Terraces, reefs, and valleys along the Brazil northeast outer shelf: deglacial sea-level archives?

摘要

Morphological features on low-latitude continental shelves have recorded past sea level fluctuations. This study aims to recognize and interrogate, on the Rio Grande do Norte (RN, NE Brazil) outer shelves, specific seabed morphologies that could have been produced by the punctuated relative sea level rise of the last deglaciation. These sea floor morphologies, imaged by single-beam bathymetric data from two areas of the North and East outer Shelves, similar to 160 km apart, reveal terraces, submerged paleo-shorelines, reefs, and incised valleys, which act as regional archives of past sea levels. The North and East outer shelves, covered by carbonate sediments, display steep gradients (similar to 0.5 degrees), variable widths (up to 13 km), and are bound between a well-defined shelf break at 75 m water depth and a beachrock ridge at 25 m. A steep step on the sea floor occurs between depths of 60 and 70 m in both areas. Though interrupted by the Acu Incised Valley, two distinct continuous terraces, 3 and 4 km in width, at depths of 49 m (+/- 5 m) (T1n) and 33 m (+/- 3 m) (T2n), respectively, extend for 60 km along the North Shelf. Three nearly continuous terraces occur along the East Shelf on either the north or south sides of the Natal Canyon. A first, 6 km wide, lower terrace occurs at a depth of 54 m (+/- 4 m) (T1e). A second terrace, 2 km wide, at 40 m (+/- 2 m) (T2e), and a third 2 km wide upper terrace at 30 m (+/- 2 m) (T3e) are also found. Patch reefs scattered over the terraces rise in average to 3 m in height. Based upon their depth occurrences, the established limited neotectonics in the study areas, and correlations with well-established archives from other morphological features of similar outer shelves, we hypothesize that these reefs and terraces were likely formed during the last deglacial interval spanning from the end of the MWP-1A (70-60 m), through the MWP-1B (50-40 m), and to subsequent punctuated millennial sea level rise events (30-25 m) occurring prior to the 8.2 kyr cooling event.