Epiclastic volcanic debrites-evidence of flow transformations between avalanche and debris flow processes, Middle Ordovician, Baie Verte Peninsula, Newfoundland, Canada

The Balsum Bud Cove Formation of the Snooks Arm Group, eastern Baie Verte Peninsula, Newfoundland contains intervals of epiclastic mafic volcanic debrites characterized by textures indicative of gravity flow evolution in a submarine setting. These proximal to distal and aggradational deposits record a downslope change from debris avalanche to cohesionless debris flow processes. This change involves the mechanical formation of small cobble to granular matrix due to apparent crushing and grinding of large, angular basalt clasts during downslope movement. This flow evolution has been studied in two measured sections in the Upper Debrite Member of the Balsum Bud Cove Formation along Snooks Arm. Avalanche and proto-debris flow deposits have been studied in the stratigraphically lower and more proximal section at Haggis Point. An upper and more distal section, Devil’s Splitting Table (:20–30 m above Haggis Point), contains mostly channelized basalt-clast debris flow deposits which are chaotic or crudely inversely graded and amalgamated with the underlying flow units. Some inversely graded, cohesionless, debris flows observed at Devil’s Splitting Table contain basaltic clasts, 20–100 cm in diameter, supported at or near their tops. In an effort to better understand the flow dynamics of the debrites observed at Haggis Point and Devil’s Splitting Table, individual flow units in both sections were analysed to determine original flow thickness at initiation, slope angle at flow initiation, and flow velocity. Using equations from Takahashi (1978) (Mechanical characteristics of debris flow, J. Hydraul. Div. Am. Soc. Civ. Eng., 104 (HYB 8), 1153–1159) and Takahashi (1981) (Debris flow, Ann. Rev. Fluid Mech., 13, 57–77), which are applicable to both to subaerial and submarine debris flows, flow velocities ranging from 6.22 to 7.78 m/s were calculated for protoand cohesionless debris flows at Haggis Point. Initiation slopes range from 12.3 to 16.7°. An avalanche deposit at Haggis Point attained a velocity of 28.87 m/s. Cohesionless debris flows at Devils Splitting Table had a range of flow velocities of 5.86–12.32 m/s at calculated slopes of 4.8–18.8°. With the exception of the avalanche velocity these examples are consistent with velocities observed or calculated for other high slope submarine debris flow deposits. The calculated initiation slopes and velocities indicate 5-11 km of potential avalanche /debris flow run-out in the Upper Debrite. The epiclastic volcanic apron observed in the Upper Debrite outcrops along Snooks Arm is progradational. It consists of a proximal apron dominated by avalanche and proto-debris flow deposits and a distal apron dominated by highly channelized cohesionless debris flows and sandy turbidites. © 2000 Elsevier Science B.V. All rights reserved. www.elsevier.com/locate/precamres * Corresponding author. 0301-9268/00/$ see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0301 -9268 (99 )00086 -8 L.G. Kessler, J.H. Bédard / Precambrian Research 101 (2000) 135–161 136