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 Bale Verte P eninsula, Newfoundland contains intervals of epiclastic mafic volcanic debr ites characterized by textures indicative of gravity flow evolution in a su bmarine setting. These proximal to distal and aggradational deposits record a downslope change from debris avalanche to cohesionless debris flow proce sses. This change involves the mechanical formation of small cobble to gran ular matrix due to apparent crushing and grinding of large, angular basalt clasts during downslope movement. This flow evolution has been studied in t wo measured sections in the Upper Debrite Member of the Balsum Bud Cove For mation 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 (approxim ate to 20-30 m above Haggis Point), contains mostly channelized basalt-clas t debris flow deposits which are chaotic or crudely inversely graded and am algamated with the underlying flow units. Some inversely graded, cohesionle ss, debris flows observed at Devil's Splitting Table contain basaltic clast s, 20-100 cm in diameter, supported at or near their tops. In an effort to better understand the how dynamics of the debrites observed at Haggis Point and Devil's Splitting Table, individual flow units in both sections were a nalysed 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. Sec. Civ. E ng., 104 (HYB 8), 1153-1159) and Takahashi (1981) (Debris flow, Ann. Rev. F luid Mech., 13, 57-77), which are applicable to both to subaerial and subma rine debris flows, flow velocities ranging from 6.22 to 7.78 m/s were calcu lated for proto- and cohesionless debris flows at Haggis Point. Initiation slopes range from 12.3 to 16.7 degrees. An avalanche deposit at Haggis Poin t attained a velocity of 28.87 mis. Cohesionless debris flows at Devils Spl itting Table had a range of flow velocities of 5.86-12.32 m/s at calculated slopes of 4.8-18.8 degrees. With the exception of the avalanche velocity t hese examples are consistent with velocities observed or calculated for oth er high slope submarine debris flow deposits. The calculated initiation slo pes and velocities indicate 5-11 km of potential avalanche /debris flow run -out in the Upper Debrite. The epiclastic volcanic apron observed in the Up per Debrite outcrops along Snooks Arm is progradational. It consists of a p roximal apron dominated by avalanche and proto-debris flow deposits and a d istal apron dominated by highly channelized cohesionless debris flows and s andy turbidites. (C) 2000 Elsevier Science B.V. All rights reserved.