High-Frequency Sequences in the Quaternary of Pelotas Basin (coastal plain): a record of degradational stacking as a function of longer-term base-level fall

Sequências deposicionais de alta frequência no Quaternário da Bacia de Pelotas (planície costeira): registro de empilhamento degradacional em função de queda do nível de base em longo período

ABSTRACT:

The aim of this work was to analyze the sedimentary record of the coastal plain of Pelotas Basin, placing it in the context of temporal and spatial scales, and proposing a stratigraphic hierarchical framework. The coastal plain, located in southern Brazil and in northern Uruguay, is mainly formed by four Quaternary barrier-lagoon systems. Three of these systems were studied through the integration of surface and subsurface data (geomorphological and topographical mapping, outcrops description, geoprocessing, dating and Ground Penetrating Radar - GPR - records) and interpreted as the preserved, onshore portion of depositional sequences mainly controlled by glacioeustatic cycles of about 100 kyr. The stacking pattern comprising these sequences is progradational with seaward downsteping (highest sea-levels measured: 9.5, 8 and 3 m, respectively), comprehending a degradational sequence set, or the regressive/falling stage systems tract of a higher order depositional sequence identified in seismic sections, with about 0.5 Ma. The youngest sequence has diachronous systems tracts - while some sectors have already transitioned from the transgressive to the highstand/falling stage systems tract, others are still under a transgressive context. It indicates that, in this time and scale, the sea level is not the main control of coastal evolution, and autogenic factors are fundamental in the run of geological record.

KEYWORDS:
Barrier-Lagoon; Glacioeustasy; Diachronism of Systems Tracts; Ground Penetrating Radar; Rio Grande do Sul coastal evolution