Macroids from mixed siliciclastic-carbonate high-frequency sequences of the Agrio Formation (Lower Cretaceous), Neuquén Basin: Palaeoecological and palaeoenvironmental constraints

Macroids, cm-sized biogenically coated grains made of associations metazoans or protozoans, accumulate in some high-frequency sedimentary sequences the upper Hauterivian to lower Barremian Agua de la Mula Member Agrio Formation Neuquén Basin (northern Patagonia, Argentina). The macroids studied are sub-spheroidal and sub-discoidal shape, 2–3 cm diameter 1–2 height. Each macroid is characterised by a rough, weakly bored surface, encrusted oysters, serpulids, small, dome-shaped bryozoan colonies. growth arrangement consists locally concentric, agglutinated encrusting foraminifers growing above around skeletal nuclei. accumulations on top starvation hemi-sequences during latest 3rd-order transgressive systems tract help understanding palaeoenvironmental conditions at onset regressions, low but persistent sedimentation rates generally low-energy environment. Mineralisation sand-sized glauconite apatite suggests ephemeral periods reduced non-deposition. Pyrite framboid size-distribution indicates dysoxic zone with oxygen restrictions. re-establishment terrigenous sediment input following transgression promoted eutrophication water column. Under such conditions, benthic life was represented mainly species able thrive eutrophicated, oxygen-poor waters sediment-water interface. This setting excludes activity borers formation macroids. Macroids result pivotal palaeoecology inherent communities refining interpretation parameters mixed siliciclastic-carbonate ramp systems.