High Frequency Cyclical Isostatic Adjustments and Incised Valleys: Examples from the Gulf of Mexico
Geology and Geophysics, Louisiana State University, Baton Rouge, LA
Isostatic adjustment to changes in water and sediment loads are rarely considered in high-resolution stratigraphic interpretations. This presentation uses Gulf of Mexico examples to illustrate two possible high-frequency (Milankovitch-scale) cyclical isostatic adjustments that may be intrinsic to fluvial systems and incised valleys.
First, geophysicists recognize that isostatic adjustments must accompany sea-level change. It is also known that sea-level fall forces channel extension across emergent shelves, and the formation of incised valleys, whereas sea-level rise forces river mouth backstepping, flooding of the shelf, and valley filling. New 1D steady-state modeling of the Texas Gulf of Mexico shelf illustrates that isostatic uplift in response to sea-level fall will impact river long profiles, and may serve as the driving force for incision itself. With a shelf width of ~100 km, and sea-level fall of 100 m, isostatic uplift may be 20-30 m at the shelf margin, with flexural effects extending 10’s of km upstream from the highstand shoreline. Sea-level rise and flooding of the shelf will have the opposite effect.
Second, deltaic loading and subsidence are widely recognized, but development of incised valleys results in sediment unloading as well, which likely produces a cyclical pattern of isostatic uplift and subsidence. New 1D steady-state and 3D visco-elastic modeling of the Mississippi delta region indicates that sediment volumes removed and replaced were sufficient to induce >12 m of uplift in the valley center, and >9 m along valley margins, followed by subsidence of the same magnitude, with flexural effects extending ~150 km along the Gulf of Mexico coast. Cyclical uplift and subsidence should amplify valley incision and filling, whereas spatial patterns of uplift and subsidence should play a key role in development of valley fill architecture.
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