COMPARISON OF GLAUCONITIC UNITS OF THE NORTHERN AND SOUTHERN MORONDAVA BASIN, CRETACEOUS OF WESTERN MADAGASCAR

A diverse suite of glauconitic units from the Cretaceous of western Madagascar provides insight into the paleoenvironmental and diagenetic histories of the northern versus southern deposits of the Morondava Basin. Cretaceous greensands have been well-known and studied for decades, but there have been no detailed petrographic reports on glauconite from Madagascar, despite its well-known Cretaceous deposits and paleo-proximity to other glauconite-rich deposits.

Glauconite in the northern Morondava Basin exhibits a wide range of colors and morphologies. It commonly replaces pellets, fossils (e.g., echinoids), and micas, sometimes with partial replacement. Glauconite in a regionally extensive, fossiliferous concretionary sandstone is shown in our samples to be autochthonous in origin. It is abundant (~30%), has a range of maturities, and is associated with open-marine fossils, such as Globotruncana forams. The autochthonous glauconite is typical of the transgressive systems tract. Small amounts (<10%) of rounded and well-sorted glauconite in superjacent mudstones and evaporitic sandstones suggest an allochthonous origin. Additionally, some samples show evidence of recent alteration, potentially from heating due to wildfires.

Glauconite in the southern Morondava Basin has a limited range of colors and morphologies. It commonly replaces or partially replaces pellets, fossils (e.g., forams, echinoids, and bryozoans), micas, and calcite, and is often associated with chlorite. Glauconite is most abundant (~20%), exhibits a range of maturities, and is likely autochthonous in several fossiliferous sandstone units with ammonites and belemnites. The coarsest, most poorly sorted shell beds also have siderite or hematite cements, and their glauconite suggests an allochthonous origin. The greatest concentrations of glauconite are again typical of transgressive deposits. Small amounts (<10%) of immature glauconite in associated mudstones are likely mixed in origin.

While glauconite color and morphology do not seem to be directly related to water depth, this may be complicated by reworking of allochthonous glauconite. The greatest concentrations of glauconite are associated with the most fossiliferous and locally sequence stratigraphically significant surfaces.