The modern mixed carbonate-siliciclastic Abrolhos Shelf; implications for a mixed depositional model

High-resolution seismic facies analysis and stratigraphy of a mixed carbonate-siliciclastic shelf yields insights into sedimentation controlled by interactions among carbonate production, siliciclastic sediment supply, and changes in relative sea level. High-resolution seismic, core, and surface samples collected along the northern area of the Abrolhos shelf (Brazil), including the inner, middle, and outer shelf, constrain the genesis of the shelf. Two sets of seismic sequences were identified: a Pleistocene sequence and Pleistocene-Holocene sequence. A subaerial unconformity from subaerial exposure during the last glacial maximum separates the two sequences. The upper seismic sequence, the focus of this study, comprises retrograding-aggrading (transgressive) and aggrading-prograding (regressive) seismic geometries. Carbonate sedimentation predominated within the last postglacial transgressive phase. The subsequent regressive phase was dominated by nearshore siliciclastic sedimentation at shallow depths (up to approximately 12 m). Below 12 m depth, a transition occurs to mixed siliciclastic-carbonate facies, and ultimately, carbonate sedimentation farther offshore. Seismic data indicate that Holocene nearshore coral reef growth was influenced by the positive antecedent relief of a Pleistocene reef platform and terrigenous sedimentation. Although siliciclastic-carbonate mixed sedimentation has been reported in other successions during periods of regression, siliciclastic sedimentation in northern Abrolhos shelf is limited to nearshore settings rather than bypassing the shelf.