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.
Volcanic Influence over Fluvial Sedimentation in the Cretaceous Mcdermott Member, Animas Formation, Southwestern Colorado
Volcanic processes during and after an eruption can
impact adjacent fluvial systems by high influx rates of
volcaniclastic sediment, drainage disruption, formation and failure
of natural dams, changes in channel geometry and changes in channel
pattern. Depending on the magnitude and frequency of disruptive
events, the fluvial system might “recover” over a period of years
or might change to some other morphology. The goal of this study is
to evaluate the preservation potential of volcanic features in the
fluvial environment and assess fluvial system recovery in a
probable ancient analog of a fluvial-volcanic system. The McDermott
Member is the lower member of the Late Cretaceous - Tertiary Animas
Formation in SW Colorado. Field studies were based on a
southwest-northeast transect of six measured sections near Durango,
Colorado. In the field, 13 lithofacies have been identified
including various types of sandstones, conglomerates, and mudrocks
interbedded with lahars, mildly reworked tuff, and primary
pyroclastic units. Subsequent microfacies analysis suggests the
lahar lithofacies can be subdivided into three types based on clast
composition and matrix color, this might indicate different
volcanic sources or sequential changes in the volcanic center. In
addition, microfacies analysis of the primary pyroclastic units
suggests both surge and block-and-ash types are present. Several
trends can be noted: (1) there is an overall fining-upward trend
seen throughout the McDermott Member in a transition from lahars
and fluvial conglomerates at the base to isolated sandstone-rich
channels and extensive mudrock deposits near the top of the unit,
(2) there is a lateral trend change in the grain size and thickness
of lahars, and the thickness of well-developed fluvial deposits,
and (3) there is evidence of drainage disruption following the
deposition of lahar deposits by the deposition of fine-grained
silt/mudstones and sandstones which may indicate the creation of
natural dams. Stratigraphic relationships and paleoflow directions
suggest lahars and other volcanic sediments were transported SSE
from the La Plata Mountains through one or more paleovalleys. In
comparing modern and ancient volcanically-influenced environments,
it is evident the thin sheets of volcanic ash which dominate modern
environments have low preservation potential. Rather, the volcanic
signature in the geologic record consists of: (1) lahars
interbedded with fluvial deposits, (2) volcaniclastic sediment such
as reworked tuff or tuffaceous sandstone, and (3) paleocurrent and
provenance data.
Characteristics of dune-bedded pyroclastic surge bedsets
Abstract The grainsize characteristics of dune-bedded pyroclastic surge bedsets are surveyed. The variance between coarsest and finest beds ranges from 1 to 6 phi in different surge bedsets, and it increases as the grainsize of the coarsest bed increases, reflecting an increasing velocity of emplacement. Deposits of wet surges, identified as those which contain accretionary and ash-coated lapilli, tend to be finer and show less variance, this partly because wet ash is cohesive, but mainly because wet surges tend to be weaker. Dry surge bedsets are strongly fines-depleted, wet ones less so. The lack of erosion of underlying ash layers shows that the environment is a strongly depositional one. Individual bedsets are demarcated by thin intervening fine ash-fall layers, which are the complementary ash-cloud deposits settled or flushed out after the passage and decay of each turbulent surge. Surge deposits are generally less coarse than the coarsest associated airfall deposits, which shows that they are formed by generally weaker events. This study helps interpret the dune-bedded parts of the landscape-mantling May 18th 1980 “blast” deposit of Mount St. Helens. The blast was a very violent event, but the variance and the grainsize of the coarsest bed are those of a relatively weak surge. This suggests that the dune-bedding was produced by a weak effect, such as minor turbulence in a thin pyroclastic flow coming to rest in a mountainous terrain roughened by tree stumps and fallen logs.