Abstract

Basalt talus breccias and debris-flow conglomerates are interbedded with terrestrial redbeds in the Lower Jurassic McCoy Brook Formation in the synrift Fundy basin. The redbeds formed in a semiarid to arid climate in alluvial-fan, eolian, fluvial, playa, and lacustrine environments. The basalt talus breccias formed adjacent to cliffs of North Mountain Basalt produced by synsedimentary faulting on the northern side of the basin. The breccias lack bedding planes, contain basalt boulders up to 8 m in length, and are interstratified with fluvial, playa, eolian, and lacustrine sandstones and mudstones. At Wasson Bluff, 24 m of talus breccia with a matrix of sandy mud comprise a sequence that is matrix-supported at the base and rapidly grades upward to the typical clast-supported fabric. The matrix of clast-supported breccia locally contains horizontal to inclined sandy gravel layers and, in a few places, abundant bones of small reptiles that lived within the talus, indicating that matrix infiltrated open spaces around the boulders. A slope of 34 degrees to the southeast of the paleosurface of the talus is inferred from the orientations of the lower surfaces of discoidal, and plate-shaped boulders. The basalt debris-flow conglomerates total 21 m in thickness and comprise 51 beds. Some beds are separated by centimeter-scale sandstone layers deposited by streamflows. The conglomerates have a sandy-mud matrix and a fabric that varies from matrix-supported to clast-supported. The mean orientation of discoidal clasts is subhorizontal, with substantial scatter of individual clasts. The base of each flow is non-erosional, and many boulder project above flow surfaces. We attribute the debris flows to remobilization of debris during heavy rainstorms in the semiarid to arid climate.

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