Influence of low amplitude/high frequency relative sea-level changes in a wave-dominated estuary (Miocene), São Luis Basin, northern Brazil

Abstract

Miocene deposits exceptionally well exposed along several coastal cliffs between the towns of Alcântara and Guimarães, Maranhão State, northern Brazil, record the history of low-amplitude/high-frequency sea-level changes in a wave-dominated incised valley estuarine setting. These deposits are interpreted as estuarine in origin, based on: (i) depauparated ichnological assemblage and occurrence of Gyrolithes, both typical of highly stressed brackish water conditions; (ii) dominance of sedimentary structures diagnostic of tidal processes (e.g. alternating thicker/thinner sand bundles marked by reactivation surfaces and/or mud drapes); and (iii) presence of widespread channel deposits. This estuarine fill consists of three stratigraphic units (Units 1–3): (1) Unit 1—shoaling shoreline (SH) and tidal channel (CH); (2) Unit 2—shoaling shoreline (SH), tidal channel (CH) facies associations, lagoon (LG), and flood–tidal delta/washover (FTD/W) facies associations; and (3) Unit 3—tidal channel (CH), estuarine bay (EB), and fluvial-influenced channel (FCH) facies associations. The individual units internally show a dominantly prograding character, a pattern also reflected throughout the succession by the overall upward stacking of units showing successively more restricted conditions. These characteristics, combined with the position in the uppermost portion of the valley fill succession, led to suggest that the deposits exposed in the study area likely formed during the transition from rising to highstand stages of relative sea level. Mapping of regionally significant discontinuity surfaces led to the recognition of a sequence boundary (i.e. discontinuity surface DS1) marked by erosion and subaerial exposure at the valley floor. This surface separates the Miocene succession from underlying Cretaceous rocks, and it is attributed to a major period of incision that led to the genesis of the incised valley during relative sea level fall. Unit 1 onlaps against DS1 and is bounded above by discontinuity surface DS2, which probably formed by tidal ravinement during a period of transgressive erosion. This ravinement surface is the boundary between two shoaling up successions (i.e. Units 1 and 2) interpreted as parasequences from a same depositional sequence. Discontinuity surface DS3 between Units 2 and 3 is attributed to a sequence boundary marked by significant erosion and subaerial exposure, and is interpreted as a reflex of a renewed period of incision within the estuary due to a strong drop in relative sea level. The top of Unit 3 is defined by another discontinuity surface (i.e. DS4) with pronounced erosion and subaerial exposure, which is attributed to a sequence boundary that ended the estuarine deposition in the study area. Thus, Unit 3, defined at the base and top by unconformities, is a complete depositional sequence resulting from a low amplitude/high frequency relative sea level cycle superposed upon the overall highstand stages of the estuary evolution.