Incursion of a large-volume, spatter-bearing pyroclastic density current into a caldera lake: Pavey Ark ignimbrite, Scafell caldera, England

Abstract

The Scafell caldera-lake volcaniclastic succession is exceptionally well exposed. At the eastern margin of the caldera, a large andesitic explosive eruption (>5 km 3 ) generated a high-mass-flux pyroclastic density current that flowed into the caldera lake for several hours and deposited the extensive Pavey Ark ignimbrite. The ignimbrite comprises a thick (≤125 m), proximal, spatter-and scoria-rich breccia that grades laterally and upwards into massive lapillituff, which, in turn, is gradationally overlain by massive and normal-graded tuff showing evidence of soft-state disruption. The subaqueous pyroclastic current carried juvenile clasts ranging from fine ash to metre-scale blocks and from dense andesite through variably vesicular scoria to pumice (<10 3 kg m −3 ). Extreme ignimbrite lithofacies diversity resulted via particle segregation and selective deposition from the current. The lacustrine proximal ignimbrite breccia mainly comprises clast-to matrix-supported blocks and lapilli of vesicular andesite, but includes several layers rich in spatter (≤1.7 m diameter) that was emplaced in a ductile, hot state. In proximal locations, rapid deposition of the large and dense clasts caused displacement of interstitial fluid with elutriation of low-density lapilli and ash upwards, so that these particles were retained in the current and thus overpassed to medial and distal reaches. Medially, the lithofacies architecture records partial blocking, channelling and reflection of the depletive current by substantial basinfloor topography that included a lava dome and developing fault scarps. Diffuse layers reflect surging of the sustained current, and the overall normal grading reflects gradually waning flow with, finally, a transition to suspension sedimentation from an ash-choked water column. Fine to extremely fine tuff overlying the ignimbrite forms ∼25% of the whole and is the water-settled equivalent of co-ignimbrite ash; its great thickness (≤55 m) formed because the suspended ash was trapped within an enclosed basin and could not drift away. The ignimbrite architecture records widespread caldera subsidence during the eruption, involving volcanotectonic faulting of the lake floor. The eruption was partly driven by explosive disruption of a groundwaterhydrothermal system adjacent to the magma reservoir.

Eruptive history of the Barombi Mbo Maar, Cameroon Volcanic Line, Central Africa: Constraints from volcanic facies analysis

“…Also, rapid deposition from dense suspension in the granular-fluid-based PDC with subsequent tractional transport would have been possible [10,11,41]. Such stratification was found in scoria cones in Western Mexico and Central Libya [24], and was interpreted as resulting from repeated explosions driven by magmatic degassing in an open conduit, because of the scoriaceous nature of the beds.…”

Mecanismos eruptivos y procesos depositacionales del Grupo Choiyoi en el área de Las Caletas, Cordillera Frontal de San Juan, Argentina

“…Por otra parte, el desarrollo de estratificación difusa sería el resultado de pequeñas inestabilidades en las corrientes y la depositación a partir de zonas límite de flujo transicionales entre aquellas dominadas por el escape de fluidos y aquellas dominadas por flujo granular ('fluid escape-to granular flow-dominated flow-boundary zone'; e.g., Brown et al, 2007). El desarrollo espontáneo de tasas de depositación levemente más bajas que aquellas concebidas para la depositación de facies macizas habrían favorecido la interacción entre granos y así permitido la segregación por tamaños, dando lugar a una incipiente estructuración en los depósitos (e.g., Kokelaar et al, 2007). En alguna proporción, los bloques sobredimensionados mayores muestran evidencias de haber sido emplazados por caída directa, proceso que habría operado de manera simultánea con las corrientes de densidad.…”

“…Debe tenerse en cuenta que no se han reconocido litofacies subácueas de carácter diagnóstico, por lo que no puede descartarse esta última posibilidad. Las deducciones realizadas no se oponen a las interpretaciones anteriores, ya que la terminología empleada es aplicable en corrientes soportadas por agua, gases o mezclas entre ellos (e.g., Kokelaar et al, 2007). Por otra parte, la presencia de bloques depositados por caída, sumada a otras características texturales y composicionales, indican el emplazamiento sineruptivo de las unidades, de manera tal que pueden ser concebidos como productos depositados directamente por erupciones explosivas ('eruption-fed pyroclastic turbidity currents' de White, 2000).…”

Tectonostratigraphic analysis of the Late Triassic-Early Jurassic syn-rift sequence of the Neuquen Basin in the Sanico depocentre, Neuquen Province, Argentina

“…5B) may be interpreted as a coarse facies of ignimbrite, deposited through the lower flow boundary of a PDC in which turbulence and traction were suppressed (Branney and Kokelaar, 2002). The absence of tractional structures or diffusive stratification with linear fabric indicates rapid progressive aggradation from the fluid escape-dominated to grain flow-dominated depositional flow boundary of a PDC (Branney and Kokelaar, 2002;Kokelaar et al, 2007). The massive lapilli-tuff facies (mLT; Fig.…”

“…The diffuse-stratified lapilli-tuff facies (dsLT; Fig. 5E) is deposited when flow-boundary zone conditions are intermediate between a fluid escape-dominated and a traction-dominated flow-boundary zone, causing the granular flow-dominated boundary of a PDC (Branney and Kokelaar, 2002;Kokelaar et al, 2007). Stratified and cross bedded tuff facies (//sT, xsT; Fig.…”