A COMPLEX MARINE SUCCESSION REVEALED BY AUTHIGENIC MINERALS AND MICROFOSSILS IN DINOSAUR-BEARING DEPOSITS OF THE LATE CRETACEOUS OF THE MORONDAVA BASIN, MADAGASCAR

A field and petrographic study of mixed clastic and chemical sedimentary units in the Late Cretaceous (Turonian-Coniacian) of the Ampolipoly area, northern Morondava Basin, western Madagascar reveals a complex nonmarine to marine post-rift succession.

Turonian beds in the Ampolipoly area commence with a thick package of interbedded heterolithic clay-rich mudstones, volcanic ash, and channel sandstones. These were deposited largely in nonmarine settings colonized by plants, preserved as compression fossils, to marginal marine environments that incorporate glauconite. Overall, these represent a regressive phase in this area. The overlying unit is a distinctive, glauconite-rich concretionary sandstone that hosts dinosaur, ammonite, and other vertebrate and invertebrate fossils. Its sedimentary and paleontological features indicate that this unit represents a sediment-starved condensed section deposited during a transgression, with the maximum flooding surface hosting a faunally condensed assemblage dominated by unfragmented calcitic shells. Superjacent to the dinosaur-bearing strata, a succession of siderite-cemented coarse sandstones lacking fossils is capped by a carbonate concretionary horizon rich in well-preserved planktic foraminifera. These units have allochthonous glauconite, relict evaporites, and pyrite. Deposition of the nearshore sands under suboxic conditions was apparently followed by a Coniacian? marine incursion that deposited the microfossil-rich carbonates.

The upper units exposed in the Ampolipoly area continue through clay-rich mudstones containing benthic and planktic foraminifera, calcispheres, and small gastropods, along with allochthonous glauconite, indicative of open marine conditions. Finally, there is a rapid transition to immature sandstones with chamosite coated grains, relict anhydrite, gypsum replacing anhydrite, and rare algae. This potentially represents an influx of coarser sediment due to local tectonic factors, and a regression to a very shallow to evaporitic nearshore environment.

Overall, these units reveal a transition from continental to marine to evaporitic environments, rich in authigenic mineralization, a diverse fauna, and overprinted by multiple stages of diagenesis.