STRATIGRAPHY OF SHELL BEDS FROM HIGH-SEDIMENT, HIGH-PRODUCTIVITY DEPOSITS OF THE LATE JURASSIC ILONA RIVER, MORONDAVA BASIN, WESTERN MADAGASCAR

Previously undocumented shell beds from Ilona River deposits, Morondava Basin, western Madagascar, record a key chapter in the island’s history during the time of its early isolation. This study investigates modes of accumulation, stratigraphic context, and paleoenvironments of shell beds in a unique, high-sediment and high-productivity setting. Field data and thin sections were used to assess changes in sedimentation, discontinuity associations (omission or erosion surfaces), diagenetic alteration, and taxa associated with each shell bed type.

1. During transgressions, deep-water shales accumulated with rare belemnites. These are capped by shell beds associated with iron-cemented, poorly-sorted sandstones, with a diverse assemblage of ammonites, belemnites, gastropods, pectinids, oysters, and phosphatized burrows, all commonly bored, fragmented, and associated with glauconitic minerals. The shell beds are 0.05-1.0 m thick with sharp, planar contacts, and are characterized by an interval of low sediment dilution producing a discrete layer of cemented fossils. They are associated with major discontinuities (maximum flooding zones), and are interpreted as accumulating during periods of sediment omission in low-energy, moderately to poorly oxygenated, deeper shelf deposits.
2. In contrast, regressive shell beds are associated with thick, coarse-grained, carbonate-cemented sandstone packages dominated by nested oyster shells and in situ pectinids, commonly encrusted by serpulids and bored. These shell beds are 0.5-1.2 m thick with sharp to deeply scoured bases, and are characterized by increasing sediment dilution, with rare shells in the surrounding beds. They are associated with diastems, and are interpreted as high-energy, well-oxygenated, nearshore deposits that recorded periodic major storms.

During intervals of maximum flooding, with accompanying nutrient influxes, protracted exposure on the seafloor resulted in condensation, bioclast fragmentation, multiple generations of bioturbation, and iron cementation. During regressive phases, increased clastic input and continued nutrient availability produced prograding packages of shelly sandstones; storms winnowed away clastics, concentrating shells and allowing oyster reefs to thrive until the next clastic influx.