migratedlaterally (at least since 1960).<br />
This may in partbe due to <strong>the</strong> transgressive (retrogradational) natureof<strong>the</strong> headlandin responseto<br />
erosion because of a lack of longshore sediment transport This <strong>the</strong>ory is also supported by <strong>the</strong><br />
beachprofiledatawhich indicatesverticalaccretion of sandin <strong>the</strong> inlet due to washoverdeposition.<br />
The inletrepresents a topographic low (less than 1 m above msl) compared to <strong>the</strong><br />
surroundingbarriershoreline;it is easily recognized as a "gap" in <strong>the</strong> barrier,extending from <strong>the</strong><br />
ocean to lagoon without interruption as a sandy, vegelationless low plain. The inlet thalwag or<br />
channel is slightly sinuous in nature, and as stated above, is only active during shore periods of<br />
time in response to storms. This inlet is <strong>the</strong>reforeclassified asephemeraldue to its limited role as<br />
an active inlet and its non-migratory nature. Additionally, its preservation potential is considered<br />
minimal due to rapidshorelineretreat, erosion,andreworkingof<strong>the</strong> shoreface.<br />
425