FINE-GRAINED DEBRIS FLOWS AND EXTRAORDINARY VERTEBRATE BURIALS IN THE LATE CRETACEOUS OF MADAGASCAR

Vertebrate fossils are remarkably abundant and exceptionally well preserved within the Upper Cretaceous Maevarano Formation of northwestern Madagascar. The vast majority of these fossils, including all of the presently known bonebeds, are entombed within deposits of fine-grained cohesive debris flows in the Anembalemba Member. These debris flow deposits are typically massive, and are characterized by very poor sorting and a significant montmorillonite-dominated silt-clay (mud) fraction ranging from 17% to 46% by weight. Some of the more clay-rich beds are matrix supported. Soft sediment deformation features are relatively common, and reflect the metastable nature of the beds after deposition.

Deposition of debris flows is attributed to recurrent exceptional rainfall events that prompted erosion and flooded the ancient channel belts of the Anembalemba Member with sediment-laden slurries rich with clay and sand. Debris flow deposits were repeatedly intercalated with cross-bedded facies that accumulated via traction currents in dilute flows. The variable and distinct flow conditions that characterized the alluvial system of the Maevarano Formation are attributed to strong seasonality in the hydrologic cycle of this Late Cretaceous ecosystem.

The abundance of bone beds preserved within debris flow deposits, as exemplified by the multiple bone-bearing layers of quarry MAD93-18, is truly remarkable and affords novel insights into the nature of sedimentation events. Taphonomic and ichnologic attributes of bone concentrations suggest that associated debris flows had limited transport capability in relation to vertebrate bioclasts and that they generally entombed subaerially exposed assemblages of dead animals, and perhaps even the occasional live individual. The recurring association of bonebeds and debris flow deposits prompts the question: “Why did diverse assemblages of vertebrate skeletal debris accumulate again and again in and around the channel belt prior to the initiation of debris flows?” Arguably the best answer hinges upon a single overarching theme—seasonality, with prolonged dry spells and their attendant hardships prompting mortality, and subsequent rains setting debris flows in motion to capture accumulated death records.