Arquitetura sísmica do sistema de canais de maré Galinhos-Guamaré (NE do Brasil) como evidência às variações do nível do mar Holocênico

Abstract

This study aims to reconstruct the seismic stratigraphy of a Holocene tropical tidal channel system through the interpretation of shallow seismic data associated with vibracores analyses. Therefore, it was necessary to recognize the chronostratigraphic limits, the termination patterns of the seismic reflectors, the analysis of the facies seismic and the external geometry of the packages. This shallow system, with a maximum depths around 10 m, is located on the north coast of the state of Rio Grande do Norte - Brazil, having developed from a system of spit-barrier islands (~ 5900 BP) on a coast dominated by tide, with semidiurnal mesotidal regime. 42km of high-resolution seismic data were collected, with an average spacing of 30m, using a Chirp seismic (0.5kHz-7kHz). The data were processed using the softwares ReflexWin and Petrel. The seismic analysis allowed the identification of five Holocene seismic facies (U01-U05) and five boundaries or strata surfaces (SF01-SF05). The basal unit (U01) was formed in an environment of moderate energy with thickness ranging from ~ 1.4 to 3.5 m. The U02 unit indicates a high energy environment, with an average thickness between ~ 1.4m to 3.5m. The U03 unit represents a moderate energy environment with an average thickness between ~ 0.35m to 2.45m. The shallower surface (SF05) is ~ 7 m below the current sea level. Unit U04 (1227 Cal year AP) and U05 are formed by tidal flat muds facies. The geometry of the units and their respective surfaces were examined in detail in this work using a series of isopach maps, designed to understand their development in time and pace. These results and interpretation indicate new insights: (1) the formation of paleochannels located in the current spit may have started before the middle Holocene; (2) there was a network of preexisting channels with a different configuration from the current ones formed by units U02 and U03, constituting the transgressive and highstand systems tracts (7500 – 5000 Cal yrs BP); (3) the closure of the paleochannels and the formation of the spit occurred during deposition; (4) the SF04 surface marks the transition from transgressive to regressive system tract (within the past 5000 Cal yrs BP); (5) the current configuration of the system has been active since the formation of the U03 unit, that is, since the closure of the paleochannels; and (6) the Galinhos region shows the progressive sedimentary infilling of a lagoon behind a coastal barrier via sediment accumulation from marine sources and its ultimate transformation into a coastal swamp with tidal channels.