THE INTERPLAY BETWEEN LATE QUATERNARY SEA-LEVEL CHANGES AND TECTONICS IN THE ASSU RIVER COSTAL PLAIN, NE BRAZIL: IMPLICATIONS FOR SEDIMENT DEPOSITION AND LANDFORMS

Analysis of geomorphology, borehole stratigraphy, and trench excavations west of the mouth of the Assu River, northeast Brazil, reveals a geologic framework shaped by the interplay of neotectonics and sedimentation. The Assu River occupies a low in the Potiguar Basin, a Mesozoic-Cenozoic extensional basin consisting of a series of SW-NE oriented half-grabens separated by basement highs. The west side of the Assu River coastal plain is bounded by the highlands of the Serra do Mel. Two fault systems cross the region of the mouth of the Assu River, the NW-SE-oriented Afonso Bezerra fault on the west side and the NE-SW-oriented Carnaubais fault, in the eastern portion. Structures related to these faults affected sedimentation in the coastal plain and geomorphology of the area.

Geomorphological evidence for neotectonic activity includes 1) development of alluvial fans on the margins of the Serra do Mel, 2) a scarp associated with the Afonso Bezerra fault that cuts alluvial fan deposits, and 3) sharp changes in direction in drainage patterns. Pronounced changes in stream orientation occur in the Serra do Mel near the margin of the Assu River plain where E-W-oriented stream trends are commonly interrupted by sharp bends that divert the streams into N-S-oriented valleys.

Sediment distribution in the Pleistocene and Holocene also reflect neotectonic activity, Miocene deposits of the Barreiras Formation are exposed in the highlands of the Serra do Mel. Alluvial fan deposits occur on the northeast margin the Serra do Mel and were dated as late Pleistocene age, ranging from 18.5 and 59.95 kyr BP by optically stimulated luminescence (OSL) dating. The main channel of the Assu River appears to have migrated eastward during the Holocene, likely as a response to uplift of the Serra do Mel along its western margin. At the base of the Serra do Mel, along the trace of Afonso Bezerra fault, a narrow belt of cemented sands and silts occurs which is interpreted as beachrock formed along a paleoshoreline. It has been dated as ca 5 kyr cal BP, probably associated with the maximum Holocene transgression. The modern shoreline is nearly 7 km NNE of this paleoshoreline marker, indicating rapid late Holocene in fill of the Assu River valley by fluvial plain and associated marginal-marine mangrove deposits and resulting rapid seaward advance of the shoreline.