Holocene Submarine Bayhead Delta Deposits - an example from the Southern Brazil

Shallow geophysical investigations in the upper reaches of North Bay, Florianópolis reveal four seismic facies. These are interpreted as crystalline basement, Quaternary sediments, a bayhead delta and Holocene lagoon/embayment sediments, respectively. The 15 mthick Bayhead delta unit was deposited on top of a flooded, undulating land surface, suggesting rapid flooding during sea level rise. This created accommodation space for accumulation of bayhead delta sediments. The Bayhead delta unit contains abundant seaward-dipping foresets and small delta-to, channel-fill sequences. The upper surface is planar suggesting tidal ravinement during subsequent sea level rise to the late Holocene highstand. The upper surface is overlain by up to 10 m of fine-grained sediments typical of tranquil water deposition, but close to river mouths, a more proximal unit contains bedded gravel and sand, representing contemporary fluvial deposition. The present absence of bayhead delta sediments is attributed to lack of accommodation space.