The Modern Mixed Carbonate-Siliciclastic Abrolhos Shelf: Implications For A Mixed Depositional Model

Abstract

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 retrogradingaggrading (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 , 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 siliciclasticcarbonate 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.

Seabed Morphology and Sedimentary Regimes defining Fishing Grounds along the Eastern Brazilian Shelf

“…The shelf width varies from 50 to 200 km. Its widest part is known as the Abrolhos Shelf or the Abrolhos Bank [8][9][10]. To the south of the Abrolhos Bank, the shelf width averages 50 km and the shelf break occurs at 60 to 80 m water depth.…”

Late Pleistocene evolution of the mixed siliciclastic and carbonate southwestern New Caledonia continental shelf/lagoon

“…This scenario has been observed in Mayotte (Zinke et al, 2001), along the northern Brazilian Abrolhos shelf (D'agostini et al, 2015), and for the Great Barrier Reef (Larcombe and Carter, 1998;Francis et al, 2007;Dunbar and Dickens, 2003). As mentioned by D'Agostini et al (2015), this paradox shows that the response of mixed tropical siliciclastic/carbonate systems to relative sea level changes varies considerably, and they point out that the evolution of such mixed shelves and the relative roles of lagoon backfilling and paleo-drainage systems with regard to sediment transfer needs to be more documented and constrained. The Upper Pleistocene evolution of the New Caledonia barrier reef-lagoon complex offers the opportunity to complete this approach.…”

“…In theAtlantic, previous studies of the Brazilian Abrolhos shelf in Brazil (D'Agostini et al, 2015) and the Mayotte lagoon in the SW Indian Ocean (Zinke et al, 2001(Zinke et al, , 2003a(Zinke et al, , 2003b platform and terrigenous sedimentation. This point is also verified for the Belize lagoon where a feedback mechanism between tectonics, reefal buildups, and incised-valley location controls the mixed carbonate-siliciclastic shelf (Choi and Homes, 1982;Esker et al, 1998;Purdy et al, 1998;McNeill et al, 2010;Gischler et al, 2010, Gischler, 2015.…”

“…The lithofacies succession of such systems and the related continental print are strongly controlled by amplitude, rate of sea-level change, clastic inputs, sediment transfer, erosion, and oceanographic conditions (Van Siclen, 1958;Schlager et al, 1994;Catuneanu et al, 2011;Droxler and Jorry, 2013). Nevertheless, compared to pure siliciclastic or carbonate systems, the evolution of sediment transfer and depositional models along shelf-margin mixed systems remains poorly documented and represents challenging in terms of understanding the cyclicity of reciprocal sedimentation (carbonate vs siliclastic) and the detailed sedimentary response of such shelves to glacialinterglacial sea-level cycles (Wilson, 1967;Mount, 1984;Doyle and Roberts, 1988;Schlager et al, 1994;Francis et al, 2007;Droxler and Jorry, 2013;D'Agostini et al, 2015).…”

Sedimentary microfacies and organic geochemical characterization of mudstones in the Keluke Formation in northeastern Qaidam, China

“…Our analysis of the Keluke Formation argues for a carbonate ramp of epeiric sea above storm wave‐base, for the featured mixed characterization and no verified sedimentation of carbonate platform edge and patch reef development during this period. The mixed sedimentation in this setting is controlled by the carbonate production, siliciclastic sediment supply, and changes in relative sea level (D'agostini, Bastos, & Reis, ). This can be indicated by the frequent rhythmicity between carbonates and siliciclastic sediments (Figure ) and the storm‐generated punctuation between the offshore carbonates and mudstone (Figure a).…”