The cohesive Naranjo debris-flow deposit (10 km 3):
Mass movement processes on volcanic terrains such as landslides and debris avalanches can cause the obstruction of main drainages producing the formation of temporary dams. A good example of this occurred 18.5 ka ago when the eastern flank of the Nevado de Colima Volcano collapsed producing a debris-avalanche deposit that was previously described as one of the largest in the world. The deposit extended from the volcanic summit as far as the Pacific coast, 120 km away. New stratigraphic, sedimentological, and componentry data suggest that the volcanic collapse of Nevado de Colima resulted in a debris avalanche that traveled 20 km southeast to the Naranjo River. There it crashed against a topographic barrier consisting of Cretaceous limestones (Cerro la Carbonera) and the flow direction was diverted to the south down the Naranjo River channel for another 25 km before the avalanche came to a halt. The obstruction of the drainage produced a temporary dam that stored ca. 1 km3 of water and deposited fluvial and slack-water sediments. Some time after the damming, the accumulated water-sediment load was able to overtop the obstructing material and to release a breakout flow with a calculated initial flow discharge of 3.5 million m3/s. The resulting flood (cohesive debris flow) followed the channel of the Naranjo River and, due to the high erodibility of the channel and introduction of substrate material, the debris flow progressively increased its volume up to 10 km3, six times its initial volume. This study highlights the relevance of evaluating the potential remobilization of debris-avalanche deposits to initiate large magnitude cohesive debris flows. Therefore, the hazard and risk analysis of future potential events of this nature must consider the pre-eruption conditions and the topography surrounding a volcano. 7 2002 Elsevier Science B.V. All rights reserved.
TAPHONOMY OF A JUVENILE LAMBEOSAURINE BONEBED FROM THE TWO MEDICINE FORMATION (CAMPANIAN) OF MONTANA, UNITED STATES
Abstract The Sun River Bonebed is a monodominant assemblage of late juvenile lambeosaurine elements from the Upper Cretaceous Two Medicine Formation of north-central Montana, United States. Detailed excavation revealed an unusual paleobiologic and depositional signature. Although the bonebed occurs in a succession of beds representing anastomosing stream deposits in a seasonal paleoenvironment, the assemblage consists of a conglomerate of bone and calcareous clasts in a matrix of silty mud and free-floating sand grains. Internally, the bed exhibits normal grading of bone and calcareous clasts, poor sediment sorting, and preferred orientation of elongate elements, all characteristics common to debris flow deposits. The mud-rich matrix, poor sorting, and graded clasts of the bonebed suggest the assemblage was entrained and deposited by a cohesive debris flow, perhaps initiated through entrainment of fine overbank sediment by a seasonal flood. Nearly complete skeletal disarticulation and weathering of some bones indicate a brief period of postmortem exposure prior to debris flow entrainment. Fracture styles suggesting fresh breaks and frequent abrasion may reflect pre-flow trampling or chaotic flow transport. A significant number of elements also exhibit wet rot. The uniformity in taphonomic effects among elements suggests a mass mortality event, a rarity for debris-flow-hosted bonebeds, though the specific cause of death is uncertain. The age class dominance is interpreted to reflect original paleobiology, rather than abiotic postmortem selection, and establishes the Sun River Bonebed as the first bonebed of predominantly late juvenile material, with no adult material, in the Two Medicine Formation.
May 18, 1980, Mount St. Helens deposits in South Coldwater Creek, Washington
Beginning at 0832 Pacific Daylight Time (PDT), Mount St. Helens, Washington, produced a rockslide debris avalanche and a lateral explosion (blast) that generated a laterally directed pyroclastic surge, called here a blast surge. The resulting deposits are thickest in the east-west trending valley of North Fork Toutle River immediately north of the volcano but are almost as thick in South Coldwater Creek, a small tributary of the Toutle River, in the lee of an east-west trending ridge (Johnston Ridge) just north of the North Fork Toutle River. The avalanche surmounted Johnston Ridge (300 m relief) and flowed down the valley of South Coldwater Creek. Closely following the avalanche and in part contemporaneously with it, the blast surge left thin (up to ∼0.5 m) deposits on Johnston Ridge and thick (up to ∼ 15 m) deposits in South Coldwater Creek. These deposits have a lower layer (layer A1) which is friable, massive, poorer in fines (particles < 1/16 mm), and more poorly sorted than the upper layer (layer A2). Layer A2 is less friable because it contains more silt- and clay-sized material. Beneath layer A1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface. This zone, designated as layer A0, is a poorly mixed layer containing schlieren of soil, shattered volcanic fragments, and considerable quantities of organic material. Layer A1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge; layer A2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head; layer A0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge. It is concluded that a strongly fines-depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high-velocity particulate density current.