Eocene resedimented limestone deposits from the Osa Peninsula, Costa Rica: slope-apron accumulation in a volcanic forearc environment

Long-term non-erosive nature of the south Costa Rican margin supported by arc-derived sediments accreted in the Osa Mélange

“…This demonstrates that the volcaniclastic sequences in the NW Osa Mélange cannot represent significant volumes of ocean island debris formed in an intra-oceanic setting (Model C in Fig.3, Vannucchi et al, 2006;Clarke et al, 2018). This result is also consistent with the origin of Lithofacies 3 in the San Pedrillo Unit, which is composed of breccia of Middle to Upper Eocene shallow-marine limestone with petrographic and palaeontological evidence for reworking from the shelf of the volcanic arc (Buchs and Baumgartner, 2007;Bolz and Calvo, 2019).…”

“…The age of the mélange's matrix is constrained by radiolarian assemblages to the Late Palaeocene-Mid Eocene, whereas blocks of siliceous and carbonate pelagic sediments range in age between the Late Cretaceous and Mid Eocene (Buchs et al, 2009 and references therein). Clasts of shallow-marine limestone include larger foraminifera of Mid to Late Eocene ages (Buchs et al, 2009;Bolz and Calvo, 2019). These biostratigraphic constraints ascribe the age of formation of the San Pedrillo Unit to the Late Eocene.…”

“…Similarly, the composition of oceanic plateau basalts in the inner OIC is sufficiently distinctive (e.g., flat patterns in normalised multielement diagrams, Buchs et al, 2016) to determine whether the Osa Mélange includes material reworked from this unit. Because the inner OIC is separated from the Osa Mélange by the outer OIC, recognising oceanic plateau material in the NW Osa Mélange would support the p. 5 occurrence of sedimentary material that sourced in the margin (Models A and D; Buchs et al, 2009;Bolz and Calvo, 2019).…”

“…The Cabo Matapalo and Salsipuedes Units are predominantly composed of a greywacke matrix that includes blocks of Palaeocene to Middle Miocene pelagic limestone (Di Marco et al, 1995). The San Pedrillo Unit that is the focus of this study is composed of an assemblage of pelagic to hemipelagic sediments, including locally abundant igneous blocks (sometimes >10 m in size) and breccias of shallow-marine limestone (Di Marco et al, 1995;Buchs et al, 2009;Clarke et al, 2018;Bolz and Calvo, 2019). The age of the mélange's matrix is constrained by radiolarian assemblages to the Late Palaeocene-Mid Eocene, whereas blocks of siliceous and carbonate pelagic sediments range in age between the Late Cretaceous and Mid Eocene (Buchs et al, 2009 and references therein).…”

“…Most of the mélange is com-posed of silty-sandy hemipelagic sediments (55% of the mélange, Lithofacies 1 in Fig.2), with subordinate igneous breccia to mega-breccia (∼25% of the mélange, Lithofacies 2 in Fig.2), and minor breccia of shallow-marine limestone (∼20% of the mélange, Litho-facies 3 in Fig.2). Four main models have been proposed to ex-plain the formation of this lithological assemblage, as illustrated in Fig.3: (A) Accretion of trench-fill deposits predominantly derived from sedimentary erosion/gravitational collapse of the mar-gin (Di Marco et al, 1995;Buchs et al, 2009); (B) Dismemberment of the base of the outer margin and formation of a block-in-matrix (mélange) fabric through subduction erosion (Meschede et al, 1999); (C) Local, temporally restricted accretion of ocean island igneous to volcaniclastic material along a predominantly erosive margin (Vannucchi et al, 2006;Clarke et al, 2018); and (D) In situ deformation of forearc trench slope sediments controlled by fore-arc tectonics in the absence of accreted oceanic sequences (Bolz and Calvo, 2019). To test the validity of these models we synthesise previous regional geological constraints and study the geo-chemical affinity of igneous minerals (blocks, clinopyroxenes and amphiboles) in Lithofacies 1 and 2 to determine the provenance of the volcaniclastic fraction that is found in ∼80% of the NW Osa Mélange.…”