Wednesday, September 06, 2006
Rapidly-deposited fine-grained mudflows
Fine-grained sediments are usually interpreted as accumulating slowly in tranquil environments. However this report of extensive mudflows in the Upper Cretaceous of Madagascar, bearing abundant and well-preserved fossil vertebrates, reminds us that fine-grained sediments can be rapidly deposited.
Rogers R. R., ‘Fine-grained debris flows and extraordinary vertebrate burials in the Late Cretaceous of Madagascar’, Geology 2005;33(4):297-300.
Abstract. 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 currently known bone beds, are entombed within deposits of fine-grained cohesive debris flows. These 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. Deposition is attributed to recurrent exceptional rainfall events that prompted erosion and flooded ancient channel belts with sediment-laden flows. These extraordinary burial events shielded vertebrate remains from destructive surface processes and also afforded protection for soft tissues. Taphonomic attributes of associated bone concentrations suggest that debris flows had limited transport potential and generally entombed subaerially exposed bone assemblages. The remarkable and recurrent association of bone beds and debris-flow deposits likely reflects marked seasonality in this Late Cretaceous terrestrial ecosystem, with prolonged dry spells prompting mortality and subsequent rains setting debris flows in motion.
Rogers R. R., ‘Fine-grained debris flows and extraordinary vertebrate burials in the Late Cretaceous of Madagascar’, Geology 2005;33(4):297-300.
Abstract. 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 currently known bone beds, are entombed within deposits of fine-grained cohesive debris flows. These 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. Deposition is attributed to recurrent exceptional rainfall events that prompted erosion and flooded ancient channel belts with sediment-laden flows. These extraordinary burial events shielded vertebrate remains from destructive surface processes and also afforded protection for soft tissues. Taphonomic attributes of associated bone concentrations suggest that debris flows had limited transport potential and generally entombed subaerially exposed bone assemblages. The remarkable and recurrent association of bone beds and debris-flow deposits likely reflects marked seasonality in this Late Cretaceous terrestrial ecosystem, with prolonged dry spells prompting mortality and subsequent rains setting debris flows in motion.
Monday, September 04, 2006
Dinosaur trackways in marine limestone
This report of dinosaur trackways in a marine limestone will be of interest to those developing catastrophist models of Earth history. The two reported track sites are located near the village of Coisia in the French Jura. Both exposures are subvertical bedding planes showing sauropod footprints impressed in a Jurassic (Tithonian) limestone referred to the 'Couches du Chailley' Formation. The Couches du Chailley are bioturbated by Thalassinoides burrows and have yielded ammonites, gastropods, bivalves, algae and foraminifers. Conventionally they represent a subtidal environment such as a lagoon separated from the open sea by a coral reef. The dinosaur footprints are referred to the ichnogenus Parabrontopodus, attributed to sauropods or diplodocoids.These animals evidently left the tracks on a surface temporarily exposed between marine incursions.
Le Loeuff J., Gourrat C., Landry P., Hautier L., Liard R., Souillat C., Buffetaut E., Enay R., ‘A Late Jurassic sauropod tracksite from Southern Jura (France)’, Comptes Rendus Palevol 2006;5(5):705-709.
Abstract. The discovery of sauropod trackways in the Late Jurassic (Tithonian) of the Jura department (eastern France) is reported. More than 170 footprints (pes and manus prints) comprise at least nine trackways. The footprints are referred to the ichnogenus Parabrontopodus, characterized by a narrow-gauge trackway. The locality of Coisia is the most important sauropod tracksite in France.
Le Loeuff J., Gourrat C., Landry P., Hautier L., Liard R., Souillat C., Buffetaut E., Enay R., ‘A Late Jurassic sauropod tracksite from Southern Jura (France)’, Comptes Rendus Palevol 2006;5(5):705-709.
Abstract. The discovery of sauropod trackways in the Late Jurassic (Tithonian) of the Jura department (eastern France) is reported. More than 170 footprints (pes and manus prints) comprise at least nine trackways. The footprints are referred to the ichnogenus Parabrontopodus, characterized by a narrow-gauge trackway. The locality of Coisia is the most important sauropod tracksite in France.
